Ferromagnetism in laser-deposited GaMnAs layers

The properties of the GaMnAs layers grown by laser deposition on semi-insulating GaAs(100) substrates at temperatures from 300 to 650°C have been investigated. A strong anisotropy of the hysteretic curve of the angle of rotation of the plane of polarization with a change in the magnetic field direction in the sample plane was found during investigation of the magneto-optical Kerr effect (300 K). The domain structure in GaMnAs layers has been observed for the first time at room temperature by magnetic-force microscopy.     

Magnetization processes in the narrow domain structures of amorphous ferromagnetic alloys

The magnetization processes within the narrow domain laminae of amorphous ferromagnetic alloys have been investigated by means of the magneto-optical Kerr effect. Changes of the domain width of the closure domains by a magnetic field applied perpendicular to the laminae have been determined for the alloys Fe₈₀B₂₀ and Fe₄₀Ni₄₀P₁₄B₆. These results are compared with theoretical calculations, assuming that wall displacements within the closure domains and rotations of the magnetization in the bulk domains take place simultaneously and a stray field free domain structure is developed. It turned out, that the closure domain structure on the surface of the sample vanishes at the same magnetic field where magnetic saturation is approached.     

Magnetization reversal asymmetry in Fe/MgO(0 0 1) thin films

We study the in-plane magnetization process in 200 Å Fe(0 0 1) thin films grown by sputtering at normal incidence. In spite of this growth geometry, a small uniaxial in plane magnetic anisotropy, whose origin is not totally understood, is found superimposed to the expected cubic biaxial one. This has a dramatic effect both on the reversal process and the domain structure. A combined longitudinal and transversal Kerr study shows the different switching processes (180° walls along the main easy axis versus 90° along the secondary easy axis) depending on the relative orientation of the magnetic field with respect to the Fe crystallographic axes. Remarkably, this two- and sometimes three-step switching process appears only when the field is applied along certain crystallographic directions. These findings are corroborated by domain observations.     

Measuring the Magnetooptic Kerr effect by diffraction

A method is presented to measure the Magnetooptic Kerr Effect (MOKE) by diffraction. This is accomplished by giving the magnetized surface a magnetization distribution which functions like a linear diffraction grating. It is shown that the first and higher orders of the light diffracted by such a grating are produced by MOKE whereas the zero order approximately represents the reflectivity of the surface (given by the Fresnel formulae assuming zero magnetization). The procedure used here to form a grating is restricted to thin soft magnetic films, and has been demonstrated on evaporated Fe-Ni films. It uses the magnetic field of a pulse-driven meandered stripe-conductor placed in close contact with the film in the presence of a homogeneous external magnetic field. Experimentally determined Kerr intensities were compared with those measured by ellipsometric methods. Good agreement was found in the case of films with uniaxial anisotropy. For films with no anisotropy, the relative dependence of Kerr intensity vs wavelength, polarisation and angle of incidence could be derived.     

MnBi磁性多层膜磁光科尔效应的数值模拟

基于各向异性的平面电磁波传输理论和4×4矩阵法，计算了MnBi，Mn_0.53Bi_0.47，Mn_0.52Bi_0.44Sb_0.04磁性多层膜系列的磁光科尔旋转角、椭圆率随波长、入射角、磁性层厚度变化曲线.计算结果表明，模拟的科尔旋转角、椭圆率随波长变化规律与实验结果吻合很好；在一定波长的垂直入射下，模拟的科尔旋转角、椭圆率随磁性层厚度变化曲线出现科尔谱极大；在磁性层厚度一定的情况下，当入射光以某一角度 关键词： 4×4矩阵法 磁性多层膜 磁光科尔谱 介电张量     

Theory for nonlinear magnetooptics in metals

The electronic theory for nonlinear magnetooptics in metals is presented. Characteristic results are given for the enhancement of the Kerr rotation, its dependence on light polarization and direction of the magnetization, on surface and film structure and on material-specific properties. The nonlinear susceptibility tensor χ_ijl(2ω) is calculated and analyzed. We discuss also how second harmonic generation is used to investigate magnetic domain structure and nonequilibrium magnetic behavior as analyzed by pump-probe spectroscopy. Finally, the theory is extended to describe generally the time-dependent nonlinear response to external fields.     

Co/Cr/Pd多层膜的磁性和磁光特性的研究

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Direct observation of magnetization reversal of epitaxial MnAs films

The magnetization reversal of MnAs epitaxial films on GaAs (0 0 1) substrates was investigated using a Kerr microscope. The direct observation of the change in domain structure under magnetic fields revealed characteristic magnetization reversal process of MnAs films with a ladder-type domain structure. The nucleus of the magnetization reversal region appears and propagates to neighboring α-MnAs lines, and then the ladder-type structures cover all over the surface. Finally the domain wall displacement occurs to expand the domain. The change in magnetic domain reflects the characteristic ridge/groove structures of MnAs films.     

Magnetic domain structure of anisotropic Nd2Fe14B-based magnets produced via the hydrogenation,decomposition, desorption and recombination (HDDR) process

Lorentz transmission electron microscopy and optical Kerr microscopy studies of the hydrogenation, decomposition, desorption and recombination (HDDR)-processed anisotropic Nd₂Fe₁₄B-based magnets show that a magnetic domain structure develops in which a unit domain includes a number of submicron Nd₂Fe₁₄B grains coupled by the intergrain-exchange interaction. Grain boundaries and particles of an intergranular phase have been observed to pin the domain boundary walls, and their magnetic characteristics and distribution may determine the size of magnetic domains forming in a thermally demagnetized state. Measurements of differential susceptibilities indicate that the motions of the domain walls become sluggish after magnetization and dc-field demagnetization, compared to that in a thermally demagnetized state.     

Needle-like domain structure in Co films deposited on Mo (1 1 0)

Magnetization reversal processes and domain structures have been studied in Mo(1 1 0)/Co(0 0 0 1)/Au(1 1 1) structures grown by molecular beam epitaxy on monocrystalline (11–20) sapphire substrates. Wedge-shaped samples with different Co thickness gradients relative to the Mo [0 0 1] direction were fabricated. Observation of the domain structure was performed at room temperature using Kerr microscopy in a Co thickness range varying from 5 to 50 nm, where the magnetization is oriented in the plane of the sample. A Co thickness-dependent coercivity field was determined through analysis of the domain wall position during the reversal process. A preferential orientation of magnetic domain walls was found, with the domains being needle-like. The orientation, as well as the size of the needles, depends on the Co thickness and the orientation of the magnetic field applied in the sample plane.     

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.     

Magneto-optical linear dichroism in threshold photoemission electron microscopy of polycrystalline Fe films

Magnetic linear dichroism in threshold photoemission has been exploited to obtain magnetic contrast in a photoemission electron microscope using a mercury arc lamp. The dichroism at threshold can be described similar to the magneto-optical Kerr effect in the region of visible light. The asymmetry of electron intensity observed for a 100 nm polycrystalline Fe film on silicon is A=(0.37+/-0.05)%. The asymmetry occurs for the geometry of the transverse Kerr effect. For unpolarized light the asymmetry was about half the value observed for linearly polarized light. Threshold photoemission microscopy has a large potential for high resolution magnetic domain imaging with fast data acquisition.     

PdMn/Co多层膜的磁性和磁光特性研究

用磁控溅射法制备了不同Mn含量的PdMn/Co磁性多层膜，通过x射线衍射对该多层膜系列进行结构分析；测定了不同Mn含量系列样品的磁滞回线、垂直各向异性及磁力显微镜图，分析了饱和磁化强度、磁畴和垂直各向异性变化的原因；通过测定该多层膜体系的克尔谱，简要分析了一定波长下克尔角随Mn含量增加而变化的物理机制. 关键词： 多层膜 磁性 磁光     

An analysis of the mechanism of Kerr effect enhancement in Mn/Dy/Bi

A study is reported of the structural, magnetic, and magneto-optic properties of Mn/Dy/Bi films obtained by multilayer technology. The maximum Kerr rotation angle in such films is shown to be θ _k =2.25°. Possible reasons for such a large Kerr effect enhancement are considered, namely, an increase in the 6p–3d transition probability caused by symmetry distortion, polarization of the Bi6p band, and a change in the density of states near the Fermi level. The latter reason has been analyzed by simulating the electronic structure of Mn/Dy/Bi through superposition of Dy levels on the MnBi band structure. This approach has revealed possible additional transitions which may be induced by the presence of a Dy buffer and could contribute to the Kerr magneto-optic effect. Fiz. Tverd. Tela (St. Petersburg) 41, 91–97 (January 1999)     

New possibilities for tuning ultrathin cobalt film magnetic properties by a noble metal overlayer

Complementary multiscale magneto-optical studies based on the polar Kerr effect are carried out on an ultrathin cobalt wedge covered with a silver wedge and subsequently with the Au thick layer. A few monolayers of Ag are found to have a substantial effect on magnetic anisotropy, the coercivity field, and Kerr rotation. The silver overlayer thickness-driven magnetic reorientation from easy axis to easy plane generates a new type of 90 degrees magnetic wall for cobalt thicknesses between 1.3 and 1.8 nm. The tuning of the wall width in a wide range is possible. Tailoring of the overlayer structure can be used for ultrathin film magnetic patterning.     

Versatile UHV system for combined far- and near-field magneto-optical microscopy of thin films

We have developed a UHV system for in situ studies of magnetic domains and magnetization reversal of thin films in the presence of external magnetic fields and at variable temperature. The system comprises a setup for magneto-optical Kerr effect measurements of magnetization curves, a Kerr-microscope for far-field magnetic-domain imaging, and a magneto-optical scanning near-field microscope in combination with a Sagnac interferometer (Sagnac-SNOM) for high-resolution imaging on a sub-μm scale. All components have successfully been tested, and the feasibility of studying ultrathin films has been demonstrated.     

180° domain structure in a cubic U3P4 crystal

In a thick plate cut from a U₃P₄ single crystal perpendicularly to the 111 direction, changes in the domain structure under the influence of a strong magnetic field perpendicular to the plate surface were studied. Experiments were carried out at T = 28 K, applying the Kerr effect technique to make the domain structure visible. In the cubic crystal studied, the applied field was found to force the appearance of a 180° branched maze domain structure, typical of thick plates of highly anisotropic uniaxial materials. The Hubert-Bodenberger model of this structure was improved and extended by considering the change in the shape of domains occurring with increase in the distance from the sample surfaces, by describing the sample in a non-zero magnetic field and by analyzing the sample in the state of remanence. The model was applied to the case of U₃P₄ and the energy density of 180° domain walls was estimated (γ = 12 erg/cm²).     

Magnetization curves and domain structures of a 38.7% NiFe (110) single crystal disk

Magnetization curves of the volume of a (011) oriented 38.7% Ni?Fe disk with magnetic fields applied in [100], [011] and [111] directions are compared with the corresponding domain structure of the surface observed by means of the magneto-optical Kerr effect. The intensity reflected from the surface yielded Kerr magnetization curves, which give information about the magnetization of the surface and allow to distinguish between boundary displacements and magnetization rotation. Besides some similarities to SiFe differences were found which are mainly due to the low anisotropy energy. The magnetization of the surface is different from that of the volume.     

Scaling properties of magnetic domain walls in Pt/Co/Pt trilayers on MgO (1 1 1)

We present a scaling analysis of the time evolution of domain walls in ultrathin magnetic films that are subject to different forms of uncorrelated and correlated disorder caused by the microstructure of the underlying template. The study is performed on ultrafine modulated Pt/Co/Pt trilayers grown on as-supplied and structured MgO (1 1 1) substrates employing polar Kerr microscopy for the imaging of the magnetic domains.     

Novel trends in the study of magnetically soft Co-based amorphous glass-coated wires

An overview of the recent progress and state-of-the-art results in the investigation of the amorphous glass-coated wires with nearly zero magnetostriction is presented. These versatile microwires display enhanced soft magnetic properties, which make them suitable as sensing elements in various sensors for biomedical and automotive applications. Current results on their magnetic characteristics refer to a major refinement of their core-shell magnetic structure by taking into account the interdomain wall and to the thorough analysis of the magnetization within the outer shell. Experimental techniques such as giant magneto-impedance, magneto-resistance, and magneto-optical Kerr effect measurements are employed to prove the outcome of the theoretical calculations. The impact of the magnetic structure of the outer shell on the propagation of domain walls in bistable amorphous wires is analyzed. Very recent results on the magnetization process in nearly zero magnetostrictive amorphous glass-coated wires with submicron dimensions are also reviewed.