鈷,鎳/氧3x3/鎢(111) 的成長、結構、熱穩定性及磁性之研究

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(1)國立台灣師範大學物理研究所碩士論文. 鈷,鎳/氧 3x3/鎢(111) 的成長、結構、熱穩定性及磁性之研究 Growth, Crystalline Structure, Thermal Stability and Magnetism of Ni, Co/O-3x3/W(111). 指導教授 : 林文欽博士研究生. : 薛坤仁撰. 中華民國一百零二年六月.

(2) 摘要本研究目的在觀察，於超高真空系統中，鈷和鎳分別在氧 3x3/ 鎢(111)基板上的晶格結構、熱穩定性、磁性及成長。藉由歐傑電子能譜與低能量電子繞射研究這些物理特性。實驗中我們發現，在鎢 (111)上曝氧後再以適當的溫度加熱，氧氣會吸附在鎢(111)表面上，並產生 3x3 的表面重構現象。特別的是，在薄膜成長及升溫過程中，我們可以從歐傑電子能譜儀中得知氧的訊號一直存在，而且訊號強度幾乎是固定的，此一結果暗示氧一直存在於表面上，類似介面活性劑。此外，我們也證明在鈷、鎳薄膜的成長及熱穩定性實驗中，氧扮演著非常重要的角色。在鎳/氧 3x3/鎢(111)系統中，鎳的成長方式為島狀成長；在鈷/氧 3x3/鎢(111)系統，鈷則是層狀成長。在熱穩定性方面，鎳/氧 3x3/鎢(111)的實驗結果顯示，即使在鍍了大量(9 PML) 的鎳後，還是不能明顯地觀察到潤濕層的存在；但在鈷/氧 3x3/鎢(111) 系統中發現了潤濕層的存在(0.33 PML)。此結果不同於在鈷/鎢(111) 及鎳/鎢(111)系統之結果，鈷、鎳薄膜在加熱到凱氏溫標 700 度時，會形成潤濕層(1 PML)，這些結果證明了氧 3x3/鎢(111)的介面對於鈷、鎳薄膜的成長及熱穩定性有極大的影響。在磁性行為上，鎳/氧 3x3/鎢(111)為傾斜磁化；而鈷/氧 3x3/鎢(111)則是平面方向上的磁化。. 關鍵字：成長、熱穩定性、磁性.

(3) Abstract We studied the growth, crystalline structure, thermal stability and magnetic properties of Co,Ni thin films deposited on O-3x3 /W(111) in a multifunctional ultra-high vacuum (UHV) system. Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) were used to investigate the growth, crystalline structure, and thermal stability. After oxygen exposure and proper annealing, oxygen adsorbed on the surface of W(111), and formed a well-ordered 3x3 super structure. Auger signal of oxygen was always observable and nearly invariant after either thin film deposition or thermal annealing. It implies that oxygen always on the top of surface, as a surfactant. We also demonstrated that oxygen played an important role in the growth and thermal stability of Co and Ni ultrathin films. For Ni/O-3x3/W(111), AES signal of W was always observable, implying the Vollmer-Weber growth mode of Ni. For Co/O-3x3/W(111), Co was layer by layer growth. About the thermal stability of Ni,Co/O-3x3/W(111), the experimental result reveals that the wetting layer was not formed even after large amount of deposition in the case of Ni/O-3x3/W(111). In Co/O-3x3/W(111), the wetting layer is about 13 PML which is smaller than Co/W(111). Our experimental results are different to the cases of Ni/W(111) and Co/W(111) in which the 1 PML wetting layer was formed after 700 K annealing. Apparently, from the investigations of growth and thermal stability, oxygen on the surface actually played an important role in affecting the mobility of Co and Ni atoms on W(111) crystal. Concern with the magnetism, Ni/O-3x3/W(111) shows canted magnetization, and Co/O-3x3/W(111) shows in plane magnetization.. 2.

(4) Contents 1 Introduction. 1. 2 Basic Concepts 2.1 Magnetic material . . . . . . . . . . . . 2.2 Hysteresis loop . . . . . . . . . . . . . 2.3 Kerr rotation and intensity . . . . . . . 2.4 Magnetic anisotropy . . . . . . . . . . 2.4.1 Shape anisotropy . . . . . . . . 2.4.2 Magneto-crystalline anisotropy 2.4.3 Stress anisotropy . . . . . . . . 2.5 The growth of the films . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. 3 3 3 5 5 7 7 8 8. 3 Experimental Apparatus 3.1 Ultra high vacuum(UHV) system . . . . 3.2 Low Energy Electron Diffraction (LEED) 3.3 Auger Electron Spectroscopy (AES) . . . 3.4 Magneto-Optical Kerr Effect (MOKE) .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 11 11 14 18 21. 4 Results 4.1 O-3x3/W(111) . . . . 4.2 Growth and structure 4.3 Thermal stability . . 4.4 Magnetism . . . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 25 25 28 31 34. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 5 Discussion. 39. 6 Summary. 43. 3.

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(6) Chapter 1 Introduction An important issue in surface science is how we can clearly observe the atoms on surface. We can use SEM, AFM, TEM...etc. For these microscopes, a sharp tip is always required. Usually, the tip is made of tungsten. Why W(111) interesting? As we know, tungsten is non-magnetic and thus electrons tunneling from tungsten are unpolarized. tungsten is a good material for tips of scanning tunneling microscopy(STM). Recently, the fabrication of single atom tips have been demonstrated with a W(111) single crystal. tungsten tips coated with thick, bulk-like films of Fe, Co, Ni and Gd can be the probe of spin-polarized scanning tunneling microscopy. Therefore knowledge of the structural and magnetic properties of magnetic thin films on W(111) would be useful for future applications in scanning probe microscopy(SPM). On the other hand, adsorbed atoms can influence surface structure and chemistry, including faceting of metallic and model bimetallic catalyst surfaces. The importance of bimetallic catalysts based on Pt-group metals has been increasingly recognised in recent decades, as they display important advantages over classical reforming catalysts. In particular, refractory metals(W, Mo, Re ) alloyed with Pt-group metals are active catalysts for hydrogenation and hydrogenolysis reactions. The W(111) surface covered with monolayer-thick films of some metals (Pt, Pd, Rh, Ir, Au) and annealed at temperatures higher than 700 K undergoes a massive reconstruction to form three-sided pyramids of nanometre-scale dimensions with mainly 211 facetsthe metal (1.7 x 1015 atoms/cm2 in case of Pt) is necessary for the surface to become completely faceted. The pyramids are composed of tungsten, completely covered with the thin metal film. The driving force for faceting is attributed to the difference in surface energy of the different facets, but ther1.

(7) mal annealing is needed to achieve sufficient surface atom mobility for mass transport Similarly, oxygen adsorbs on tungsten surface and induces surface faceting after proper temperature annealing. Moreover, oxygen causes surfactant effect which has been reported in recent score. The relation between the MR(magneto-resistance) ratio and the flatness at interface of multilayer has been observed in several studies[23-25]. Besides, it is well known that surfactant effect in the growth of ultra-thin film can improve the interface flatness of the multilayer[26]. The surfactant atoms are sometimes effective to stabilize the non-equilibrium phase at room temperature. The pre-adsorption of oxygen on the Cu(110) surface leads to layer-by-layer growth of Co at 350 K, and the Co(110) film on this highly anisotropic Cu(110) substrate appears to be stabilized by an oxygen induced (3x1) reconstruction of the topmost Co layer[20]. Due to the hybridization with ferromagnetic film (Fe, Co, Ni), the oxygen atoms acquire an induced magnetic moment that can be probed by XMCD at the O edge[16,18]. Hence, the behavior of tungsten system which combine the oxygen effect with magnetic material deposition is interesting for us to explore. In our study, the thermal stability of the Co and Ni deposit is also important for the future applications. For example, will high temperature thermal annealing of Co, Ni/O-3x3/W(111) induce faceted surface, which might be applied in fabrication of magnetic single atom tips for spin-polarized field emission source? Besides, due to the novel properties for applications, graphene-related studies have attracted much attention in recent years. Co or Ni thin films on a tungsten substrate were frequently used as a catalytic template for the fabrication of single layer or multilayer graphene[22], because high temperature annealing is usually unavoidable for the chemical vapor deposition processes. Thus we are also curious about the thermal stability of Co, Ni films on O-3x3/W(111).. 2.

(8) Chapter 2 Basic Concepts 2.1. Magnetic material. Magnetic materials can be magnetized with applying an external mag− → netic field. The magnetic moment in a unit volume is named magnetization(M ). The different materials which have different atomic structure and particular arrangement of magnetic moment lead to the different magnetic behaviors. Generally, we use magnetic susceptibility χ for discussion, where χ is the ratio of magnetization to magnetic field(χ= M/H). According to the magnitude of χ, magnetic materials can be separate into 4 kinds, Paramagnetism(χ> 0, χ≈10−3 ∼ 10−5 ), Diamagnetism(χ< 0, χ≈10−5 ), Ferromagnetism(χ> 0, χ≈101 ∼ 106 ), and Anti-ferromagnetism(χ> 0).. 2.2. Hysteresis loop. Hysteresis loop shows the relationship between the induced magnetic flux density(B) and the magnetizing force(H). The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetized or has been demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point ”a” almost all of the magnetic domains are aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. 3.

(9) Figure 2.1: Hysteresis loop, Coercivity, and Retentivity[27].. When H is reduced to zero, the curve will move from point ”a” to point ”b.” At this point, it means that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred in this point, some of the magnetic domains remain aligned but some have lost their alignment. As the magnetizing force is reversed, the curve moves to point ”c”, where the flux has been reduced to zero. This is called the point of coercivity on the curve. The force required to remove the residual magnetism from the material is called coercivity. As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point ”d”). Reducing H to zero brings the curve to point ”e.” It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point ”f” back to the saturation point where it with complete the loop. 4.

(10) 2.3. Kerr rotation and intensity. Figure 2.2: Kerr rotation angle and Kerr rotation curve[27].. Kerr rotation angle (θk ) is the angle of original linear polarized light and the long axis of elliptically polarized light. By measuring Kerr rotation curve, we needed to define the magnetic field. Customarily, we define right-handed is the positive direction (clockwise) and left-handed is negative (counterclockwise). So in the process of rotating the polarization (Accuracy is about 0.0163) with an external magnetic field for opposite direction. We can get two Kerr rotation curves. And the corresponding intensity is the subtraction of two opposite magnetic field lock-in signal at the same polarization angle.. 2.4. Magnetic anisotropy. Magnetic anisotropy is the energy difference of substances when the magnetization is changed to different directions. When a fixed external magnetic field is applied along different direction of the magnetic material, the magnetization processes in each direction is different. In all the directions, the easiest saturated direction is named easy axis, the most difficult direction to get saturated is named hard axis. This kind of internal energy is called magnetic anisotropy energy(MAE). We usually use MAE to discuss the direction of easy and hard axis. MAE means the work needed about transfer self direction of magnetization to 5.

(11) external magnetic field direction. And the easy axis is the direction which has minimum E. Generally the total energy expressed as: E=Ksin2 θ K: The constant of MAE. θ: The angle of magnetization direction and normal direction. Figure 2.3: Structure and Magnetic anisotropy of block Fe, Co and Ni[27]. From this equation, when K>0, and E decreases with θ. It means that the easy axis prefers the direction perpendicular to the surface plane. On the other hand, when K<0, and E increases with θ. The easy axis prefers the direction parallel to the surface plane. There are three kinds of Magnetic anisotropy listed as below. 6.

(12) Figure 2.4: Shape anisotropy and Magneto-crystalline anisotropy[27].. 2.4.1. Shape anisotropy. When spin arrangement has to be affected by magneto crystalline, the sample surface will become magnetic dipole. Those magnetic dipoles have interaction by itself, and the energy of diploe-dipole interaction results in a net magnetic moment. Without external magnetic field, if all the spin arrangement in the same direction, it has the maximum net magnetic energy. In reality, spin will prefer magnetic domain and decrease net magnetic energy. If the sample is a long and narrow stripe, the long axis will become an easy axis and it has bigger MAE. Therefore, the interaction of magnetic dipole moment produces magnetic anisotropy. It has affected by shape, and called shape anisotropy.. 2.4.2. Magneto-crystalline anisotropy. In the magneto-crystalline structure, magnetic anisotropy energy and self magnetization are related to the direction of crystal axis. The symmetry of the materials and crystalline structure is called magneto-crystalline anisotropy. It comes from spin orbital coupling. Spin orbital interaction will let the materials self magnetized along the crystal axis, so the easy magnetization axis of magneto-crystalline anisotropy is the crystal axis of the crystal. If the crystal only has single magnetization axis, it is called uniaxial anisotropy. We often used magneto-crystalline anisotropy Fc to describe the anisotropy of thin films. By uniaxial magnetic anisotropy sample, it can be represented as Fc =Kc sin2 θ Kc : The constant of magneto-crystalline anisotropy. θ : The angle between magnetization direction and easy magnetization axis. 7.

(13) 2.4.3. Stress anisotropy. Put a magnetic material into a magnetic field, it can be magnetized by external magnetic field. The coupling effect between atoms are changed by the external field, create a small deformation. The size of the deformation is also anisotropic, so the symmetry of the magneto-crystalline axis is destroyed. The direction of self magnetization is changed. This phenomenon is called magneto-elastic anisotropy or magnetostriction anisotropy, or stress anisotropy.. 2.5. The growth of the films. Magnetic properties such as magnetic moment and anisotropy are very sensitive to the surface morphology and crystalline structure which may change with different growth condition. Hence, the growth mode is important for magnetism. In a phenomenological analysis, the growth mode of thin films can be divided into three types, as show in Fig. 2.6. The growth mode depends on the interaction between film and surface atoms.. Figure 2.5: Schematic display of the ideal growth mode.. Layer by layer(Frank-van der Merwe) growth occurs when the interaction between the film and the surface atom is stronger than the interaction between film atoms, resulting in atomically smooth film. The interaction between the film and the surface atoms will decrease with increasing layer coverage.. 8.

(14) Island(Volmer-Weber) growth occurs when the interaction of the atom within the film is stronger than the interaction between the film and surface atoms, resulting in three dimensional clusters or islands on the surface. Layer plus island(Stranski-Krastanov) growth which may be occurred when the lattice mismatch, or different symmetry or reorientation between the substrate and the film. Initially, the film grows in layer by layer growth until a critical layer thickness, and then grows in island growth through the nucleation and coalescence.. 9.

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(16) Chapter 3 Experimental Apparatus 3.1. Ultra high vacuum(UHV) system. In surface science study, ultra high vacuum chamber is necessary. In our Lab, we have three different chambers, each chamber has different purpose. First is the purely chamber, in the Fig 3.1. We only use this chamber to preparing samples, it does not have any measurement device in this chamber. This chamber is disposed with a mechanical pump and a turbo pump, and the pressure can achieve about 3x10−9 torr. We generally use this chamber to deposit ferromagnetic materials (Fe, Co, Ni) with a capping layer (Au, Pd, Cu) to protect under layer films. After a sample is prepared, we take it out off vacuum and measure MOKE, XRD, EXAFS extra. Second is STM chamber, it is set up on the optic table. To avoid external vibrate, the optic table can float by the four pneumatic pillars. By using load-leg we can transfer our sample to STM. Third is the complicated and the biggest chamber. It disposes for a mechanical pump, a turbo pump, an ion pump, and a titanic sublimation pump (TSP). With all pumps, the bass pressure can achieve 10x−10 torr. Furthermore we have AES, LEED, MOKE in this chamber. By gas kinetic theory, at a constant temperature and pressure we can estimate the frequency for gas atoms to collide the sample. The frequency is r=. nva 4. Consider a gas molecular of mass m, at temperature T, the root mean square velocity is 11.

(17) Figure 3.1: STM chamber is on the float optical table[27]. 2 = vrms. 3kB T m. Where kB is Boltzmanns constant, and the average velocity is va = vrms. q. 8 3π. =. q. 8kB T mπ. And ideal gas formulation P = nkB T Compare all the equations. we can get r = P/(2kB mT ) P is in mbar, T is in Kelvin (K), and m is in molecular mass(M) P r=2.63x1022 √M ( cmm2 ·s ) T. For example, N2 molecular mass is 28 grams, at room temperature (300K), with pressure of 1 mbar, the frequency of gas atoms colliding with the sample surface is 2.91x1022 molecules/(cm2 ·s), consider a 10x10 mm2 sample. 1ML have about 1x1015 atoms, assume all the atoms will stay on the surface, under 1 mbar, about 5x10−6 second will become one atom layer ; under 10−6 mbar , about after 5s; under 10−9 mbar, about after 1hr. Its means that, if we want the sample is clean enough, almost no pollution, the pressure were below 10−10 mbar. 12.

(18) Figure 3.2: Main chamber, it equip with AES, LEED, and MOKE[27]. 13.

(19) Figure 3.3: Sample preparing chamber, the base pressure is 1x10−9 torr[27].. 3.2. Low Energy Electron Diffraction (LEED). In surface science study, the popular device to observe the atoms arrangement and the structure of thin films is LEED. By LEED images, we can know whether the atoms arrange are regular or not. Generally used 20∼1000 eV for the incidence electronic energy. From de Broglie equation: λ(˚ A) =. v u u (150.6 t. E(eV ). (3.1). E: Incidence electronic energy λ: Corresponding the incidence electronic wave length When the incidence electronic energy is 20eV∼1000 eV, the corresponding wave length is 2.74∼0.388 ˚ A. The materials lattice constant is close to the wave length, so the reflection will produce diffraction pattern. 14.

(20) a: The distance of two atoms, is also the materials lattice constant. θ: The angle of incidence electron beam and reflex electron beam. For 1D space, the distance of two reflex electron beam is d = a sin θ, if two reflex electron beam become to complete constructive interference, then the distance has to be integer multiple of the wavelength of the electron. d = nλ = a sin θ(n = 0, ±1, ±2 · · ·) sin θ =. nλ a. (3.2) (3.3). λ and a are known, from the angle of different incidence electron beam and reflected electron beam, it can correspond the first maximum diffraction pattern. 2π (3.4) |k0 | = λ k0 is incidence electron wave vector h h ,λ = λ mv to make up equation(3.4) and (3.5) p = mv =. (3.5). 2π mv (3.6) h 2π is a constant, mv is an incidence electron momentum, wave vector |k0 | h is relate to mv. From equation(3.3) and (3.4), we can get |k0 | =. 2π n (3.7) a From eq.(3.7) , with different n, we can get the difference of the parallel wave vector. |k0 | sin θ =. 15.

(21) |kk | is the direction which is parallel to the materials. 2π n a Used 1D diffraction theory to extend to 2D. By the same reason ∆kk = |k0 | sin θ =. (3.8). 2π m, m = 0, 1, 2 . . . (3.9) b θb : In 2D space, the angle of incidence and reflected electron beam. b: The lattice constant in 2D space. ∆kk = |k0 | sin θb =. Actuality we measure LEED, it is 2D space, if we needed diffraction pattern, then eq.(3.8),and (3.9) are both satisfied. Used 2D reciprocal lattice vector G, it to regard as the combination of two independent 1D space. 2π 2π n+ m, (3.10) a b Real space and K-space (reciprocal lattice space) transform type.(1D space) G = ∆kk =. |a∗ | =. 2π ∗ 2π , |b | = , a · b ∗ = a∗ · b = 0 |a| |b|. (3.11). a, b is the crystal lattice in real space, a∗ and b∗ is the crystal lattice in K-space. From eq.(3.10) and (3.11) G = ∆kk = na∗ + mb∗ 0. ∆kk = kok − kok = G kok : Wave vector of incidence electron beam. 0 kok : Wave vector of relax electron beam. 16. (3.12) (3.13).

(22) The diffraction pattern needed Elastic Collision electrons, so the wave vector of incidence is equal to reflected. 0. kok = kok. (3.14). Ewald sphere is using the value of the incidence electron wave vector for radius. The atoms in the sample surface are a 2D real space, corresponding to K-space is reciprocal lattice column. Like the Fig 3.4.. Figure 3.4: Ewald Sphere[27].. When reciprocal lattice column and Ewald sphere intersect a point, this point occur diffraction pattern. In this point, the electron is perfect elastic collision, so incidence electron wave vector are equal to reflex. When the incidence electron beam is perpendicular to the sample surface, electrons are collide with the surface atoms, and reflex everywhere. In all reflected atoms, some atoms are elastic collision, some are not. By used LEED measurement, only elastic collision atoms are useful. So the device has to filter out unwanted electron signal, and keep the signal of elastic collision atoms.. 17.

(23) Figure 3.5: LEED device[27]. E-gun energy (EP ) is from 0 eV∼750 eV, when we measure LEED, the sample must to connect with ground. It can avoid sample surface charge by huge electrons. The reflex atoms will go through 3 network structure, and arrive to screen. About the device structure, Grid 1 and Grid 3 are grounding; Grid 2 connects to retarding voltage, which is kept in negative voltage(−EP + ∆V ) ,V is 0 ∼ 10 V. Retarding voltage is used to filter unnecessary electrons. To ensure elastic collision electrons can arrive to the screen, we give a positive high voltage (5keV) to screen. Let electrons to accelerate and collides screen to arouse fluorescence. To analyze the spot position, we can know the structure of the sample.. 3.3. Auger Electron Spectroscopy (AES). Auger electron spectroscopy is a popular analytical technique in the study of surface science. It is because that different atoms has different energy of an Auger electron. The Auger effect is an electronic process, at the heart of AES resulting from the inter and intrastate transitions of electrons in an excited atom. When an atom is probed by an external mechanism, such as a photon or a beam of electrons with high energies about 3KeV, a core state electron 18.

(24) Figure 3.6: Auger Electron[27]. can be removed leaving behind a hole. As this is an unstable state, the core hole can be filled by an outer shell electron, by the electron moving to the lower energy level loses an amount of energy equal to the difference in orbital energies. The transition energy can be coupled to a second outer shell electron which will be emitted from the atom if the transferred energy is larger than the orbital binding energy. An emitted electron will have a kinetic energy of EA = EK − EL1 − EL2,3 EK , EL1 , EL2,3 are respectively the core level, first outer shell, and second outer shell electron energies. In the Fig. 3.7, it is our AES circuit diagram at NTNU. Blue lines are output signal, and green lines are input signal. First, we used PC control NI card to output a negative electric potential to CMA analyzer controller. This signal was transfer to a high-voltage module and amplified by a high electric potential. In the same times, Lock-in sends out a sine wave with an oscillation 5V, frequency 10 KHz to controller. Inside the controller, it has 19.

(25) an isolation transformer, it can combine AC and DC signal and transfer to OC.. Figure 3.7: Connected Line of AES[27].. Simultaneously analyzer controller gave a positive high-voltage to Collector port, in order to let collector receive electron much easier. We also want to get AC signal, so we make a high pass filter after Collector port. High pass filter can filter DC high-voltage and let small AC signal pass through, and then protect Pre-amplifier, Lock-in etc. Small AC signal goes through a pre-amplifier, Lock-in amplifier, then to NI-card and at last we using VB to have Auger spectrum. DPCMA is a device to select Auger electron. In AES system, E-gun ejects an electron beam to the sample. Lots of electrons are stimulated and a part 20.

(26) Figure 3.8: DPCMA[27]. of electrons fly into the concentric column. This two concentric columns can select specific energy electrons. The principle is very simple, when an electron goes through a parallel electroplate, the electrons orbit is a parabolic. We fix the electric field and the incidence angle, than we can fix the range. By the principle, if we change the OC and IC electric field, we can choose the specific energy electron. Because of the collector has higher voltage than OC, so the specific energy electrons go through the focus and channeltron (like amplifier) to the collector.. 3.4. Magneto-Optical Kerr Effect (MOKE). A linear polarization laser beam was used for MOKE measurement. The linearly polarized light is consists of the left-hand circularly polarized light and right-hand circularly polarized light component. The linearly polarized light would change in both phase and reflected intensity after reflecting from a magnetized surface. The different propagating velocity of left-hand circu21.

(27) Figure 3.9: The apparatus of MOKE[27]. larly polarized light and right-hand circularly polarized light results in the phase difference, and the reflected intensity changes due to the different absorption of left-hand circularly polarized light and right-hand circularly polarized light. Hence, the linearly polarized light transforms into elliptically polarized light, and the effect is called Magneto-Optical Kerr Effect. The angle between original linear polarized light and the long axis of elliptically polarized light is called Kerr rotation angle (θk ), the ellipticity of the elliptically polarized light is called Kerr elliptically(εk ). In general description, θk and εk are very small ( 1◦ ), and they are proportional to magnetization of the magnetic material (M). Magneto-optical Kerr effect can be divided into three types, which are depending on the direction of the magnetization vector (M) with respect to the plane of incidence and the sample surface, as shown in Fig. 3.11. When the magnetization vector is perpendicular to the reflection surface and parallel to the plane of incidence, the effect is called the polar Kerr effect(Polar MOKE). To simplify the analysis, near normal incidence is usually employed when doing experiments in the polar geometry.. 22.

(28) Figure 3.10: The illustration of MOKE[27]. In the longitudinal MOKE(In-Plane MOKE), the magnetization vector is parallel to both the reflection surface and the plane of incidence. The longitudinal setup involves light reflected at an angle from the reflection surface and not normal to it, as above in the polar MOKE case. In the same manner, linearly polarized light incident on the surface becomes elliptically polarized, with the change in polarization directly proportional to the component of magnetization that is parallel to the reflection surface and parallel to the plane of incidence. When the magnetization is perpendicular to the plane of incidence and parallel to the surface, it is said to be in the transverse configuration(Transverse MOKE). In this case, the incident light is also not normal to the reflection surface but instead of measuring the polarity of the light after reflection, the reflectivity is measured. This change in reflectivity is proportional to the component of magnetization that is perpendicular to the plane of incidence and parallel to the surface.. 23.

(29) Figure 3.11: Longitudinal (In-Plane), Polar (Perpendicular) and Transverse MOKE[27].. Figure 3.12: The experimental setup of MOKE[27].. 24.

(30) Chapter 4 Results 4.1. O-3x3/W(111). The sample preparation and investigation were performed in an ultrahigh vacuum chamber with a base pres sure 3x10−10 torr. The Co and Ni films were deposited at room temperature (RT) by e-beam heated thermal evaporation. The magnetic properties of the n ML Co, Ni thin films on O-3x3/W(111) were detected in air using polar and longitudinal magneto-optical Kerr effect (MOKE) at room temperature, after covering a protective Pd layer.. Figure 4.1: Preparation conditions and procedures of O-3x3/W(111). Left and right axes are O2 pressure and annealing temperature, respectively. 25.

(31) In previous reports, oxygen on W(111) formed different faceted structures with different annealing temperature. In our experiment, we find a 3x3 structure for oxygen on W(111). How could we form the 3x3 structure? Usually, we cleaned W(111) with 2000 K annealing under O2 atmosphere, in order to get ride of carbon contamination. In this case, the procedure of sample preparation as shown in Fig.4.1. The first step, we cleaned W(111) by cyclic 1500 K annealing under 1x10−6 torr O2 for removing carbon. The second step, after pumping out the oxygen and recovering the base pressure lower than 5x10−10 torr, a well order 3x3 reconstructed surface was formed on the W(111) surface by 1700 K post annealing. Both the two steps of annealing procedure are essential for the 3x3 reconstruction.. Figure 4.2: Low energy electron diffraction (LEED) patterns of W(111) after (a) the first step annealing under 10−6 torr Oxygen and (b) the second step annealing in an ultrahigh vacuum with a base pressure better than 5x10−10 torr. The inset in (a) exhibits the magnified image of the LEED spot, indicated by the rectangle. Why the two steps of annealing procedure are necessary for forming the 3x3 reconstruction? Fig. 4.2(a) shows the LEED patterns taken after the 1st annealing of W(111) under 1x10−6 torr O2 with various annealing temperature. For the 1200 K annealing, as exhibited in the magnified LEED 26.

(32) spot image of Fig. 4.2(a), the triangular shape of diffraction spot was observed, indicating the formation of faceted surface, which was composed of both planar (111) surface and (112)-3-sided pyramids. From Fig. 4.2(a), we know that the LEED image has no significant change with the increase of annealing temperature. It also pointed out that there was no any 3x3 reconstruction without the post annealing. Fig. 4.2(b) is LEED image of the second step annealing in an ultrahigh vacuum with a base pressure better than 5x10−10 torr. It shows that the post annealing lead to the evolution of surface structure. We can clearly see that six-fold satellites around the 1x1 spots appears with post annealing between 1000 and 1600 K. While gradually increase the annealing temperature to 1200-1400 K, the 3x3 reconstruction was more clear to observe. It means that 1200-1400 K is the most suitable annealing temperature for the formation of O-3x3/W(111) surface.. Figure 4.3: Auger electron spectrum measured from the reconstructed O3x3/W(111) surface. The intensity ratio of O(510 eV) to W(170 eV) is around 1/3. (b) LEED patterns of O-3x3/W(111) and clean W(111). 27.

(33) We used AES to investigate the surface chemical composition of the O3x3/W(111) structure. Fig. 4.3(a) shows the AES spectrum taken from the O-3x3/W(111) surface. From Fig. 4.3(a), there are several peaks exist in the AES spectrum. The peak 170 eV and 353 eV belong to W(111). The other peak 510 eV is oxygen. Besides of the W(170 eV) and W353eV peaks, only the O(510 eV) peak is observable. The possible contamination of CO can be excluded, since the C(272 eV) peak is indiscernible. Thus, the 3x3 reconstruction caused by oxygen effect can be demonstrated. In each O-3x3/W(111) preparations, the ratio of O(510 eV) /W(170 eV) is always close to the value of 1/3. It indicates that the required oxygen quantity and the covering rate for this 3x3-reconstruction is quite stable and nearly invariant in the repeated preparations. Fig. 4.3(b) shows more clearly the LEED patterns of O-3x3/W(111) and clean 1x1-W(111) surfaces.. 4.2. Growth and structure. Bulk tungsten is of body-centered cubic structure. The surface atomic structure of W(111) was plotted in Fig. 4(a). As indicated by the three different colors in Fig. 4(a), the W(111) surface is composed of three topmost atomic layers, which are exposed to the vacuum. The edge (lattice constant) of the W(111) hexagonal atomic cell is 4.47 ˚ A. From Fig. 4(b) and (c), the structure of Ni(111) is face-centered cubic and its lattice constant is 2.48 ˚ A. For Co(0001), it is of hexagonal-closed packing structure and the lattice constant is 2.51 ˚ A. The lattice mismatch is too large to form a ideal stack for Ni and Co. However, after 30 degree rotation, as shown in Fig. 4(d) and (e), the six-fold atomic structures of h.c.p.-Co(0001) and f.c.c.-Ni(111) can match the unit cell of b.c.c.-W(111), with the misfit values of 2.76 % and 3.95 % respectively. We expect that the Co thin films may sustain a better crystalline structure and smoother layer-wised morphology due to the smaller lattice mismatch. In order to investigate the growth behavior of Co and Ni thin films on O-3x3/W(111), the AES spectra of these thin films were measured. The AES intensity of Co(776 eV) , W(170 eV) , C(272 eV) and O(510 eV) as a function of Co coverage, as shown in Fig. 4.5(a). The W(170 eV) signal monotonically decreased to nearly zero and Co(776 eV) signal monotonically increased with Co coverage. This indicates that the Co deposit uniformly covered the W(111) substrate, and thus W atoms were buried deeper and 28.

(34) Figure 4.4: (a) Top view of the atomic structure on body-centered-cubic (b.c.c.) W(111) surface. The three different colors indicate the three exposed atomic layers with different height. (b) and (c): Top-most surface atomic structure of hexagonal-close-packed (h.c.p.) Co(0001) and face-centeredcubic (f.c.c.) Ni(111). (d) and (e): The atomic stacking and lattice mismatch for hcp-Co(0001) and fcc-Ni(111) on bcc-W(111), respectively.. deeper below the surface with increasing deposited Co atoms. Specifically, the O(510 eV) signal increased at first and then kept invariant. This observation implied that the oxygen was always not buried deeper and deeper with the W substrate. It means that the oxygen was always on top of the surface, even after large amount of Co deposition . on the other hand, we can exclude the possibility of CO contamination during thin film deposition because the C(272 eV) signal was always within the noise level and nearly unobservable. The Auger signal ratio of Co(776 eV) /W(170 eV) is plotted in Fig. 4.5(b) as a function of Co deposition. Both RT(300 K) and LT(250 K) growth revealed the exponential-like evolution of Co(776 eV) /W(170 eV) ratio. The Co(776 eV) /W(170 eV) ratio increased more and more quickly with Co deposition. This result also implies the 2-dimensional growth behavior of Co on O-3x3/W(111).. 29.

(35) Figure 4.5: (a) Auger signals of Co(776 eV) , W(170 eV) , O(510 eV) and C(272 eV) plotted as functions of the deposited Co coverage. (b) Auger signal ratio of Co(776 eV) /W(17o eV) plotted as a function of the deposited Co coverage. Square and circle denote the Auger ratio measured from RT(300 K) and LT(200 K)-grown Co films on O-3x3/W(111). In contrast, for Ni/O-3x3/W(111), the AES intensity of Ni(848 eV) , W(170 eV) , C(272 eV) and O(510 eV) as a function of Ni coverage as shown in Fig. 4.6(a). The behavior of C(272 eV) and O(510 eV) are similar to the case of Co/O-3x3/W(111) from the AES results. Since the C(272 eV) signal was always within the noise level and nearly unobservable, we can exclude the possibility of CO contamination during thin film deposition. The Ni(848 eV) AES signal increased slowly and the W(170 eV) was always observed with Ni coverage. Furthermore, Fig. 4.6(b) shows that the Ni(848 eV) /W(170 eV) Auger ratio increased slowly with more and more Ni deposition. This implies that Ni preferred the 3-dimensional growth behavior on O-3x3/W(111). Even after large amount of Ni deposition, the Ni atoms forms 3-d islands, instead of uniform films, still exposing some areas of W substrate in the surface region. To demonstrate the 3-dimensional growth in Ni/O-3x3/W(111), we do an lower temperature growth for Ni/O3x3/W(111). When the substrate was cooled at a temperature (LT=250 K) lower than RT, the mobility of deposited Ni atoms would slow down and thus the 3-d islands formation would be deuced. Actually, the LT-grown Auger data indeed shows the much higher ratio of Ni(848 eV) /W(170 eV) while the substrate O-3x3/W(111) was simply cooled down from RT to LT. One thing deserves to be mentioned is that the O(510 eV) was still clearly observable 30.

(36) Figure 4.6: (a) Auger signals of Ni(848 eV) , W(170 eV) , O(510 eV) and C(272 eV) plotted as functions of the deposited Ni coverage. (b) Auger signal ratio of Ni(848 eV) /W(17o eV) plotted as a function of the deposited Ni coverage. Square and circle denote the Auger ratio measured from RT(300 K) and LT(200 K)-grown Ni films on O-3x3/W(111). after Ni deposition, and the intensity was nearly the same as the signal measured before Ni deposition. This indicates that the similar surfactant effect played by oxygen in both Co and Ni thin films on O-3x3/W(111).. 4.3. Thermal stability. In previous literature, the thermal stability of Co and Ni on W(111) had been investigated, as shown in Fig. 4.7. From Fig. 4.7(a), we know that the AES signal ratio of Co(775 eV)/W(169 eV) decreases as the annealing temperature is increased to 1200 K for an initial AES ratio > 3.0. The AES ratio decreases dramatically from 500 K to 700 K, and almost keep invariant up to 1100 K. This implies that the wetting layer formed between 700 K and 1100 K. On the other hand, the AES signal ratio of Ni(848 eV)/W(169 eV) decreases from 300 K to 700 K, and nearly keep invariant up to 1000 K for an initial AES ratio > 3.0. Similarly, Ni on W(111) formed wetting layer between 700 K and 1100 K. In contrast, for Co,Ni/O-3x3/W(111), the results revealed different behavior in thermal stability. The Auger spectrum of Fig. 4.8, Fig. 4.9 and 31.

(37) Figure 4.7: (a) Plot of the Co(775 eV)/W(169 eV) AES ratio as a function of annealing temperature for several different Co coverage. (b) Plot of the Ni(848 eV)/W(169 eV) AES ratio as a function of annealing temperature for several different Ni coverage. The data is quoted from [2]. Fig. 4.10 were measured after cooling down from various annealing temperatures. Fig. 4.8 shows the Auger signals of Co(776 eV), W(170 eV), C(270 eV) and O(510 eV) for RT and LT-grown n ML Co/O-3x3/W(111) after postannealing at different temperatures. From Fig. 4.8(a)-(c), the Co(776 eV) Auger sinal quickly dropped down while annealing to about 600 K, decreased slowly after 600-830 K annealing, and finally dropped to zero after 1000-1200 K annealing. Similarly but inversely, W(170 eV) Auger sinal quickly raised while annealing to 600 K, increased slowly after 600-830 K annealing. The Auger ratio of Co(776 eV)/W(170 eV) was summarized in the lower panel of Fig. 4.8. Independent of growth temperature and Co coverage,after 600 K annealing, the Co(776 eV) /W(170 eV) ratio always quickly dropped to 1, which means that the apparent thickness of 1 ML, and then gradually decreased to zero. The AES ratio value kept invariant up to 1000 K, it means 1 ML wetting layer formed between 600 and 1000 K. In the system of Ni/O-3x3/W(111), Fig. 4.9 and Fig. 4.10 show that the annealing-temperature dependent evolution of Ni(848 eV), W(170 eV), C(270 eV) and O(510 eV) Auger signal for RT and LT-grown n ML Ni/O3x3/W(111), respectively. The AES signal of Ni(848 eV) dropped down gradually with annealing temperature increasing. Contrary to Ni(848 eV), the AES signal of W(170 eV) increased gradually with annealing temperature increasing. The Auger ratio of Ni(848 eV)/W(170 eV) quickly decreased to 1 while annealing to about 800 K, and dropped to zero at about 1100 K. From 32.

(38) Figure 4.8: (a)-(c) Evolution of Co(776 eV) , W(170 eV) , O(510 eV) and C(272 eV) Auger signals plotted as functions of annealing temperature for n ML Co films grown on O-3x3/W(111). (d)-(f) Auger ratio of Co(776 eV) /W(170 eV) deduced from (a)-(c) respectively and plotted as a function of annealing temperature.. Figure 4.9: (a)-(c) Evolution of Ni(848 eV) , W(170 eV) , O(510 eV) and C(272 eV) Auger signals plotted as functions of annealing temperature for n ML RT-grown Ni films on O-3x3/W(111). (d)-(f) Auger ratio of Ni(848 eV) 33 /W(170 eV) deduced from (a)-(c) respectively and plotted as a function of annealing temperature..

(39) this result, we know that no wetting layer forms after annealing. Interestingly, during the multi-step post-annealing, O(510 eV) Auger signal always kept invariant. No matter how the Co and Ni aggregated and formed 3d islands, oxygen is always on top of the surface.. Figure 4.10: (a)-(c) Evolution of Ni(848 eV) , W(170 eV) , O(510 eV) and C(272 eV) Auger signals plotted as functions of annealing temperature for n ML LT-grown Ni films on O-3x3/W(111). (d)-(f) Auger ratio of Ni(848 eV) /W(170 eV) deduced from (a)-(c) respectively and plotted as a function of annealing temperature.. 4.4. Magnetism. Fig. 4.11 and Fig. 4.12 show the magnetization of Ni/O-3x3/W(111) and Co/O-3x3/W(111). From Fig. 4.11, Ni/O-3x3/W(111) reveals in plane magnetization and weak perpendicular magnetization. It means that Ni/O3x3/W(111) is canted magnetization. As shown in Fig. 4.12, Co/O-3x3/W(111) performs in plane magnetization.. 34.

(40) Figure 4.11: Hysteresis loop of Ni/O-3x3/W(111). Figure 4.12: Hysteresis loop of Co/O-3x3/W(111). 35.

(41) Figure 4.13: (a) Hysteresis loop of 9.61 PML Co/O-3x3/W(111) in different angle. (b) Coercivity of 9.61 PML Co/O-3x3/W(111) as a function of rotating angle.. 36.

(42) Fig. 4.13 and Fig. 4.14 show different angle hysteresis loop of 9.61 PML and 6 PML Co/O-3x3/W(111) respectively. Coercivity of 9.61 PML Co/O3x3/W(111) as a function of rotating angle, as shown in Fig. 4.13(b). The largest coercivity point appears repeatedly after 60 degree rotating. It means that Co/O-3x3/W(111) reveals six-fold symmetry of magnetization. The 6 PML Co/O-3x3/W(111) shows similar behavior in magnetism, as shown in Fig. 4.14. The 6 PML Co/O-3x3/W(111) also reveals six-fold symmetry of magnetization.. 37.

(43) Figure 4.14: (a) Hysteresis loop of 6.0 PML Co/O-3x3/W(111) in different angle. (b) Coercivity of 6.0 PML Co/O-3x3/W(111) as a function of rotating angle.. 38.

(44) Chapter 5 Discussion Q1: Why Ni performs the island growth and Co performs the layerwise growth on O-3x3/W(111)? In chapter 4.2, the result reveals that Ni performs the island growth on O3x3/W(111) and Co performs the layerwise growth on O-3x3/W(111), why? It may be due to the lattice mismatch. As shown in Fig. 4.4, the lattice mismatch of Ni/W(111) and Co/W(111) are 3.95% and 2.76% respectively. The larger lattice mismatch of Ni/W(111) results in the island growth, and the suitable lattice constant of Co causes the layerwise growth for Co/W(111). There may exist other reasons to affect growth mode, but we need more experimental data to demonstrate. Q2: How does the growth mode affect the magnetic behavior? The different growth mode of Ni and Co film causes different magnetic behavior. From Fig. 4.11 and Fig. 4.12, the Ni/O-3x3/W(111) reveals the canted magnetization, and Co/O-3x3/W(111) reveals the in plane magnetization. Owing to the island growth of Ni/O-3x3/W(111), the magnetic moment of Ni can be separate into two parts, in plane magnetic moment and perpendicular magnetic moment, and it results in the canted magnetization. However, for Co/O-3x3/W(111), it reveals layerwise growth, and the direction of the magnetic moment is in plane direction which causes the in plane magnetization. The six-fold symmetry in magnetism of Co is also due to the layer growth because the Co films grows follow the W(111) substrate. Hence, the Co films has the same symmetry as W(111) six-fold symmetry. Q3: Why Co and Ni are different in thermal stability of wetting layer? About the thermal stability, a wetting layer is an initial layer of atoms that is epitaxially grown on a surface. The thickness of wetting layer is due to the lattice mismatch in theoretically. Co/O-3x3/W(111) exists 13 PML 39.

(45) wetting layer result from a smaller lattice mismatch 2.76 %, and the wetting layer does not exist for Ni/O-3x3/W(111) because of a larger lattice mismatch 3.95 %. Q4: How do you demonstrate the exist of surfactant effect of oxygen? As mentioned in chapter 4.3, O(510 eV) Auger signal always keep invariant after deposition, even the thickness increases over 10 PML. The similar behavior is observed after annealing, as shown in the result of thermal stability. These results implies that the oxygen atom is always on the top of surface after deposition and annealing, as a surfactant. The surfactant effect of oxygen in other system had been demonstrated in previous literature[1621]. These literatures indicates that the oxygen gas adsorbs on surface and the oxygen atom should float on the surface in order to decrease the surface energy. Q5: Why the post-annealing is necessary? The oxygen molecule adsorbs on the W(111) surface, and then it breaks up into atoms. With the oxygen exposing time increasing, the AES signal of O(510 eV)/W(170 eV) increases quickly and finally saturates about a maximum of 0.5. It implies that the quantity of oxygen is limited on the W(111) surface. Moreover, we also do the experiment about annealing times. The results show that the AES signal of O(510 eV)/W(170 eV) decreases with annealing times, and it reaches a minimum of about 13 as the O-3x3 surface reconstructed. It means that the oxygen is too much to form the 3x3 surface after oxygen exposing. Hence, we must reduce the quantity of oxygen by post-annealing. Therefore, a well-ordered O-3x3 surface is formed after post-annealing with proper temperature. Q6: Does the change of AES signal ratio of Co/W and Ni/W result from alloy effect? From the phase diagram of Co-W and Ni-W, as shown in Fig. 5.1, it shows that no alloy forms when the percentage of Ni and Co is lower than 10 %. Hence, we can conclude that no alloy effect occurs in Co/O-3x3/W(111) and Ni/O-3x3/W(111).. 40.

(46) Figure 5.1: The phase diagram of Co-W[28].. Figure 5.2: The phase diagram of Ni-W[29]. 41.

(47) 42.

(48) Chapter 6 Summary In previous literature, the oxygen induced facetting on W(111), and we are interesting in the magnetism of magnetic material deposited on facetting surface which induced by oxygen. From our results, a 3x3 reconstruction was formed after oxygen exposing and annealing with proper temperature. Without the two-step of annealing, no 3x3 reconstruction formed. In the other reports, Ni deposited on a clean W(111) crystal reveals Vollmer-Weber growth mode, and Co deposited on a clean W(111) was layer by layer growth. In contrast, for Ni/O-3x3/W(111), AES signal of tungsten was always observable, implying that Ni undergoes island-growth on O-3x3/W(111). For Co/O-3x3/W(111), Co undergoes layerwise growth on O-3x3/W(111).. 43.

(49) About the thermal stability, the wetting layer formed after about 700 K annealing in the case of Co/W(111) and Ni/W(111). However, our experimental result reveals that the wetting layer is not formed after annealing, even after large amount of deposition in the case of Ni/O-3x3/W(111). Oppositely, the smaller wetting layer was observed after annealing in Co/O-3x3/W(111). Our experimental result also indicates that the Co and Ni films which grew on O-3x3/W(111) formed 3D islands after annealing process. The AES signal as a function with annealing temperature. As we see, no wetting was formed in Ni/O-3x3/W(111). Differently, the wetting layer whose thickness is 31 PML(1 ML) formed after about 600 K annealing in Co/O-3x3/W(111), and the wetting layer of Co/O-3x3/W(111) is thinner than Co/W(111) whose thickness is 1 PML(3 ML).. Finally, about the magnetism, Co/O-3x3/W(111) reveals the in plane magnetization, and Ni/O-3x3/W(111) reveals a canted magnetization.. 44.

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