具垂直異向性之一維磁性多層奈米線與磁性穿隧接面奈米元件

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(1)國立台灣師範大學光電科技研究所博士論文 Institute of Electro-Optical Science and Technology National Taiwan Normal University. 具垂直異向性之一維磁性多層奈米線與磁性穿隧接面奈米元件 One dimensional magnetic multilayer nanowires and magnetic tunneling junction nanometer device with perpendicular anisotropy. 指導教授：黃昭淵. 博士. 研究生：陳柏源. 中華民國. 一○○. 年十月.

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(4) 摘要具垂直磁異向性之奈米材料於發展下一代磁紀錄媒體與磁電阻式隨機存取記憶體扮演極重要的腳色。結合電化學電沉積技術與具奈米孔洞之氧化鋁模板可達成大量製造、低成本與高密度之目標。本研究所製備之鈷與鎳鐵合金之奈米線被證實具備垂直磁異向性且可透過磁晶異向性與形狀異向性來調整。結合具垂直磁異向性之鎳鐵合金奈米線與鈷鐵硼薄膜之磁性穿隧接面元件已成功被製造與探討。於低溫 10K 的環境下，鈷鐵硼薄膜厚度為 1.5 奈米時，其磁阻為 104%，而鈷鐵硼薄膜厚度為 1.0 奈米時，其磁阻為 110%，且在鈷鐵硼薄膜厚度小於 1.0 奈米時，於無固定層的條件下元件呈現出自旋閥的特性。. 關鍵字: 奈米線、磁阻、磁性穿隧接面與垂直磁異向性.

(5) Abstract. The nanometer size magnetic materials with perpendicular anisotropy are more important to develop the next generation magnetic recording media or magnetoresistive random access memory. The magnetic nanowire via electrochemical deposition into the anodic alumina oxide template is one possible method to achieve the goal of massive fabrication, low cost and high density. The magnetic nanowires of cobalt and permalloy have demonstrated that the perpendicular anisotropy can be tunable by controlling the magnetocrystalline and shape anisotropy. The nanometer perpendicular magnetic tunnel junctions are prepared and the feasibility is also has been demonstrated. The TMR ratio of MTJs with 1.0 nm thick CoFeB layer are 110% at 10K and with 1.5 nm thick CoFeB layer are 104% at 10K. Below the 1.0 nm thick of CoFeB layer, the MTJs display the spin valve like properties without pinned layer and the spin quantum limitation was observed at 10K.. Keywords: Nanowires, Magnetoresistance, Magnetic Tunneling Junction and Magnetic Perpendicular Anisotropy.

(6) Contents. Chapter 1. Introduction……………..…………….……….………………………..…..1 1.1 Nanomaterials…………………………………………………….…………….1 1.2 Synthesis of nanomaterials by electrochemical deposition…….……....………2 1.3 Principle of magnetic materials..……………………………………………4 1.3.1 Ferromagnetic materials……………….…………………………………..5 1.3.2 Magnetoresistance…………………….…………………………………6 1.3.3 Magnetic anisotropy………………………….……………….…………...7 1.3.3.1 Magnetocrystalline Anisotropy……………………………..…………...8 1.3.3.2 Shape Anisotropy……………….……………………….……………....9 1.4 Literature Review.………………………………..........................................10 1.5 Research Motivation.…………………………………………….…………12 Chapter 2. Characterization Techniques………………….……………………….....14 2.1 Anodic aluminum oxide template and nanowires preparations..…………...14 2.2 Cyclic voltammetry (CV).........................................………………………….15 2.3 Electrochemical Quartz Crystal Microbalance (QCM)……….………….….16 2.4 X-Ray Diffraction (XRD)………………………………………..………….17 2.5 Scanning Electron Microscope (SEM)…..………………………………….19 2.6 Transmission Electron Microscopy (TEM)…………………………………20 2.7 Energy-dispersive X-ray spectroscopy (EDS)………………………………21 2.8 Vibrating Sample Magnetometer (VSM)……….……………………………22 2.9 Superconducting Quantum Interference Device (SQUID)…………………23 Chapter 3. Magnetic Nanowires with Perpendicular Anisotropy….…..…………25.

(7) 3.1 Enhancement of Perpendicular Anisotropy of Co/Cu Multilayer Nanowires by Phase Doping…………………………………….…………………………25 3.1.1 Introduction………………………………………………………………..25 3.1.2 Experimental………..…………………………………………………...26 3.1.3 Results and discussion……..…………………………………………….28 3.1.4 Conclusions………………………………………………………………..32 3.2 Controlling Magnetic Anisotropy of Permalloy/Cu by Electrochemical Deposition…………………………………………….…………….………34 3.2.1 Introduction………………………………………………………………..34 3.2.2 Experimental………..…………………………………………………...36 3.2.3 Results and discussion……..…………………………………………….37 3.2.4 Conclusions………………………………………………………………..42 3.3 Spin-Valve Like Perpendicular Magnetic Tunnel Junctions without pinned layer in Nano-Dimensioned……………..……….…………………………43 3.3.1 Introduction………………………………………………………………..43 3.3.2 Experimental………..…………………………………………………...44 3.3.3 Results and discussion……..…………………………………………….45 3.3.4 Conclusions………………………………………………………………..49 Chapter 4. Concluding Remarks……………………………..……………….………51 Chapter 5. Future Prospect……………………………………………………………52 References……………………………………………………………………………..53 Personal publications………………………………..…………………………………60.

(8) Chapter 1. Introduction. 1.1 Nanomaterials. Over the past years, nano-scale materials have been the more interesting subject because of their small size showing the potential for applications in electronic, biomedical, and electronic areas due to their small sizes and novel properties which are generally not seen in their bulk counterparts. Nano-scale materials have more surface area to volume ratio than their bulk counterparts, which forms the novel physical and chemical properties exhibited by these nano-scale materials. At the nano-scale, which is defined as at least one hundred nanometers or less, quantum effects play more important role in determining the materials properties and characteristics, leading to novel optical, electrical and magnetic behavior. The novel properties at the nano-scale can be attributed to the lack of symmetry at the interface or to electron confinement and the nanomaterial behavior is also explained by surfaces and interfaces at the nano-scale. Surface properties including energy levels, electronic structure and reactivity are quite different and new properties in the nano-scale are reliable and continual change can be achieved using a single material. Nano-scale materials can be divided into three types on their dimension space. Firstly, the zero-dimensional (0-D) nanomaterial is defined as that all of their dimension size is below than 100 nm, and such as the quantum dots which the morphology is sphere and all the dimensions are in nano-scale. Secondly, one-dimensional (1-D) nanomaterial is defined as that one of their dimension is smaller than 100 nm, and such as rod or wire nano-scale materials. The two-dimensional (2-D) nano-scale 1.

(9) material indicates that only one of dimension is confined in nano-scale. As a result, morphology of 2-D nano-scale materials is the film like. It is possible to prepare nano-scale material with various properties. Nano-scale materials can be metals, ceramics, polymeric materials, or composite materials. Mass fabrication of the nano-scale materials with uniform morphology is most important when the many chemical or physical properties of nano-scale materials have not been elucidated clearly. Metal nanowires can be prepared by various methods including the physical or chemical processes. The physical methods involve vapor deposition depend on the principle of subdividing bulk precursors to nanowires. The chemical methods involve the reduction of metal ions to the metal elements. The chemical process is more suitable to obtain small and uniform nanowires than the physical methods. From the viewpoint of mass production of metal nanowires the chemical method is more important and effective than the physical methods.. 1.2 Synthesis of nanomaterials by electrochemical deposition. A porous anodic alumina membrane is a commercially available template with nano scale hole. Since applying of two anodization processes can obtain a hexagonal close-packed highly ordered alumina. template, many applications with membrane. under the nano scale were existed. In most cases various inorganic and organic materials were arranged in such one dimension ordering nanowires arrays. Another interesting characteristic of these membranes is the growth of arrays of well ordered hexagonal cells with central pores parallel to each other and with a symmetry axis perpendicular to the substrate surfaces. Anodic alumina templates combined with 2.

(10) various approaches, including electrochemical and electroless depositions, sol–gel, chemical vapor deposition, sputtering, wetting and supercritical fluid deposition, have been exploited to deposit a wide range of one dimensional materials with single crystalline, polycrystalline, or amorphous structures. As one of the important domains of one dimensional nanostructures, metal nanowires are often used to study transport properties of electrons and phonons and thus they are ideal nanoscale electronic and photonic devices. Up to now, different kinds of metal nanowires, including Au, Ag, Al, Bi, Co, Cu, Fe, Ni, Pb, Pd, Sb and Zn nanowires, were successfully deposited into the pores of anodic alumina templates. Here, the Ag nanowire arrays as an example shown in Fig. 1 to illustrate the general fabrication method. The oxide materials can be made as one dimensional nanowires which were inside the anodic alumina template. Zinc oxide is a known direct wide band gap semiconductor and also is one of the most important compounds useful for fabricating photonic, optical and electronic devices. It has a large binding energy about 60 meV and is suitable for short wavelength optoelectronics. Over the last years, the synthesis of ZnO nanowire arrays has been of growing interest owing to their promising applications in nanoscale technology and devices. One dimensional ZnO nanowires are synthesized successfully and shown in Fig. 2. Selenide-based nanowires as silver selenide is a well-known superionic conductor. At low temperature, its phase is a semiconductor with a narrow direct band gap and high carrier mobility. Such properties are attractive in various fields of nanoelectronics. Recently, it has been reported that displays large positive magnetoresistance. At room temperature, the significant magnetoresistance effect and its linear dependence on the magnetic field could make it a promising material for 3.

(11) applications in the magnetic field sensing devices. Here, the Ag2Se nanowire arrays as an example shown in Fig. 3 to illustrate the general fabrication method. Bismuth telluride nanowires are one of the most famous telluride nanowires and it is well known for their good thermoelectric properties for near-room-temperature applications. Theoretical studies suggest that one dimensional bismuth telluride nanowires may have a higher figure of merit than that of the bulk material. This has stimulated a study on the fabrication of bismuth telluride nanowires. Here, the bismuth telluride nanowire arrays as an example shown in Fig. 4 to illustrate the general fabrication method. Organic nanostructures are more important in the field of nano scale science and nanotechnology. In recent years many synthesis methods for controlling the growth and uniformity of one dimensional organic nanowires arrays and nanotubes have been developed. Herein, Fig. 5 describes the organic materials of hexaphenylsilole nanowires as a typical example illustrating the fabrication of one dimensional organic nanowires in templates.. 1.3 Principle of magnetic materials. Magnetism is a property of materials that respond at spin direction under the applied magnetic field. The best known form of magnetism in our life is ferromagnetism and below the curie temperature such that some ferromagnetic materials produce their own persistent magnetic field. All materials are influenced by the presence of the magnetic field. Some materials which are attracted to the magnetic field called paramagnetism; others are resisted by the magnetic field called 4.

(12) diamagnetism; others have a much more complex relationship with an applied magnetic field. The materials that are negligibly affected by magnetic fields are known as non-magnetic materials.. 1.3.1 Ferromagnetic materials. Ferromagnetic materials are the most magnetically active force in the world, and so they have very high magnetic susceptibilities. The materials build by the atoms have the permanent dipole moments. If the materials in the state of lower energy, it means that the atomic dipole moments disorder with each other. In the state of high energy, entire sample were to be made of aligned dipoles and a strong magnetic field would be created. The ferromagnetic material under the too high a temperature, it ceases to be ferromagnetic. The reason is that when the ferromagnetic material above a certain critical temperature which was called the Curie temperature, the thermal motion of the atoms is so violent that the electrons in the bonds are no longer able to keep the dipole moments aligned. The ferromagnetic material will change into a paramagnetic material. If a ferromagnetic sample under a strong magnetic field, the domains can be forced to coalesce into large domains aligned with the external field. When the external field is removed, the spin directions of electrons maintain the alignment and the magnetism remains which is called hysteresis. Some regions have the same spin direction and are called magnetic domains. These magnetic domains and their behavior gives ferromagnetic materials their distinctive properties. The magnetic domain can be described a region within a magnetic material which has uniform magnetization. This indicates that the individual 5.

(13) magnetic moments of the atoms are aligned with one another and they have the same direction. Magnetic domain structure is the source for the magnetic behavior of ferromagnetic materials like iron, nickel, cobalt and their alloys. The regions which are separated between the magnetic domains are called domain walls, where the magnetisation rotates from the direction in one domain to that in the next domain.. 1.3.2 Magnetoresistance. Magnetoresistance is the property of changing the value of the electrical resistance when an external magnetic field is applied. It can be classified according the value of the magnetoresistance ratio. One is called ordinary magnetoresistance (OMR).. More. recent. researchers. indicated. that. materials. showing. giant. magnetoresistance (GMR), colossal magnetoresistance (CMR) and magnetic tunnel effect (TMR). The Tunnel magnetoresistance is one kind of the magnetoresistive effect that occurs in magnetic tunnel junctions as shown in Fig. 6. The structure is consisting of two ferromagnets separated by a thin insulator. If the insulator layer is thin enough, the electrons can tunnel from one ferromagnet into the other. The direction of the two magnetizations for the ferromagnetic films can be switched individually by an external magnetic field. If the magnetizations are in a parallel direction it is more likely that electrons will tunnel through the insulating film than if they are in the oppositional direction. Consequently, such a junction can be switched between two states of electrical resistance, one with low and one with very high resistance. The up spin direction electrons are those with spin orientation parallel to the external 6.

(14) magnetic field, whereas the spin-down electrons have anti-parallel alignment with the external field. The relative resistance change is defined by the spin polarizations of the two ferromagnets and cab be explained by two current model as shown in Fig. 7. If no voltage is applied to the junction, electrons tunnel in both directions with equal rates. With a voltage, electrons tunnel to the positive electrode. With the assumption that spin is tunneling, the current can be described in a two-current model. The total current is split in two partial currents, one for the spin-up electrons and another for the spin-down electrons. These vary depending on the magnetic state of the junctions.. 1.3.3 Magnetic anisotropy. The ferromagnetic materials are subdivided into many small grains which were called magnetic domains. Each domain can be magnetized to saturation state under the magnetic field, but each direction of magnetization of a domain is quite different not the same. The total magnetization is defined that the net direction of the domains and the preferred direction is called magnetic anisotropy. Under the applied magnetic field, the magnetic materials with anisotropic properties will align their direction of domains with the easy axes. An easy axis also can be defined that the stable energy state of favorable direction of domains magnetization. The shape of hysteresis loops define the coercivity and squarness, they are strongly affected by magnetic anisotropy. Anisotropy is also important in the design of many commercial magnetic devices. The magnetic anisotropy is influenced by the crystal structure and the shape of grains on the direction of magnetization. There are several different types of anisotropy:. 7.

(15) 1. Magnetocrystalline anisotropy: the atomic structures of the crystal introduce favored directions for the magnetization of domains. 2. Shape anisotropy: when a sample whose morphology is not spherical, the demagnetizing field is not equal for all directions and the one direction of magnetization will be created.. 1.3.3.1 Magnetocrystalline Anisotropy. Magnetocrystalline anisotropy is an intrinsic property of the ferromagnetic materials. Magnetocrystalline anisotropy is defined that the energy to change the magnetization of domain in a single crystal from the easy axis to the hard axis direction. The magnetization is aligned with an easy direction and in order to switch from easy axis to hard axis direction, the magnetization has to traverse a path over an energy barrier which is the difference between the energy in the easy direction and the hard direction. The formula of magnetocrystalline anisotropy energy density can be given by:. where K1 and K2 are determined magnetocrystalline anisotropy constants and α1, α2 and α3 are the cosines of the angles between the crystal axes. Table 1 gives the value of E when the vector of the saturated magnetization lies in a particular direction. The values of the two constants, K1 and K2, determine the directions of easy, medium and 8.

(16) hard magnetization is shown in Table 2 and the values of iron, cobalt and nickel bulk materials are show in table 3. It also can be most easily observed by measuring magnetization curve along the different crystal directions. The Fig. 8 shows the magnetization curves of different magnetic single crystal. For the iron element, the [100] is the easy axis and the [111] is the hard axis, for the nickel element, the [111] is the easy axis and the [100] is the hard axis and for the cobalt element, the [0001] is the easy axis and the [1010] is the hard axis.. 1.3.3.2 Shape Anisotropy. The shape anisotropy is due to the shape of the irregular grains. Under the magnetic field, the magnetic charges or poles will be existed on the surface of the magnetized grains. The magnetic charges or poles will produce a magnetic field and according to the surface charge distribution, the intensity of the induced magnetic field is different. It is called the demagnetizing field because it will be produced in opposition to the magnetization that produces it. For example is shown in Fig. 9, take a long thin needle shaped grain. The demagnetizing field will be smaller if the magnetization is along the long axis than if is along one of the short axes. This produces the easy axis of magnetization along the long axis direction. For magnetic materials, if smaller than 20 microns, shape anisotropy is the dominant form of anisotropy. In larger sized particles, shape anisotropy is not important than magnetocrystalline anisotropy. the demagnetizing field is given by:. 9.

(17) where N is a demagnetizing factor determined by the shape. In fact, the demagnetizing factors depend on the orientation of M within the crystal. For example, the cobalt element with a uniaxial crystal structure was discussed at room temperature and the curve of the relationship between the shape-anisotropy constant and the c/a is shown in Fig. 10.. 1.4 Literature Review. In 2003, Sort et al. indicated that the magnitudes of the exchange bias fields of both anti-parallel layers can be tuned at room temperature by simply varying the relative number of (Co/Pt) repeats in each multilayer. This effect can be quantitatively explained by the different energy contributions involved during magnetization reversal. An extended sample with a virtually constant magnetization can be observed around zero field when the number of Co/Pt repeats in one anti-parallel layer is equal or larger than in another anti-parallel layer as shown in Fig. 11. This is very suitable for field sensor or memories applications using spin valve structures or tunnel junctions with perpendicular anisotropy. In 2005, Dijken et al. indicated that the sensor with Pt/CoFe sensing layers, the sensor response depends critically on the perpendicular magnetic anisotropy of the CoFe film. The sensitivity increases with thickness of CoFe layer and it reaches its maximum value just below the spin reorientation transition in the CoFe sensing layer as shown in Fig. 12.. 10.

(18) In 2006, Seki et al. indicated that FePt layers exhibited strong perpendicular magnetic anisotropy of the order of 107 erg/cm3 as shown in Fig. 13 and it showed that magnetization reversal induced by current from an antiparallel to a parallel alignment occurred at the current density of the order of 108 A/cm2 with the assistance of magnetic field. In 2007, Lacour et al. indicated that the magnetic properties of ultrathin Co layers sandwiched between Pt and Al layers are studied as a function of the Al layer oxidation time as shown in Fig. 14. The coercive field of ultrathin Co was reduced by their oxidation process layers with perpendicular anisotropy and can even induce transition from a ferromagnetic to a superparamagnetic state. In 2008, Kim et al. indicated that magnetic tunnel junctions using L10 ordered CoPt electrodes with perpendicular magnetic anisotropy. The L10 chemical order parameter of 0.82 was obtained for the bottom CoPt electrode which was deposited at substrate temperature of 600°C. The transport measurements with magnetic field perpendicular was applied to the film plane showed a tunnel magnetoresistance ratio of 6% at room temperature and 13% at 10 K as shown in Fig. 15. In 2009, Wei et al. indicated that a large linear magnetoresistance of up to 22% can be achieved in a magnetic tunnel junction that consists of two ferromagnetic layers as shown in Fig. 16, one with out of plane and one with in-plane magnetic anisotropy. The tunneling magnetoresistance was measured with the electrical current perpendicular to the film plane. In 2009, Mizunuma et al. indicated that TMR ratio in MTJs with CoFeB/MgO/Fe stack reached 67% at annealing temperature of 200 °C and degradation of the TMR ratio may be related to crystallization of CoFe(B) into fcc (111) or bcc (011) texture resulting from diffusion of B into Pd layers. 11.

(19) In 2010, Ikeda et al. indicated that the perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter.. 1.5 Research Motivation. Our research is focus on studying characteristic of nanowires and also includs novel synthesis of nanowires and perpendicular magnetic tunnel junctions. In the part of magnetic nanowires, the materials in nanometer are usually prepared by lithography and etching technique of physical fabrication, but the process need the more time and cost. So I apply the chemical deposition to prepare the multilayer structure of magnetic nanowires and this method can offer some advantages, easy fabrication process, less process time and massive fabrication. The larger perpendicular magnetic anisotropy is more important in high density of magnetoresistive random access memory (MRAM) and magnetic recording media and combining the electrochemical deposition and anodic aluminum oxide (AAO) template can achieve the goal of enhancement of the magnetic perpendicular anisotropy more easily. The characteristic of magnetic nanowires are studyied and a magnetic tunnel junction (MTJ) in nanometer is fabricated to demonstrate the feasibility of next generation magnetic spintronic devices. The fabrication of functionalized ma terial is analyzed from synthesis to application. This work describes the fabrication of Co/Cu magnetic multilayer nanowires by the electrodeposition in porous alumina templates and a novel method for enhancing perpendicular anisotropy by doping nano grains with the hcp phase into the fcc phases, 12.

(20) Co segments in multilayer nanowires, shown the evidence of enhancement in magnetocrystalline anisotropy due to the crystal orientation transformation and demonstrate the magnetic perpendicular anisotropy can be controlled by the methods of phase doping and shape variation. This work explores a simple method that is based on anodic aluminum oxide (AAO) template synthesis for preparing Ni3Fe1 (permalloy)/Cu magnetic nanowires with controllable segment lengths and tunable perpendicular magnetic anisotropy. The magnetic dipolar interactions can be incorporated by a factor that is applied to the shape anisotropy energy. The improvement in perpendicular magnetic anisotropy matches theory, and the results herein were also demonstrated by simulation of the nano-scale magnetism using a phenomenological model that was recently developed elsewhere. This work explores that nano magnetic tunnel junctions (MTJs) consist of magnetic nanowires and ultra-thin film with perpendicular anisotropy are fabricated. The permalloy nanowires is synthesized by electrodeposition into nano-pore template and the insulator (MgO) and magnetic thin film (CoFeB) are capped by sputtering. The magnetic perpendicular anisotropy is demonstrated for CoFeB ultra-thin film and permalloy nanowires and the TMR ratio of nano perpendicular MTJs at 10K is 110% along the field parallel with nanowires at 3000 Oe. The spin quantum limitation between nano magnetism is observed without the antiferromagnetic pinned layer. The realization of spin dynamics in exchange interaction should have a large impact on the development of spintronic devices.. 13.

(21) Fig. 1. (a) FESEM image for micrograph of the silver nanowire arrays; (b) TEM image of the silver nanowire; (c) XRD pattern of the silver nanowire arrays which were embedded in templates[1]..

(22) Fig. 2. XRD patterns of the Zn nanowire arrays which were embedded in templates with the diameters of 40 nm. (a) without heat treatment, and heated at 300℃ (b) for 8 h, (c) for 15 h, (d) for 25 h, and (e) for 35 h, respectively[2]..

(23) Fig. 3. XRD of the Ag2Se nanowires embedded in templates with pore diameters of about 50 nm: (a) as-deposited and (b) annealed in Ar atmosphere[3]..

(24) Fig. 4. XRD pattern of the Bi2Te3 nanowire array with diameter of 40 nm fabricated by (a) pulsed electrodeposition and (b) direct-current deposition[4]..

(25) Fig. 5. Schematic illustration of the hexaphenylsilole (HPS) nanowire growth mechanism in templates for nanowires with diameters of (a) 35 nm and (b) 250 nm[5]..

(26) Fig. 6. The magnetic tunnel junctions with structure consisting of two ferromagnets separated by a thin insulator[6].. Fig. 7. Two-current model for parallel and anti-parallel alignment of the magnetizations[6]..

(27) Fig. 8. Magnetization curves for a single crystal of iron, nickel and cobalt[7]..

(28) Fig. 9. Magnetization vector for the long shape materials[7].. Fig. 10. Shape-anisotropy constant for saturated cobalt in the form of a prolate spheroid[7]..

(29) Fig. 11. Dependence of the exchange bias field of anti-parallel layer , where nAP1 designates the number of Co layers in anti-parallel layer[8].. Fig. 12.. The curves is function of resistance and applied magnetic field for sensors. with different CoFe sensing layer thickness[9]..

(30) Fig. 13. Magnetization curves were measured for a 4 nm thick FePt thin film at (a) room temperature and (b) 77 K. The magnetic field was applied perpendicular and parallel to the film plane, respectively[10].. Fig. 14. Coercive field left axis and tunnel magnetoresistanceright axis as a function the oxidation time both recorded at room temperature. Inset: The curve is the function of resistance and applied magnetic field for a MTJ with an Al layer oxidized for 95 s[11]..

(31) Fig. 15. Dependence of the TMR ratio on temperature for a pMTJ prepared with an epitaxial CoPt bottom and top electrodes. The inset shows the TMR curve at 10 K[12].. Fig. 16. Resistance and TMR at RT. The spin configuration of the two magnetic layers are indicated by the double arrows. Details near zero magnetic field. The temperature dependence of TMR as measured and for a hypothetical antiparallel state. The curve is a function of resistance and field at different temperatures[13]..

(32) Fig. 17. TMR ratio at RT as a function of annealing temperature, with and without CoFeB layer[14].. Fig. 18. MTJ structure. (a) Schematic of an MTJ device for TMR, (b) Top view of an MTJ pillar SEM image[15]..

(33) Fig. 19. Annealing temperature dependence of the TMR ratio[15]..

(34) Table 1. Crystal anisotropy energies for various directions in a cubic crystal [7]. a. b. c. α1. α2. α3. E. [100]. 0. 90. 90. 1. 0. 0. K0. [110]. 45. 45. 90. 𝟏⁄√𝟐. 𝟏⁄√𝟐. 0. K0 + 𝑲𝟏⁄𝟒. [111]. 54.7. 54.7. 54.7. 𝟏⁄√𝟑. 𝟏⁄√𝟑. 𝟏⁄√𝟑. K0+𝑲𝟏⁄𝟑 + 𝑲𝟐⁄𝟐𝟕. Table 2. Directions of east, medium and hard magnetization in a cubic crystal [7]. K1. +. +. +. -. -. -. K2. +∞. −𝟗𝑲𝟏⁄𝟒. -9K1. -∞. 𝟗|𝑲𝟏|⁄𝟒. 𝟗|𝑲𝟏|. to. to. to. to. to. -𝟗|𝑲𝟏|⁄𝟒. -9K1. -∞. 𝟗|𝑲𝟏|⁄𝟒. 𝟗|𝑲𝟏|. +∞. Easy. <100>. <100>. <111>. <111>. <110>. <110>. Medium. <110>. <111>. <100>. <110>. <111>. <100>. Hard. <111>. <110>. <110>. <100>. <100>. <111>. to.

(35) Table 3. Anisotropy constants of various crystal structure [7]. Structure. Element. K1. K2. Cubic. Fe. 4.8. Cubic. Ni. -0.5. -0.2. Hexagonal. Co. 45. 15.

(36) Chapter 2. Characterization Techniques. This chapter is discussed about the technical appraisement used for characterization and determination of magnetic nanowires fabricated at the present studies. The fundamental principles and details of these techniques are elucidated further for understanding. Methodologies used for analysis of magnetic nanowires are listed in the following.. 2.1 Anodic aluminum oxide template and nanowires preparations. In this study, anodic aluminum oxide (AAO) is chosen as a template which has an isolating, uniform and parallel pore structure and also including the tunable pore diameters. The AAO membranes used to be prepared by anodic oxidation of Al foil and the process. In order to have the smooth surface, the Al foil was first electropolished in sulfuric acid and phosphoric acid mixed solutions at room temperature. After the process of electropolishing, the Al foils were anodized in 0.3 M oxalic acid at low temperature for few hours to form the pours structure template. The voltage of anodization was kept at 30 V to keep the pore size of AAO template at about 50 nm and the inter-distance between pores are also constant. The AAO template was dipped into HgCl2 solution to remove the remaining Al foil, and then dipped in H3PO4 to dissolve the barrier layer. Finally, after above processes the AAO template is ready for deposition. Electrochemical deposition process was performed at room temperature, using a three-electrode potentiostatic control with an Ag/AgCl 14.

(37) reference electrode and a platinum wire as a counter electrode. The metal thin film as silver and gold was coated by thermally evaporated on AAO template which was used as the working electrode in an electrochemical cell, and with the porous side exposed to the electrolyte. The nanowires were grown in an electrolyte with the alternative constant potentials to deposit.. 2.2 Cyclic voltammetry (CV). Cyclic voltammetry (CV) [1] is one of the technique which was widely used technique for acquiring qualitative information about electrochemical reactions. The method includes a reference electrode, working electrode, and counter electrode which in combination are sometimes referred to as a three-electrode setup. To ensure sufficient conductivity, ectrolyte is usually added to the test solution. The range of the potential was determined by the combination of the solvent, electrolyte and specific working electrode material. Common materials for working electrodes were used including glassy carbon, platinum, and gold. These electrodes are generally encased in a rod of inert insulator with a disk exposed at one end. A regular working electrode has a radius within an order of magnitude of 1 mm. Having a controlled surface area with a defined shape is important for interpreting cyclic voltammetry results. During the cyclic voltammetry measurement, the voltage is swept between two values at a fixed rate, however now when the voltage reaches V2 the scan is reversed and the voltage is swept back to V1. This ramping is known as the experiment's scan rate (V/s). The potential was applied between the reference electrode and the working electrode and the current is measured between the working electrode and the counter 15.

(38) electrode. This data is then plotted as current vs. potential. As the waveform shows, the forward scan produces a current peak for any analytes that can be reduced (or oxidized depending on the initial scan direction) through the range of the potential scanned. As the potential reaches the reduction potential of the analyte, the current will increase, but then falls off as the concentration of the analyte is depleted close to the electrode surface. If the redox couple is reversible then when the applied potential is reversed, it will reach the potential that will reoxidize the product formed in the first reduction reaction, and produce a current of reverse polarity from the forward scan. This oxidation peak will usually have a similar shape to the reduction peak. As a result, the information including the redox potential and electrochemical reaction rates of the compounds are obtained.. 2.3 Electrochemical Quartz Crystal Microbalance (QCM). A quartz crystal microbalance [2] which is called QCM measures a mass per unit area by measuring the change in frequency of a quartz crystal resonator. The quartz crystal frequency of oscillation is partially dependent on the thickness of the crystal. During standard operation, all the other influencing variables remain constant; thus a change in thickness correlates directly to a change in frequency. As mass was coated on the surface of the crystal, the thickness increases; consequently the frequency of oscillation decreases from the initial value. With some simplifying assumptions, this frequency change could be calculated and correlated precisely to the mass change using Sauerbrey's equation. The Sauerbrey equation was first derived by G. Sauerbrey in 1959 and calculated changes in the oscillation frequency of a piezoelectric crystal 16.

(39) with mass deposited on it. A method developed for measuring the resonance frequency and its changes by using the crystal as the frequency-determining component of an oscillator circuit. The method continues to be used as the useful tool in QCM experiments for conversion of frequency to mass. The Sauerbrey equation is defined as:. f0 – Resonant frequency (Hz) Δf – Frequency change (Hz) Δm – Mass change (g) A – Piezoelectrically active crystal area (Area between electrodes, cm2) ρq – Density of quartz (ρq = 2.648 g/cm3) μq – Shear modulus of quartz for AT-cut crystal (μq = 2.947x1011 g/cm.s2) νq – Transverse wave velocity in quartz (m/s). 2.4 X-Ray Diffraction (XRD). X-Ray Diffraction [3] is a technique used to characterise the crystallographic structure, crystallite size (grain size), and preferred orientation in polycrystalline or powdered solid samples. Powder diffraction was commonly used to identify unknown materials, by comparing diffraction data against a database maintained by the International Centre for Diffraction Data. It may also be used to analysis 17.

(40) heterogeneous solid mixtures to determine relative abundance of crystalline compounds and, when coupled with lattice refinement techniques, such as Rietveld refinement, can provide structural information on unknown materials. Powder diffraction is also a common method for measuring strains in crystalline materials. An effect of the finite crystallite sizes is calculated as a broadening of the peaks in an X-ray diffraction as is explained by the Scherrer Equation. The advents of synchrotron sources have drastically changed this picture and caused powder diffraction methods to enter a whole new phase of development. Not only that there is a much wider choice of wavelengths which is available, the high brilliance of the synchrotron radiation makes it possible to observe changes in the pattern during chemical reactions, temperature ramps, changes in pressure. Bragg diffraction was first found by William Lawrence Bragg and William Henry Bragg in 1913 in response to their discovery that crystalline solids produced surprising patterns of reflected X-rays. They found that these crystals, with certain specific wavelengths and incident angles, produced intense peaks of reflected radiation (known as Bragg peaks). X-rays interact with the atoms in a crystal. When the phase shift is a multiple of 2π, the interference is constructive; this condition can be expressed by Bragg's law. where an integer is n, λ is the wavelength of incident wave, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes. Bragg diffraction will occur when electromagnetic radiation or subatomic particle waves with wavelength comparable to atomic spacings are incident 18.

(41) upon a crystalline sample, scattered in a specular fashion by the atoms in the system, and undergo constructive interference in accordance to Bragg's law. For a crystalline materials, the waves are scattered from lattice planes separated by the interplanar distance d. Where the waves were scattered interfere constructively; they remain in phase since the path length of each wave is equal to an integer multiple of the wavelength. The difference of path between two waves undergoing constructive interference is given by 2dsinθ, where θ is the scattering angle. This results in Bragg's law which describes the condition for constructive interference from successive crystallographic planes (h,k,l) of the crystalline lattice.. 2.5 Scanning Electron Microscope (SEM). Scanning electron microscope (SEM) [4] is a microscope which uses electrons instead of light to form an image. The SEM has many advantages over traditional microscopes. The SEM has a large depth of field, which allows more of a specimen to be in focus at one time. Due to the higher resolution of SEM, so closely spaced specimens can be magnified at much higher levels. Because using electromagnets rather than lenses for the SEM, the researcher has much more control in the degree of magnification. All of these advantages including that actual strikingly clear images, make the scanning electron microscope one of the most useful instruments in research today. The SEM is an instrument that offers a largely magnified image by using electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by an electron gun. The electron beam passes along a vertical path through the microscope, which is held within a vacuum. The beam passes through 19.

(42) electromagnetic fields and lenses, which focus the beam down toward the sample. Once the beam hits the sample, from the sample the electrons and X-rays are ejected. These X-rays, backscattered electrons, and secondary electrons were collected by detectors and convert them into a signal that is sent to a screen similar to a television screen. This produces the final image.. 2.6 Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) [5] is one of the microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. At smaller magnifications TEM image contrast is due to the thickness and composition of the material, due to the absorption of electrons in the material. The intensity of the image was modulated at higher magnifications complex wave interactions, requiring expert analysis of observed images. Alternate modes of use allow for the TEM to observe modulations in chemical identity, crystal orientation, electronic structure and sample induced electron phase shift as well as the regular absorption based imaging. The transmission electron microscope runs on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light. TEMs use electrons as "light source" and their much lower wavelength makes it possible to get a resolution a thousand times better than with a 20.

(43) light microscope. Contrast formation in the TEM depends greatly on the mode of operation. Complex imaging techniques, which utilise the unique ability to change lens strength or to deactivate a lens, allow for many operating modes. These modes may be used to discern information that is of particular interest to the investigator. Selected area electron diffraction (SAED), is a crystallographic experimental technique that can be performed inside a transmission electron microscope. As a diffraction technique, SAED can be used to identify crystal structures and examine crystal defects. It is similar to x-ray diffraction, but unique in that areas as small as several hundred nanometers in size can be examined, whereas x-ray diffraction typically samples areas several centimeters in size. A diffraction pattern was made under broad, parallel electron illumination. An aperture in the image plane is used to select the diffracted region of the specimen, giving site-selective diffraction analysis. SAED patterns are a projection of the reciprocal lattice, with lattice reflections showing as sharp diffraction spots. By tilting a crystalline sample to low-index zone axes, SAED patterns can be used to identify crystal structures and measure lattice parameters. SAED is essential for setting up DF imaging conditions. Other uses of SAED include analysis of: lattice matching; interfaces; twinning and certain crystalline defects.. 2.7 Energy-dispersive X-ray spectroscopy (EDS). The interaction between an electron beam and a sample target produces a variety of emissions, including x-rays. An energy-dispersive (EDS) [6] detector can be used to separate the characteristic x-rays of different elements into an energy spectrum, and 21.

(44) EDS system software is used to analyze the energy spectrum in order to determine the abundance of specific elements. EDS can be used to find the chemical composition of materials at a spot size of a few microns, and to create element composition maps over a much broader raster area. Together, these capabilities provide fundamental material compositional information for a wide variety of materials. An EDS detector contains a crystal which can absorb the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The x-ray absorption thus converts the energy of the x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element.. 2.8 Vibrating Sample Magnetometer (VSM). A vibrating sample magnetometer (VSM) [7] runs on Faraday's Law of Induction, which tells us that a changing magnetic field will produce an electric field. This electric field can be measured and can offer us information about the changing magnetic field. A VSM is used to study the magnetic properties of magnetic materials. A VSM operates by first placing the sample to be studied in a constant magnetic field. If the sample is magnetic materials, this constant magnetic field will magnetize the sample by aligning the magnetic domains, or the individual magnetic spins, with the field. As the constant field is stronger, the magnetization will be large. The magnetic dipole moment of the sample will generate a magnetic field around the sample, sometimes called the magnetic stray field. As the sample is moved up and down, this magnetic stray field is changing as a function of time and can be sensed by a set of 22.

(45) pick-up coils. The alternating magnetic field will generate by an electric field in the pick-up coils according to Faraday's Law of Induction. This current will be proportional to the magnetization of the materials. The greater the magnetization, the greater the induced current.. 2.9 Superconducting Quantum Interference Device (SQUID). The Superconducting Quantum Interference Device (SQUID) [8] consists of a superconducting loop interrupted by one or more Josephson junctions. Understanding of SQUID requires both the physical phenomena of flux quantization and Josephson tunneling. The temperature is controlled by the helium gas and the heater. To cool down, the variable impedance inlet, which is controlled by the impedance heater, would be opened to let cold helium gas flow into sample space. When the temperature is close to the setting point, the gas flow and the heater will complement each other to achieve thermal equilibrium. A superconducting magnet is an electromagnet generated from coils of superconducting wire. During operation, they must be cooled to cryogenic temperatures. Superconducting magnets can produce larger magnetic fields than all but the strongest electromagnets and can be cheaper to operate because no energy is dissipated as heat in the windings. The Resistivity option of the SQUID adds a configurable resistance bridge board, called the user bridge board, to the Model 6000 SQUID Controller. None of the four channels on the user bridge board are dedicated to a specific system operation, so all four channels are available to perform four-wire resistance measurements on the SQUID. The Resistivity option can record resistance as well as resistivity, conductance, and conductivity. Using four wires to 23.

(46) attach a sample to a sample puck greatly reduces the contribution of the leads and joints to the resistance measurement. In a four-wire resistance measurement, current is passed through a sample via two current leads, and two separate voltage leads measure the potential difference across the sample. The voltmeter has a very high impedance, so the voltage leads draw very little current. In theory, a perfect voltmeter draws no current whatsoever. Therefore, by using the four-wire method, it is possible to know, to a high degree of certainty, both the current and the voltage drop across the sample and thus calculate the resistance with Ohm’s law.. 24.

(47) Fig. 1. The AAO template anodization process. Fig. 2. The nanowires electrochemical deposition process.

(48) Fig. 3. The cyclic voltammetry measurement was applied by Autolab PGSTAT 100. Fig. 4. The crystalline structure measurement was applied by X’Pert PRO.

(49) Fig. 5. The morphology measurement was applied by FESEM-JSM6700F. Fig. 6. The morphology and selected area electron diffraction of nanowires were.

(50) applied by JEM－2010F Field Emission TEM. Fig. 7. The magnetic hysteresis loop was measured by alternating gradient magnetometer.

(51) Fig. 8. The magnetoresistance measurement under various was applied by superconducting quantum interference device.

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(53) Chapter 3. Magnetic Nanowires with Perpendicular Anisotropy. 3.1 Enhancement of Perpendicular Anisotropy of Co/Cu Multilayer Nanowires by Phase Doping. 3.1.1 Introduction. The fabrication and characterization of a one-dimensional magnetic nanoscale structure. have. received. increasing. interest. owing. to. its. application. in. ultra-high-density magnetic recording media [1,2] and electronic devices [3,4]. The magnetic characteristics of a nanoscale ferromagnetic material depends on its size, shape and crystallinity. Nanowires with a high aspect ratio not only have the advantage of a greater surface-to-volume ratio than both bulk material and thin films, but. also. exhibits. favorable dipolar magnetic properties. Synthesis. using. electrochemical templates is a versatile method for producing single component and multisegment nanowires or nanorods [3,4]. Ferromagnetic nanowires of materials such as Fe, Co and Ni [5-8], and multilayer nanowires such as NiFe/Cu, Ni/Cu, Co/Cu and CoPt/Pt [9-11], have been fabricated using this method. Ferromagnetic nanowires typically exhibit very high coercivity and remanence along the magnetic easy axis, parallel to the wire axis, owing to the inherent shape anisotropy and reduced dimensions [12]. Co- or Fe-based multilayer films prepared by the alternating deposition of a transition metal (Co or Fe) and a nonmagnetic element (Pd, Ag, Pt, Au, 25.

(54) etc.) have reportedly exhibiting strong perpendicular magnetic anisotropy when the thickness of the transition metal Co or Fe was less than a few monolayers [13-15]. In the past year, the ferromagnetic multilayer nanowires was interesting in studying effects related to spin transfer torque from a spin-polarized current in a nanosize structure and it was also shown that the high density spin-polarized current passed through to multilayer nanowires can be induced the torque [16,17]. The magnetic anisotropy of Co/Cu multilayer nanowires can be controlled by changing in magnetocrystalline anisotropy [18], the easy magnetization direction perpendicular to the nanowires axis due to the separation of cobalt layers [19], the magnetization reversal appeared to be a combination of rotation and spin flipping, which was dependent on the copper thickness [20] and the spin-flip diffusion length was measured to be about 21 nm for Co/Cu system [21]. This work describes the fabrication of Co/Cu magnetic multilayer nanowires by the electrodeposition in porous alumina templates and a novel method for enhancing perpendicular anisotropy by doping nano grains with the hcp phase into the fcc phases, Co segments in multilayer nanowires, shown the evidences of enhancement in magnetocrystalline anisotropy due to the crystal orientation transformation and demonstrate the magnetic perpendicular anisotropy can be controlled by the methods of phase doping and shape variation.. 3.1.2 Experimental. Anodic aluminum oxide (AAO) is herein as the template because AAO has a structure of isolated, non-connected, and parallel pores with tunable pore diameters. 26.

(55) The AAO templates that are used in the experiments herein were prepared by anodic oxidation of Al foil (99%, 0.25 mm-thick). The Al foil was initially electropolished in a mixed sulfuric acid / phosphoric acid solution and then anodized in 0.3 M oxalic acid at 10 ºC for 16 hours to form a nano-sized channel template. The voltage of anodization was maintained at 30 V to keep a constant pore diameter and a constant interpore distance in the AAO template. The template was dipped in saturated HgCl2 solution to remove the remaining aluminum, and then dipped in 5 wt% H3PO4 to dissolve the barrier layer. Electrodeposition in a single solution of both Co2+ and Cu2+ ions was performed to synthesize Co/Cu nanowires. First, a 5000 Å -thick silver film was thermally evaporated onto one side of AAO to serve as the back electrode. The electrodeposition was carried out using a three-electrode method. The counter electrode was Pt wire and the Ag/AgCl electrode was the reference electrode. The deposition solution contained 2 M CoSO4, 0.02 M CuSO4 and 0.6 M H3BO3 and the concentration ratio of Co2+/Cu2+ was 200/1 to prevent the formation of the copper impurities in cobalt segments during the electrodeposition process. To form a multilayer nanowires structure, the reduction voltage was set to -1 V for Co2+ ions and -0.16 V for Cu2+ ions. The duration of each pulse can be adjusted to tune the thickness of each layer. After the nanowires were formed, the silver electrode was removed using 1 M HNO3 and then the AAO was dissolved in 1 M NaOH to obtain Co/Cu nanowires. The morphology of AAO templates was characterized by field-emission scanning electro microscopy (FE-SEM, JEOL JSM-6700F). The Co/Cu multilayer nanowires that were embedded in AAO were observed by transmission electron microscopy (TEM, Hitachi H-7100), X-ray powder diffraction (XRD, BL-01C2 NSRRC, Taiwan) was performed to obtain the morphology and crystallographic structures. The deposition 27.

(56) rates of cobalt and copper were measured using a electrochemical quartz crystal microbalance (EQCM, D3100 Digital Instrument) and the magnetic hysteresis loops were carried out using a commercial vibrating sample magnetometer (VSM) with fields of up to 8 T at room temperature. Measurements were made on a silica micro-slide perpendicular and parallel to the magnetic field. The magnetoresistance measurements were performed made using a four-point probe station in magnetic fields from 8T to -8T at room temperature.. 3.1.3 Results and discussion. The morphologies of the formed structures of nanoporous AAO membranes were characterized by FE-SEM. Figure 1 presents a plan-view SEM image of nanoporous AAO templates, which contain uniform pores (50 nm in diameter). The pore interval length of AAO is 20 nm, the pore density is 12.1 × 1010 cm-2, and the aspect ratio (height/diameter) of the pores is maintained at around 700. In a previous investigation [22] self-organized pore growth results in the formation of a densely packed hexagonal pore structure for particular sets of parameters. The diameter and the interval of the pores depend on the applied voltage and electrolytes. Any repulsive interaction between the pores explains the self-organized arrangement of neighboring pores in hexagonal arrays. The pores of the front surface of the AAO template are also etched during the process of barrier layer-removal, tending to form a spherical array structure. Because of this, the magnetic anisotropy effect in spherical shape of the Co/Cu multilayer nanowires can be neglected. Figure 2 (a) presents a typical deposition current and the corresponding EQCM 28.

(57) responses of the cobalt and copper layer. Two deposition potentials (–0.16 V and –1 V versus Ag/AgCl) were applied to deposit Co/Cu multilayer nanowires. The deposition rates of cobalt and copper, calculated from mass versus time data, for a unit area and the average length of the individual thickness of the nanowires, was also confirmed by the TEM image in Fig. 2(b). The Sauerbrey equation [23] yielded the thickness of the multilayer structure: Δm = -Δf/Cf, where Δm is the change of mass per unit area in g cm-2; Δf is the shift in the resonance frequency in Hz; Cf is the sensitivity factor of the crystal in Hz ng-1cm2 and the deposition rates of cobalt and copper are 1.6 nm/s and 0.1 nm/sec, respectively. Figure 2(b) presents a TEM bright field image of part of a 50nm diameter [Co(50 nm)/Cu(20 nm)]n multilayer nanowire in which the Co segments have an aspect ratio of 1. The measured diameter of the nanowires closely to approximates the pore diameter. To prevent inaccuracies in the average length of the nanowires (by breaking), the nanowires that were used to measure the deposition rate were not grown longer than 1 μm. Based on these results, multilayer nanowires with controlled compositions and lengths were fabricated by manipulating the applied-potential waveforms and pulse durations. Figure 3 presents the XRD profile of a specimen with a lenth of 1 μm and diameter of 50 nm Co/Cu multilayer nanowires. All of the observed peaks fit the fcc phase (PDF#015-0806) and hcp face (PDF#089-4308) of standard cobalt material. At room temperature, the only stable phase in bulk (or bulklike) Co is hcp, while in a nanostructure or very thin film of Co, an fcc phase can also appear [24]. According to the analysis herein, the fcc and hcp phases dominate the cobalt layer and the fcc phase dominates the copper layer. The result is similar to that concerning the hcp phase in cobalt nanowires, published by Li et al. [25], whose preferred orientation is (100). However, a significant transformation of the preferred orientation from (100) to (002) 29.

(58) was observed, probably by the cobalt hcp phase with (002) preferred orientation was induced to form by the cobalt and copper fcc phases. Rietveld refinement was applied using the general structure analysis system (GSAS) program to determine the ratio of the fcc to hcp phases in the crystal structure and the results reveal that the ratio of the fcc to hcp phases is almost 1:1. The grain sizes, as determined from the full width at half-maximum of the fcc-Co (111) and hcp-Co (101) peaks, all exceed approximately 30 nm. The ferromagnetism of multilayer nanowires is well modulated by changing the aspect ratio of the length of the cobalt segments to the wire diameter. Figure 4 presents M–H curves for the Co/Cu multilayer nanowires whose ferromagnetic Co segments have aspect ratios of 2.0 (rod-shaped), 1.0 (intermediate), and 0.1 (disk-shaped), and various crystal structure, fcc and fcc and hcp mixed. The value of coercivity (Hc), saturation magnetization (Ms), residual magnetization (Mr), and squarness (Mr/Ms) of hysteresis loops were shown in table 1 and table 2. The external magnetic field was applied parallel (∥) and perpendicular (⊥) to the axes of the nanowires. Figure 4(a) and (b) present the magnetization curves in an applied parallel and perpendicular field for multilayer nanowires with diameters of 50 nm [Co(fcc)/Cu(fcc)] and [Co(fcc+hcp)/Cu(fcc)]. The coercivity of nanowires with fcc phase or fcc and hcp mixed phase in cobalt segment in an applied parallel field were both higher than perpendicular field and the gain ratios of coercivity and the squarness of fcc and hcp mixed phase were higher than fcc phase. The magnetocrystalline anisotropy energy density in hcp phase [19] (K1 = 5 × 106 erg cm-3) was higher than fcc phase [19] (K1 = 6.3 × 105 erg cm-3) of cobalt and the gain ratios of coervivity and squarness in mixed phase were raised significantly than single fcc phase, result from the effective magnetocrystalline anisotropy energy density 30.

(59) increasing by doping a hcp phase into a fcc phase in the cobalt segment. Fig. 4(c) presents the magnetization curves in an applied parallel field for multilayer nanowires with diameters of 50 nm [Co(5, 50, 100 nm)/Cu(4 nm)]. The coercivity reveals the change in shape of the Co segments from disk-shaped to rod-shaped and the change in the easy axis from perpendicular to parallel to the wire axis (from 489 Oe to 621 Oe). Figure 4(d) presents the magnetization curves when a perpendicular field is applied to multilayer nanowires with a diameter of 50 nm [Co(5, 50, 100 nm)/Cu(4 nm)]. Similarly,the easy axis of the multilayer nanowires tends to become parallel to the wire axis, as revealed by the decline in coercivity (from 368 Oe to 109 Oe) as the aspect ratio of the Co segments increases. The results reveal that the magnetic easy axis tends rapidly to become parallel to the axis of nanowires as the aspect ratio increases and the enhancement in magnetic perpendicular anisotropy can be achieved by doping a hcp phase into fcc phase in the cobalt segment. Based on the above investigation, this work proposes a model of the enhancement of perpendicular anisotropy by doping the hcp phase into the fcc phase structure of cobalt nano grains, as shown in Fig. 5. The shape anisotropy energy density of the pure phase of the fcc [25] (πMs2 = 6 × 106 erg cm-3) and hcp19 (πMs2 = 6 × 106 erg cm-3) crystal were almost equal and can be neglected to calculate the enhancement in magnetic perpendicular anisotropy in the cobalt segment of multilayer nanowires. The magnetocrystalline anisotropy energy density can be increased by doping hcp phase with higher energy density into fcc phase and the preferred orientation of hcp phase was transformed from (101) to (002) which induced by fcc cobalt nano grains and copper layers, result in enhancement of the magnetic perpendicular anisotropy. This model helps to elucidate the magnetic dipole interaction between the shape and magnetocrystalline anisotropy on the nanoscale 31.

(60) structures. Figure 6 presents the magnetoresistive hysteresis loop of 1 μm Co(40 nm)/Cu(4 nm) multilayer nanowires electrodeposited into AAO templates and 35 μm-lenth copper nanowire was electrodeposited as the connector between the top electrode and the multilayer nanowires and the top electrode, Au pad, was evaporated on the AAO template before electrodepositon. The measurement conditions were a source current of 1 μA, a temperature of 298 K and magnet fields from 8k Oe to -8k Oe. The 1 μm-lenth. multilayer. Co(40. nm)/Cu(4. nm). nanowires. were. obtained. an. magnetoresistance ratio (MR ratio) of 0.65 % at 8000 Oe when the magnetic field is parallel to the nanowires and 0.2% when the magnetic field is perpendicular to the nanowires. The resistance of nanowires include 1 μm Co(40 nm)/Cu(4 nm) multilayer nanowires and 35 μm-lenth copper nanowire, without considering the contribution of the copper resistance, the MR ratio which was calculated to multiply the ratio [36 μm-length(Co/Cu + Cu)/1μm-length(Co/Cu)] is 23.4 % when the magnetic field is parallel to the nanowires and 7.2 % when the magnetic field is perpendicular to the nanowires.. 3.1.4 Conclusions. This work systematically investigated the fabrication of Co/Cu multilayer nanowires by anodization and electrodeposition. The magnetism can be modulated by controlling the aspect ratio and the crystalline structure of the Co segments in the multilayer nanowires. Doping nano grains with the hcp phase into the fcc phase of the cobalt segment of multilayer nanowires via controlling pH can efficiently increase the 32.

(61) magnetic perpendicular anisotropy and this method also overcomes the obstacle to the processing of multilayer nano-scale structures. The magnetic/non-magnetic multilayer nanowires have potential applications as key components in the next generation nanoscale perpendicular magneto-electronic devices, including perpendicular magnetoresistive random access memory devices.. 33.

(62) 3.2. Controlling. Magnetic. Anisotropy. of. Permalloy/Cu. by. Electrochemical Deposition. 3.2.1 Introduction. Chemically synthesized nano crystals are the focus of increasing research interest, because these materials can behave as artificial atoms or quantum dots and because they have unique self-assembly properties that can be exploited in the formation of two-dimensional (2D) and three-dimensional (3D) superlattices [1-6]. Nanowires are a focus of research interest because they can be used in one-dimensional nanosystems and potentially have exciting applications in nanotechnology, ranging from nanoelectronic devices, through cell separation, to magnetic labeling in biomedicine [7,8]. Numerous methods for preparing nanowires with novel optical, electrical, catalytic and magnetic properties from magnetic [9], semiconductor [10], inorganic [11], organic [12], polymer [13], metallic [14] and dielectric materials [15]. Magnetic multilayers are of particular interest because of their unique magnetic properties and their potential use in magneto-optic recording [16-18]. Currently, interest in the effects of spin transfer torque (STT) that is generated by a spin-polarized current in a nanosized ferromagnet is great. The main advantages of multilayer nanowires are that they exhibit zero-field microwave absorption that can be easily tuned over a large range of frequencies [19], and they are both low-cost and can be reproduced rapidly over large areas, unlike standard ferrite devices. A microwave circulator has been demonstrated and discussed in the systems of nanowires embedded in a porous polymer membrane. The device thus fabricated also has shortcomings, the polymer 34.

(63) membranes have large dielectric losses [20,21], however alumina has been known to exhibit low dielectric losses [22]. Early experimental studies of Ni3Fe1/Cu nanowires with a high magnetoresistance ratio at 4.2 K of 78% [23], have indicated that the ratio depends on the magnetic anisotropy, the thickness of the nonmagnetic Cu layer and the state of spin alignment between pairs of two ferromagnetic layers [24,25]. The magnetoresistance ratio can be increased by the highly ordered alignment of spins in the ferromagnetic segments. The most effective method of so doing is to modulate the shape of the magnetic segments to increase the energy of perpendicular anisotropy and the spin diffusion length of Ni3Fe1 , calculated using the Valet-Fert model, is between 3.3 and 5.3 nm [26]. Multilayer nanowire systems of single element of ferromagnetic segment, as example, nickel, cobalt and iron, have been examined. Cho et al. [27] and Chen et al. [28] have developed the simple and fast method to study the relationship between the shape morphology and nano-scale magnetic properties. They demonstrate that the nano-scale magnetic properties is strongly dependent on the aspect ratio as well as on dipolar coupling and enhancement in the anisotropy due to modulate the shape morphology. The dependencies of coercivity and remanence of the multilayer nanowires on the diameter and aspect ratio were demonstrated by both experimental data and micromagnetic simulations. However the role of dipolar interactions in ferromagnetic alloy multilayer nanowire arrays, such as Ni3Fe1/Cu, is not yet fully understood. This work explores a simple method that is based on anodic aluminum oxide (AAO) template synthesis for preparing Ni3Fe1 (permalloy)/Cu magnetic nanowires with controllable segment lengths and tunable perpendicular magnetic anisotropy. The magnetic dipolar interactions can be incorporated by a factor that is applied to shape anisotropy energy. The improvement in perpendicular magnetic anisotropy matches theory, and the results herein were also demonstrated by 35.

(64) simulation of the nano-scale magnetism using a phenomenological model that was recently developed elsewhere [29,30]. Multilayer nanowires that are synthesized using an AAO template are suited to use in microwave devices with a large saturation magnetization and high resonance frequencies.. 3.2.2 Experimental. Herein, an AAO template is applied to deposit multilayer nanowires. Porous alumina templates with a diameter of 50 nm were fabricated using a two-step anodizing process. An aluminum sheet (thickness: 0.1 mm and purity: 99%) underwent anodic oxidation at 30 V in 0.3 M aqueous oxalic acid solution at 10 ℃ for 16 h to prepare an AAO film. The as-prepared AAO film was dipped into 5 wt% phosphoric acid for three hours to eliminate the obstructing film, and then a 500nm-thick silver film was evaporated onto one side of the template to act as a conductive contact. Electrodeposition was performed at room temperature, using a three-electrode potentiostatic control with an Ag/AgCl reference electrode and a platinum wire as a counter electrode. The Ag-coated AAO template was used as the working electrode in an electrochemical cell, and with the porous side) exposed to the electrolyte. Solution was freshly prepared from NiSO4·6H2O, FeSO4·7H2O, CuSO4.5H2O and boric acid. Ni3Fe1/Cu multilayered nanowires were grown in an electrolyte that contained 0.057 M Ni2+ ion, 0.008 M Fe2+ ion, 0.001 M Cu2+ ion and 0.1 M H3BO3. Alternative constant potentials of -1.3 V to deposit Ni3Fe1 segments and -0.3 V to deposit Cu segments were applied. 36.

(65) The morphology of AAO templates was characterized by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The Ni3Fe1/Cu multilayer nanowires that were embedded in AAO were observed by transmission electron microscopy (TEM, Hitachi H-7100). X-ray powder diffraction (XRD, BL-01C2 NSRRC, Taiwan) was performed to elucidate the morphology and crystallographic structures. The deposition rates of permalloy and copper were measured using an electrochemical quartz crystal microbalance (EQCM, D3100 Digital Instrument). The magnetic hysteresis loops were measured using a commercial vibrating sample magnetometer (VSM) with fields of up to 4.5 T at room temperature. Measurements were made on a silica micro-slide that was held perpendicular and parallel to the magnetic field.. 3.2.3 Results and discussion. Nanoscale templates have an important role in fabricating arrays of nanowires because the templates have porous with ordered hexagonal honeycomb structures. Figure 7 shows an FE-SEM image of the porous template that is used in this work. The pore diameter and interpore distance of the homemade template are 50 nm and 10 nm, respectively. The nano-scale pores were formed under the electrical field in the acid electrolyte, the pore diameters can be controlled by applied voltage and the length of the template is determined by the process time. The morphology of the homemade template is rough and tendency of the pore shape to be spherical rather than hexagonal, caused by etching of the barrier layer by the alkali solution. To modulate the magnetic anisotropy, the nanoscale template was fabricated with the 37.