Phenomenological theory of current driven exchange switching in ferromagnetic nanojunctions

Phenomenological approach is developed in the theory of spin-valve type ferromagnetic junctions to describe exchange switching by current flowing perpendicular to interfaces. Forward and backward current switching effects are described and they may be principally different in nature. Mobile electron spins are considered as being free in all the contacting ferromagnetic layers. Joint action of the following two current effects is investigated: the nonequilibrium longitudinal spin-injection effective field and the transverse spin-transfer surface torque. Dispersion relation for fluctuations is derived and solved for a junction model having spatially localized spin transfer torque: depth of the torque penetration into the free layer is assumed much smaller than the total free layer thickness. Some critical value of the well known Gilbert damping constant is established for the first time. Spin transfer torque dominates in the instability threshold determination for small enough damping constants, while the spin-injection effective field dominates for high damping. Fine interplay between spin transfer torque and spin injection is necessary to provide a hysteretic behavior of the resistance versus current dependence. The state diagram building up shows the possibility of non-stationary (time dependent) nonlinear states arising due to instability development. Calculations lead to the instability rise time values of the order of 0.1 ns. Spin wave resonance frequency spectrum softening occurs under the current growing to the instability threshold. Magnetization fluctuations above the threshold rise oscillating with time for low damping, but rise aperiodically and much more rapid for high damping.Comment: 16 pages, 7 figures. Corrected typos. Submitted to Phys. Rev.

Disturbance of spin equilibrium by current through the interface of noncollinear ferromagnets

Boundary conditions are derived that determine the penetration of spin current through an interface of two non-collinear ferromagnets with an arbitrary angle between their magnetization vectors. We start from the well-known transformation properties of an electron spin wave functions under the rotation of a quantization axis. It allows directly find the connection between partial electric current densities for different spin subbands of the ferromagnets. No spin scattering is assumed in the near interface region, so that spin conservation takes place when electron intersects the boundary. The continuity conditions are found for partial chemical potential differences in the situation. Spatial distribution of nonequilibrium electron magnetizations is calculated under the spin current flowing through a contact of two semi-infinite ferromagnets. The distribution describes the spin accumulation effect by current and corresponding shift of the potential drop at the interface. These effects appear strongly dependent on the relation between spin contact resistances at the interface.Comment: 10 pages, 2 figures. Submitted to J. Magn. Magn. Mater. Corrected typo

Possibility of exchange switching ferromagnet - antiferromagnet junctions

Current flowing is studied in magnetic junctions consisting of a ferromagnetic metal (FM), antiferromagnetic conductor (AFM) and a nonmagnetic metal closing the electric circuit. The FM layer with high anisotropy and pinned spins of the magnetic atoms in the lattice acts as a spin injector relative to the AFM layer. To obtain resulting magnetization in the AFM layer, magnetic field is applied, which may be varied to control the magnetization. The spin-polarized current from the FM layer creates a torque and makes it possible to switch the magnetization. A possibility is shown to lower the threshold current density by the orders of magnitude by means of the magnetic field.Comment: 10 pages, 1 figur

Switching magnetic junction by joint action of current pulse and magnetic field: Numerical simulation

The results are presented of a numerical simulation of the switching magnetic junction by a spin-polarized current pulse under applied magnetic field with the current density and field below the threshold values. A possibility is shown of the switching with controllable time delay relative to the current pulse.Comment: 7 pages, 3 figure

Current-induced exchange switching magnetic junctions with cubic anisotropy of the free layer

The stability is analyzed of the equilibrium configurations of a magnetic junction with a free layer that has cubic symmetry and two anisotropy axes in the layer plane. Different variants of the switching between various configurations are considered. A possibility is shown of the substantial lowering of the threshold current density needed for the switching. Numerical simulation is made of the switching dynamics for various configurations.Comment: 14 pages, 7 figure

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.Comment: 7 pages, 2 figure

A possibility of increasing spin injection efficiency in magnetic junctions

Nonequilibrium electron spin polarization is calculated under spin injection from one ferromagnet to another in magnetic junction. It is shown that the nonequilibrium spin polarization can be comparable with equilibrium one if the material parameters are chosen appropriately. This leads to lowering the threshold current density necessary for the junction switching and opens a perspective to creating THz laser based on the spin-polarized current injection.Comment: 6 pages, 1 figure. Corrected Typos. Submitted to Physica status solidi

Spintronics of metal ferromagnetic structures: New approaches in the theory and experiments

Two channels of the sd exchange interaction are considered in magnetic junctions. The first channel describes the interaction of transversal spins with the lattice magnetization. The second one describes the interaction of longitudinal spins with magnetization. We show the longitudinal channel leads to a number of significant effects: 1) drastic lowering of the current instability threshold down to three (or even more) orders of magnitude; 2) creation of large enough distortion of equilibrium due to current driven spin injection leading to inversion of energy spin subband populations and laser-like instability in THz frequency range at room temperature. External magnetic field may tend to lower additionally the instability threshold due to the proximity effect of purely magnetic reorientation phase transition. This effect demonstrates the new properties: the giant magnetoresistance (GMR) becomes strongly current dependent and the exchange switching becomes of very low threshold. We derived some matching condition that should be satisfied to achieve high spin injection level. Some characteristic quantities were appeared in the condition. We investigated also the junctions having variable lateral dimensions of the layers, for example, a ferromagnetic rod contacting with a very thin ferromagnetic film. Large enhancement of the current density may appear near the contact region leading to the spin injection luminescence.Comment: 13 pages, 7 figure

Current-driven spin injection from a probe to a ferromagnetic film

The distribution is calculated of the electron spin polarization under current-driven spin injection from a probe to a ferromagnetic film. It is shown that the main parameters determining difference of the spin polarization from the equilibrium value are the current density and the spin polarization of the probe material, while the relation between the probe diameter and the spin diffusion length influences the result very weakly, to a certain extent. A possibility is shown of reaching inverse population of the spin subbands at distances from the probe boundary comparable with the spin diffusion length.Comment: 8 pages, 2 figure

Current-induced resonance in a ferromagnet - antiferromagnet junction

We calculate the response of a ferromagnet - antiferromagnet junction to a high-frequency magnetic field as a function of the spin-polarized current through the junction. Conditions are choused under which the response is zero in absence of such a current. It is shown that increasing in the current density leads to proportional increase in the resonance frequency and resonant absorption. A principal possibility is indicated of using ferromagnet - antiferromagnet junction as a terahertz radiation detector.Comment: 10 pages, 4 figure