Spin-valve Nanopillars Research Articles

Vortex dynamics in a spin valve nanopillar having hybrid polarizer and magnetostatic coupling

This study investigates the observation and control of vortex gyration in the free layer by utilizing polarized current. Modifying the current alone proves challenging in adjusting the frequency and amplitude, as the orbit radius of the vortex core remains constant. To address this, a spin valve nanopillar model with hybrid polarizer is introduced and analyzed, enabling support and gyration of the vortex core in the nanodisk under spin polarized current. Manipulating current density and fine-tuning of hybrid polarizer parameters adjusts system frequency and amplitude. However, with hybrid polarizer, while vortex gyration and higher-frequency signal production increase at higher current densities, the amplitude generally decreases. To mitigate this behavior, the study explores the vortex interactions influenced by magnetostatic coupling in the free layers of spin valve nanopillar. The objective of this paper is to advance the understanding of vortex physics for applications that require greater amplitude microwave signals.

Magnetization states driven by spin-transfer torque in spin-valve nanopillars in presence of four-fold magnetocrystalline anisotropy

In this work, the magnetization states under the spin-transfer switching effect are investigated in a nanopillar device composed of a half-metallic Heusler Co1.5Fe1.5Ge alloy with a four-fold in-plane magnetocrystalline anisotropy. A comparative study with a half-metallic Heusler alloy of Co2FeAl0.5Si0.5 and a typical material of Fe is realized by numerically solving the Landau-Lifshitz-Gilbert (LLG) equation with the Spin-Transfer Torque (STT) contribution, using micromagnetic simulations. Our ultimate goal is to elucidate the origins of the intermediate state (IS) that occurs during magnetization switching by deeply examining the effects of magnetocrystalline anisotropy and shape anisotropy on the magnetization states through analyzing and comparing various switching processes. Our simulation results showed that the magnetization switching via a single-step under low current densities is possible at certain critical cross-sectional dimensions of the ellipse that are adjusted to increase the demagnetization field against the four-fold in-plane magnetocrystalline anisotropy. As well, we provided in this paper a set of appropriate geometric dimensions which allow the magnetization to reverse via a single-step by overcoming IS with minimal spin-polarized current densities.

Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

Rapid creation and reversal of skyrmion in spin-valve nanopillars

Abstract Skyrmions are nanoscale and topologically protected objects, which are promising candidates for information carriers of magnetic storage devices. To this end, the rapid creation and manipulation of a single skyrmion is required. In this work, a spin-valve device with a single-skyrmion reference layer is considered. Using micromagnetic simulations, we find that a skyrmion is likely to nucleate in the free layer as a picosecond current pulse is applied. In addition, we also demonstrate that the skyrmion in the free layer can be reversed by injecting an alternating current. It is promising that such a study can benefit the development of skyrmion-based storage devices.

Creation of localized skyrmion bubbles in Co/Pt bilayers using a spin-valve nanopillar

We fabricate devices in which a magnetic nanopillar spin valve makes contact to a Co/Pt bilayer thin film with perpendicular magnetic anisotropy, in order to achieve local control of domains in the Co/Pt bilayer underneath the nanopillar. The goal is to develop the ability to nucleate, detect, and annihilate magnetic skyrmions in the Co/Pt using spin-polarized currents from the nanopillar. We demonstrate the ability to distinguish the local behavior of the Co/Pt film beneath the nanopillar from the extended film and show that the two can switch independently of each other. This allows us to isolate a localized domain under the pillar that can be controlled separately from the rest of the Co/Pt film using applied currents and magnetic fields. Micromagnetic simulations indicate that this localized domain has skyrmion symmetry. Our results represent a first step toward controlling room-temperature skyrmions using localized spin-transfer torque.

Micromagnetic modeling of magnetization switching and oscillation modes in spin valve with tilted spin polarizer

Materials with perpendicular magnetic anisotropy have been intensively investigated due to their potential applications in the nonvolatile magnetic memory and spin-torque oscillators. Hear in this paper, we report a special interesting spin-transfer-driven magnetic behavior in perpendicularly magnetized (Co/Ni) -based spin-valve nano-pillars due to the reduced symmetry of easy axis in the free layer. The micromagnetic simulations indicate that a dip in the average magnetization curve can take place due to the reduced symmetry such as tilt of the magnetic field as well as the easy axis of the free and polarizer layers. In order to further clarify the physics mechanism of the dip, we carry out a series of new simulation studies. In our simulations, we consider a spin-valve nano-pillar with perpendicular anisotropy free layer and a 3 tilted polarizer layer. A negative perpendicular magnetic field and a positive perpendicular current are both applied simultaneously. In the average magnetization curves mz as a function of the magnetic field with various currents, three dips are observed. Note that although the spin-transfer torque is essential to the appearance of the dips, the position of the dips is less affected by the current in a certain current range. For three dips, we notice that the mz values are almost identical at a special magnetic field for different currents. At this special magnetic field, the magnetization oscillation modes in the free layer are similar to each other for different currents. The corresponding frequency spectra show that the amplitude of the main frequency peak decreases with the increasing of current due to the enhanced spin-transfer torque. In addition, the frequency shows a blue-shift with the increasing of applied current. Our simulations show that the main frequency f1 corresponding to the highest peak is approximately equal to the precession frequency of the local magnetization in the free layer. Several high-order frequency peaks are also observed in the frequency spectrum with fn=nf1, where n is an integer. Therefore the periodic oscillation of mz is a harmonic oscillation. Further simulations indicate that the dip appearance is also affected by the thickness of free layer. The spin-transfer torque effect decreases with the thickness of the free layer increasing. As a consequence, the dips shift to a low magnetic field range with the increase of the thickness. And for larger thickness t=8.0 nm, no dip appears. This result suggests that the spin-transfer torque is necessary for the dip, rather than the unique effect factor, to occur. In the dip region, the magnetic oscillation modes of the free layer show interesting frequency spectrum characters:harmonic frequency or inter-harmonic frequency. As a consequence, the periodic oscillation of the free layer is accompanied by the harmonic waves.

Efficiency of Spin-Transfer-Torque Switching and Thermal-Stability Factor in a Spin-Valve Nanopillar with First- and Second-Order Uniaxial Magnetic Anisotropies

Spin-valve nanopillars are under extensive development to realize spintronic nonvolatile memory. Here highly efficient spin-transfer-torque switching is required, but cannot be enhanced by simply adjusting the first-order uniaxial magnetic anisotropy. The authors find that, by introducing second-order anisotropy and manipulating the ratio of first- and second-order quantities, efficiency can be raised dramatically, or the thermal stability against accidental switching can be maximized. These results provide an indispensable guide for the design of magnetoresistive random-access memory.

Current-Induced Pinwheel Oscillations in Perpendicular Magnetic Anisotropy Spin Valve Nanopillars

Nanopillar spin valve devices are typically comprised of two ferromagnetic layers: a reference layer and a free layer whose magnetic orientation can be changed by both an external magnetic field and through the introduction of spin-polarized electric current. Here we report the continuous repeated switching behavior of both the reference and free layers of a perpendicular spin valve made of Co/Pd and Co/Ni multilayers that arises for sufficiently large DC currents. This periodic switching of the two layers produces an oscillating signal in the MHz regime but is only observed for one sign of the applied current. The observed behavior agrees well with micromagnetic simulations.

Modified High Frequency Radial Spin Wave Mode Spectrum in a Chirality-Controlled Nanopillar

Circular magnetic spin valve nanopillars in a dual vortex configuration have dynamic characteristics strongly dependent on the interlayer dipole coupling. We report here on frequency domain properties of such nanopillars obtained by micromagnetic simulations. After the free layer is chirality switched with spin transfer torque, a radial spin wave eigenmode spectrum forms in the free layer with unusually large edge amplitude. The structure of these modes indicate a departure from the magnetostatic processes typically observed experimentally and treated analytically in low aspect ratio isolated disks. Our findings give new details of dynamic chirality control and relxation in nanopillars and raise potential signatures for experiments.

Angular Dependence of Spin Transfer Switching in Spin Valve Nanopillar Based Heusler Alloy

The spin transfer induced magnetization switching in current perpendicular-to-the-plane spin valve nanopillar based Co2FeAl0.5Si0.5Heusler alloy with varying the initial angles of the magnetization of sensing layer,θ0, was investigated via macrospin simulations. The effects of an in-plane magnetic field,Hi, on the switching behavior were also evaluated. The magnetization switching was excited by spin polarized switching current,Is. The time varying magnetization was computed by the Landau-Lifshitz-Gilbert-Slonczewski equation, while the spin transfer induced noise was examined by using the power spectral density analysis. It was found thatθ0should be narrowly initialized since this configuration produces the small noise during the switching. Also, the negativeIsproduced more uniform switching than the positiveIsdue to existence of ferromagnetic exchange coupling. WhenHiwas presented, the noise generated at low frequencies could be suppressed, and then the switching behavior became more uniform. In addition, the results indicated that the noise configuration could be explained by the physical dynamic of magnetization behavior. Hence, the spin transfer induced noise needs to be minimized in order to improve the performance of spin transfer torque random access memory for high density recording.

Non-linear mode interaction between spin torque driven and damped modes in spin torque nano-oscillators

The influence of dynamic coupling in between magnetic layers of a standard spin torque nano-oscillator composed of a synthetic antiferromagnet (SyF) as a polarizer and an in-plane magnetized free layer has been investigated. Experiments on spin valve nanopillars reveal non-continuous features such as kinks in the frequency field dependence that cannot be explained without such interactions. Comparison of experiments to numerical macrospin simulations shows that this is due to non-linear interaction between the spin torque (STT) driven mode and a damped mode that is mediated via the third harmonics of the STT mode. It only occurs at large applied currents and thus at large excitation amplitudes of the STT mode. Under these conditions, a hybridized mode characterized by a strong reduction of the linewidth appears. The reduced linewidth can be explained by a reduction of the non-linear contribution to the linewidth via an enhanced effective damping. Interestingly, the effect depends also on the exchange interaction within the SyF. An enhancement of the current range of reduced linewidth by a factor of two and a reduction of the minimum linewidth by a factor of two are predicted from simulation when the exchange interaction strength is reduced by 30%. These results open directions to optimize the design and microwave performances of spin torque nano-oscillators taking advantage of the coupling mechanisms.

Verification of the Thomson-Onsager reciprocity relation for spin caloritronics

We investigate the Thomson-Onsager relation between the spin-dependent Seebeck and spin-dependent Peltier effect. To maintain identical device and measurement conditions we measure both effects in a single ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Cu}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanopillar spin valve device subjected to either an electrical or a thermal bias. In the low bias regime, we observe similar spin signals as well as background responses, as required by the Onsager reciprocity relation. However, at large biases, deviation from reciprocity occurs in the voltage-current relationships, dominated by nonlinear contributions of the temperature-dependent transport coefficients. By systematic modeling of these nonlinear thermoelectric effects and measuring higher-order thermoelectric responses for different applied biases, we identify the transition between the two regimes as the point at which Joule heating starts to dominate over Peltier heating. Our results signify the importance of local equilibrium (linearity) for the validity of this phenomenological reciprocity relation.

Comparison between thermal and current driven spin-transfer torque in nanopillar metallic spin valves

We investigate the relation between thermal spin-transfer torque (TSTT) and the spin-dependent Seebeck effect (SDSE), which produces a spin current when a temperature gradient is applied across a metallic ferromagnet, in nanopillar metallic spin valves. Comparing its angular dependence (aSDSE) with the angle dependent magnetoresistance (aMR) measurements on the same device, we are able to verify that a small spin heat accumulation builds up in our devices. From the SDSE measurement and the observed current driven STT switching current of 0.8 mA in our spin valve devices, it was estimated that a temperature difference of 230 K is needed to produce an equal amount of TSTT. Experiments specifically focused on investigating TSTT show a response that is dominated by overall heating of the magnetic layer. Comparing it to the current driven STT experiments we estimate that only ~10% of the response is due to TSTT. This leads us to conclude that switching dominated by TSTT requires a direct coupling to a perfect heat sink to minimize the effect of overall heating. Nevertheless the combined effect of heating, STT and TSTT could prove useful for inducing magnetization switching when further investigated and optimized.

Bimodal switching field distributions in all-perpendicular spin-valve nanopillars

Switching field measurements of the free layer element of 75 nm diameter spin-valve nanopillars reveal a bimodal distribution of switching fields at low temperatures (below 100 K). This result is inconsistent with a model of thermal activation over a single perpendicular anisotropy barrier. The correlation between antiparallel to parallel and parallel to antiparallel switching fields increases to nearly 50% at low temperatures. This reflects random fluctuation of the shift of the free layer hysteresis loop between two different magnitudes, which may originate from changes in the dipole field from the polarizing layer. The magnitude of the loop shift changes by 25% and is correlated to transitions of the spin-valve into an antiparallel configuration.

Switching field distributions with spin transfer torques in perpendicularly magnetized spin-valve nanopillars

We present switching field distributions of spin-transfer assisted magnetization reversal in perpendicularly magnetized Co/Ni multilayer spin-valve nanopillars at room temperature. Switching field measurements of the Co/Ni free layer of spin-valve nanopillars with a 50 nm x 300 nm ellipse cross section were conducted as a function of current. The validity of a model that assumes a spin-current dependent effective barrier for thermally activated reversal is tested by measuring switching field distributions under applied direct currents. We show that the switching field distributions deviate significantly from the double exponential shape predicted by the effective barrier model, beginning at applied currents as low as half of the zero field critical current. Barrier heights extracted from switching field distributions for currents below this threshold are a monotonic function of the current. However, the thermally-induced switching model breaks down for currents exceeding the critical threshold.

Modelling current-induced magnetization switching in Heusler alloy Co2FeAl-based spin-valve nanopillar

We investigated the current-induced magnetization switching in a Heusler alloy Co2FeAl-based spin-valve nanopillar by using micromagnetic simulations. We demonstrated that the elimination of the intermediate state is originally resulted from the decease of effective magnetic anisotropy constant. The magnetization switching can be achieved at a small current density of 1.0 × 104 A/cm2 by increasing the demagnetization factors of x and y axes. Based on our simulation, we found magnetic anisotropy and demagnetization energies have different contributions to the magnetization switching.

Micromagnetic study of high-power spin–torque oscillator with perpendicular magnetization in half-metallic Heusler alloy spin valve nanopillar under external magnetic fields

We investigated the high-power spin–torque oscillator in a half-metallic Heusler alloy Co 2 MnSi spin-valve nanopillars with perpendicular magnetization under external magnetic field using micromagnetic simulations. Our simulations show that the narrow optimum current of magnetization precession in the Heusler-based spin valve is broadened by introducing the surface anisotropy. The linear decrease of frequency with the out-of-plane magnetic field is obtained in our simulation. Additionally, the in-plane magnetic field dependence of frequency shows a parabolic curve which is explained by the magnetization trajectory tilting. Furthermore, we also discussed the decrease of output power using the excitation of non-uniform magnetization precession in the in-plane magnetic fields. • We investigated spin–torque oscillator in Co 2 MnSi spin-valve under magnetic fields. • The narrow optimum current is broadened by introducing the surface anisotropy. • The frequency dependences of out-of-plane and in-plane magnetic fields show linear and parabola. • The results may give the guidance for designing Heusler-based spin–torque oscillator.

Temperature dependent nucleation, propagation, and annihilation of domain walls in all-perpendicular spin-valve nanopillars

We present a study of the temperature dependence of the switching fields in Co/Ni-based perpendicularly magnetized spin-valves. While magnetization reversal of all-perpendicular Co/Ni spin valves at ambient temperatures is typically marked by a single sharp step change in resistance, low temperature measurements can reveal a series of resistance steps, consistent with non-uniform magnetization configurations. We propose a model that consists of domain nucleation, propagation, and annihilation to explain the temperature dependence of the switching fields. Interestingly, low temperature (<30 K) step changes in resistance that we associate with domain nucleation have a bimodal switching field and resistance step distribution, attributable to two competing nucleation pathways.

Modelling high-power spin-torque oscillator with perpendicular magnetization in half-metallic Heusler alloy spin valve nanopillar

We investigated the high-power spin-torque oscillator in a half-metallic Heusler alloy Co2MnSi spin-valve nanopillars with perpendicular magnetization using micromagnetic simulations. A stable high output power spin transfer precession was obtained in the condition of zero external magnetic field, and the narrow current of oscillation due to the high spin polarization of Heusler alloy is significantly widened by introducing the surface anisotropy. Furthermore, we discussed the current dependence of oscillation frequency and explained the blue frequency shift using the trajectories and spatial magnetic domains.

Investigations on the Magnetization Switching Dynamics in Spin Valve Multilayers Under Periodic Applied Magnetic Field

Magnetization switching dynamics in a spin valve nanopillar, induced by spin transfer torque in the presence of a periodic applied field is investigated by solving the Landau–Lifshitz–Gilbert–Slonczewski equation. Under steady state conditions, the switching of magnetization occurs in the system, above a threshold current density value Jc. A general expression for the critical current density is derived and it is shown that this further reduces when there is magnetic interface anisotropy present in the free layer of the spin valve. We also investigated the chaotic behavior of the free layer magnetization vector in a periodically varying applied magnetic field, in the presence of a constant DC magnetic field and spin current. Further, it is found that in the presence of a nonzero interfacial anisotropy, chaotic behavior is observed even at much smaller values of the spin current and DC applied field.