氧化鋅壓電層對鎳鐵薄膜磁性調控之研究

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(1)國立高雄大學應用物理研究系碩士班碩士論文. 氧化鋅壓電層對鎳鐵薄膜磁性調控之研究 Voltage-control of magnetic properties of NiFe films through a ZnO layer. 研究生：柯辛樺指導教授：余進忠. 撰博士. 中華民國一百零四年七月.

(2) 致謝兩年的碩士生涯終於要告一段落，首先我要感謝的是我碩士班的指導教授，余進忠老師，老師為人與學生相處總是很親切，有問題時常常會不厭其煩地陪你一起解決，在這兩年的求學生涯，老師真的給予學生很大的幫助，還有要感謝我的口試委員，胡裕民老師與駱芳鈺老師，謝謝老師們在口試時的指教與碩論上的指導。再來我要謝謝我的同學與學弟妹們，謝謝 518 實驗室的各位，實驗上煩悶時跟你們一起聊聊天總是能我笑得很開心，謝謝阿繆、舒婷、乃乃與的阿宏，在薄膜製備上的幫忙還有實驗與生活上的討論與協助，謝謝政遠與他實驗室學弟妹的 XRD 量測協助，感謝新諺學弟在成大幫忙量測 SEM，還有感謝屏東大學賴老師實驗室的 EDS 量測協助，謝謝你們的幫忙。最後我要感謝我的家人，若沒有你們的支持與鼓勵，我很難可以沒後顧之憂地走完這兩年，謝謝你們，這兩年雖然有辛苦但也有許多的歡笑，再辛苦撐過去就是你的了，謝謝這兩年給予過我幫助的朋友們，謝謝大家！. I.

(3) 氧化鋅壓電層對鎳鐵薄膜磁性調控之研究指導教授：余進忠博士學生：柯辛樺國立高雄大學應用物理學系碩士班. 摘要. 在此研究中，我們利用射頻磁控濺鍍系統(RF magnetron sputtering system)來製備氧化鋅與鎳鐵薄膜在 n-type 導電矽基板 Si(100)上，形成 Ni80Fe20/ZnO/n-typeSi(100)的複合膜結構。利用此結構量測壓電力顯微鏡與磁光柯爾效應來觀察薄膜的壓電性質與磁性質，發現施加一電壓差於氧化鋅薄膜，會使薄膜因為電場產生逆壓電效應來形變，交互作用下引致鎳鐵薄膜層的磁特性產生變化。藉由一連串的磁光柯爾效應量測可得到，鎳鐵薄膜層的柯爾訊號強度與矯頑場 Hc 均會隨著施加電場的提升而下降，故鎳鐵的磁特性可藉由電場來控制。. 關鍵字：氧化鋅,鎳鐵,壓電力顯微鏡,磁光柯爾效應 II.

(4) Voltage-control of magnetic properties of NiFe films through a ZnO layer Advisor: Dr.Chin-Chung Yu Student: Hsin-Hua Ko Institute of Department of Applied Physics National University of Kaohsiung. ABSTRACT. In this study, we prepared ZnO and Ni80Fe20 films on highly doped n-type Si(100) using RF magnetron sputtering system. We obtained Ni80Fe20/ZnO/n-typeSi(100) multilayer structures. We observed the piezoelectric properties and magnetic properties of samples using piezoresponse force microscopy and mangeto optical Kerr effect. We applied a voltage on ZnO films. The films would strain by inverse piezoelectric effect. Then, the effect could change magnetic properties of Ni80Fe20 films. The MOKE measurement was executed, while the applied voltage was increased from 0 to 10V. We discovered the Kerr intensity and magnetic coercivity would decrease as applied voltage was increased. So we could control magnetic properties of Ni80Fe20 by electric field.. Key words: ZnO, Ni80Fe20, PFM, MOKE III.

(5) Table of Contents Table of Contents .........................................................................................................IV List of figures ...............................................................................................................VI List of tables .................................................................................................................IX Chapter 1 Motivation ..................................................................................................... 1 Chapter 2 Literature Review .......................................................................................... 3 2-1 Ferroelectric and Piezoelectric materials ................................................. 3 2-1-1 basic properity .............................................................................. 3 2-1-2 Piezoelectric properties of ZnO ................................................... 9 2-2 Permalloy and Magnetic materials ........................................................ 12 2-2-1 Magnetic materials ..................................................................... 12 2-2-2 Basic properties of permalloy .................................................... 14 2-3 Measured MOKE by applying voltage .................................................. 17 Chapter 3 Experimental Equipment ............................................................................. 20 3-1 Atomic Force Microscope ..................................................................... 20 3-1-1 Principle of AFM [12][13] ............................................................. 21 3-2 Piezoresponse Force Microscopy .......................................................... 23 3-2-1 Vertical piezoresponse force microscopy .................................. 24 3-2-2 Hysteresis loop [20]...................................................................... 27 3-2-3 Piezoelectric coefficient ............................................................. 28 3-3 X-ray diffraction system ........................................................................ 30 3-4 Mangeto Optical Kerr Effect ................................................................. 32 3-5 RF magnetron sputtering system ........................................................... 36 Chapter 4 Results and Discussion ............................................................................. 38 4-1 Sample Preparation ................................................................................ 38 4-2 Piezoresponse force microscopy analysis of ZnO ................................. 44 4-2-1 ZnO PFM analysis...................................................................... 45 4-2-2 ZnO piezoelectric coefficient analysis ....................................... 48 4-3 MOKE analysis ..................................................................................... 51 4-3-2 10nm thick Ni80Fe20 MOKE analysis ......................................... 59 IV.

(6) 4-3-3 50nm thick Ni80Fe20 MOKE analysis ......................................... 66 Chapter 5 Conclusion ................................................................................................ 72 Appendixes .................................................................................................................. 74 Reference ..................................................................................................................... 76. V.

(7) List of figures Fig. 1 Various polarization [4] ......................................................................................... 4 Fig. 2 32 kinds of point group s ..................................................................................... 6 Fig. 3 Relationship of piezoelectricity, pyroelectricity and Ferroelectricity .................. 6 Fig. 4 piezoelectric effect ............................................................................................... 7 Fig. 5 inverse piezoelectric effec ................................................................................... 8 Fig. 6 X-ray diffraction pattern of ZnO/Zn/glass [6] ....................................................... 9 Fig. 7 (a) Topography (b) OOP (c)(d) IP piezoresponse images (e) Distribution of ‘positive’ and ‘negative’ ZnO grains reconstructed by comparison of OOP and IP signals (f) Cross sections of IP and OPP images. [6] .................................................... 10 Fig. 8 table of ZnO piezoelectric coefficient [6] ........................................................... 10 Fig. 9 ZnO piezoresponse hysteresis loop [6] ............................................................... 11 Fig. 10 Relationship of FCC structure permalloy magnetostriction constant λs at RT [8] ..................................................................................................................................... 15 Fig. 11 phase diagram of permalloy magnetostriction constant λs [8]........................... 15 Fig. 12 Relationship of saturated magnetization, curie temperature, magnetocrystalline anisotropy constant, magnetostriction constant and nickel content [8]......................... 16 Fig. 13 Normalized Kerr magnetic loops at room temperature measured at different applied electric fields [11] .............................................................................................. 17 Fig. 14 (a) MOKE hysteresis loops measured at RT, and various bias voltages.(b) Summarized Hc values plotted as a function of bias voltage. The solid lines are guides for the eye.[10] ............................................................................................................... 18 Fig. 15 The summarized Hc values were plotted as a function of bias voltage for as-deposited sample and the samples after applying 8V, 10V, and 12V to the Fe/ZnO junction for 10 min. The solid lines are guides for the eye.[10] .................................... 19 Fig. 16 Entity diagramof AFM [12] ............................................................................... 20 Fig. 17 AFM systems [14] .............................................................................................. 21 Fig. 18 Diagram of probe height to PSPD [14]. ............................................................. 22 Fig. 19 distance and force of probe and sample surface [14]. ........................................ 22 Fig. 20 Left is XE-100AFM and Bottom right is SR830 lock-in amplifier. ................ 23 Fig. 21 Strain behavior of ferroelectric materials (a) no voltage is applied (b) applying a positive voltage (c) applying a negative voltage.[17] ................................................. 24 Fig. 22 Form of outputting pulse DC voltage by measuring hysteresis loop [20]. ........ 27 Fig. 23 (a) piezoresponse signal image (b) amplitude image (c) phase image [21] ....... 28 Fig. 24 Diagram of Bragg’s law [15] ............................................................................. 30 VI.

(8) Fig. 25 X-ray diffraction system [15] ............................................................................. 31 Fig. 26 (a) change of reflected light in polarization direction (b) P-MOKE (c) L-MOKE (d) T-MOKE ................................................................................................ 33 Fig. 27 L-MOKE system .............................................................................................. 34 Fig. 28 MOKE signal ∝ Ex2 ......................................................................................... 34 Fig. 29 Left is L-MOKE system and right is P-MOKE system. .................................. 35 Fig. 30 Diagram of radio frequency magnetron sputtering system. ............................. 37 Fig. 31 Picture of radio frequency magnetron sputtering system. ............................... 37 Fig. 32 Simple flowchart of preparing ZnO thin film .................................................. 39 Fig. 33 Appearance of ZnO film .................................................................................. 39 Fig. 34 Simple flowchart of analyzing ZnO thin film .................................................. 40 Fig. 35 Simple flowchart of preparing Ni80Fe20 film ................................................... 41 Fig. 36 correction fluid on glass ................................................................................... 42 Fig. 37 measurement of Ni80Fe20 film thickness .......................................................... 42 Fig. 38 Simple flowchart of analyzing ZnO thin film .................................................. 43 Fig. 39 No. 20141227 XRD spectra ............................................................................. 44 Fig. 40 No. 20141227 AFM topography (4μm × 4μm) ............................................... 44 Fig. 41 Diagram of PFM .............................................................................................. 45 Fig. 42 ZnO PFM images of (a) topography (b) amplitude (c) phase. ......................... 46 Fig. 43 Measured hysteresis loop in red cross ............................................................. 47 Fig. 44 (a) phase image (b) amplitude image ............................................................... 47 Fig. 45 Piezoresponse of ZnO and quartz .................................................................... 48 Fig. 46 Chart of ZnO .................................................................................................... 49 Fig. 47 Diagram of MOKE .......................................................................................... 51 Fig. 48 agilent 33220A function generator .................................................................. 52 Fig. 49 Diagram of silver plastic and copper wires ..................................................... 52 Fig. 50 Forming Set of ZnO ......................................................................................... 53 Fig. 51 three times switching ....................................................................................... 53 Fig. 52 Diagram of ZnO P-MOKE .............................................................................. 54 Fig. 53 Diagram of ZnO L-MOKE .............................................................................. 54 Fig. 54 ZnO cross-section SEM ................................................................................... 55 Fig. 55 Topography of 5nm Ni80Fe20 ........................................................................... 56 Fig. 56 XRD spectra of 5nm Ni80Fe20 .......................................................................... 57 Fig. 57 5nm Ni80Fe20 L-MOKE ................................................................................... 57 Fig. 58 5nm Ni80Fe20 Kerr intensity of various voltage ............................................... 58 Fig. 59 Topography of 10nm Ni80Fe20 ......................................................................... 59 Fig. 60 XRD spectra of 10nm Ni80Fe20 ........................................................................ 59 Fig. 61 10nm Ni80Fe20 L-MOKE ................................................................................. 60 VII.

(9) Fig. 62 10nm Ni80Fe20 Hc in various applied voltage................................................... 61 Fig. 63 10nm Ni80Fe20 Kerr intensity of various voltage ............................................. 61 Fig. 64 10nm Ni80Fe20 L-MOKE in the negative voltage ............................................ 62 Fig. 65 10nm Ni80Fe20 Hc in various applied voltage ................................................... 63 Fig. 66 10nm Ni80Fe20 Kerr intensity of various voltage ............................................. 63 Fig. 67 10nm Ni80Fe20 P-MOKE .................................................................................. 64 Fig. 68 10nm Ni80Fe20 Hc in various applied voltage ................................................... 64 Fig. 69 Diagram of square wave signal ........................................................................ 65 Fig. 70 10nm Ni80Fe20 relative Kerr intensity record ................................................... 65 Fig. 71 Topography of 50nm Ni80Fe20 ......................................................................... 66 Fig. 72 XRD spectra of 10nm Ni80Fe20 ........................................................................ 66 Fig. 73 50nm Ni80Fe20 L-MOKE ................................................................................. 67 Fig. 74 50nm Ni80Fe20 Hc in various applied voltage................................................... 67 Fig. 75 50nm Ni80Fe20 Hc in various applied voltage ................................................... 68 Fig. 76 50nm Ni80Fe20 P-MOKE .................................................................................. 69 Fig. 77 Diagram of square wave signal ........................................................................ 70 Fig. 78 10nm Ni80Fe20 relative Kerr intensity record ................................................... 70 Fig. 79 List of magnetostriction constant ..................................................................... 71. VIII.

(10) List of tables Table 1 ZnO sample number and composition ............................................................. 39 Table 2 Ni80Fe20 sample number and composition ....................................................... 43 Table 3 Wt% of 50nm NiFe EDS ................................................................................. 71 Table 4 Wt% of 10nm NiFe EDS ................................................................................. 71 Table 5 List of AFM probe[23][24] .................................................................................. 74 Table 6 PDF card of ZnO ............................................................................................. 75 Table 7 PDF card of Si ................................................................................................. 75. IX.

(11) Chapter 1 Motivation The magnetic materials has been widely applied, such as magnetoresistive random access memory, magnetoresistive head of hard drives et al. About properties and mechanism of magnetic materials are worth to explore and research. In traditional technology, magnetic field was often used to control magnetic domain. And it can’t use magnetic field to let magnetic properties of magnetic materials be changed. In this study, we used Piezoresponse Force Microscope and Magneto-Optical Kerr Effect. [1]. to observe sample, and permalloy films with 80 at% of Ni (Ni80Fe20). were fabricated on ZnO(0001)/highly doped n-type Si(001). Then, we applied a voltage at Ni80Fe20 and n-type silicon, it would let ZnO generate a inverse piezoelectric effect. This effect might change magnetic properties of Ni80Fe20 further. Therefore it could let electric field control hysteresis behavior. Magnetoelectric effect, ME, refers coupling of magnetic and electric. It is the phenomenon of inducing magnetic (electric) polarization by applying an external electric (magnetic) field. The magnetoelectric effect was first conjectured by P. Curie [1]. [2] in 1894, while the term magnetoelectric was coined by P. Debye. And the. effect was observed for the first time in Cr2O3. [3] Generally, magnetoelectric effect is very small, and it is no value in application. Recent, composite materials and multilayers were discovered giant magneto electric effect. These composite materials are multilayers of ferromagnetic and ferroelectricity materials. Recently, it was studied widely by academia of physics. On the other hand, the materials have multiferroic properties so it could use eectric field to control direction of magnetization. For example, MRAM always need a current to memorize units. When density of device exceeded a certain standard, it would generate some problem of 1.

(12) thermal. If we can apply electric field to control magnetization, this problem will be solved.. 2.

(13) Chapter 2 Literature Review 2-1 Ferroelectric and Piezoelectric materials 2-1-1 basic properity (a) Polarization mechanism [4] The polarization is using applied electric field to arrange messy dipole moment to be ordered. Polarization mechanisms had four kinds of, there are atomic polarization, ionic polarization, dipolar polarization and space charge polarization, as shown in Fig. 1[4]. (1) atomic polarization Applying electric field on materials to make charged particles within atoms offset relatively, such as atomic nucleus and electron. Because this mechanism is related to electron, it also is called electronic polarization. (2) ionic polarization The ionic polarization appeared in crystals of having ionic bonding. Because cations and anions have certain bonding rules, cations and anions offset relatively to form electric dipole moments. The ion with charges are more than atom with charges so ionic polarization is stronger than atomic polarization. (3) dipolar polarization The materials of permanent electric dipole moments generated easily this polarization. These materials have messy Arrange electric dipole moments. When applying electric field, electric dipole moments reversed toward the electric field direction to arrange orderly. But intensities of electric dipole moments didn’t be affected by applied electric field.. 3.

(14) (4) space charge polarization When materials themselves had defects or different crystal interfaces, they would obstruct charge carriers to move. These charge carriers were bound to the different grains and generate space polarization phenomena because grain boundary and phase boundary had bounding charge carriers characteristics. Therefore it generated these polarization phenomena easily.. Fig. 1 Various polarization [4]. 4.

(15) (b) Ferroelectricity The nature of the materials could be sort to 32 kinds of point groups at present. They had 11 kinds of that didn’t have offset of anions and cations so they had centrosymmetric structure and didn’t have polarity. There were 21 kinds of non-centrosymmetric point groups. There were 20 kinds of piezoelectricity. There were 10 kinds of piezoelectricity which had characteristics of non-central symmetry and a single axis of rotation so they had spontaneous polarization phenomenon. The 32 kinds of point groups were showed in Fig. 2 . If materials had spontaneous polarization characteristics and direction and polarization of dipole moment would change with applied electric field, it could be called ferroelectric materials. When ambient temperature increased, the spontaneous polarization of ferroelectric materials would decrease. While spontaneous polarization was zero, we called this temperature to be Curie temperature. The ferroelectric materials had ferroelectricity, besides pyroelectricity and piezoelectricity. When ambient temperature changed, the spontaneous polarization of materials would also change which was called pyroelectricity. When materials were strained by mechanical stress, the spontaneous polarization of materials would also change which was called piezoelectricity. The relationship of them was showed in Fig. 3.. 5.

(16) Fig. 2 32 kinds of point group s. Fig. 3 Relationship of piezoelectricity, pyroelectricity and Ferroelectricity 6.

(17) (c) Piezoelectricity In 1880, Pierre Curie and Jacques Curie. [5]. discovered that some crystals were. strained by mechanical stress to generate charges. This phenomenon they called piezoelectric effect. Then, they also discovered that crystals were put in an electrical field and it would strain. This reverse mechanism they called inverse piezoelectric effect. In other words, these effects could transform between mechanical energy and electrical energy. (1) Piezoelectric effect： The piezoelectric effect is property of transforming mechanical energy into electrical energy. When applying mechanical stress on piezoelectric materials, the spontaneous dipole moment of materials would strain by external force. In order to resist force, surface of materials would produce bound charges to keep shape of materials.. Fig. 4 piezoelectric effect 7.

(18) (2) Inverse piezoelectric effect： The inverse piezoelectric effect is property of transforming electrical energy into mechanical energy. If we applied a electric field in the same direction with bound charges, the polarization of internal electric dipole would be strong. Then piezoelectric materials would elongate. Conversely, if we applied a electric field in the reverse direction with bound charges, the polarization of internal electric dipole would be reduced. Then, piezoelectric materials would shorten. The deformation of piezoelectric materials would change with direction and intensity of applied electric field, this effect we called inverse piezoelectric effect.. Fig. 5 inverse piezoelectric effec. 8.

(19) 2-1-2 Piezoelectric properties of ZnO In 2010, I. K. Bdikin et al.. [6]. prepared Zn and ZnO film on glass substrate by. using pulsed-laser deposition, and Zn as an electrode. It formed a film structure of ZnO/Zn/glass. Author measured piezoresponse images, piezoelectric coefficients and piezoresponse hysteresis loop by using PFM measurements (Fig. 9)。 Fig. 6 shows the representative x-ray diffraction pattern of a ZnO thin film, where only two strong peaks are observed in the 2θ range between 30∘ and 40∘. The strongest peak consists of two superimposed peaks observed at 2θ=36.322∘ and 36.762∘. They can be attributed to the (0002) plane of metallic Zn and the (1011) plane of hexagonal ZnO, respectively. The second one, less intense, is observed at 2θ=34.38∘and can be attributed to the (0002) plane of the hexagonal ZnO.. Fig. 6 X-ray diffraction pattern of ZnO/Zn/glass [6] 9.

(20) Fig. 7 (a) Topography (b) OOP (c)(d) IP piezoresponse images (e) Distribution of ‘positive’ and ‘negative’ ZnO grains reconstructed by comparison of OOP and IP signals (f) Cross sections of IP and OPP images.. [6]. Then, author measured longitudinal and transverse PFM, he obtained three-dimensional piezoresponse images (Fig. 7) and Summed up distribution of different ZnO grains reconstructed. In order to obtaine piezoelectric coefficients, author used known piezoelectric coefficient calibration of LiNbO3 single crystals.[7] And he calculated for each piezoelectric coefficient of lattice planes. Fig. 8。. Fig. 8 table of ZnO piezoelectric coefficient [6]. 10.

(21) Fig. 9 ZnO piezoresponse hysteresis loop [6]. 11.

(22) 2-2 Permalloy and Magnetic materials 2-2-1 Magnetic materials The materials could be magnetized in magnetic field (H), we called magnetic materials. We usually described materials of magnetized level as magnetization intensity. We also called magnetization (M), it was defined as the magnetic moment of material per unit volume. The relationship between applied magnetic field and magnetization were showed as magnetic susceptibility, χ=M/H. If magnetization was higher, it meant that materials would be magnetized easily. The source of magnetism was caused by atom within and electron motion. We could sort materials by magnetic susceptibility as paramagnetism, diamagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. (a) paramagnetism In a paramagnetic material there are unpaired electrons, i.e. atomic or molecular orbital with exactly one electron in them. While paired electrons are required by the Pauli Exclusion Principle to have their intrinsic “spin” magnetic moments pointing in opposite directions, causing their magnetic fields to cancel out, an unpaired electron is free to align its magnetic moment in any direction. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, thus reinforcing it. (b) diamagnetism Diamagnetism appears in all materials, and is the tendency of a material to oppose an applied magnetic field, and therefore, to be repelled by a magnetic field. However, in a material with paramagnetic properties (that is, with a tendency to enhance an external magnetic field), the paramagnetic behavior dominates. Thus, despite its universal occurrence, diamagnetic behavior is observed only in a purely diamagnetic 12.

(23) material. In a diamagnetic material, there are no unpaired electrons, so the intrinsic electron magnetic moments cannot produce any bulk effect. In these cases, the magnetization arises from the electrons' orbital motions. (c) ferromagnetism A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons' intrinsic magnetic moment's tendency to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered-energy state. Thus, even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to one another. Every ferromagnetic substance has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order. Ferromagnetism only occurs in a few substances; the common ones are iron, nickel, cobalt, their alloys, and some alloys of rare earth metals. (c) antiferromagnetism In an antiferromagnet, unlike a ferromagnet, there is a tendency for the intrinsic magnetic moments of neighboring valence electrons to point in opposite directions. When all atoms are arranged in a substance so that each neighbor is 'anti-aligned', the substance is antiferromagnetic. Antiferromagnets have a zero net magnetic moment, meaning no field is produced by them. Antiferromagnets are less common compared to the other types of behaviors, and are mostly observed at low temperatures. In varying temperatures, antiferromagnets can be seen to exhibit diamagnetic and ferrimagnetic properties. 13.

(24) In some materials, neighboring electrons want to point in opposite directions, but there is no geometrical arrangement in which each pair of neighbors is anti-aligned. This is called a spin glass, and is an example of geometrical frustration. (d) ferrimagnetism Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins like to point in opposite directions. These two properties are not contradictory, because in the optimal geometrical arrangement, there is more magnetic moment from the sublattice of electrons that point in one direction, than from the sublattice that point in the opposite direction. Most ferrites are ferrimagnetic. The first discovered magnetic substance, magnetite, is a ferrite and was originally believed to be a ferromagnet; Louis Neel disproved this, however, after discovering ferrimagnetism.. 2-2-2 Basic properties of permalloy In this study, we chose Ni80Fe20 as ferromagnetic materials, it belongs to one kind of permalloy and magnetic permeability is higher. Permalloy is 35~82% ferronickel alloy. To control ferronickel component proportion and we could change anisotropy constant ( K1) and magnetostriction constant (λs) , as shown in Fig. 10[8]. The crystal structure was showed in Fig. 11. Most of the proportion is fcc structure. We usually could sort permalloy to three kinds of nickel content, 50%, 65% and 78% [9]. (1) The alloy of 50% nickel, it would have stronger magnetic flux density ( Bs=1.6T ) and square loop after annealing.. 14.

(25) (2) The alloy of 65% nickel, it would make magnetic anisotropy constant be zero after annealing. (3) The alloy of 78% nicke, its magnetostriction constant was zero and it had good magnetic permeability.. Fig. 10 Relationship of FCC structure permalloy magnetostriction constant λs at RT [8]. Fig. 11 phase diagram of permalloy magnetostriction constant λs [8]. 15.

(26) Usually affecting the factor of magnetic anisotropy constant are lattice structure, mechanical force and heat treatment. In order to reduce affection, we could know when nickel content is 45%～95%, the Curie temperature could reach more than 400 ℃. with observing relationship of anisotropy constant, Curie temperature,. magnetostriction constant and nickel content in Fig. 12 . About 75%, magnetocrystalline anisotropy constant (Ku) is zero ; about 35%～45% and 80%, magnetostriction constant is zero so we obtain ideal permalloy in 80% nickel content. Because magnetostriction constant is zero and anisotropy constant is low, it has good coercivity and magnetic properties.. Fig. 12 Relationship of saturated magnetization, curie temperature, magnetocrystalline anisotropy constant, magnetostriction constant and nickel content [8] 16.

(27) 2-3 Measured MOKE by applying voltage In 2007, Sarbeswar Sahoo et al. [11] prepared 10nm thick Fe film on BaTiO3(100) substrate using molecular beam epitaxy. Then, author applied a electric field at 0.5mm thick BTO substrate and measured MOKE at Fe film. The electric field was changed from E=−10 kV/cm up to E=10 kV/cm in ascending steps and then descending back to E=−10 kV/cm along the arrows. We could observe coercivity Hc changing along with electric field.（Fig. 13）。. Fig. 13 Normalized Kerr magnetic loops at room temperature measured at different applied electric fields [11]. 17.

(28) In 2014, Wen-Chin Lin et al.[10] prepared multilayer structures of Au/Fe/ZnO/Au on Al2O3(0001) substrates to detect how voltage affected magnetism. The top and bottom Au layers were used as the electrodes when applying the voltage. The Au over-layer protects the Fe film from oxidation and contamination. The MOKE measurement was executed at RT using a magnetic field along the in-plane direction, while the applied bias voltage was gradually increased from 0 to 6 V. As shown in Fig. 14, the magnetic coercivity (Hc) of the MOKE hysteresis loops decreased as the bias voltage increased. The Hc exhibited the same variation, whether the applied voltage was positive or negative, indicating that the Hc reduction is not related to the electric field direction. And it had a reversible behavior.. Fig. 14 (a) MOKE hysteresis loops measured at RT, and various bias voltages.(b) Summarized Hc values plotted as a function of bias voltage. The solid lines are guides for the eye. [10] 18.

(29) The Hc of Fe/ZnO heterostructure was significantly enhanced by 2–3 times after applying a suitable direct heating current. This Hc enhancement is irreversible and originates from the Fe-oxidation at the Fe/ZnO interface induced by direct current heating while the bias voltage is applied. Depth-profiling XPS analysis confirmed the formation of FeO, Fe3O4, and Fe2O3 close to the interface region, depending on the Fe thickness and annealing process. To investigate whether interface conditions changed how voltage reversibly affected on Hc, author applied large voltages of 10 and 12V to generate more Fe-oxide at the interface. Fig. 15 shows that after applying 10V for 10 min, the Hc value irreversibly increased from 112Oe to 147Oe (measured when V=0). After the 10 V-annealing, not only the Hc was enhanced but also the resistance of Fe/ZnO junction was decreased.. Fig. 15 The summarized Hc values were plotted as a function of bias voltage for as-deposited sample and the samples after applying 8V, 10V, and 12V to the Fe/ZnO junction for 10 min. The solid lines are guides for the eye.[10] 19.

(30) Chapter 3 Experimental Equipment 3-1 Atomic Force Microscope In this study, our AFM equipment is Park System XE-100, and cantilever used in Table 5. The force constant, resonance frequency and tip radius are noted in different mode when measuring samples. Atomic Force Microscope is composed of piezoelectric scanner that has three axes perpendicularly to each other, position sensitive photodector, feedback system and external probe.. Fig. 16 Entity diagramof AFM [12]. 20.

(31) 3-1-1 Principle of AFM [12][13] Atomic force microscope is composed of piezoelectric scanner that has three axes perpendicularly to each other, position sensitive photodector, feedback system and external probe. As shown in Fig. 17. The basic principle of AFM is using laser to irradiate on cantilever, and when probe moved on a sample, cantilever would bend along with height of sample surface. And bending variation of cantilever would let laser shift on position sensitive photodector, as shown in Fig. 18. The position sensitive photodector generated signals by shifting, and then transferred to feedback system generating feedback signals. Finally, using feedback signals to control movement of piezoelectric scanner and image with controlling distance of probe and sample.. Fig. 17 AFM systems [14]. 21.

(32) Fig. 18 Diagram of probe height to PSPD [14].. We could know relationship of force and distance of probe and sample surface in Fig. 19. According to distance of probe and sample surface and using force between atom and atom, there are three kinds of operating modes. There are contact mode, non-contact mode and tapping mode, respectively. When using contact mode to scan samples, it is several Å in length of probe and sample surface. The force of probe and sample is repulsive force between atoms. Because van der Waals force is short-range force, it can touch sample surface effectively. The probe touched sample surface closely so contact mode would obtain atomic-level resolution easily.. Fig. 19 distance and force of probe and sample surface [14]. 22.

(33) 3-2 Piezoresponse Force Microscopy In 1991, H. Birk,J. Glatz-Reichenbach et al. used this technique to measure ferroelectric polymers firstly. [16] At the time they used scanning tunneling microscopy with lock-in amplifier to measure it then using Au film and Al film as electrodes. They applied AC voltage signal of 20Hz and 10V on electrodes. It made sample strain and shock, and measured deformation of surface by probe, and obtained piezoresponse hysteresis loop by lock-in amplifier. The principle of piezoresponse force microscopy built on inverse piezoelectric effect. It is composed of atomic force microscope and lock-in amplifier. While measuring samples, probe is as upper electrode and detecting deformation of the sample surface to obtain piezoresponse signal by lock-in amplifier. According to different polarization directions of samples, it can be differentiate as vertical and lateral signal of piezoresponse force microscopy. In this study, we only used vertical piezoresponse force microscopy.. Fig. 20 Left is XE-100AFM and Bottom right is SR830 lock-in amplifier. 23.

(34) 3-2-1 Vertical piezoresponse force microscopy When we didn’t apply DC voltage, it had two ferroelectric domains of opposite polarization in ferroelectric materials. At that time, two ferroelectric domains didn’t deform, as shown in Fig. 21(a). [17]. . We assumed that two ferroelectric domains of. ferroelectric materials are c+ ferroelectric domain and c- ferroelectric domain respectively. The c+ ferroelectric domain is ferroelectric domain of upward polarization and out of plane. The c- ferroelectric domain is ferroelectric domain of downward polarization and out of plane. When we applied positive voltage on two ferroelectric domains, c+ ferroelectric domain would shrink and c- ferroelectric domain would expand, as shown in Fig. 21(b)[17] . Conversely, we applied negative voltage on two ferroelectric domains, c+ ferroelectric domain would expand and cferroelectric domain would shrink, as shown in Fig. 21 (c)[17] .. Fig. 21 Strain behavior of ferroelectric materials (a) no voltage is applied (b) applying a positive voltage (c) applying a negative voltage.[17]. From the foregoing, we applied electric field to make ferroelectric materials strain and we could calculate its deformation. Deformation of sample showed following equation. ΔZ= -d＊V ．．．．．．．．．．．．．．．．．．．．．．．．．．．．(3-1) 24.

(35) Eq.:. Z is deformation of surface in vertical. d＊ is piezoelectric coefficient of ferroelectric materials. V is DC voltage. In equation (3-1) negative could show that deformation of c in vertical is for. what strain contributing. For example, when we applied positive voltage on ferroelectric materials, direction of electric field is opposite of polarization of c+ ferroelectric domain. We could obtain ΔZ is negative from equation and it could show that deformation in vertical is contributed by shrinking. Assuming a d33 of ferroelectric materials is 50pm/V. If we applied a 2V of DC voltage, we could measure deformation of 0.1nm in vertical. The roughness is approximately 10~102nm so small deformation would be covered by topography signal. Therefore using DC voltage is not suitable to measure samples of the ferroelectric materials with rough surfaces. So we need to use AC voltage with lock-in amplifier to obtain oscillation signal of sample surface. The equation of AC modulation signal isＶ＝Ｖ０cos(ωt + ϕ ) and deformation of sample surface showed following equation. [18] ΔZ=ΔZ0cos(ωt + ϕ )．．．．．．．．．．．．．．．．．．．．．．．．(3-2) Eq.:. ΔZ0=d33 V0 d33 is piezoelectric coefficient along electric field in vertical. V0 is amplitude signal of AC voltage. ϕ is phase of AC voltage signal and ferroelectric domain polarization. When phase ϕ = 0, directions of ferroelectric domain polarization and electric. field were forward so deformation of samples was expansive. But phase ϕ = π, directions of ferroelectric domain polarization and electric field were reverse so deformation of samples was shrinking. 25.

(36) Generally atomic force microscope is using laser to irradiate on cantilever , and when probe moved on a sample, cantilever would bend along with height of sample surface. And bending variation of cantilever would let laser shift on position sensitive photodector. We analyzed this displacement to obtain height of surface. But position sensitive photodector of piezoresponse force microscopy would receive topography signals and oscillation signals by AC voltage. In order to distinguish two signals, we used lock-in amplifier to filter topography signals and receive oscillation signals. We received first harmonic is piezoelectric constant. We obtained piezoresponse signal that was related to oscillation amplitude and phase. We measured piezoresponse signal that using probe on one point optionally and showed following equation [19]. vω(ϕ) = Rcos(ϕ) = δd33Vaccos(ϕ) ．．．．．．．．．．．．．．．．．．．．(3-3). Eq. ：vω(ϕ) is piezoresponse signal. R is amplitude of piezoresponse signal. δ is sensitivity of position sensitive photodector d33 is piezoelectric coefficient along electric field in vertical. Vac is amplitude signal of AC voltage.. 26.

(37) 3-2-2 Hysteresis loop [20] The circuits of piezoresponse force microscopy were refit. Besides we could measure ferroelectricity domain, we could fix point to measure electricity. The way of measuring hysteresis loop is series connection with DC voltage and AC voltage. We otputted a DC voltage by using high voltage amplifier and computer system, and computer system changed DC voltage signals with pulse wave and residence time step by step. Then measuring piezoresponse signal at voltage was zero, as shown in Fig. 22. And we recorded piezoresponse signal in every residence time. Finally we obtained piezoresponse amplitude and phase images with different voltage, as shown in Fig. 23 (b) and (c). Then we could use equation (3-3) to obtain piezoresponse signal with piezoresponse amplitude and phase images. In order to obtain good hysteresis loop, we could judge correct domain by using piezoresponse force microscopy to obtain amplitude and phase images. From Fig. 23(b) and (c) we could know that amplitude would have twice low point and it was overturn position of phase. We obtained amplitude and phase images by piezoresponse force microscopy, it had a shadoweave in middle of two bright areas. At the same position, it had two area of bright and dark in phase images. We could use this way to obtain ferroelectric domain location.. Fig. 22 Form of outputting pulse DC voltage by measuring hysteresis loop [20]. 27.

(38) Fig. 23 (a) piezoresponse signal image (b) amplitude image (c) phase image [21]. 3-2-3 Piezoelectric coefficient We measured piezoresponse signals instead of exact displacements by PFM. If we wanted to know about relationship of applying voltage and ferroelectric materials deformation, we must detect piezoelectric coefficient. In equation (3-3), the piezoresponse signal is related with sensitivity, piezoelectric coefficient d33 and applying voltage Vaccos(ϕ). If we wanted to obtain piezoelectric coefficient, we needed a standard sample to calibrate d33. 28.

(39) Firstly, we used a standard sample of known piezoelectric coefficients, we choose x-cut quartz (d11 = 2.3pm/V) as standard sample. We coated Ag on x-cut quartz as upper electrode and stainless steel sheet as bottom electrode. Then we applied a continuous AC voltage on electrodes, we could obtain a piezoresponse signal. The equation showed in (3-4-1)： vω(ϕ) = Rcos(ϕ) = aVac cos(ϕ) + b (理論上，b = 0) ．．．．．．．．．．．．(3-4-1) Where a is transformation factor of piezoresponse signal and applying AC voltage ( a is transformation factor of x-cut quartz ) , therefore be regarded as： vω(ϕ) = Rcos(ϕ) = aVac cos(ϕ) ．．．．．．．．．．．．．．．．．．．(3-4-2) We compare eq. (3-4-2) and eq. (3-3), we could obtain eq. (3-5)： δd11 Vac cos(ϕ) = a Vac cos(ϕ) ．．．．．．．．．．．．．．．．．．．．．(3-5) We knew d11 = 2.3pm/V and we could obtain that： δ = a/d11 ．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．(3-6) We changed from standard sample to the sample. Then we applied a continuous AC voltage on electrodes and changed amplitude, we could obtain a piezoresponse signal. The equation showed in eq. (3-4-2) and eq. (3-5-1) δd33Vac cos(ϕ) = a’Vac cos(ϕ) ．．．．．．．．．．．．．．．．．．．．．．(3-5-1) Where a’ is transformation factor of piezoresponse signal and applying AC voltage. Because parameters were not changed and sensitivity was same, we could obtain piezoelectric coefficient d33 by taking eq. (3-6) into eq. (3-5-1) ： d33 = d11 × (a’/ a) ．．．．．．．．．．．．．．．．．．．．．．．．．(3-7). 29.

(40) 3-3 X-ray diffraction system We analyze crystal structure of film by using X-ray diffraction system and we emitted X-ray by using electron to hit metal target (Cu, Fe et al.). Because wavelength of X-ray is smaller than distance among crystal planes, it is suitable for light source of structural analysis. According to Bragg’s law (Fig. 24) , when the atomic planes of a crystal caused an incident beam of X-rays to interfere with one another as they leave the crystal. It made wave path difference of reflection beam 2dsinθ be equal to wavelength or integral multiple and formed different intensity constructive interference with diffraction beam. The detectors received on specific angle and obtained intensity signal in different crystal plane. 2dsinθ= nλ d： Distance of two adjacent planes θ： The angle between incident beam and plane n： Arbitrary integer when wave path is integer multiple of wavelength λ： Wavelength of X-ray. Fig. 24 Diagram of Bragg’s law [15]. In this study, we measured structure and composition of samples using X-ray diffraction system (Rigaku Ultima IV) in NUK Professor Chiou Lab. (Fig. 25). 30.

(41) The power is 40KV, 40mA. The target is Cu and Kα wavelengthλ is 1.5418 Å . We selected step was 0.02°, scan speed was 2.4°/min, scan range was 30°~80° and power is 1.6KW.. In measuring process, it had declination with pressing sample, so. we would do θ scan to fix the declination.. Fig. 25 X-ray diffraction system [15]. 31.

(42) 3-4 Mangeto Optical Kerr Effect Principle of mangeto-optical effect was when a incident light penetrate or reflect a ferromagnetic material, electric field and magnetic field of light and magnetization of spontaneity of ferromagnetic materials had a interaction affect. It made polarization state of light change. In nineteenth century, Kerr found that a linearly polarized light reflected with magnetic materials, then reflected light became elliptically polarized light and long axis deviate polarization plane of original incident light, we called it Kerr rotation angle ϕk, as shown in Fig. 26(a). This is due to the angle between direction of the light and direction of magnetization in materials would affect refractive index of materials. We called mangeto-optical Kerr effect, MOKE [1]. According to the different experiment system, there are three kinds of mangeto-optical Kerr effect： (1) Polar-MOKE, P-MOKE：The direction of magnetization is perpendicular with sample surface and it is parallel with plane of incident light. Fig. 26 (b) (2) Longitudinal-MOKE, L-MOKE：The direction of magnetization is parallel with sample surface and it is parallel with plane of incident light. Fig. 26 (c) (3) Transverse-MOKE, T-MOKE：The direction of magnetization is parallel with sample surface and it is perpendicular with plane of incident light. Fig. 26 (d)[22]. 32.

(43) (a). (b). (c). (d). Fig. 26 (a) change of reflected light in polarization direction (b) P-MOKE (c) L-MOKE (d) T-MOKE. L-MOKE system is showed in Fig. 27. We used He-Ne laser in our lab and its wavelength is 633nm. This wavelength is equivalent to infrared light and the power is 10mW. The laser was changed to linearly polarized light by polarizer, then it reflected by sample and elliptically polarized light went through the analyzer to let photo detector received signal. Finally, we used digital multimeter and computer to analyze data.. 33.

(44) Fig. 27 L-MOKE system. Fig. 28 MOKE signal ∝ Ex2. As Fig. 27, when the laser went through the polarizer, just electric field of y component (linearly polarized light) went through. The reflected light became elliptically polarized light via magnetic sample and long axis deviate polarization plane of original incident light. We called it Kerr rotation angle. When the elliptically polarized light went through the analyzer, just electric field of x component went 34.

(45) through. Finally, photo detector received signals and transformed signals into MOKE signals, as shown in Fig. 28. Because Kerr rotation angle signal was similar to magnetization, we could obtain hysteresis curve. The Kerr effect was sensitive to deflection of magnetic moment so we often measured hysteresis properties with magnetic ultrathin film and laser penetration depth is approximately 30nm. Besides we could use Helmholtz coils for our magnetic field source, we also could use horseshoe electromagnet. The following is MOKE systems in our lab, as shown in Fig. 29.. Fig. 29 Left is L-MOKE system and right is P-MOKE system.. 35.

(46) 3-5 RF magnetron sputtering system The radio frequency magnetron sputtering system is composed of high vacuum chamber system, pressure sensors, gas controller, specimen holder, AC voltage power and sputtering guns. We prepared samples using the radio frequency magnetron sputtering system in our lab. Its principle is the following. We passed into working gas such as argon or oxygen in low vacuum pressure. When chamber pressure was stable, we applied AC voltage to sputtering gun. We made target and sample distinguish to cathode and anode respectively. Then part of working gas would be dissociated to plasma. The positive ion of plasma would hit cathode target. The kinetic energy of positive ion could be transform into target so the molecule of target would get out from target surface and attached on the substrates. In order to let it sputter efficiently, we configured magnet in sputtering guns. Besides it could fix target, the magnetic field would make electrons hit working gas and ions to strike target efficiently. it could increase plasma density and coating rate. Fig. 31 is Diagram of radio frequency magnetron sputtering system in our lab. Our permalloy samples were prepared in this system, and we introduce process in 4-1.. 36.

(47) Fig. 30 Diagram of radio frequency magnetron sputtering system.. Fig. 31 Picture of radio frequency magnetron sputtering system.. 37.

(48) Chapter 4. Results and Discussion. 4-1 Sample Preparation In this study, we prepared ZnO films using radio-frequency magnetron sputtering by NUK Professor Hu Lab. We hoped to have good epitaxy and conductivity as bottom electrode so we chose n-type Si(100) to prepare films. The prepared process was that we pickle substrates first. The pickling solution was HF with diluting. We took out substrates after standing in solution two minutes. This step was in order to remove SiO2 on silicon substrates. Next, substrates were individually and ultrasonically cleaned in acetone, isopropyl alcohol and deionized water for 10 min. We put substrates into chamber and controlled to be equilibrium in 10-8～10-9 Torr. Then, a quartz tube is used to heat substrates to 600℃ for 50 min. The purpose is to remove moisture and let the surface atoms of substrates rearrange neatly. Then, we poured into two working gases in 2×10-2Torr with argon 30sccm and oxygen 10sccm. Finally, we turned on RF power to 100W. After plasma was stable and pre-sputter for 10 min, we opened the shutter and coated for 2 hours. As shown in Fig. 32.. 38.

(49) Fig. 32 Simple flowchart of preparing ZnO thin film. Fig. 33 Appearance of ZnO film. Because substrates was two inches in diameter, we cut samples to be 1cm×0.5cm in order to measure conveniently. The sample number and composition as shown in Table 1. Then, we analyzed sample with atomic force microscope, piezoresponse force microscopy and X-ray diffraction system, as shown in Fig. 34.. sample number. sample composition. 20141227-c. ZnO/n-type Si(100). Table 1 ZnO sample number and composition 39.

(50) Fig. 34 Simple flowchart of analyzing ZnO thin film. And part of cut sample, we prepared prmalloy films on ZnO films using radio-frequency magnetron sputtering in our lab. First, we put ZnO film into chamber. We let vacuum level reach about 2×10-5 Torr by using mechanical pump and turbo pump. Next, we poured into working gas argon and let vacuum level reach about 6× 10-3 Torr. Finally, we turned on RF power to 40W. After plasma was stable and pre-sputter for 10 min, we opened the shutter and coated .. 40.

(51) Fig. 35 Simple flowchart of preparing Ni80Fe20 film. In this study, we prepared three kinds of thickness with Ni80Fe20 film, 50nm, 10nm and 5nm respectively. Because no thickness detection meter was on sputtering position, we dropped correction fluid on glass, as shown in Fig. 36 . We sputtered for 30 minute in the same process parameters. After putting sample into acetone for one day, we shocked the sample about 10 second with ultrasound oscillator. The correction fluid of glass surface would peel off and surface of glass would expose out. We could measure a height difference using AFM, as shown in Fig. 37. 41.

(52) Fig. 36 correction fluid on glass. Fig. 37 measurement of Ni80Fe20 film thickness. We obtained that the height was about 200 nm between film and substrates. Because we coated film for 30 minute, we obtained that the ratio of coating was about 6.67nm/min. Then, we controlled the film thickness of permalloy by this ratio. 42.

(53) Finally, we analyzed these three samples by using atomic force microscope, X-ray diffraction system and Mangeto Optical Kerr Effect, as shown in Table 2.. sample number. sample composition. 20141227-Py50. Ne80Fe20(50nm)/ZnO/n-type Si(100). 20141227-Py10. Ne80Fe20(10nm)/ZnO/n-type Si(100). 20141227-Py5. Ne80Fe20(5nm)/ZnO/n-type Si(100). Table 2 Ni80Fe20 sample number and composition. Fig. 38 Simple flowchart of analyzing ZnO thin film. 43.

(54) 4-2 Piezoresponse force microscopy analysis of ZnO First, we could observe preferred crystal orientation ZnO(002) in 34.42∘, as shown in Fig. 39 XRD spectra. And Si(400) in 69.16∘ was primary diffraction peak of silicon substrate. We could observe that film surface was composed of intensive and small grains in the topography (4μm×4μm) and we obtained root mean square roughness was 9.156nm, as shown in Fig. 40. It meant that the surface was not a too rough film.. Fig. 39 No. 20141227 XRD spectra. Fig. 40 No. 20141227 AFM topography (4μm × 4μm) 44.

(55) 4-2-1 ZnO PFM analysis Our conductive probe we chose was NSC36C, Our measurement parameters set point was 15nN, amplitude was 3V and frequency was 10 kHz voltage, then SR830 time const was 3ms and sensitivity was 20mV. We scanned in 1μm×1μm. Finally, we could obtain image of topography, amplitude and phase.. Fig. 41 Diagram of PFM. 45.

(56) Fig. 42 ZnO PFM images of (a) topography (b) amplitude (c) phase.. From Fig. 42(b) and (c), we could observe amplitude image that it had a shadoweave in middle of two bright areas. The shadoweave showed the piezoresponse was almost zero. Then, it was a domain wall in this position. At the same position, it had two area of bright and dark in phase images. It showed that there were two different ferroelectric domain structures. It also proved the shadoweave in amplitude image. Therefore, we could obtain ferroelectric domain of sample with PFM. It also proved this sample had ferroelectric property, and we would measure hysteresis loop and piezoelectric coefficient. 46.

(57) Fig. 43 Measured hysteresis loop in red cross. Fig. 44 (a) phase image (b) amplitude image. Next, we chose one point on ZnO film to measure hysteresis loop, position as shown in Fig. 43. We obtained successfully hysteresis loop, and its coercivity was about ±2V, as shown in Fig. 44.. 47.

(58) 4-2-2 ZnO piezoelectric coefficient analysis. We used standard sample x-cut quartz (d11=2.3pm/V) to measure piezoelectric coefficient d33. We chose frequency of 10kHz and applied AC voltage Vac = 0V～3V to measure piezoelectric coefficient of ZnO and quartz respectively. We calculated the slope respectively with fitting. We obtained slope of ZnO was a’=2.27946×10-4 and slope of quartz was a=1.04536×10-4. Finally, we used equation (3-7) to calculate and we could obtain d33 of ZnO was 5pm/V d33 = d11 × (a’/ a) ．．．．．．．．．．．．．．．．．．．．．．．．．(3-7). Fig. 45 Piezoresponse of ZnO and quartz. We used d33=5pm/V, and assumed c-axis lattice length c1=520pm and a-axis lattice length a1=325pm, as shown in Fig. 46. Therefore, hexagonal column volumes were calculated as follows. 48.

(59) Fig. 46 Chart of ZnO. V = 3 × a1 2 × cos(. 30 π) × c1 180. V1 = 3 × (0.325nm)2 × cos(. 30 π) × 0.520nm = 0.143nm3 180. This volume is 0.143nm3 we obtained. If we applied voltage 10V on this crystal in vertical direction, this crystal would deform about 50pm (ZnO d33=5pm/V). We assumed it shortened 50pm. If volume unchanged, and a2=342pm, calculated as follows. a2 = √. a2 = √. v1 30 3 × c2 × cos(180 π) 0.143nm3 30 3 × 0.470nm × cos(180 π). = 0.342nm. 49.

(60) Finally, lattice deformed ratio was calculated as follows： ratioc =. |c2 −c1 | = 9.6% c1. ratioa =. |a2 −a1| = 5.2% a1. We could obtain deformed rate whether c-axis or the a-axis direction were less than 11%. In accordance with the thin film growth theory, When the crystal deformed more than 11%, it must have destroyed structure behavior. We took 11% into equation and calculated, we would obtain maximum voltage was 11.44V. So it was within the safe range we applied voltage of 10V. And it had a reversible effect.. 50.

(61) 4-3 MOKE analysis This Part measurement of mainly three samples (Table 2). The Ni80Fe20 and silicon substrate as electrode we applied voltage. First, we dropped silver plastic on Ni80Fe20 and silicon substrate and each one connected to the copper out (Fig. 49), as shown in Fig. 47. We used agilent 33220A function generator (Fig. 48) to applied DC voltage and generated a electric field to make ZnO lattice deform. In order to cause magnetic properties of Ni80Fe20 be changed. We applied voltage from 0V to 10V and measured mangeto-optical Kerr effect to observe change of coercivity and Kerr intensity.. Fig. 47 Diagram of MOKE. 51.

(62) Fig. 48 agilent 33220A function generator. Fig. 49 Diagram of silver plastic and copper wires. In order to avoid film generate RRAM behavior by applying voltage too high. It made electric field weaken and the piezoelectric layer cannot cause magnetic property change of Ni80Fe20 layer. So we do a simple measurement of RRAM, as shown in Fig. 50. 52.

(63) Fig. 50 Forming Set of ZnO. We could observe the set voltage Vset=14.5V. Next, we do three times switching, as shown in Fig. 51. The set voltage were larger than 10V. So our applied voltage were in save range when measuring MOKE.. Fig. 51 three times switching 53.

(64) In order to determine hysteresis curve we measured were not contributed by ZnO films. We measured MOKE with ZnO, as shown in Fig. 52 and Fig. 53. The phenomenon could be seen no hysteresis curve so the effect were contributed mostly by Ni80Fe20 layer.. Fig. 52 Diagram of ZnO P-MOKE. Fig. 53 Diagram of ZnO L-MOKE. 54.

(65) Fig. 54 ZnO cross-section SEM. In order to obtain thickness of ZnO film and calculate electric field by applying voltage on ZnO film. We measured cross-section SEM of ZnO by Student Zheng in NCKU. From Fig. 54 we could see, the thickness of ZnO film was about 319nm. In our study, we applied voltage of 1V to 10V. We calculated by formula, E = V / d, then we obtained electric field as follows.. Voltage(V) Field. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 31.3 62.7 94.0 125.4 156.7 188.1 219.4 250.8 282.1 313.5. (kV/cm). 55.

(66) 4-3-1 5nm thick Ni80Fe20 MOKE analysis We could observe that film surface was composed of intensive and small grains in the topography (3μm×3μm) and we obtained root mean square roughness was 8.872nm, as shown in Fig. 55. Because the roughness was higher than film thickness, the film might have nanoparticles and be poor continuity. In XRD spectra (Fig. 56), the peak of Ni80Fe20 fcc(111) was not obvious. It was possible that the thickness was too thin.. Fig. 55 Topography of 5nm Ni80Fe20. 56.

(67) Fig. 56 XRD spectra of 5nm Ni80Fe20. Fig. 57 5nm Ni80Fe20 L-MOKE 57.

(68) We made y-axis Kerr intensity be normalized. It was convenience to analyze. We could observe that L-MOKE hysteresis loop didn’t have obvious magnetic coercivity. It was hard axis. While the applied voltage was gradually increased from 0 to 10V, the hysteresis loop tended to paramagnetism, as shown in Fig. 57.. Fig. 58 5nm Ni80Fe20 Kerr intensity of various voltage. Next, we compared Kerr intensity where the maximum minus the minimum, as shown in Fig. 58. We could observe that the intensity would reduce while the applied voltage increased gradually. There was a turning point in 2V. The proportion of variation is 48.6%.. 58.

(69) 4-3-2 10nm thick Ni80Fe20 MOKE analysis We could observe that film surface was composed of intensive and small grains in the topography (3μm×3μm) and we obtained root mean square roughness was 8.605nm, as shown in Fig. 59. It was similar to 5nm Ni80Fe20. In XRD spectra (Fig. 60), the peak of Ni80Fe20 fcc(111) was not obvious. It was also possible that the thickness was too thin and we would obtain conspicuous peak in 50nm Ni80Fe20. Fig. 59 Topography of 10nm Ni80Fe20. Fig. 60 XRD spectra of 10nm Ni80Fe20 59.

(70) Fig. 61 10nm Ni80Fe20 L-MOKE. It had obvious ferromagnetism in L-MOKE measurement, as shown in Fig. 61 The magnetic coercivity Hc decreased from 3.5 to 1.3 Oe as the applied voltage increased from 0 to 10 V. This obvious variation which we thought ZnO generated inverse piezoelectric effect by electric field to cause strain of Ni80Fe20 and change magnetic properties.. 60.

(71) Fig. 62 10nm Ni80Fe20 Hc in various applied voltage. As shown in Fig. 65, it was a diagram of the magnetic coercivity Hc in different applied voltage. After fitting, we could obtain the Hc decreased gradually in 2.2V and reduced drastically in 5.7V. We could obtain a correspondence between this result and hysteresis loop (Fig. 44). The proportion of variation is 65.6%.. Fig. 63 10nm Ni80Fe20 Kerr intensity of various voltage. Next, we compared Kerr intensity where the maximum minus the minimum, as shown in Fig. 63. We could observe that the intensity would reduce while the applied voltage increased gradually. There was a turning point in 2 to 3V. The proportion of variation is 33.5%. 61.

(72) Fig. 64 10nm Ni80Fe20 L-MOKE in the negative voltage. As shown in Fig. 64, we could obtain that the magnetic coercivity Hc decreased as the applied negative voltage increased from 0 to -10V. This result was same with previous measurement (Fig. 61). There were a symmetry with each other.. 62.

(73) Fig. 65 10nm Ni80Fe20 Hc in various applied voltage. As shown in Fig. 65, there was a slowdown trend in -2V until it started to decrease drastically in -6V.. It decreased from 3.4Oe to 0.7Oe and the proportion of. variation is 77.6%.. Fig. 66 10nm Ni80Fe20 Kerr intensity of various voltage. Next, we compared Kerr intensity where the maximum minus the minimum, as shown in Fig. 66. We could observe that the intensity would reduce while the applied voltage increased gradually. The fitting curve was a linear distribution and proportion of variation is 25.4%. 63.

(74) Fig. 67 10nm Ni80Fe20 P-MOKE. In P-MOKE measurement, it also had ferromagnetism. But its magnetic coercivity Hc was greater than result of L-MOKE, about 59Oe.. Fig. 68 10nm Ni80Fe20 Hc in various applied voltage. Next, we observed each of magnetic coercivity Hc in various applied voltage, we obtained the magnetic coercivity Hc reduced as applied voltage increase. The magnetic coercivity Hc decrease from 59 to 27 Oe and the proportion of variation is 60.5%. 64.

(75) Fig. 69 Diagram of square wave signal. Fig. 70 10nm Ni80Fe20 relative Kerr intensity record. Finally, we measured relative Kerr intensity, we used function generator to apply two different voltage, 0V and 10V respectively. It was a square wave signal and 30 seconds for a cycle. We could obtain that the relative Kerr intensity would be modulated by voltage. It would prove previous result. 65.

(76) 4-3-3 50nm thick Ni80Fe20 MOKE analysis We could observe that film surface was composed of intensive and small grains in the topography (3μm×3μm) and we obtained root mean square roughness was 8.386nm, as shown in Fig. 71. In XRD spectra (Fig. 72), the peak of Ni80Fe20 fcc(111) was obvious than 5nm and 10nm.. Fig. 71 Topography of 50nm Ni80Fe20. Fig. 72 XRD spectra of 10nm Ni80Fe20 66.

(77) Fig. 73 50nm Ni80Fe20 L-MOKE. Fig. 74 50nm Ni80Fe20 Hc in various applied voltage. We obtained the magnetic coercivity Hc reduced as applied voltage increase from 37Oe to 23Oe, as shown in Fig. 74. After fitting, we could obtain the Hc decreased gradually in 5.1V and reduced drastically in 8V. It was greater than previous result. We thought that the Ni80Fe20 thickness was thicker than previous one. If we would drives Hc to change, we had to apply greater voltage. Ｔhe proportion of variation is 45%. 67.

(78) Fig. 75 50nm Ni80Fe20 Hc in various applied voltage. Next, we compared Kerr intensity where the maximum minus the minimum, as shown in Fig. 75. We could also observe that the intensity would reduce while the applied voltage increased gradually. The proportion of variation is 17.4%.. Finally, we compare the proportion of variation. It is variation of L-MOKE Kerr intensity in different Ni80Fe20 thickness. It was the maximum proportion minus the minimum proportion. We obtain that proportion decreased as thickness thickening, as the follow table.. ΔKerr intensity(%). 5mm-think NiFe. 10mm-think NiFe. 50mm-think NiFe. 48.6%. 33.5%. 17.4%. 68.

(79) Fig. 76 50nm Ni80Fe20 P-MOKE. The P-MOKE measurement result was in hard axis hysteresis loop. We thought the magnetic moment like to stay in horizontal. So it generated a platform in zero applied voltage and it appeared two loops in Fig. 76.. 69.

(80) Fig. 77 Diagram of square wave signal. Fig. 78 10nm Ni80Fe20 relative Kerr intensity record. Finally, we measured relative Kerr intensity, we used function generator to apply two different voltage, 0V and 10V respectively. It was a square wave signal and 30 seconds for a cycle. We could obtain that the relative Kerr intensity would be modulated by voltage. We compared previous diagram (Fig. 70). It was more like square, because different thickness made it have crystal relaxation behavior in 10 nm thickness Ni80Fe20. 70.

(81) Fig. 79 List of magnetostriction constant. Element. Weight%-1. Weight%-2. Weight%-3. Average ratio. Ni. 2.63. 3.03. 2.12. 50.8. Fe. 2.28. 2.89. 2.34. 49.2. Table 3 Wt% of 50nm NiFe EDS. Element. Weight%-1. Weight%-2. Weight%-3. Average ratio. Ni. 2.21. 15.53. 18.25. 49.2. Fe. 2.26. 15.67. 19.48. 50.8. Table 4 Wt% of 10nm NiFe EDS. From Fig. 10[8] and 錯誤! 找不到參照來源。, if Weight% of NiFe had slight change, the magnetostriction constant (λ) would have great differences.. We executed EDS measurement with NiFe films in NPTU Professor Lai Lab, as shown in Table 3 and Table 4. We obtained the Weight% of 50nm thick NiFe was 50.8:49.2 and the Weight% of 10nm thick NiFe was 49.2:50.8. So we could conjecture the NiFe had magnetostriction effect. In previously measurement, the inverse piezoelectric effect of piezoelectric layer ZnO cause lattice strain to produce inverse magnetostriction effect of NiFe. The magnetic properties measurement results would change with the applied voltage.. 71.

(82) Chapter 5. Conclusion. In this study, we could control magnetic properties by applying voltage. First, we applied a voltage on ZnO, it caused inverse piezoelectric effect to make ZnO strain. We obtained the piezoelectric coefficient d33 was 5.2pm/V using PFM. We could know the applied voltage had a limit value 11.44V by calculation and theory. We obtained the set voltage was 14.5V using RRAM measurement. Therefore it was save that we applied 10V voltage in the experiment. It meant that the film structure of ZnO did not be destroyed.. Next, there were three kinds of thickness variation which we prepared Ni80Fe20 on ZnO, 5nm, 10nm and 50nm respectively. The MOKE measurement was executed, while the applied voltage was increased from 0 to 10V. We could observe the magnetic coercivity Hc of 10nm and 50nm thick Ni80Fe20 decreased as the applied voltage was increased. The proportion of variation was 45% and 65.6% respectively. The voltage of the Hc of starting to decrease was 2.2V and 5.1V respectively. This voltage 2.2V could be prove with coercivity of ZnO hysteresis loop ±2V. And we discovered while the thickness increased to 50nm, the reduced voltage increased from 2.2V to 5.1V. We conjectured due to thickness increased, we had to increase the voltage to cause films strain and obtained changes in effect. We also observed 5nm, 10nm and 50nm thick Kerr intensity decreased as the applied voltage increased. And we could modulate Kerr intensity by applying square wave voltage. In L-MOKE measurement, we obtain that Kerr intensity proportion decreased as thickness thickening. We conjecture that different thickness made different strain field. So 72.

(83) thickness increased let magnetic properties of Ni80Fe20 surface be affected small by the piezoelectric layer strain. We conjectured change of magnetic properties was related to inverse magnetostriction effect by EDS measurement and literatures.. 73.

(84) Appendixes NSC36C. Microlevers A. Length. 130um. 180um. Width. 35um. 18um. Thickness. 1um. 0.6um. Resonant frequency. 75kHz (typical). 22kHz. Force constant. 0.6N/m (typical). 0.05N/m. Tip radius. <35nm. <20nm (sharpened) <50nm (unsharpened). Tip height. 20-25um Table 5 List of AFM probe[23][24]. 74.

(85) Table 6 PDF card of ZnO. Table 7 PDF card of Si. 75.

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