Electric control of magnetic moment in a ferromagnet/semiconductor hybrid system

It is shown that the exchange coupling in a “ferromagnet/semiconductor quantum well” heterostructure allows the electric control of the orientation of magnetic moment in the ferromagnet. A highly anisotropic exchange interaction between holes in the quantum well and magnetic atoms in the ferromagnet causes the orientational transition: magnetic moment leaves the plane and becomes oriented along the normal. The normal component of magnetization can be inverted by applying voltage pulses to the structure gate.     

纯有机聚合物磁性的第一性原理研究

基于全电子势线性缀加平面波的第一性原理计算，针对在聚乙炔的侧链上挂有有机自由基团（CH₂）的典型纯有机磁性聚合物体系的能带结构、带自旋的总的和分的电子态密度、劈裂能量、磁矩等物理量进行了定量分析，发现自旋能带劈裂发生在布里渊区边界上，定量说明了这一体系是反铁磁的，同时说明有机聚合物中只有共轭π电子对磁性有贡献． 关键词：     

Magnetization of the ferromagnetic-superconductor structures

The magnetization of a structure consisting of a thin superconducting film lying on a thin ferromagnetic substrate with uniaxial anisotropy is considered. It is shown that if a ferromagnet is in a multidomain state, then due to the presence of a superconducting film, the period of its domain structures decreases, which is caused by the increase of the magnetostatic energy of the system owing to Meissner currents. In a certain range of the constant of uniaxial anisotropy, under the action of a superconducting film, a domain structure may transform from a strip structure to the structure with closure domains. It is found that due to the nonuniform magnetic field of a multidomain ferromagnet, Abrikosov vortices may exist in a thin film only at certain parameters of the magnetic field.     

Elimination of magnetic relaxation in superconductors on approaching a ferromagnet

It is found that magnetic relaxation is suppressed in a high-temperature superconductor when it approaches a ferromagnet. It is supposed that this phenomenon results from formation of a nonequilibrium magnetic structure, in which counter driving forces act in different regions.     

The magnetic barkhausen effect

When the magnetization of a ferromagnet (in this paper “ferromagnet” will often include “fernmagnet”) is changed under the influence of an applied magnetic field, the magnetization, particularly in the vicinity of the coercive force, is not smooth function of the field but rather shows a structure composed of many individual steps. This structure is evidence of irreversible changes in magnetization and is known as the barkhausen effect. The effect was named after its discoverer who, in 1919, connected a coil surrounding a ferromagnet to a newly invented amplifier and listened to the output of the amplifier with earphones.¹     

Josephson effect in SIFS with an inhomogeneous superconductor/ferromagnet interface

The Josephson effect in a superconductor-insulator-ferromagnet-superconductor structure with a stepwise change in the transparency of the superconductor/ferromagnet interface has been investigated within the formalism of the Usadel equations. It is shown that, at a certain thickness of the ferromagnet layer, this feature leads to the formation of a point contact with an anomalous dependence of the critical current on the external magnetic field.     

Probing antiferromagnetic order with heat capacity in exchange-biased structures

We explore the magnetic heat capacity in exchange-biased ferromagnet/antiferromagnet bilayers theoretically. We show that changes in the antiferromagnetic structure due to the reversal of the ferromagnet layer can be detected by distinct features in the heat capacity. This offers a method for probing antiferromagnetic domains in exchange-biased systems.     

Proximity effect and spontaneous vortex phase in planar SF structures

The proximity effect in SF structures is examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arising in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.     

Phase boundary between uniaxial and planar ferromagnets in layered perovskite manganite La2−2xSr1+2xMn2O7 (0.313⩽x⩽0.350)

We have examined magnetizations as a function of temperature and magnetic field in layered perovskite manganites La_2−2xSr_1+2xMn₂O₇ single crystals (x=0.313, 0.315, 0.318, 0.320 and 0.350) in order to determine the phase boundary between two ferromagnets (one is an uniaxial ferromagnet whose easy axis is parallel to the c-axis and the other is a planar ferromagnet whose easy axis is within the ab-plane) and following results are obtained: (i) all the present manganites exhibit magnetic transitions from a ferromagnet to a paramagnet at 76, 107, 116, 120 and 125 K for x=0.313, 0.315, 0.318, 0.320 and 0.350, respectively; (ii) for x=0.318, 0.320 and 0.350, the magnetic structure is a planar ferromagnet below Curie temperature; (iii) for x=0.313 and 0.315, the magnetic structure changes from an uniaxial to a planar ferromagnet at 66 and 85 K, respectively. From the results described above we have constructed the magnetic phase diagram of layered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (0.313?x?0.350).     

Pinning of vortices by the domain structure in a two-layered type II superconductor-ferromagnet system

The effectiveness of magnetic pinning of vortices in a layered system formed by a uniaxial ferromagnet and type II superconductor is considered. It is shown that, irrespective of the saturation magnetization of the ferromagnet, the energy of pinning at the domain structure does not exceed, in order of magnitude, the energy of artificial pinning at a column-type defect. The limitation of pinning energy is caused by the interaction of external vortices with a spontaneous vortex lattice formed in the superconducting film when the magnetization of the ferromagnetic film exceeds the critical value.     

Nonlinear wave propagation through a ferromagnet with damping in (2+1) dimensions

We investigate how dissipation and nonlinearity can affect the electromagnetic wave propagating through a saturated ferromagnet in the presence of an external magnetic field in (2+1) dimensions. The propagation of electromagnetic waves through a ferromagnet under an external magnetic field in the presence of dissipative effect has been studied using reductive perturbation method. It is found that to the lowest order of perturbation the system of equations for the electromagnetic waves in a ferromagnet can be reduced to an integro-differential equation.     

Asymmetric kinetics of magnetization reversal of thin exchange-coupled ferromagnetic films

This paper reports on the results of the investigation of the kinetics of magnetization reversal in FeNi-FeMn ferromagnet-antiferromagnet thin hybrid films grown by magnetron sputtering on silicon substrates in the presence of in-plane magnetic field, which provided unidirectional in-plane magnetic anisotropy in the ferromagnetic layer and a single-domain structure of the ferromagnet in the absence of an external magnetic field. The constructed hysteresis loops and magnetization loci have made it possible to reveal the specific features of the magnetization reversal process of an exchange-coupled ferromagnet, to establish new types of asymmetry, and to obtain new proofs for the existence of a spin spring at the ferromagnet-antiferromagnet interface. The visualization of the magnetization reversal process has allowed one to establish a one-to-one correspondence between the macrocharacteristics of the material and the real processes occurring in ferromagnet-antiferromagnet hybrid structures.     

Effect of the Semiconductor Spacer on Positive Exchange Bias in the CoNi/Si/FeNi Three-Layer Structure

JETP Letters - Films consisting of a hard magnetic ferromagnet CoNi and a soft magnetic ferromagnet FeNi interacting through a nonmagnetic Si semiconductor spacer are experimentally studied. The...     

Magnetic structure of the interface between ferromagnet and antiferromagnet with parallel anisotropy axes

The structure of a spin spiral arising in the vicinity of the interface between a ferromagnet and a compensated antiferromagnet is studied in the case where the anisotropy axes of the layers are parallel to each other, and the evolution of this spiral in an external magnetic field is investigated.     

Magnetostatic surface waves produced by an inhomogeneity of the anisotropy with a turning point of the spectral function on a ferromagnet surface

Magnetostatic surface waves with fixed frequency and wave vector are predicted to exist in a ferromagnet with an inhomogeneity of the magnetic anisotropy such that the spectral function has a turning point on the surface. This result is most important for the case when an external magnetic field magnetizes the ferromagnet perpendicular to its surface. The frequency of the surface wave is determined by the frequency of the magnetostatic volume wave at the surface of the ferromagnet, and the wave vector is determined by the surface values of the local magnetic anisotropy field and its derivative. Zh. Tekh. Fiz. 68, 118–123 (June 1998)     

Orientational phase transition into a modulated magnetic state in the weak ferromagnet FeBO<Subscript>3</Subscript>: Mg

The transition from a homogeneous into a modulated magnetic state in the easy-plane weak ferromagnet FeBO₃: Mg is studied by a magnetooptic method. At T < 135 K, the application of a magnetic field in the basal plane of the crystal is shown to excite the modulation of its magnetic order parameter, which manifests itself in a periodic deviation of the local ferromagnetism vector from the magnetization direction. The conditions for the existence of a modulated magnetic superstructure in FeBO₃: Mg are studied, and its preferred orientation in the basal plane of the crystal is analyzed. A magnetic H-T phase diagram that shows the boundaries between the homogeneous and inhomogeneous magnetic states of this weak ferromagnet is constructed. The modulation period and the azimuthal angle specifying the local ferromagnetism vector direction in the structure are studied as a function of temperature and magnetic field. The results obtained are discussed in terms of the theory of magnetic ripple using the model of anisotropic magnetic centers appearing in a crystal doped by magnesium ions.     

Optical detection in a ferromagnet

The detection of plane-polarized amplitude-modulated infrared laser radiation in a transparent yttrium-iron garnet ferromagnet at room temperature is performed experimentally. The nonlinear interaction mechanism is suggested. It is found that the magnitude and sign of the detected signal depend significantly on the magnetizing external magnetic field. The signal is observed when the magnetization of the ferromagnet approaches the saturated value. The measured dependence of the amplitude of the detected signal on the angle of polarization of the laser radiation with respect to the magnetization of the ferromagnetic sample supports the suggested nonlinearity mechanism. The largest nonlinearity appears when the magnetic field of the plane-polarized laser radiation is parallel to the magnetic moment of the magnetized ferromagnet.     

Temperature characteristics of magnetoelectric interaction in langatate-ferromagnet composite resonators

Magnetoelectric interaction in a layered langatate-hysteretic ferromagnet structure is observed with no bias magnetic field applied. The temperature characteristics of interaction are determined mainly by the properties of the ferromagnetic layer. Raising the temperature weakens the coupling between layers and lowers the efficiency of interaction.     

Effect of the transformation of the magnetic structure of a FeBO<Subscript>3</Subscript>:Mg crystal on its magnetooptical anisotropy

The field dependence of the magnetic linear birefringence (MLB) in easy-plane weak FeBO₃:Mg ferromagnet has been investigated. It is found that MLB nonmonotonically (stepwise) tends to a constant value with an increase in magnetic field at low temperatures. This feature of MLB in FeBO₃:Mg is related to the transformation of the crystal magnetic structure during magnetization.     

Structure of the magnetic-field-induced modulated phase of an α-Fe2O3: Ga single crystal

The field and orientational dependences of magnetic linear birefringence in α-Fe₂O₃: Ga single crystals are investigated. The experimental results make it possible to refine and specify the structure and reveal the features of technical magnetization of the modulated magnetic phase of this weak ferromagnet. It is shown that the model of the modulated magnetic structure used in this study adequately describes the experimental situation.