Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.     

Parameters of the transition layer in the exchange-biased NiFe/DyCo film structure

The parameters of the transition layer in exchange-biased film structures are necessary agents to understand the mechanism of formation of unidirectional anisotropy. The layer thickness in NiFe/DyCo films has been determined by comparison of signals of the polar magneto-optical Kerr effect from a reference DyCo film and a hard magnetic layer of the exchange-biased structure. The layer thickness obtained is one order of magnitude larger than that characteristic of ferromagnet-antiferromagnet bilayer films. The mechanism of magnetization reversal of the structure under study has been explained within the model suggesting the formation of 180° boundaries in the interface.     

Cyclotron resonance and screening effects in GaAs-GaAlAs heterojunctions

Cyclotron resonance absorption has been studied in several high quality GaAs-GaAlAs heterojunctions. The dependence of the effective mass upon frequency, electron concentration and temperature has been measured and gives a nonparabolicity that is some 20% smaller than that measured in bulk GaAs. This is contrary to simple theory which predicts an enhanced polaron contribution in two-dimensional systems, and we attribute this difference to strong screening of the electron-phonon interaction. The presence of a small magnetic field component parallel to the interface introduces a coupling at energies corresponding to the electric subband separations. These couplings have been studied as a function of electron concentration and the results interpreted in terms of changes in the depletion charge in the GaAs layer.     

Coupling of ferroelectric and antiferromagnetic order parameters in hexagonal RMnO3

Second-harmonic generation (SHG) is used as a probe for the coexisting ferroelectric and antiferromagnetic orders in hexagonal RMnO₃. SH contributions coupling to the electric and/or magnetic order parameters are identified on the basis of their spectral dependence and the symmetries of the corresponding order parameters. The SH signals from the ferroelectric and antiferromagnetic orders were employed to image the electric and magnetic domain structures separately. The transformation properties of electric and magnetic domains are discussed with respect to the transformation properties of the corresponding order parameters. An investigation of the mutual coupling between the coexisting electric and magnetic orders reveals apparently independent domain structures, which contradicts the symmetry and transformation properties of electric-dipole-induced SHG in this ferroelectromagnetic group of compounds. Apart from higher-order multipole contributions to SHG, interface contributions from clamped electric and magnetic domains can solve the contradiction. Received: 16 October 2001 / Published online: 24 April 2002     

Nanomagnetism in nanocrystalline multiferroic bismuth ferrite lead titanate films

Multiferroics conventionally refer to the materials exhibiting co-existing electric, magnetic, and structure order parameters. Interplay between ferroelectricity, magnetism, and ferroelasticity in a single phase makes multiferroics truly multifunctional providing control over magnetic and electric ordering by applying electric and magnetic fields, respectively. Incorporation of multiferroic-based components into nanoscale applications will enable additional degrees of freedom in manipulating with spin and charge not easily attainable otherwise. Multiferroic bismuth ferrite lead titanate has been chemically synthesized in form of nanocrystalline films. The morphology of the films revealed a single perovskite phase confined within crystalline grains of few tens of nm in size. The films were found to exhibit ferroelectricity and ferromagnetism with characteristic electric polarization and magnetization hysteresis loops, transformations associated with spin reorientation in an external magnetic field and the spin-glassy behavior well above the room temperature. High degree of magnetic frustration and disorder in the spin system spatially confined in the nanograins, distribution of the grains anisotropy axis, inter-grain interactions, and the effects of uncompensated spins on the large effective surface/interface favored by the nanocrystalline morphology were assumed to be responsible for the anomalous magnetic properties and glassy dynamics in the films.     

Effect of exchange interaction in ferromagnetic superlattices:A Monte Carlo study

The Monte Carlo simulation is used to investigate the magnetic properties of ferromagnetic superlattices through the Ising model. The reduced critical temperatures of the ferromagnetic superlattices are studied each as a function of layer thickness for different values of exchange interaction. The exchange interaction in each layer within the interface and the crystal field in the unit cell are studied. The magnetic coercive fields and magnetization remnants are obtained for different values of exchange interaction, different values of temperature and crystal field with fixed values of physical parameters.     

超导体界面层的电场电荷分布及起源

从相对论性超导场方程出发，探讨了弱磁场条件下超导体界面层的结构及形成机理.得出在超导体界面层电场、电势差、Hall效应及电荷密度“三明治”式层叠结构的存在，是源于满足相对论协变性要求的势场可观测效应.并提出了一个实验思路，以期验证该观点的正确性. 关键词： 超导 界面层电场 相对论效应 层叠结构     

Influence of Electric and Magnetic Fields on the Polaron in a Quantum Well

A combinative method of variational wavefunction and harmonic oscillator operator algebra, the ground-state energy correction to an electron confined in the quantum well of GaAs/Ga_1-xAl_x, As in the electric and magnetic fields along the growth axis has been studied by taking into account the interaction of different optical phonon modes with the electron. The ground-state energy is obtained as a function of the well width and the strength of electric and magnetic fields. The results show that the magnetic field greatly enhances the in terface-phonon part of the polaronic correction to electron ground-state energy in the well width d ≤ 300 Å. The electric field also enhances the polaron effect of interface mode, but decreases the part of bulk longitudinal mode.     

Rashba spin-orbit interaction effects on thermal and magnetic properties of parabolic GaAs quantum dot in the presence of donor impurity under external electric and magnetic fields

Based on the effective mass approximation, the magnetic and thermal properties of parabolic GaAs quantum dot have been investigated in the presence of Rashba Spin-Orbit interaction (RSOI), donor impurity and applied magnetic and electric fields. The exact diagonalization method has been used to solve the Hamiltonian of an electron confined in a quantum dot (QD) and obtain the eigenenergies and the binding energy of the donor impurity as a function of various QD physical parameters. We have shown the dependence of the average statistical energy, magnetization, magnetic susceptibility and heat capacity of the donor impurity in the QD on: the Rashba interaction parameter, the magnetic and electric fields, confining frequency, and temperature. The results reveal that these parameters can tune the magnetic properties of the GaAs quantum dot and flip the sign of magnetic susceptibility from negative (diamagnetic) to positive (paramagnetic) type material.     

Oscillation galvanomagnetic effect in double-layer metal films

A semiclassical approximation has been used to analyze theoretically the dependence of the conductivity σ (resistivity ρ) of a double-layer film consisting of plane-parallel single-crystal metal layers of different thickness d_j ≠ d_n and grade l_j ≠ l_n on the layer thickness ratio d_2,1 = d₂/d₁ and on the magnitude of the magnetic field normal to the layer interface. A general expression (for any d_2,1) and asymptotic expressions (for thick and thin (compared to the electron free path l_j) metal layers in a weak and a strong magnetic field) for the conductivity of a double-layer specimen have been obtained. A nonmonotonic relation between the conductivity of a double-layer film and the layer thickness ratio at weak magnetic fields has been predicted which is sensitive to the character of the interaction of charge carriers with the conductor boundaries. At strong magnetic fields, ρ becomes an oscillating function of magnetic field (layer thickness). A detailed numerical analysis of the resistivity of a double-layer film on the layer thickness ratio and on the magnetic field strength has been performed for arbitrary values of the parameters that describe the character of the interaction of charge carriers with the specimen boundaries. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 26–33, July, 2007.     

CEMS-study of the interface in Fe/Cr multilayer systems

The magnetic hyperfine (hf.) fields at the Fe/Cr interface were analyzed in epitaxial Fe/Cr thin film structures of (100)- and (110)-orientation with monolayer resolution by means of in-situ⁵⁷Fe Conversion Electron Mössbauer Spectroscopy (CEMS). The hf. field (300 K) in the 1st Fe-monolayer (ML) at the interface has been found to be strongly reduced to 22.0/20.9 T for (110)-/(100)-orientation, whereas the 2nd and 3rd ML reveal a slightly increased hf. field of 33.7 T as compared with the Fe-bulk value of 33.4 T. The temperature dependence of the hf. field at the interface shows aT ^3/2 spin wave law. The spin wave parameters are enlarged with respect to the bulk value indicating a reduced exchange interaction. A discontinuity in theT ^3/2-dependence is interpreted by the onset of magnetic order (Néel-temperature) of the Cr layers adjoining the⁵⁷Fe probe layer.     

Nuclear alignment of implanted160Tb in highly oriented pyrolytic graphite layers

Low-temperature nuclear alignment of¹⁶⁰Tb ions implanted in Highly Oriented Pyrolytic Graphite has been observed by the anisotropic intensity distribution of γ-rays. The data can be understood with an effective spin Hamiltonian containing combined magnetic and electric hyperfine interactions. Values deduced for the hyperfine parameters are A/k=(101±7) mK for the magnetic interaction, and P/k=(−3.7±0.9) mK for the nuclear electric quadrupole interaction, under the assumption that B=0 and Δ=0. Measurements in magnetic fields of 0.2 and 1.0 T directed along the graphite c-axis showed thermal saturation due to the strongly reduced heat conductivity of HOPG in the presence of a magnetic field.     

Motion of domain walls under the action of spin-polarized current in a magnetic junction

The effect of spin-polarized current on a domain structure in a magnetic junction consisting of two ferromagnetic metallic layers separated by an ultrathin nonmagnetic layer is studied within a phenomenological theory. The magnetization of one ferromagnetic layer (layer 1) is assumed to be fixed, while that of the other ferromagnetic layer (layer 2) can be freely oriented both parallel and antiparallel to the magnetization of layer 1. Layer 2 can be split into domains. Charge transfer from layer 1 to layer 2 is not attended with spin scattering by the interface but results in spin injection. Due to s-d exchange interaction, injected spins tend to orient the magnetization in the domains parallel to layer 1. This causes the domain walls to move and “favorable” domains to grow. The average magnetization current injected into layer 2 and its contribution to the s-d exchange energy are found by solving the continuity equation for carriers with spins pointing up and down. From the minimum condition for the total magnetic energy of the junction, the parameters of the periodic domain structure in layer 2 are determined as functions of current through the junction and magnetic field. It is shown that the spin-polarized current can magnetize layer 2 up to saturation even in the absence of an external magnetic field. The associated current densities are on the order of 10⁵ A/cm². In the presence of the field, its effect can be compensated by such a high current. Current-induced magnetization reversal in the layer is also possible.     

Dynamics of self-organization in a thin layer of a magnetic fluid placed in a constant electric field

Structuring in the form of rotating rings, which has not been observed previously in magnetic fluids, is revealed, and the parameters of these structures are determined. Also, the evolution of vortices in a magnetic fluid layer is traced. The electrical properties of the layer exposed to a constant electric field are studied as functions of the field magnitude and exposure time.     

Magnetic and magnetooptical properties of multilayer ferromagnet-semiconductor nanostructures

The magnetic and magnetooptical properties of spin-tunneling multilayer permalloy-silicon carbide nanoheterostructures deposited by rf sputtering have been studied. Magnetometric and magnetooptical methods are used to show that the magnetic-semiconducting nanostructures have a complex magnetic structure and to track the evolution of the magnetic properties of these structures as functions of the magnetizing field and the thickness and sequence order of ferromagnetic and semiconducting layers in them. The induction response and the field and orientation dependences of the transversal Kerr effect are found to have anomalies. The experimental results are interpreted under the assumption that there is exchange interaction between the ferromagnetic and semiconducting layers through a thin magnetically ordered transition layer formed inside the interface.     

Spin-wave spectroscopy and application of its methods to heterostructures of silicon dioxide with Co nanoparticles on a GaAs substrate

A method has been developed for determining magnetic and electrical characteristics of film nanostructures containing magnetic nanoparticles from dispersion curves of surface spin waves propagating in these nanostructures. The dispersion curves of spin waves are determined by the dynamics of the spin component described by the generalized Landau-Lifshitz equations and an alternating electromagnetic field induced by a spin wave. Since spin waves are very sensitive to inhomogeneity of magnetic parameters, spin disorder, and conductivity of an object near or inside which these waves propagate, they can be used for determining magnetic and electrical characteristics of the objects under investigation. The developed calculation method, which can be employed both in spin-wave spectroscopy and in analysis of dispersion curves obtained by other methods, has been used for determining parameters of heterostructures consisting of a SiO₂ film with Co nanoparticles on a GaAs substrate. It has been found from the shape of dispersion curves of the surface spin waves that, in the film near the interface, spins of the nanoparticles are close to a ferromagnetic ordering, whereas near the free surface, the spin orientation of nanoparticles is more chaotic. It has been revealed that a conducting layer is formed in GaAs, and the SiO₂(Co) film near the interface has an increased conductivity.     

单晶石墨、多晶石墨电导行为的差异

报道了单晶石墨、多晶石墨电导行为的差异.在8.15T磁场下，4.5K处单、多晶石墨的正磁电阻效应分别为69400%，170%，同时在0T，8T磁场作用下，多晶石墨的电导行为呈现类绝缘体型性质，但在单晶石墨中我们发现了与磁场相关的类半金属-绝缘体型转变，通过分析，我们认为：单、多晶石墨电导行为存在较大差异的原因可能来源于库仑相互作用在高定向热解石墨中变得不可忽视，而多晶石墨样品却存在晶界散射. 关键词： 半金属石墨 类半金属-绝缘体型转变     

Study of the current layer at the boundary between the plasma and a magnetic obstacle in a laboratory experiment

The behavior of the plasma and magnetic and electric fields in the current layer that separates oppositely-directed magnetic fields is studied experimentally. The layer is formed by the interaction of a magnetic dipole with a magnetized plasma flow at Mach numbers lower than 10.     

Effect of optical blooming of a nanocrystal monolayer and the interface between two media

It is shown that several parameters of the near-field interaction of particles effectively control the reflectance of the interface between vacuum and a semi-infinite medium in a structured monolayer of nanoparticles, changing it from 0 to 1. The formulas derived for the electric field amplitudes inside and outside the nanostructural layer take into account the periodic arrangement of nanoparticles. The ideal blooming condition of the boundary of the underlying medium is derived, under which the reflectance of the interface tends to zero in a wide range of wavelengths in the visible optical range.     

Tunneling conductance of Al-oxide-Al junctions doped with Ni at the oxide-Al interface

The effect of a Ni impurity layer at the Oxide-Al interface has been studied for thickness in the range between 0.3 nm and 100 nm. The results obtained from the temperature dependence of the zero bias conductance and the voltage dependence of the dynamic resistance exhibit structures being due to the dynamics of spin-flip scattering and impurity interaction. We show that at a layer thickness of 1.5 nm Ni the behaviour of the tunnel junctions changes drastically because of the occurence of the magnetic interaction between the impurity atoms.