Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

Dependence of Interlayer AF Coupling on Ferromagnetic Layer Thickness in [Pt/Co]5/Ru/[Co/Pt]5 Multilayers

We study magnetization reversal in the interlayer coupled [Pt/Co]5/Ru/[Co/Pt]5 multilayers （MLs） by means of the measurement of extraordinary Hall effect （EHE）. Fitting experimental data to a simple model, we determine the interlayer coupling strength for various thicknesses of the ferromagnetic layers at a fixed Ru spacer thickness. It is found that the dependence of interlayer coupling strength on the Pt layer thickness is much stronger than the previous report in the ferromagnetic/nonmagnetic/ferromagnetic multilayers.     

High-Temperature Characteristics of Ti/Al/Ni/Au Ohmic Contacts to n-GaN

We present the high-temperature characteristics of Ti/Al/Ni/Au（15 nm/220 nm/40 nm/50 nm） multiplayer contacts to n-type GaN （Nd = 3.7 × 10^17 cm^-3, Nd = 3.0 × 10^18 cm^-3）. The contact resistivity increases with the measurement temperature. Furthermore, the increasing tendency is related to doping concentration. The higher the doped, the slower the contact resistivity with decreasing measurement temperature. Ti/Al/Ni/Au ohmic contact to heavy doping n-GaN takes on better high temperature reliability. According to the analyses of XRD and AES for the n-GaN/Ti/Al/Ni/Au, the Au atoms permeate through the Ni layer which is not thick enough into the AI layer even the Ti layer.     

Stability and Growth Modes of Ni--C Clusters: A Study based on All-Electron Density Function Theory

Growth modes of the free-standing NiCN （N ≤ 8） and Ni2CN （N ≤ 8） dusters are investigated by the allelectron density functional theory. The results reveal that there are two competing modes for the growth of these clusters： the linear chain and the ring structure without transannular bonds. The lowest-energy geometries of NiCN （N ≤ 8） are the linear chains with the Ni atom at one end, except for NiC2 and NiCT. The Ni2CN （N ≤ 8） clusters all prefer to the linear chains with the two Ni atoms at the two ends. Miilliken population analysis indicates that the total spin of the lowest-energy cluster show significant odd-even alternation. The NiMCN （M = 1,2） clusters with the even N are one and those with the odd-N are zero.     

Hydrogen Sensors Based on AlGaN/AlN/GaN Schottky Diodes

Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3 V at 300 K is obtained at a fixed forward current after switching from N2 to 10%H2＋N2. The sensor responses under different concentrations from 50ppm H2 to 10%H2＋N2 at 373K are investigated. Time dependences of the device forward current at 0.5 V forward bias in N2 and air atmosphere at 300 and 373K are compared. Oxygen in air azcelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity Of the sensor are calculated.     

Microstructure and magnetic properties of multilayered [Fe/Pt]n structures prepared by successive deposition

The structure and magnetic properties of multilayered [Fe/Pt]n structures prepared by successive magnetron sputtering of Fe and Pt plates and deposition of Fe and Pt layers on a preliminarily heated glass substrate have been studied as functions of the number n and thickness of the layers. Mössbauer studies and measurements of magnetic hysteresis loops (MH) have established that [Fe/Pt]n films for n = 16 exhibit primarily magnetic anisotropy normal to the film plane. Data obtained by X-ray photoelectron spectroscopy (XPS) strongly suggest that the films have an interface between the substrate and the multilayered structure. Our micromagnetic modeling leads to the conclusion that the magnetic anisotropy oriented normal to the [Fe/Pt]n film plane (for n = 16) is induced by formation of an anisotropic interface.     

In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.     

Fe40Ni40P12B8非晶合金的冲击晶化实验研究

 本文研究了Fe₄₀Ni₄₀P₁₂B₈非晶合金冲击波加载下的晶化行为，冲击波由二级轻气炮发射的告诉弹丸撞击靶产生。实验结果表明：Fe₄₀Ni₄₀P₁₂B₈非晶合金在冲击波加载下，晶化可在加载时间（微秒量级）内发生；晶化的阈值压力在30~50 GPa之间，相应的冲击温度约为510~800 K，晶化析出相与冲击压力有关，低压下析出相是面心立方γ-(Fe, Ni)固溶体和Fe₃(P_0.37B_0.63)化合物，高压下（大于60 GPa）析出相除了面心立方γ-(Fe, Ni)固溶体和Fe₃(P_0.37B_0.63)化合物之外，还包括(Fe, Ni)₃P化合物。     

Magnetic Behaviour of Sm2Co7/Fe/Sm2 Co7, Nanocomposite Trilayers with Cr and Ti Additions

Trilayered Sm2Co7/Fe/Sm2Co7 spring exchange magnets are fabricated by dc magnetron sputtering on MgO substrates. Very thin layers （0.3-0.7 nm） of Cr and Ti are added at the interfaces of the two magnetic phases. The thickness of StucCo7 is kept at 20nm and Fe at 6nm while the thickness of Cr and Ti are varied as 0.3, 0.5, and 0.7nm. The base pressure of sputtering chamber is kept below 10^-7 Torr and Ar pressure at 3-8m Torr. The samples are characterized by x-ray diffraction （XRD） and SQUID magnetometer. We report improvement in exchange coupling of nonacomposite magnets by addition of thin layers of Cr at interfaces.     

Effect of diffusion and stresses on the magnetic properties of multilayer Fe/Pd and Fe/Ge films

Multilayer [Fe/Pd]₁₀ and [Fe/Ge]₁₀ films were studied using ferromagnetic resonance, magnetic-force and atomic-force microscopies, magneto-optical Kerr effect, and X-ray diffraction. It was shown that an increase in the interlayer thickness causes changes in magnetic parameters, such as the induced anisotropy, effective magnetization, coercive force, and the parameters of the fine magnetic structure. Reasons of the changes in the magnetic parameters were established.     

Spin wave relaxation and magnetic properties in [M/Cu] super-lattices; M=Fe, Co and Ni

In this work, we study the elementary excitations and magnetic properties of the [M/Cu] super-lattices with: M=Fe, Co and Ni, represented by a Heisenberg ferromagnetic system with N atomic planes. The nearest neighbour (NN), next nearest neighbour (NNN) exchange, dipolar interactions and surface anisotropy effects are taken into account and the Hamiltonian is studied in the framework of the linear spin wave theory. In the presence of the exchange alone, the excitation spectrum E(k) and the magnetization 〈S^z〉/S analytical expressions are obtained using the Green's function formalism. The obtained relaxation time of the magnon populations is nearly the same in the Fe and Co-based super-lattices, while these magnetic excitations would last much longer in the Ni-based super lattice. A numerical study of the surface anisotropy and long-ranged dipolar interaction combined effects are also reported. The exchange integral values deduced from a comparison with experience for the three super-lattices are coherent.     

Study of the magnetism and the magnetic anisotropy of Fe layers in Ni/Fe/Ni(1 1 1) by Mössbauer spectroscopy

The hyperfine field and the magnetic anisotropy of a Fe layer as a function of thickness have been investigated in several Ni/⁵⁷Fe_x/Ni(1 1 1) trilayers with relatively thick Ni layers by Mössbauer spectroscopy. For Fe layers with thickness below 16 Å, the Mössbauer spectra show always the presence of two ferromagnetic phases with high-spin state. In the range between 6 and 8 Å, also a ferromagnetic phase with low-spin state and a paramagnetic phase have been found. The evolution of the mean hyperfine field of the ⁵⁷Fe nuclei is used to study the Fe growth. A structural FCCBCC phase transition is found to begin with an iron thickness of 8 Å. The easy direction of the magnetization is found out-of-plane for Fe interlayer with FCC structure, and perfectly in plane for Fe interlayer with BCC structure.     

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

Domain morphology in ultrathin ferromagnetic films with perpendicular magnetization

We determine the minimal domain structure for the equilibrium thickness of stripes as well as for the minimal energy of the domain configuration in ultrathin films of ferromagnetically coupled spins, where the easy direction of magnetization is perpendicular to the film. It is found that the equilibrium thickness of stripes and walls depend on the exchange energy. The normalized anisotropy, f, depends on interplay between the magnetic and anisotropy energies and is almost independent of the exchange energy inside the wall. The results are compared with the experimental data for thin Ag/Fe/Ag (0 0 1) films and a good coincidence is obtained between both results.     

Magnetic anisotropy of multilayer [Fe/Pt]n structures

The possibility of controlling the magnetic anisotropy of multilayer [Fe/Pt]n structures grown by magnetron sputtering of Fe and Pt plates by varying number n of layers is studied. Mössbauer spectroscopy data and measured magnetic hysteresis loops demonstrate that the multilayer [Fe/Pt]n structures at n = 16 have a predominantly perpendicular magnetic anisotropy. The results of X-ray photoelectron spectroscopy and micromagnetic simulation point to the presence of intermediate layers enriched in iron ions in the structures. The magnetic anisotropy perpendicular to the surface of the [Fe/Pt]n films at n = 16 is found to be caused by the anisotropy of the intermediate layers.     

Spin-engineering in the Co75Fe25/Cu(1 1 0) system

We present results on the growth and magnetic anisotropies of Co₇₅Fe₂₅ films grown on a Cu(1 1 0) single crystal. Angular dependent MOKE measurements show a thickness dependent, in-plane rotation of the easy axis of magnetisation of up to 60° from the [0 0 1] direction (towards [−1 1 0]). For a film thickness of 5 ML, just greater than that required for the onset of ferromagnetism, uniaxial anisotropy is observed with the easy axis along the [0 0 1] direction. As the film thickness increases this is seen to rotate in-plane towards the [−1 1 0] direction as the contribution from the cubic anisotropy constant grows. At a film thickness of 9 ML there is predominantly cubic anisotropy and at 10 ML the easy axis is rotated to 150° with respect to the [1 −1 0] axis, where it is stabilised.     

Structural and magnetic properties of BCC Fe/Co (0 0 1) superlattices

In thin layered Fe/Co (0 0 1), grown on MgO (0 0 1), both Fe and Co crystallize in the body-centered cubic (BCC) structure, as seen in a series of superlattices where the layer thickness of the components is varied from two to twelve atomic monolayers. These superlattices have novel magnetic properties as observed by magnetization and polarized neutron reflectivity measurements. There is a significant enhancement of the magnetic moments of both Fe and Co at the interfaces. Furthermore, the easy axis of the system changes from [1 0 0] for films of low cobalt content to [1 1 0] for a Co content exceeding 33%. No indication of a uniaxial anisotropy component is found in any of the samples. The first anisotropy constant (K₁) of BCC Co is found to be negative with an estimated magnitude of 110 kJ/m³ at 10 K. In all cases, the magnetic moments of Fe and Co have parallel alignment.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

FMR investigation of magnetic anisotropies in Fe/Au superlattices

Fe(xML)/Au(xML) superlattices (1⩽x⩽4, ML: monatomic layer thickness) have been investigated by the ferromagnetic resonance method at room temperature. It has been confirmed that out-of-plane anisotropy field H_u shows oscillatory behavior as a function of Fe layer thickness with a period of 1 ML as reported previously from magnetization measurements. In addition, we have found that the in-plane fourfold anisotropy field H₂ oscillates in a similar manner. The easy magnetization axis for x⩾2.25 is Fe [1 1 0] in contrast with the case of bulk Fe, and the values of H₂ show maxima for x=2.5 and 3.5, suggesting that the atomic steps at interfaces are formed along the Fe [1 1 0] direction. Furthermore, the interface roughness for x=non-integer causes wide distributions of H_u and H₂ compared to those for x=integer with flat interfaces.     

High-frequency excitations of stripe-domain lattices in magnetic garnet films

Magnetic garnet films of composition (Y,Bi)₃(Fe,Al)₅O₁₂ have been grown by liquid phase epitaxy on [111] and [110] oriented substrates of gadolinium gallium garnet. The domain wall resonance and the two branches of the domain resonance of periodic stripe domains are measured as function of the bias induction applied in the film plane parallel to the stripes. Resonance frequencies up to 7.5 GHz are observed. An improved version of the hybridization model is developed to describe these resonances. It turns out that hybridization of the domain resonance branches is determined by the cubic anisotropy for [111] oriented films, while for [110] oriented films coupling of the domain resonances is mainly caused by the orthorhombic anisotropy. The theoretical model is in excellent agreement with experiments, no fitting parameters are used. It is also used to derive the phase relation between the precessing magnetizations of neighbouring domains.