D. Gusakova

Amplitude and phase noise of magnetic tunnel junction oscillators

The microwave emission linewidth of spin transfer torque nano-oscillators is closely related to their phase and amplitude noise that can be extracted from the magnetoresistive voltage signal V(t) using single shot time domain techniques. Here we report on phase and amplitude noise studies for MgO based magnetic tunnel junction oscillators. The analysis of the power spectral densities allows one to separate the linear and nonlinear contributions to the phase noise, the nonlinear contribution being due to the coupling between phase and amplitude. The coupling strength as well as the amplitude relaxation rate can be directly extracted.

Injection locking of tunnel junction oscillators to a microwave current

Injection locking of a spin transfer nano-oscillator, based on an in-plane magnetized magnetic tunnel junction and generating the frequency f0, to an external signal of varying frequency fe is studied experimentally and with macrospin simulations. It is shown, that if the driving signal has the form of a microwave current, the locking effect is well-pronounced near fe≅2f0, but is almost completely absent near fe≅f0, confirming predictions of analytical theory. It is also shown that noise plays an important role in the locking process, causing the linewidth of the locked oscillation to substantially exceed that of the driving signal.

Linewidth reduction in a spin-torque nano-oscillator caused by non-conservative current-induced coupling between magnetic layers

We demonstrate by macrospin simulations that in a spin-torque nano-oscillator having synthetic antiferromagnet fixed layer, the non-conservative dynamic coupling between the free and fixed layers caused by spin-torque effect leads to a substantial reduction of the linewidth of the current-induced spin wave mode, involving oscillations in all three magnetic layers. By analysing the phase and amplitude noise extracted from the simulated signal, we prove that the obtained linewidth reduction is related to the reduction of the dimensionless non-linear amplitude-phase coupling parameter ν.

Spin torque driven excitations in a synthetic antiferromagnet

Spin polarized current induced self oscillations have been investigated in both the free layer and the synthetic antiferromagnetic (SAF) pinned layer of spin valve nanopillars. Compared with free layer excitations, the acoustic type SAF excitations are characterized by high emitted power and much narrower linewidth. Furthermore, in contrast to free layer excitations, the SAF in-plane precession mode exhibits an unexpected crossover from redshift (df/dI 0) in frequency f versus current I as the in-plane magnetic field is increased. From simulations we identify this crossover as a signature of large amplitude spin current induced precessional dynamics in the SAF.

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.

Redshift and Blueshift Regimes in Spin-Transfer-Torque Nano-Oscillator Based on Synthetic Antiferromagnetic Layer

The spin-torque-driven in-plane precession mode of synthetic antiferromagnets (SAFs) is characterized by a frequency current dependence that changes its slope from negative redshift to positive blueshift upon increasing the applied magnetic field. Here, we show that the transition from redshift to blueshift is due to additional torques arising from the dynamic interlayer exchange coupling of the SAF. These torques depend on the precession amplitude that changes with current and field. We present an analysis of the corresponding energy surface that is able to predict such behavior for any spin-torque-driven excitation of a coupled two-layer system.

Current driven magnetization dynamics of a self-polarised synthetic ferrimagnet

Spin torque driven excitations in spin valves and tunnel junctions are often investigated for a two magnetic layer system for which a polarizer (fixed magnetization) and a free layer can be distinguished. In the search for improved microwave properties and to understand the role of different coupling mechanisms between the magnetic layers, here, the excitation spectrum of an exchange coupled two layer synthetic ferrimagnet (SyF) system is investigated numerically with spin momentum transfer acting on both layers simultaneously. This self-polarised two layer system does not contain an external polarizer, and excitation of coupled modes arises due to the mutual spin transfer torque and the Ruderman-Kittel-Kasuya-Yosida interlayer exchange coupling. The current-field state diagrams of static and dynamic states are reported as a function of the interlayer exchange coupling strength. The numerically determined critical boundaries are well reproduced by an analytical stability analysis. The dynamic steady state...

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.

Spin transfer torque nano-oscillators based on synthetic ferrimagnets: Influence of the exchange bias field and interlayer exchange coupling

A comprehensive numerical study of the spin toque driven dynamic states is presented for a synthetic ferrimagnet. For this, the Landau-Lifshitz-Gilbert equation has been solved simultaneously for the two coupled layers of the synthetic ferrimagnet in a macrospin approach including the spin transfer torque term from an external polarizer for one of them. It is shown that a large variety of dynamic modes (in-plane precession (IPP) and out-of-plane precession) can be established, upon varying the strength of the exchange bias field that pins one of the layers of the SyF as well as the Ruderman-Kittel-Kasuya-Yosida interlayer coupling strength. The current—field state diagrams are presented as well as the frequency current dependencies of the most important mode which is the IPP mode. A characteristic feature of the IPP mode for the coupled system (as compared to single layer excitations) is the change, increase or decrease of the frequency, with current upon increasing field. It is shown that this strongly d...

Hall Effect Induced by Spin-Wave Excitation in Metal/Ferromagnetic Insulator Bilayer

We investigate Anomalous Hall effect in nonmagnetic metal/ferromagnetic insulator bilayer with rotating magnetization of the magnetic insulator. Spin-orbit interaction of Rashba type takes place near metal/insulator interface. Magnetization of the ferromagnetic insulator rotates with some frequency w by microwave radiation under ferromagnetic resonance condition. This rotation together with spin-orbit interaction in non-magnetic metal layer induced Hall current along the interface. The Hall current appears under zero bias in the system. The dependence of Hall current on the exchange splitting, the magnetization rotation frequency and the barrier height is calculated. We analyze various contributions in Hall current and discuss the limit of small frequencies.