arXiv subjects

R. J. Elliott

Publications and source records attributed to R. J. Elliott.

11 recordsLinked to original sources

arXiv2005

Surface spin-transfer torque and spin-injection effective field in ferromagnetic junctions: Unified theory

We consider theoretically a current flowing perpendicular to interfaces of a spin-valve type ferromagnetic metallic junction. For the first time an effective approach is investigated to calculate a simultaneous action of the two current effects, namely, the nonequilibrium longitudinal spin injection and the transversal spin-transfer surface torque. Dispersion relation for fluctuations is derived and solved. Nonlinear problem is solved about steady state arising due to instability for a thick enough free layer.

cond-mat.mtrl-sci↗

arXiv2004

Current induced spin injection and surface torque in ferromagnetic metallic junctions

Joint influence of two effects, namely, nonequilibrium spin injection by current, and current induced surface torque, on spin-valve type ferromagnetic metallic junctions is considered theoretically. The CPP configuration is assumed. The consideration is based on solving a coupled set of equations of motion for the mobile electron and lattice magnetizations. Boundary conditions are derived from the total magnetization flux continuity condition. A dispersion relation is derived for current dependent spin-wave fluctuations. The fluctuations become unstable under current density exceeding some threshold value. Joint action of the longitudinal spin injection and the torque lowers the instability threshold. The spin injection softens spin wave frequency near the threshold and can pin magnetization at the injecting contact. The pinning rises under the current increasing, so that the appearance of new spin-wave resonance lines can be observed.

cond-mat.mtrl-sci↗

arXiv2003

Resonant excitation of the domain wall oscillations by a parallel current under spin injection

A possibility is discussed of observing spin injection effect on the ferromagnet domain structure by means of resonant excitation of the domain wall oscillations by a spin-polarized ac injection current. The natural frequency of the domain wall oscillations in a thin ferromagnetic film with parallel anisotropy is calculated. Amplitude of the domain wall forced oscillations excited by the spin-polarized ac current is determined. Then effect of such oscillations on the current is considered and appearance of nonlinear phenomena such as rectification of the ac current and second harmonic generation is predicted.

cond-mat.mtrl-sci↗

arXiv2003

Nonuniform switching ferromagnetic layers by spin polarized current

The magnetic reversal by spin-polarized current of a magnetic junction consisting of two ferromagnetic layers and a nonmagnetic spacer in between is considered. Initially, the free layer is magnetized antiparallel to the pinned layer by an external magnetic field. Under current flowing, a nonequilibrium spin polarization appears in the free layer. The interaction between the injected spins and the lattice leads to instability of the antiparallel orientation at the high enough current density and to switching the free layer to a state with magnetization parallel to one in the pinned layer. If the free layer thickness and the external magnetic field strength are large enough, then a nonuniform switching is favorable, so that only a part of the free layer near the injector switches. Such a switching is accompanied with appearance of a domain wall between the switched and non-switched regions. The domain wall can oscillate around the equilibrium position with some natural frequency depending on the external magnetic field.

cond-mat.mtrl-sci↗

arXiv2003

Magnetization reversal and current hysteresis due to spin injection in magnetic junction

Magnetic junction is considered which consists of two ferromagnetic metal layers, a thin nonmagnetic spacer in between, and nonmagnetic lead. Theory is developed of a magnetization reversal due to spin injection in the junction. Spin-polarized current is perpendicular to the interfaces. One of the ferromagnetic layers has pinned spins and the other has free spins. The current breaks spin equilibrium in the free spin layer due to spin injection or extraction. The nonequilibrium spins interact with the lattice magnetic moment via the effective s-d exchange field, which is current dependent. Above a certain current density threshold, the interaction leads to a magnetization reversal. Two threshold currents are found, which are reached as the current increases or decreases, respectively, so that a current hysteresis takes place. The theoretical results are in accordance with the experiments on magnetization reversal by current in three-layer junctions Co/Cu/Co prepared in a pillar form.

cond-mat.mtrl-sci↗

arXiv2003

Magnetism in the Hubbard model-An improved treatment

We propose a new treatment of the Hubbard model that is based both on the coherent-potential approximation (CPA) and the virtual-crystal approximation (VCA). It is well known that the equilibrium found using the one-particle CPA Green's functions does not predict an ordered magnetic ground state, while Stoner's mean-field treatment, which is equivalent to the VCA on the Hubbard model, does so for a wide range of parameters. A hybrid treatment, the tau-CPA, is developed, in which a particle is assumed to be scattered from an array of static opposite spins for a time tau related to the inverse of the band width. The propagation is treated in the CPA over this period; thereafter the particle sees the time-averaged effect of the scatterers and hence can be treated in the VCA. This model, with suitable approximations, does predict magnetism for a modified Stoner criterion.

cond-mat.str-el↗

arXiv2003

Two-particle propagator and magnetic susceptibility in the Hubbard model- An improved treatment

We treat the two-particle Green's function in the Hubbard model using the recently developed tau-CPA, a hybrid treatment that applies the coherent-potential approximation (CPA) up to a time tau related to the inverse of the band width, after which the system is averaged using the virtual-crystal approximation (VCA). This model, with suitable approximations, does predict magnetism for a modified Stoner criterion. The evaluation of the two-particle propagator in the tau-CPA requires the solution of the pure CPA, within whose formalism the vertex correction and the weighted Green's functions are obtained. The dynamical susceptibility, including the vertex correction and the weighted scattering by the residual interaction, is calculated and shows a spin wave spectrum in the ferromagnetic regime.

cond-mat.str-el↗

arXiv2000

Current driven switching of magnetic layers

The switching of magnetic layers is studied under the action of a spin current in a ferromagnetic metal/non-magnetic metal/ferromagnetic metal spin valve. We find that the main contribution to the switching comes from the non-equilibrium exchange interaction between the ferromagnetic layers. This interaction defines the magnetic configuration of the layers with minimum energy and establishes the threshold for a critical switching current. Depending on the direction of the critical current, the interaction changes sign and a given magnetic configuration becomes unstable. To model the time dependence of the switching process, we derive a set of coupled Landau-Lifshitz equations for the ferromagnetic layers. Higher order terms in the non-equilibrium exchange coupling allow the system to evolve to its steady-state configuration.

cond-mat.mes-hall↗

arXiv1996

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction across a tunneling junction out of equilibrium

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic $s$-$d$ spin impurities across a tunneling junction is studied when the system is driven out of equilibrium through biasing the junction. The nonequilibrium situation is handled with the Keldysh time-loop perturbation formalism in conjunction with appropriate coupling methods for tunneling systems due to Caroli and Feuchtwang. We find that the presence of a nonequilibrium bias across the junction leads to an interference of several fundamental oscillations, such that in this tunneling geometry, it is possible to tune the interaction between ferromagnetic and antiferromagnetic coupling at a fixed impurity configuration, simply by changing the bias across the junction. Furthermore, it is shown that the range of the RKKY interaction is altered out of equilibrium, such that in particular the interaction energy between two slabs of spins scales extensively with the thickness of the slabs in the presence of an applied bias.

cond-mat↗

arXiv1995

Approximation of excitonic absorption in disordered systems using a compositional component weighted CPA

Employing a recently developed technique of component weighted two particle Green's functions in the CPA of a binary substitutional alloy $A_cB_{1-c}$ we extend the existing theory of excitons in such media using a contact potential model for the interaction between electrons and holes to an approximation which interpolates correctly between the limits of weak and strong disorder. With our approach we are also able to treat the case where the contact interaction between carriers varies between sites of different types, thus introducing further disorder into the system. Based on this approach we study numerically how the formation of exciton bound states changes as the strengths of the contact potentials associated with either of the two site types are varied through a large range of parameter values.

cond-mat↗

arXiv1995

Weighted two particle Green's functions in the CPA

We extend the two particle theory of disordered systems within the coherent potential approximation CPA to obtain weighted contributions to averaged two particle resolvents which arise from separate alloy components. Starting from first principles in a model of diagonal disorder and the single site approximation for a binary substitutional alloy $A_cB_{1-c}$ we extend the approach of a fundamental paper by Velick{ý} to evaluate various weighted forms of a general class of two particle Green's functions. Applications in a wide range of linear response theory are discussed in detail as well as the behavior of the weighted functions in a strong disorder limit. To exemplify our analytic calculations the optical absorption in a disordered model- alloy is studied numerically.

cond-mat↗