Asymmetry and separation of spin tunneling time in ZnSe/Zn1-xMnxSe multilayers

We investigate characteristics of spin tunneling time in ZnSe/Ze_1-xMn_xSe multilayers under the influence of both an electric field and a magnetic field. The results indicate that the tunneling time shows complicated oscillations and significant spin separation for electrons with different spin orientations traversing semimagnetic semiconductor heterostructures. It is also shown that the tunneling time exhibits obvious asymmetry in opposite tunneling directions for electrons tunneling through asymmetric heterostructures, which mainly occurs in resonant regions. The degree of the asymmetry of the tunneling time is not only spin-polarization dependent but also external-field induced. Received 10 July 2001     

Spin-polarized current produced by a double barrier resonant tunneling diode

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) made with a semimagnetic semiconductor is studied theoretically. The calculated spin-polarized current and polarization degree are in agreement with recent experimental results. It is predicted that the polarization degree can be modulated continuously from +1 to −1 by changing the external voltage such that the quasi-confined spin-up and spin-down energy levels shift downwards from the Fermi level to the bottom of the conduction band. The RTD with low potential barrier or the tunneling through the second quasi-confined state produces larger spin-polarized current. Furthermore a higher magnetic field enhances the polarization degree of the tunneling current.     

Many-body Green's function theory for thin ferromagnetic anisotropic Heisenberg films: treatment of the exchange anisotropy

The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier work, on the magnetic properties of thin ferromagnetic films are compared. Received 31 October 2002 Published online 7 May 2003     

Spin-dependent shot noise in diluted-magnetic-semiconductor/ semiconductor heterostructures

We investigated the shot noise properties in the diluted-magnetic-semiconductor/semiconductor heterostructures, where the sp-d exchange interaction gives rise to a giant spin splitting when an external magnetic field is applied along the growth direction of the heterostructures. It is found that the noise becomes strongly spin-dependent and can be greatly modulated not only by the external magnetic and electric fields, but also by the structural configuration and geometric parameters. Both the spin-up and spin-down components of the noise spectral density can be greatly suppressed by the magnetic field. The Fano factor is notably sensitive to the transmission probabilities, which varies greatly with the spin-polarization, the external magnetic field, and the structural configuration.     

Magnetism of alloyed models of Cr(Mn) V and Mn Cr overlayers on V (001) substrates

The magnetism is calculated for substitutional alloyed of Cr_x(Mn_x)V_1-x monolayers (MLs) and Cr_xMn_1-x on V (001) surface with a variety of concentrations (x = 0.25, 0.50 and 0.75). The substitutional surface alloys were treated by an artificial super cell construction. Parallel magnetic ordering is obtained for all the considered structures. The surface net magnetization increases in terms of Cr (Mn) concentration in Cr_x(Mn_x)V_1-x/V (001) system, while no serious variations occur in Cr_xMn_1-x/V (001). Vanadium atoms at the interfacial layers acquire appreciable magnetic moments antiferromagneticlly (AF) coupled with the surface moments. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: jkalifa@sci.ju.edu.jo     

Tilt-induced ferromagnetic ordering in anisotropic molecular monolayers

It is shown that orientational ordering of anisotropic organic molecules with permanent magnetic dipoles in a tilted film should result in a macroscopic magnetisation in the plane of the film. The important requirement here is that the molecules are strongly biaxial, and the corresponding biaxial orientational order parameter in the tilted phase is sufficiently large. The molecules should also be characterised by a reduced symmetry of the magnetic core compared with existing “single molecular magnets". Possible symmetry groups of the molecular magnetic core, which allow for the existence of nonzero average magnetic moment, are discussed in detail. The tilt-induced ferromagnetic ordering of such molecules may be determined by nonmagnetic intermolecular interactions including, for example, quadrupole-quadrupole electrostatic interaction or dispersion interaction between molecules of particular symmetry. Magnetic intermolecular interactions are not important here, and as a result the induced ferromagnetic state may be stable in any temperature range where the corresponding tilted film is stable. These general conclusions, which form a theoretical foundation for the existence of novel fluid low-dimensional magnetic materials, are based on symmetry arguments and are supported by a simple mean-field molecular model. We also discuss how such induced ferromagnetic ordering may be observed in Langmuir-Blodgett films which seem to be the best candidates for preparing these magnetic materials.     

Magneto-optical Faraday and Kerr effect of orthoferrite thin films at high temperatures

Orthoferrites present, as bulk materials, reorientation transitions of their magnetic moment alignment at temperatures depending on the rare-earth (RE) ion. In particular, orthoferrites (REFeO₃) with RE = Sm, Dy, present this transition at T _SRT = 443 K and 36 K, respectively. The spectra of the complex Kerr and Faraday angle have been measured on orthoferrite thin films (RE = Sm, Dy, Y), which were prepared by pulsed laser deposition on amorphous quartz substrates. The obtained spectra exhibit contributions of both surfaces and interfaces. Propagation effects of the polarized light in the magneto-optical medium which is interpreted in terms of a simplified theoretical formalism, is also observed. For selected photon energies, temperature dependent Faraday rotation measurements, , on orthoferrite thin films (RE = Sm, Dy, Y) have been performed. A quite different thermal variation compared to the bulk magnetization has been observed. Curie temperatures are found to be close to the bulk values or slightly larger by 10 K to 20 K as in the case of DyFeO₃ and YFeO₃. For RE = Sm and Dy, increases with increasing temperature contrary to the saturation magnetization, passes through a maximum at about 460 K and vanishes with a T _C of 647±18 K, 695 K for RE = Sm and Dy respectively. Received 28 July 2000     

Many-body Green's function theory for the magnetic reorientation of thin ferromagnetic films

The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term, an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms. The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good approximation for realistic coupling strengths. We apply the new method to monolayers with spin and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some significant differences. Received 19 June 2000 and Received in final form 2 August 2000     

Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the spin components (), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches. Received 6 April 1999 and Received in final form 9 July 1999     

Calculations of ferromagnetic surface dynamics using the Green's functions theory and matching procedure

A Heisenberg model is employed to study the spin fluctuation dynamics on a (001) ferromagnetic surface using a new theoretical formalism. The solution of the full magnetic problem arising from the absence of magnetic translation symmetry in one dimension due to the presence of a magnetic surface is presented. The calculations are described using simultaneously a closed form of the spin-wave Green's function and the matching procedure in the random-phase approximation. Analytic expressions for the Green's functions are also derived in a low-temperature spin-wave approximation. The theoretical approach determines the bulk and evanescent spin fluctuation fields in the two dimensional plane normal to the surface. The results are used to calculate the localised modes of magnons associated with the surface. Numerical examples of the modes are given and they are found to exhibit various effects due to the interplay between the bulk and surface modes. It is shown that there may be surface spin-waves that decay in amplitude with distance into the bulk domain. Also the bulk spin fluctuations field as well as the magnons localised at the surface depend on the nature of the bulk-surface coupling exchange. The unstable surface magnetic configurations are illustrated and discussed. The results derived from the dynamic correlation functions between a pair of spin operators at any two sites are employed to evaluate the spin deviation in the ferromagnet due to localised surface modes obtained by the matching procedure as a function of temperature. Received 21 April 2002 / Received in final form 25 October 2002 Published online 14 March 2003     

Mechanism of grain-boundary magnetoresistance in Fe O films

The magnetotransport properties of magnetite films with different microstructures were investigated in order to identify prerequisites for the attainment of a large tunnelling magnetoresistance in polycrystalline samples. Epitaxial films on MgAl₂O₄, polycrystalline films on Al₂O₃ and rough MgAl₂O₄ substrates and a polycrystalline La_0.7Ca_0.3MnO₃ film on MgO were compared. Although grain boundaries induce a large high-field magnetoresistance in magnetite films, the low-field magnetoresistance characteristic for spin-polarized tunnelling was virtually absent in these samples. Two factors might be responsible for this behaviour: (1) grain boundaries in magnetite are conducting and do not form tunnelling barriers and (2) the spin-polarization near grain boundaries is suppressed due to non-stoichiometry. Received 15 April 2002 Published online 13 August 2002     

Resonant tunneling in magnetic semiconductor tunnel junctions with arbitrary magnetic alignments

Combining an extended Julliere model with transfer matrix method, we study the spin-polarized resonant tunneling in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double barrier ferromagnetic semiconductor (FS) tunnel junctions with the arbitrary angle θ between the magnetic directions of two FS's. It is shown that tunneling magnetoresistance (TMR) ratio linearly varies with sin²(θ/2). We also demonstrate that for the heavy and light holes, the properties of the spin-polarized resonant tunneling are obviously different. The present results are expected to be instructive for manufacturing the relevant semiconductor spintronic devices.     

Phase diagrams of Ising films with competing interactions

The axial next-nearest-neighbour Ising (ANNNI) model of finite thickness is studied. Using mean-field theory, Monte Carlo simulations, and low-temperature analyses, phase diagrams are determined, with a distinct phase diagram for each film thickness. The robustness of the phase diagrams against varying the couplings in the surface layers is analysed. Received 19 March 2002 and Received in final form 2 April 2002 Published online 6 June 2002     

Magnon scattering by a symmetric atomic well in free standing very thin magnetic films

A theoretical model is presented for the study of the scattering of magnons at an extended symmetric atomic well in very thin magnetic films. The thin film consists of three cubic atomic planes with ordered spins coupled by Heisenberg exchange, and the system is supported on a non-magnetic substrate, and considered otherwise free from magnetic interactions. The coherent transmission and reflection scattering coefficients are derived as elements of a Landauer type scattering matrix. Transmission and reflection scattering cross sections are hence calculated specifically, as a function of the varying local magnetic exchange on the inhomogeneous boundary. Detailed numerical results for the individual incident film magnons, and for the calculated overall magnon conductance, show characteristic transmission properties, with associated Fano resonances, depending on the magnetic boundary conditions and on the magnon incidence.     

Strain effect on transport properties of <Emphasis Type="Bold">SrRuO</Emphasis><Subscript><Emphasis Type="Bold">3</Emphasis></Subscript> films grown by laser MBE

Transport properties of SrRuO₃ thin films were studied as a function of the epitaxial strain. SrRuO₃ films were grown on (100) SrTiO₃ substrates by the Pulsed Laser Deposition technique equipped with Reflection High Energy Electrons Diffraction (RHEED). Samples thickness has been varied from a few unit cells to above 1000 ? while monitoring RHEED intensity oscillations. In thicker films epitaxial strain was found to be progressively relaxed. SrRuO₃ relaxed films (thickness 1000 ?) show metallic behavior for the whole temperature range with a ferromagnetic ordering at about 150 K. For thinner films, ferromagnetic ordering occurs at progressively lower temperatures, until in films thinner than 400 ? it disappears. Films thinner than 80 ? show a semiconducting behavior at low temperatures. Our results provide direct evidence of the crucial role of the strain effect for conducting and magnetic properties of SrRuO₃. Received 16 July 2001 and Received in final form 22 October 2001     

Coupling effects of layers on spin transport in ZnSe/Zn 1 - x Mn x Se heterostructures

By use of the scattering matrix method, we investigate the coupling effects of layers on spin-polarized transport through semimagnetic semiconductor heterostructures with triple paramagnetic layers. Due to the coupling between double non-magnetic layers or among triple paramagnetic layers, spin tunneling exhibits interesting and complex features, which are determined by the structural configuration, the external fields as well as the spin orientations. It is shown that for electrons with either spin orientation tunneling through the symmetric or asymmetric heterostructures with triple paramagnetic layers, transmission resonances can approach the optimum under several biases. Moreover, for asymmetric structures, the resonant enhancement can occur under both several positive and negative biases. The spin-dependent resonant enhancement is also clearly reflected in the current density. In addition, for spin electrons traversing the multilayer heterostructure, the resonant splitting occurs in the transmission, which shows rich variations with the bias. These interesting results may be helpful to the development of spintronic devices. Received 28 April 2001     

Spin-dependent current in resonant tunneling diode with ferromagnetic GaMnN layers

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) with ferromagnetic GaMnN emitter/collector is investigated theoretically. Two distinct spin splitting peaks can be observed at current-voltage (I-V) characteristics at low temperature. The spin polarization decreases with the temperature due to the thermal effect of electron density of states. When charge polarization effect is considered at the heterostructure, the spin polarization is enhanced significantly. A highly spin-polarized current can be obtained depending on the polarization charge density.     

Shape anisotropy and magnetic domain structures in striped monolayers

We study the effect of dipolar interactions on a magnetic striped monolayer with a microscopic unit cell of square symmetry, and of size spins. Even if the aspect ratio r=N _x /N _y is very large, an in-plane shape anisotropy is always negligible, except if N_y is fairly small (N _y <40). In-plane domains are not possible, except for values of the dipolar coupling larger than the domain wall energy. Received: 11 July 1997 / Revised: 24 September 1997 / Accepted: 24 October 1997     

First order high-spin/low-spin phase transition induced by acoustic-phonons

A new interpretation of light-induced magnetization changes of a magnetic semiconductor, manganese arsenide (MnAs), observed by the authors of references [1,2], is proposed in this paper. Contrary to references [1,2], where the results of experiments were interpreted as the observation of light-induced phase transition, here we propose a completely different approach. It suggests that at least far from the vicinity of T _c, there are no real magnetization changes as in case of phase transition, but there are changes of the magnetic flux threading the MnAs-sample. These changes are due to non-equilibrium light-induced diamagnetic moments of quasi-free electrons of narrow d-subbands of the MnAs-conduction band. The other aspects of the experiments of [1,2] are also discussed and some similarity between this effect and the orbital diamgnetism due to persistent currents in mesoscopic structures is emphasised. Received 7 November 2000     

Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors. Received 9 July 2002 / Received in final form 22 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: herve.courtois@grenoble.cnrs.fr RID="b" ID="b"associated to Université Joseph Fourier