Effect of the anisotropy of the electron g-factor in spin polarization

Spin polarization in the presence of an external magnetic field and electric bias in quantum confined semiconductor structures has been studied by time- and polarization-resolved spectrometry. From measurements with angular variations of the magnetic field from the Voigt configuration (VC) it was found that both the frequency (Ω) and decay rate (β) of the oscillatory component of the polarization increase with variation of the angle from the VC. Their dependences are discussed based on the electron spin dephasing related to the spread of the electron g-factor (g_e) (i.e. unequal values of the longitudinal (g_e||) and transverse (g_e⊥) components of g_e) and the exchange interaction between the electron and hole spins. It is demonstrated that the increase in Ω upon deviation of the magnetic field from the VC relates to the anisotropy of g_e (g_e|| and g_e⊥) resulting from the quantum confinement effect. However, the angular dependence on β is related to the residual exchange interaction between the electron spin and rapidly relaxing hole spin.     

用环形试样测定软磁材料磁各向异性的研究

采用趋近饱和定律测定了纳米晶合金环形试样有效磁各向异性常数＜K＞.为了对比测量的准确度,同时测试了传统的晶态坡莫合金环形试样的磁晶各向异性.结果表明,用环形试样可以完成对低矫顽力的软磁材料进行磁各向异性的测定.     

Stabilization of spin skyrmions in magnetic materials and a two-dimensional electron gas

An integrodifferential equation for skyrmions in a magnetic material is solved by the finite-difference method with allowance for Dzyaloshinskii-Moriya and dipole-dipole interactions.     

自旋畴壁动力学的若干研究进展

磁畴壁运动是实现信息高速存储的新途径,而畴壁调控领域的研究目前尚不全面.探索磁场、电流及其他物理场对磁畴壁的作用,在理论与实际应用方面均具有重要意义.本文简述了近年来磁畴壁的自旋结构、磁畴壁电阻及磁畴壁动力学的若干研究进展,阐述了磁畴壁研究的主要方向、方法以及重要性,分析了形状结构对畴壁类型的影响和各种外场对畴壁运动的调控,提出了通过对磁畴壁的精确操控,可实现信息存储.展望了利用外场调控磁畴壁,为信息读写或逻辑控制提供新途径,在未来信息存储领域具有潜在的应用价值.     

Growth and magnetic properties of MnGe films for spintronic application

Morphological and magnetic properties of Mn _x Ge_1–x films have been investigated by scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE) measurements, respectively. Several Mn _x Ge_1–x alloys were grown by molecular beam epitaxy (MBE) on Ge(1 0 0) substrates, varying the growth temperature and alloy composition (x). STM analysis demonstrated island morphology with islands having a mean dimension ranging from about 100 to 130 nm, depending on the substrate temperature and Mn content in the film. Growth conditions also influence the island distribution. MOKE analysis, carried out on all the Mn _x Ge_1–x alloys, showed only a negligible hysteresis effect in the investigated temperature range from about 12 to 300 K. At low temperatures (below 70–110 K, depending on the sample), the MOKE signal tends to saturate at a magnetic field intensity less than about 0.5 T, indicating a superparamagnetic behavior. On the contrary, above that temperature the films do not show a magnetic character. The features of the MOKE curves depend on the growth parameters.     

晶粒间界相对纳米硬磁材料各向异性的影响

采用立方晶粒结构模型研究了晶粒间界相对纳米硬磁材料交换耦合相互作用和有效各向异性的影响.结果表明,晶粒间界相减弱了交换耦合作用,使晶粒的平均各向异性〈K〉增加,有助于提高材料的有效各向异性Keff;而非磁性晶粒间界相又会使Keff降低,导致Keff在间界相为某一厚度d时取极大值.适当厚度的晶粒间界相可以提高材料的有效各向异性和矫顽力.     

Interfacial spin polarization and magnetic structure of Co/MgO/Co magnetic tunnel junction: ab initio calculation

Using ab initio method based on the density functional theory, the equilibrium bcc-Co(001)/rocksalt-MgO(001)/bcc-Co(001) magnetic tunnel junction structure was investigated. Spin polarization and magnetic moment were calculated for each atomic slab in the equilibrium structure by spin dependent density of states analysis. Interfacial Co atoms showed significantly larger spin polarization of -88.3%, compared to the value of inner Co slabs, -82.3%, and bulk bcc Co, -82.1%. Interestingly, Mg and O atoms also showed induced spin polarizability ranged from -45.0% to -66.0%, except for O atoms in the centered slab of barrier layer, which showed relatively small polarization, -14.9%. Magnetic moments for the electrode Co atoms were calculated to be approximately 1.74 microB with no significant variation across the electrode.     

Probing materials with positrons

During the last few years a large body of research has developed around the use of positron techniques to study materials problems. The value of the positron as a probe into the structure of materials stems from the rather special characteristics of the gamma rays which result from the annihilation of the positron with its antiparticle, an electron. Positrons are positively charged particles and are attracted to regions of a solid with low nuclear density, such as defect sites. The annihilation gamma rays from positrons trapped at such sites have slightly different characteristics from those originating from annihilation events in the rest of the material, and this forms the basis of a truly non-destructive technique for detecting defects. This article describes some applications of positron annihilation to problems involving mechanical damage in alloys, with a special emphasis on the prospects for its practical use in non-destructive testing.     

Programming magnetic anisotropy in polymeric microactuators

Polymeric microcomponents are widely used in microelectromechanical systems (MEMS) and lab-on-a-chip devices, but they suffer from the lack of complex motion, effective addressability and precise shape control. To address these needs, we fabricated polymeric nanocomposite microactuators driven by programmable heterogeneous magnetic anisotropy. Spatially modulated photopatterning was applied in a shape-independent manner to microactuator components by successive confinement of self-assembled magnetic nanoparticles in a fixed polymer matrix. By freely programming the rotational axis of each component, we demonstrate that the polymeric microactuators can undergo predesigned, complex two- and three-dimensional motion.     

Photocurrent polarization anisotropy of randomly oriented nanowire networks

While the polarization sensitivity of single or aligned NW ensembles is well-known, this article reports on the existence of residual photocurrent polarization sensitivities in random NW networks. In these studies, CdSe and CdTe NWs were deposited onto glass substrates and contacted with Au electrodes separated by 30-110 microm gaps. SEM and AFM images of resulting devices show isotropically distributed NWs between the electrodes. Complementary high resolution TEM micrographs reveal component NWs to be highly crystalline with diameters between 10 and 20 nm and with lengths ranging from 1 to 10 microm. When illuminated with visible (linearly polarized) light, such random NW networks exhibit significant photocurrent anisotropies rho = 0.25 (sigma = 0.04) [rho = 0.22 (sigma = 0.04)] for CdSe (CdTe) NWs. Corresponding bandwidth measurements yield device polarization sensitivities up to 100 Hz. Additional studies have investigated the effects of varying the electrode potential, gap width, and spatial excitation profile. These experiments suggest electrode orientation as the determining factor behind the polarization sensitivity of NW devices. A simple geometric model has been developed to qualitatively explain the phenomenon. The main conclusion from these studies, however, is that polarization sensitive devices can be made from random NW networks without the need to align component wires.     

Coating cracks in materials with general anisotropy

The problem of a straight crack lying in the coating of a semi-infinite substrate is considered. The most general case of anisotropy is assumed for both materials (coating and substrate). The solution of Atkinson and Eftaxiopoulos [2] for a dislocation in an anisotropic bimaterial is modified to solve the fundamental problem of a dislocation in the coating, by including a corrective solution that accounts for the additional boundary conditions at the free surface. The coating crack is modelled as a continuous distributions. The resulting system of singular integral equations is solved by applying an inversion theorem and then using numerical integration. The stress intensity factors for modes I, II and III can be directly obtained from the dislocation densities. Numerical results are presented to compare the present analysis with existing solutions for some particular geometries and material models. For the general anisotropic case, results are presented showing the influence of a number of geometric and material parameters.     

Ferroelectric control of magnetic anisotropy

We demonstrate unambiguous evidence of the electric field control of magnetic anisotropy in a wedge-shaped Co film of varying thickness. A copolymer ferroelectric of 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE) overlays the Co wedge, providing a large switchable electric field. As the ferroelectric polarization is switched from up to down, the magnetic anisotropy of the Co films changes by as much as 50%. At the lowest Co thickness the magnetic anisotropy switches from out-of-plane to in-plane as the ferroelectric polarization changes from up to down, enabling us to rotate the magnetization through a large angle at constant magnetic field merely by switching the ferroelectric polarization. The large mismatch in the stiffness coefficients between the polymer ferroelectric and metallic ferromagnet excludes typical magnetoelectric strain coupling; rather, the magnetic changes arise from the large electric field at the ferroelectric/ferromagnet interface.     

Induced magnetic anisotropy and strain in permalloy films deposited under magnetic field

Field induced magnetic-anisotropy is a very important but poorly understood property. There have been many hypotheses on the origin of the phenomenon, e.g. strain, atomic pair ordering, etc., but little experimental evidence exist. This study prepares 100 nm thick Permalloy films having the field-induced-magnetic-anisotropy and carefully measure strains, i.e. interplanar distance of crystallographic (111) planes in various directions, using high power synchrotron radiation and precise Grazing Incidence X-Ray Diffraction method. The result delineates that the field-induced-magnetic-anisotropy has a strong correlation with the strain-anisotropy in the film.     

Magnetocrystalline anisotropy energy and spin polarization of Fe3Si in bulk and on Si(001) and Si(111) substrates

In order to achieve highly efficient spin polarized transport, first of all magnetocrystalline anisotropy energy, which determines the magnetic easy axis, must be understood. The highly precise full-potential linearized augmented plane-wave method is employed to investigate the magnetism and magnetocrystalline anisotropy energy of a ferromagnetic Heusler alloy Fe₃Si on Si(001) and Si(111) substrates. The calculated magnetocrystalline anisotropy energy of bulk D0₃ Fe₃Si was found to depend sensitively on a tetragonal distortion: The magnetization is along the z-axis at c/a < 1 and on the xy plane at c/a > 1. The out-of-plane magnetic easy axis of both Fe₃Si/Si(001) and (111) was calculated to be quite stable with enhanced magnetocrystalline anisotropy energy compared with bulk value. The magnetic easy axis of Fe₃Si/Si(001) and (111) is discussed in detail with single particle energy spectra. The degree of spin polarization is also presented at the interfaces between Fe₃Si and Si. The calculated spin polarizations of Fe₃Si/Si(111) tend to retain the spin polarization of the bulk, whereas they are reduced for the (001) interfaces.     

Induced in-plane magnetic anisotropy in YIG films

We have studied the symmetry properties of the in-plane induced magnetic anisotropy in {100} and {110} disks of single crystal YIG epitaxially grown on gadolinium gallium garnet (GGG) substrates. The ferromagnetic resonance (FMR) technique was used to determine the values of the cubic and induced magnetic anisotropies. The induced anisotropy is interpreted in terms of a magnetostrictive magnetic potential energy. We find that the effective field induced in the plane of the film is isotropic for {100} disks but for {110} disks the induced anisotropy is uniaxial within the plane.     

Rotatable magnetic anisotropy in epitaxial ferrite layers

Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg0.9Mn0.3Fe1.8O4ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K1= - 2.2 × 10⁴ergs/cc, KN= 2.3 × 10⁵ergs/cc.     

Nuclear spin echo in magnetic fluids

The two-pulse proton spin echo signal decay was studied in benzene-based magnetic fluids containing magnetite (filler) and oleic acid (surfactant). The spin echo signal decay rate increases with the magnetic filler concentration. A decrease in the transverse proton relaxation time is due to the increasing inhomogeneity of the local nuclear magnetic fields.