N. Strelkov

Spin-current vortices in current-perpendicular-to-plane nanoconstricted spin valves

The charge and spin diffusion equations, taking into account spin-flip and spin-transfer torque, were numerically solved using a finite element method in complex noncollinear geometry with strongly inhomogeneous current flow. As an illustration, spin-dependent transport through a nonmagnetic nanoconstriction, separating two magnetic layers was investigated. Unexpected results such as vortices of spin-currents in the vicinity of the nanoconstriction were obtained. The angular variations of magnetoresistance and spin-transfer torque are strongly influenced by the structure geometry.

Giant magnetoresistance in hybrid superconductor/ferromagnetic sandwich heterostructures

The giant magnetoresistance (GMR) of ferromagnetic bilayers with a superconducting contact (F1/F2/S) is calculated in ballistic and diffusive regimes. As in spin-valve, it is assumed that the magnetization in the two ferromagnetic layers F1 and F2 can be changed from parallel to antiparallel. It is shown that the GMR defined as the change of conductance between the two magnetic configurations is an oscillatory function of the thickness of F2 layer and tends to an asymptotic positive value at large thickness. This is due to the formation of quantum well states in F2 induced by Andreev reflection at the F2/S interface and reflection at F1/F2 interface in antiparallel configuration. In the diffusive regime, if only spin-dependent scattering rates in the magnetic layers are considered (no difference in Fermi wave-vectors between spin up and down electrons) then the GMR is supressed due to the mixing of spin up and down electron-hole channels by Andreev reflection.The perpendicular electron transport through a ferromagnetic bilayer (F1/F2) in contact with a superconductor (S) was investigated theoretically. The conductance is calculated in the Kubo formalism with Green functions found as the solutions of the Gorkov equations. It is shown that the giant magnetoresistance (GMR) defined as the relative difference in conductivity between the parallel and antiparallel alignments of the magnetizations in F1 and F2 behaves differently in the ballistic and diffusive regimes. In the former case, the GMR amplitude can be fairly large, whereas in the latter case, it almost vanishes. The interpretation of this behaviour is given by comparing the contributions to the total resistance of the Andreev reflection at the F1/S interface and the usual quantum reflection at the F1/F2 interface.

Numerical Simulation of Spin Torque Induced by Spin Hall Effect in CuPt/Fe Heterostructure

Spin transport and distribution of spin accumulation in CuPt/Fe heterostructure are numerically investigated, using linearized Levy-Fert model. It was shown that Spin Hall Effect in CuPt layer produces non-equilibrium spin accumulation in adjacent ferromagnetic layer. Spin accumulation vector is not collinear with the direction of magnetization in ferromagnetic layer which leads to the appearance of spin transfer torque. The absolute values and angular dependence of this torque were calculated and it was demonstrated that for the current the values of torques are sufficient for manipulation of magnetization of ferromagnetic layer.

Analytical description of ballistic spin currents and torques in magnetic tunnel junctions

In this work we demonstrate explicit analytical expressions for both charge and spin currents which constitute the 2x2 spinor in magnetic tunnel junctions with non collinear magnetizations under applied voltage. We demonstrate that spin/charge currents and spin transfer torques are all explicitly expressed through only three irreducible quantities, without further approximations. From the expressions derived, it is shown that in the case of a symmetric tunnel junction, the bias dependence is conventional and in the presence of asymmetries, additional odd parity terms appear. Furthermore, the conditions and mechanisms of deviation from the conventional sine angular dependence of both spin currents and spin torques are evidenced and discussed. Finally, it is shown in the thick barrier approximation that all tunneling transport quantities can be expressed in an extremely simplified form via Slonczewski spin polarizations and newly introduced effective spin averaged interfacial transmission probabilities and effective out-of-plane polarizations at both interfaces. It is proven that the latter plays a key role in origin of perpendicular spin torque as well as for angular dependence character of all spin and charge transport considered. It is then demonstrated directly that for any applied voltage, the parallel component of spin current at the FM/I interface is expressed via collinear longitudinal spin current components.

Finite Element Modeling of Charge- and Spin-Currents in Magnetoresistive Pillars With Current Crowding Effects

Charge- and spin-diffusion equations, taking into account spin-diffusion and spin-transfer torque, were numerically solved using a finite element method in complex noncollinear geometry. As an illustration, this approach was used to study the spin-dependent transport in a two-dimensional model giant magnetoresistance metallic pillar sandwiched between extended electrodes as is the case in magnetoresistive heads for hard disk drives. In this model system, the charge current crowding around the boundaries between the electrodes and the pillar has a quite significant influence on the spin current within the magnetoresistive pillar.

Impact of Intergrain Spin-Transfer Torques Due to Huge Thermal Gradients in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a new technology which uses temporary near-field heating of the media during write to increase hard disk drive storage density. By using a plasmonic antenna embedded in the write head, an extremely high thermal gradient is created in the recording media (up to 10 K/nm). State-of-the-art HAMR media consist of grains of L10-FePt exhibiting high perpendicular anisotropy separated by 1–2 nm-thick carbon segregant. Next to the plasmonic antenna, the difference of temperature between two nanosized FePt grains in the media can reach 80 K across the 2 nm-thick grain boundary. This represents a gigantic local thermal gradient of 40 K/nm across a carbon tunnel barrier. In the field of spincaloritronics, much weaker thermal gradients of ~1 K/nm were shown to cause a thermal spin-transfer torque (TST) capable of inducing magnetization switching in magnetic tunnel junctions (MTJs). Considering on the one hand, two neighboring grains separated by an insulating grain boundary in an HAMR media can be viewed as an MTJ, and on the other hand, the thermal gradients in HAMR are 1–2 orders of magnitude larger than those used in the conventional spincaloritronic experiments; one may expect a strong impact from these TSTs on magnetization switching dynamics in HAMR recording. This issue has been totally overlooked in the previous investigations on the development of the HAMR technology. This paper combines theory, experiments aiming at determining the polarization of tunneling electrons across the media grain boundaries, and micromagnetic simulations of the recording process taking into account these thermal gradients. It is shown that the thermal in-plane torque can have a detrimental impact on the recording performances by favoring antiparallel magnetic alignment between neighboring grains during the media cooling. Implications on media design are discussed in order to overcome the influence of these thermal torques. Suggestions of spincaloritronic experiments taking advantage of these huge thermal gradients produced by plasmonic antenna are also given.