Spin Torque Oscillator Research Articles

Origin of threshold current density for asymmetric magnetoresistance in Pt/Py bilayers

The asymmetric magnetoresistance (MR) in Py/Pt bilayers is investigated. It increases linearly with respect to the current density up to a threshold and increases more rapidly above this threshold. To reveal the origin of the threshold behavior, we investigate the magnetic field dependence of the asymmetric MR. It is found that the magnetic field strongly suppresses the asymmetric MR only above the threshold current density. Micromagnetic simulation reveals that the reduction of magnetization due to the spin-torque oscillation can be the origin of the threshold behavior of the asymmetric MR.

Mutual synchronization of spin torque nano-oscillators through a long-range and tunable electrical coupling scheme

The concept of spin-torque-driven high-frequency magnetization dynamics, allows the potential construction of complex networks of non-linear dynamical nanoscale systems, combining the field of spintronics and the study of non-linear systems. In the few previous demonstrations of synchronization of several spin-torque oscillators, the short-range nature of the magnetic coupling that was used has largely hampered a complete control of the synchronization process. Here we demonstrate the successful mutual synchronization of two spin-torque oscillators with a large separation distance through their long range self-emitted microwave currents. This leads to a strong improvement of both the emitted power and the linewidth. The full control of the synchronized state is achieved at the nanoscale through two active spin transfer torques, but also externally through an electrical delay line. These additional levels of control of the synchronization capability provide a new approach to develop spin-torque oscillator-based nanoscale microwave-devices going from microwave-sources to bio-inspired networks.

Fieldlike and Dampinglike Spin-Transfer Torque in Magnetic Multilayers

We investigate the spin-transfer torque in a magnetic multilayer structure by means of a spin-diffusion model. The torque in the considered system, consisting of two magnetic layers separated by a conducting layer, is caused by a perpendicular-to-plane current. We compute the strength of the field-like and the damping-like torque for different material parameters and geometries. Our studies suggest that the field-like torque highly depends on the exchange coupling strength of the itinerant electrons with the magnetization both in the pinned and the free layer. While a low coupling leads to very high field-like torques, a high coupling leads to low or even negative field-like torques. The dependence of the different torque terms on system parameters is considered very important for the development of applications such as STT MRAM and spin-torque oscillators.

Thickness dependence of magnetization dynamics of an in-plane anisotropy ferromagnet under a crossed spin torque polarizer

Magnetization dynamics of a soft layer (SL) with in-plane anisotropy and under a spin transfer torque (STT) from a perpendicularly magnetized reference layer is investigated. The thickness t of the SL is varied from 1nm to 4nm. It is found that uniform and damped oscillations of SL magnetization could be seen at both positive and negative applied current values. The range of each type of magnetization dynamics and the frequency values could be adjusted by changing the current density. The results obtained show the importance of optimizing the SL thickness in addition to its intrinsic properties for spin torque oscillator application with low applied current, high frequency and wide operation range.

Eigenmodes of two-dimensional conservative droplet soliton

Droplet solitons are strongly nonlinear, dynamic structures in ferromagnets with perpendicular magnetic anisotropy. For experimental observation of such structures a nanocontact spin torque oscillator (NC-STO) where spin polarized currents act as a gain source to counteract magnetic damping is utilized. We analyze eigenmode of this droplet excitations in a simplest case of conservative droplet solitons, when both Gilbert damping in magnetic films and energy gain due to spin-polarized current can be neglected. As a result of such analysis eigenmode frequencies of conservative droplet are obtained. Among main features of conservative droplet eigenmode is azimuthal degeneration of mods and localization of mode frequencies in a finite region.

Driven energy transfer between coupled modes in spin-torque oscillators

The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multi-mode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices with one vortex in each of the two magnetic layers of the oscillator. In such double-vortex system, the remarkable properties of energy transfer between the coupled modes, one being excited by spin transfer torque while the second one being damped, result into an alteration of the damping parameters. As a consequence, the oscillator nonlinear behavior is concomitantly drastically impacted. This efficient coupling mechanism, driven mainly by the dynamic dipolar field generated by the spin transfer torque induced motion of the vortices, gives rise to an unexpected dynamical regime of self-resonance excitation. These results show that mode coupling can be leveraged for controlling the synchronization process as well as the frequency tunability of spin-torque oscillators.

Phase Locking and Chaotic Dynamics in Dual-Barrier Magnetic Tunnel Junction

This work investigates theoretically the possibility of a dual-barrier magnetic tunnel junction (MTJ) to act as a spin-torque oscillator (STO) which could produce microwaves. We have identified the parameters that control the frequency of oscillations and tuned them to get a maximum frequency. To improve the output power and reduce the distortions, phase locking the STO through external alternating current is done. It is found that the synchronisation occurs rapidly leading to a reduced response time, which is a significant result. Interestingly, magnetisation dynamics becomes chaotic for specific choice of parameters and the period doubling bifurcation is studied to explore the route to chaos.

High frequency out-of-plane oscillation with large cone angle in mag-flip spin torque oscillators for microwave assisted magnetic recording

We investigated spin torque induced magnetization dynamics in the mag-flip spin torque oscillators (STOs) of diameters D from 29 to 96 nm comprising of an in-plane magnetized field generation layer (FGL) Fe67Co33 (7 nm) with high saturation magnetization, μ0Ms ∼ 2.3 T, and perpendicular FePt(10 nm)/Co2FeGa0.5Ge0.5(3 nm) highly spin polarized spin injection layers. Out-of-plane high frequency, f ∼ 21–26 GHz, spin torque induced oscillation with a large cone angle in FGL was observed under nearly perpendicular external magnetic field μ0Hext of 1.1 T for the pillar D of 29 and 42 nm. Our micromagnetic simulation results indicated that ac magnetic fields of about 0.15 to 0.2 T are obtainable from the STOs having the same stacking structure and size as the experiment, which is large enough for the applications to microwave assisted magnetic recording technology.

Spin-Torque Oscillator Read Head in Inhomogeneous Magnetic Fields

Spin-torque oscillators (STOs) are the candidates for a novel class of magnetic field sensors, suitable especially for magnetic media read head purposes. While a number of investigations on the STO readout ability have been reported, comparatively little attention has been paid to the issue of its behavior in highly inhomogeneous magnetic fields, i.e., generated by densely stored magnetic media. We examine the theoretical limits for the STO technology usage for sensor applications, using both a general auto-oscillator model and a micromagnetic simulations framework. We find that the deterioration of the output signal may lead to significant readout errors already for the bit width of a few nanometers, which is close to the current state-of-the-art in the data storage industry.

Influence of interlayer coupling on the spin-torque-driven excitations in a spin-torque oscillator

The influence of dynamic interlayer interactions on the spin torque driven and damped excitations are illustrated for a three layer macrospin model system that corresponds to a standard spin-torque oscillator. The free layer and a synthetic antiferromagnetic (SyF) pinned layer of the spin-torque oscillator are in-plane magnetized. In order to understand experimental results, numerical simulations have been performed considering three types of interlayer interactions: exchange interaction between the two magnetic layers of the SyF, mutual spin torque between the top layer of the SyF and the free layer and dipolar interaction between all three magnetic layers. It will be shown that the dynamic dipolar coupling plays a predominant role. First, it leads to a hybridization of the free layer and the SyF linear modes and through this gives rise to a strong field dependence of the critical current. In particular, there is a field range of enhanced damping in which much higher current is required to drive the modes into steady state. This results in a gap in the excitation spectrum. Second, the dynamic dipolar interaction is also responsible for the non-linear interaction between the current driven steady state mode and the damped modes of the system. Here one can distinguish: (i) a resonant interaction that leads to a kink in the frequency-field and frequency-current dispersions accompanied by a small hysteresis and a reduction of the linewidth of the steady state mode and (ii) a non-resonant interaction that leads to a strong frequency redshift of the damped mode. The results underline the strong impact of interlayer coupling on the excitation spectra of spin-torque oscillators and illustrate in a simple three mode model system how in the non-linear regime the steady state and damped modes influence each other.

Switching field reduction of a perpendicular magnetic nanodot in a microwave magnetic field emitted from a spin-torque oscillator

We demonstrate microwave-assisted magnetization switching of a perpendicular magnetic nanodot in a microwave stray field from a spin-torque oscillator (STO). The switching field decreases when the STO is operated by applying a current. The switching field reduction is almost the same as that in a microwave magnetic field generated by a signal generator despite the fluctuations of the STO oscillation. The switching field distribution, however, is broader when the STO is used. We also examine the magnetization switching process in the nanosecond region by applying a nanosecond-order pulse current to the STO and measuring the STO signal waveform. The onset of the STO oscillation and subsequent assisted switching occur within a few nanoseconds.

Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting**This article belongs to the Special Issue: Emerging Leaders, which features invited work from the best early-career researchers working within the scope of J. Phys. D. This project is part of the Journal of Physics series' 50th anniversary celebrations in 2017. Paul Keatley was selected by the Editorial Board

Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.

Influence of exchange on signal-to-noise ratio in [CoX/Pt]4 media

In longitudinal hard disk drives, the medium Signal-to-Noise Ratio (SNR) is higher with better grain segregation or lower inter-grain exchange. In current energy assisted magnetic recording system, multilayer perpendicular media are utilized; thus, it is significant to study the influence of grain segregation on SNR, as well as the relevant percolation phenomenon, to give suggestions on the recording media design. In this study, micromagnetic recording models of Microwave Assisted Magnetic Recording (MAMR) is built up to calculate SNR to find optimized [CoX/Pt]4 media parameters such as the inter-grain exchange Agb and the anisotropy orientation distribution αθ, with different field generation layer (FGL) saturation in the spin torque oscillator (STO). The constrained relationship between Agb and αθ in MAMR have been estimated, and the medium SNR will be optimized in the perpendicular [CoX/Pt]4 with a proper but not lowest exchange.

Controlling the canted state in antiferromagnetically coupled magnetic bilayers close to the spin reorientation transition

Canted magnetization is obtained in ultrathin, antiferromagnetically coupled magnetic bilayers with thicknesses around the spin reorientation transition. The canting angle is controlled by both the magnetic layer thickness and interlayer coupling strength, which are tuned independently. Hysteresis loops are obtained, where magnetization components parallel and transverse to the applied field are measured, and analyzed by comparison to micromagnetic simulations. This enables the canting angle to be extracted and the behavior of the individual layers to be distinguished. Two types of canted systems are obtained with either single-layer reversal or complex, coupled two-layer reversal, under moderate external magnetic fields. Controlling the magnetization canting and reversal behavior of ultra-thin layers is relevant for the development of magnetoresistive random-access memory and spin-torque oscillator devices.

Spin-torque oscillation in large size nano-magnet with perpendicular magnetic fields

DC current induced magnetization reversal and magnetization oscillation was observed in 500nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. A magnetic hybrid structure was achieved for the study of spin torque oscillation by applying a perpendicular field >3kOe. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight for synchronization of inhomogeneities in large sized device. The ability to manipulate spin-dynamics on large size devices could be proved useful for increasing the output power of the spin-transfer nano-oscillators (STNOs).

Vortex spin-torque oscillator stabilized by phase locked loop using integrated circuits

Spin-torque nano-oscillators (STO) are candidates for the next technological implementation of spintronic devices in commercial electronic systems. For use in microwave applications, improving the noise figures by efficient control of their phase dynamics is a mandatory requirement. In order to achieve this, we developed a compact phase locked loop (PLL) based on custom integrated circuits (ICs) and demonstrate that it represents an efficient way to reduce the phase noise level of a vortex based STO. The advantage of our approach to phase stabilize STOs is that our compact system is highly reconfigurable e.g. in terms of the frequency divider ratio N, RF gain and loop gain. This makes it robust against device to device variations and at the same time compatible with a large range of STOs. Moreover, by taking advantage of the natural highly non-isochronous nature of the STO, the STO frequency can be easily controlled by e.g. changing the divider ratio N.

Micromagnetic simulation for detection of magnetic nanobeads by spin torque oscillator

Micromagnetic simulation for detecting magnetic nanobeads is performed by using spin torque oscillation as the detector. The non-uniform stray field generated by magnetic beads can induce a detectable frequency shift of a spin torque oscillator. Simulations indicate that an 80-nm-diameter magnetic bead can be detected with a frequency shift of 1.2GHz and a maximum linewidth of 28MHz. Due to the non-uniform stray field, the frequency shift and linewidth vary with the bead position. For multiple beads detection, the oscillation frequency is linear with the number of 40-nm-diameter beads, namely 0.16GHz/bead on average.

Multiple Layer Microwave-Assisted Magnetic Recording

Simulations of recording on media with two and three distinct recording layers are presented. A high frequency field is used to select the layer on which data are recorded. Layer-selective recording was confirmed for two layer granular media with narrow distributions. For three layer media magnetostatic interactions between the layers led to a loss of selectivity in the bottom recording layer. Readout of data is possible by staggering transitions in different layers or by using a spin-torque oscillator.

Nanocontact based spin torque oscillators with two free layers

We study a spin torque oscillator based on magnetic vortices hosted by a pseudo spin valve, and generated and confined by a nanocontact configuration. In addition to the standard functioning modes of such spin torque oscillators, we present evidence for field and current conditions leading to a novel extinction regime, where the oscillator response is transiently quenched. We model the dynamics analytically for two non-interacting vortices, one in each magnetic layer. The similarity of their trajectories makes them prone to interact. We argue that they can synchronize and then follow identical orbits, which leads to the extinction of the RF voltage emission.

Scaling effect of spin-torque nano-oscillators

We have investigated the impact of device size on the characteristics of spin-torque nano-oscillators by studying the frequency and linewidth of the oscillator as a function of current density utilizing micromagnetic simulations at room temperature. Upon reduction of the device size from 40 down to 10 nm, thermal noise flattens the frequency and linewidth trends with current density and lowers the threshold current density of the oscillation. The magnetization vector trajectories reveal that irregular oscillations exist in smaller devices. Our findings suggest that a 20 × 20 nm2 spin-torque oscillator could be a viable candidate for a magnetic read sensor.