Spin Torque Oscillator Research Articles

Reduction of critical current density for out-of-plane mode oscillation in a mag-flip spin torque oscillator using highly spin-polarized Co2Fe(Ga0.5Ge0.5) spin injection layer

We study spin torque oscillators comprised of a perpendicular spin injection layer (SIL) and a planar field generating layer to reveal the influence of the spin polarization of SIL material on the critical current density, JC, to induce microwave oscillation. Two systems with different SIL are compared: one with a highly spin-polarized Heusler alloy, Co2Fe(Ga0.5Ge0.5) (CFGG), and the other a prototypical Fe2Co alloy. Cross sectional scanning transmission electron microscopy observations show the B2-ordered structure in a 3-nm-thick CFGG SIL, a prerequisite for obtaining half-metallic transport properties. Current induced microwave oscillations are found at frequencies of ∼15 GHz for both systems. However, the current needed to cause the oscillations is ∼50% smaller for films with the CFGG SIL compared to those of the Fe2Co SIL. These results are in accordance with micromagnetic simulations that include spin accumulation at the SIL.

Low-nonlinearity spin-torque oscillations driven by ferromagnetic nanocontacts

Spin-torque oscillators are strong candidates as nano-scale microwave generators and detectors. However, because of large amplitude-phase coupling (non-linearity), phase noise is enhanced over other linear auto-oscillators. One way to reduce nonlinearity is to use ferromagnetic layers as a resonator and excite them at localized spots, making a resonator-excitor pair. We investigated the excitation of oscillations in dipole-coupled ferromagnetic layers, driven by localized current at ferromagnetic nano-contacts. Oscillations possessed properties of optical-mode spin-waves and at low field ($\approx$200 Oe) had high frequency (15 GHz), a moderate precession amplitude (2--3$^\circ$), and a narrow spectral linewidth ($<$3 MHz) due to localized excitation at nano-contacts. Micromagnetic simulation showed emission of resonator's characteristic optical-mode spin-waves from disturbances generated by domain-wall oscillations at nano-contacts.

Frequency control of a spin-torque oscillator using magnetostrictive anisotropy

We report the working principle of a spin-torque oscillator, of which the frequency is efficiently controlled by manipulating the magnetostrictive anisotropy. To justify the scheme, we simulate a conventional magnetic-tunnel junction-based oscillator which is fabricated on a piezoelectric material. By applying mechanical stress to a free layer using a piezoelectric material, the oscillation frequency can be controlled to ensure a broad tuning range without a significant reduction of the dynamic resistance variation. Such controllability, which appears in the absence of an external magnetic field, will not only enable the integration of spin-torque oscillators and conventional complimentary metal-oxide semiconductor technology but will also broaden the applicability of spin-torque oscillators.

Micromagnetic Studies of Spin-Torque Oscillator Adjacent to Soft Magnetic Thin Film

We studied the precession frequency and the corresponding Fourier component intensity of a spin-torque oscillator (STO), when adjacent to a soft magnetic film or a magnetic recording write-pole tip, for a drive current range of 10-400 MA/cm 2 . The soft film interferes with the precession of the magnetic moment in the field-generation layer (FGL) of the STO, enhancing the critical current density required to drive a stable, high-frequency precession with a large in-plane component (working mode). Vibration of the magnetization in the soft film occurs when the FGL precesses nearby. A saturated pole tip can generate a large but inhomogeneous field in the STO, and the STO can achieve a working mode under a large driven current density of around 250 MA/cm 2 . However, the precession frequency is limited to below 14 GHz, which does not meet the optimum recording condition in microwave-assisted magnetic recording.

Perpendicular magnetic properties of ultrathin L10-Mn1.67Ga films grown by molecular-beam epitaxy

Materials with large perpendicular magnetic anisotropies (PMAs) have drawn great attention because of their potential applications in advanced spintronic devices such as spin-transfer-torque magnetic random access memory (STT-MRAM) and ultrahigh-density perpendicular magnetic recording. To date, a large variety of PMA materials have been investigated, such as L10-ordered FePt, CoPt granular films, Co/(Pt,Pd,Ni) multilayers, ultra-thin CoFeB alloys and perpendicularly magnetized Co2FeAl films. Among the various kinds of materials with PMA, MnGa film with L10-structure has received the most attention because it has large PMA (Ku～107 erg/cm3), ultralow Gilbert damping constant (0.008) and theoretically predicted high spin polarization (more than 70%). All these properties make L10-ordered MnGa a good candidate for spintronic devices such as STT-MRAM and spin-torque oscillators. Meanwhile, from the viewpoint of STT related spintronic device, it is necessary to fabricate ultrathin perpendicularly magnetized L10-MnxGa films to lower the critical current for magnetization reversal. However, up to now, in the main researches the ultrathin L10-MnxGa films have been grown on MgO substrates, which makes it difficult to integrate the MnGa-based magnetic tunnel junctions into the semiconductor manufacturing process.In this work, ultrathin L10-Mn1.67Ga films with different thickness values (1-5 nm) are grown on traditional GaAa (001) substrates by a molecule-beam epitaxy system. During the deposition, in situ streaky surface reconstruction patterns are observed from reflection high-energy electron diffraction, which implies high crystalline quality of the L10-Mn1.67Ga film. Only MnGa superlattice (001) and MnGa fundamental (002) peaks can be observed in the X-ray diffraction patterns in a range from 20 to 70, which means that the L10-Mn1.67Ga film is a good single-crystalline with c-axis along the normal direction. The magnetic properties of these films are measured by superconductor quantum interference device magnetometer in a field range of 5 T. The perpendicular M-H curves are almost square, while the in-plane curves are nearly hysteresis-free, each with a remnant magnetization (Mr) of around zero, which clearly evidences the PMA of the ultrathin L10-Mn1.67Ga film. Moreover, as the thickness of L10-Mn1.67Ga film decreases from 5 nm to 1 nm, the ratio of Mr/Ms also decreases from 1 to 0.72, which indicates that the PMA loses as thickness decreases. We also estimate the perpendicular anisotropy constant (Ku) from the relation Ku=Keff+2 Ms2, and the maximum Ku of 14.7 Merg/cm3 is obtained for the 5 nm MnGa film. Although the Ku decreases with thickness decreasing, a Ku value of 8.58 Merg/cm3 is observed in a 2 nm thick film. The obtained results are important for developing the L10-MnGa-based spin-transfer torque Gbit class magnetic random access memory.

Modulation of the Spectral Characteristics of a Nano-Contact Spin-Torque Oscillator via Spin Waves in an Adjacent Yttrium-Iron Garnet Film

NiFe-Cu-Co trilayer nano-contact spin-torque oscillators (NC-STOs) fabricated on an yttrium-iron garnet (YIG) film were studied in two different modes. In passive mode, i.e. without any NC-STO auto-oscillations, a microwave current through the nano-contact can excite spin waves (SW) in the YIG film, and, vice versa, antenna generated SWs in the YIG film can be detected by the nano-contact. In active mode, i.e., in the presence of auto-oscillations, significant changes appear in the NC-STO spectrum when its frequency approaches that of the SWs excited in the YIG. These results demonstrate strong coupling between NC-STOs and SWs in YIG and open new possibilities of 1) pure spin-current generation in YIG by NC-STOs; 2) mutual locking of a number of NC-STOs through SWs in YIG; and 3) improvement of NC-STO spectra through SW feedback in YIG.

Splitting of standing spin-wave modes in circular submicron ferromagnetic dot under axial symmetry violation.

The spin wave dynamics in patterned magnetic nanostructures is under intensive study during the last two decades. On the one hand, this interest is generated by new physics that can be explored in such structures. On the other hand, with the development of nanolithography, patterned nanoelements and their arrays can be used in many practical applications (magnetic recording systems both as media and read-write heads, magnetic random access memory, and spin-torque oscillators just to name a few). In the present work the evolution of spin wave spectra of an array of non-interacting Permalloy submicron circular dots for the case of magnetic field deviation from the normal to the array plane have been studied by ferromagnetic resonance technique. It is shown that such symmetry violation leads to a splitting of spin-wave modes, and that the number of the split peaks depends on the mode number. A quantitative description of the observed spectra is given using a perturbation theory for small angles of field inclination from the symmetry direction. The obtained results give possibility to predict transformation of spin wave spectra depending on direction of the external magnetic field that can be important for spintronic and nanomagnetic applications.

Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators.

The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. Not only is robust synchronization between two oscillators observed for separations larger than 1,000 nm, but synchronization of up to five oscillators, a new record, has been observed in the vertical array geometry. Furthermore, the synchronization can no longer be considered mutual in nature.

Extremely Coherent Microwave Emission from Spin Torque Oscillator Stabilized by Phase Locked Loop.

Spin torque oscillator (STO) has been attracting a great deal of attention as a candidate for the next generation microwave signal sources for various modern electronics systems since its advent. However, the phase noise of STOs under free running oscillation is still too large to be used in practical microwave applications, thus an industrially viable means to stabilize its oscillation has been strongly sought. Here we demonstrate implementation of a phase locked loop using a STO as a voltage controlled oscillator (VCO) that generates a 7.344 GHz microwave signal stabilized by a 153 MHz reference signal. Spectrum measurement showed successful phase locking of the microwave signal to the reference signal, characterized by an extremely narrow oscillation peak with a linewidth of less than the measurement limit of 1 Hz. This demonstration should be a major breakthrough toward various practical applications of STOs.

Critical damping constant of a spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized reference layer

We theoretically analyzed the effects of damping on spin torque induced oscillation in a spin torque oscillator with a perpendicularly magnetized free layer and an in-plane magnetized reference layer. It was found that there exists an unexpectedly small critical damping constant, above which no steady-state oscillation can be excited by spin torque. It was also found that the critical damping constant is almost independent of the anisotropy constants but increases with the spin polarization of the injected current.

Growth and characterization of epitaxial NiMnSb/ZnTe/NiMnSb magnetic multilayers

The half-metal ferromagnet NiMnSb, with its high spin polarization, low magnetic damping and tunable magnetic anisotropy, is a promising material for applications in spin torque devices. We develop the epitaxial growth of NiMnSb/ZnTe/NiMnSb heterostructures, aiming towards the realization of an all-NiMnSb based magnetic tunneling junction (MTJ). Layers are grown in situ by Molecular Beam Epitaxy (MBE) and Atomic Layer Epitaxy (ALE) methods. By tuning Mn content, the magnetic anisotropy of each of the two NiMnSb layers is adjusted in order to achieve mutually orthogonal uniaxial anisotropies. SQUID measurements of the magnetization along orthogonal crystal directions [110] and [11¯0] confirm that the two layers have mutually orthogonal anisotropy. High Resolution X-Ray Diffraction measurements and simulations confirm the nominal layer stack and demonstrate the high crystalline quality of the individual layers. Such layer stacks provide a potential basis for TMR-based spin-torque devices such as spin-torque oscillators.

Physical Implementation of Coherently Coupled Oscillator Networks

We present a combined experimental and simulation study of the physical implementation of coherently coupled oscillator networks composed of spin-torque oscillators (STOs). Based on published works and on our recent experiments, we review the behavior of individual oscillators and arrays of coupled oscillators. We construct models that are calibrated by experiments, and our simulations demonstrate that an array of coherently coupled STOs exhibits the basic functionality of an associative memory.

Non-Boolean Associative Processing: Circuits, System Architecture, and Algorithms

We present the design and the performance of a hierarchical associative memory (AM) based on phase locking of coupled oscillators used for pattern recognition. The use of coupled oscillators, rather than Boolean logic, provides for implementations using emerging nanotechnology, such as magnetic spin-torque oscillators and resonant body transistor oscillators, which have the potential of lower energy and higher density than CMOS solutions. We develop a model for the general behavior of weakly coupled nonlinear oscillators that perform pattern matching using a simulation of coupled CMOS ring oscillators. We derive a simple analytic model for their phase locking behavior and use this reduced model in a hierarchical AM for image recognition tasks, such as identifying handwritten numbers.

The Importance of Depth-Varying Fields for MAMR Switching-Field Reduction

Microwave assisted magnetic recording (MAMR) was proposed to increase the data storage areal density in hard disk drives [1]. With a localized AC field generated by a spin torque oscillator (STO), MAMR significantly reduces the switching field when the AC field frequency matches the ferromagnetic resonance frequency of the media elements [2]. In this study we present micromagnetic modeling of media switching-field reduction due to AC fields at GHz frequencies. We employ a 4-layer media model based on current exchange-coupled media with the top layer describing a large lateral exchange “cap” with three low lateral exchange “oxide” layers below. We examine hysteresis loops in two scenarios: depth invariant applied fields (external and AC), and depth invariant external field with depth varying AC field. We then do a DC-band write study with realistic writer field and macrospin-based STO field to gauge switching-field reduction in a scenario more similar to actual recording.

Detrimental Effects of the STO Vertical Field Component in MAMR

Microwave-assisted magnetic recording (MAMR) employs a spin-torque oscillator (STO) to assist a standard magnetic writer field in switching grains of a magnetic recording medium. The STO field has different polarization angles at different positions, resulting in different mixes of in-media-plane and out-of-media-plane (vertical) field components. We find that the vertical component of the STO field degrades the writing process, increasing jitter, decreasing low-frequency signal-to-noise ratio, decreasing magnetic write width, and decreasing areal density capability. This is not due to postwrite erasure, but rather is part of the write process itself. Simple vector considerations illuminate the cause for this degradation. Consequently, experimental and modeling approaches that estimate MAMR switching-field reduction using only in-media-plane oscillating fields will erroneously overestimate the impact of the ac fields in the magnetic recording process.

Stray Field Dependence of Threshold Current in a Spin-Torque Oscillator With a Perpendicular Spin Polarizer

We theoretically investigate the effect of stray field on the current-induced magnetization dynamics in a nanopillar with a perpendicular spin polarizer. The threshold current to excite out-of-plane (OP) magnetization procession is analytically calculated based on a simple Newton-like dynamic equation. The numerical calculations of this analytical expression are confirmed by macrospin simulations, revealing that the critical current linearly depends on the stray field. The stray field versus current phase diagram of magnetization oscillation states in such a device is presented, showing three regions of magnetization dynamics including parallel, antiparallel, and OP precession states. The physical insight of the phase transitions between the three states can be explained by calculating the energy profiles as well as the phase portraits of the free layer.

Injection-Locked Spin Hall-Induced Coupled-Oscillators for Energy Efficient Associative Computing

In this paper, we show that the dynamics of injection-locked Spin Hall Effect Spin-Torque Oscillator (SHE-STO) cluster can be exploited as a robust primitive computational operator for non-Boolean associative computing. A cluster of SHE-STOs can be locked to a common frequency and phase with an injected ac current signal. DC input to each STO from external stimuli can conditionally unlock some of them. Based on the input dc signal, the degree of synchronization of SHE-STO cluster is detected by CMOS interface circuitry. The degree of synchronization can be used for associative computing/matching. We present a numerical simulation model of SHE-STO devices based on Landau–Lifshitz–Gilbert equation with spin-transfer torque term and Spin Hall Effect. The model is then used to analyze the frequency and phase locking properties of injection-locked SHE-STO cluster. Results show that associative computing based on the injection locked SHE-STO cluster can be energy efficient and relatively immune to device parameter variations and thermal noise.

Microwave-Assisted Shingled Magnetic Recording

Simulations of shingled, microwave-assisted magnetic recording were carried out. First, a simplified model of the head field near the corner of the main pole of a write head was used to understand the effect of the spin torque oscillator (STO) position relative to the head. Next, shingled tracks were written and evaluated for two different write head and STO geometries. For shingled recording, locating a rotated STO on the side of the main pole may provide some advantages in that medium grains experience the high-frequency (HF) field for a longer time and the chirality of the HF field does not change as the grains pass by the STO. Recording simulations show that the rotated STO writes smoother track edges at lower densities, but there is little difference in signal-to-noise ratio at high linear densities.

Roadmap for Emerging Materials for Spintronic Device Applications

The Technical Committee of the IEEE Magnetics Society has selected 7 research topics to develop their roadmaps, where major developments should be listed alongside expected timelines; (i) hard disk drives, (ii) magnetic random access memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and actuators, (vi) magnetic materials and (vii) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used for example as a cell for a magnetic random access memory and spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarised electrons efficiently into a non-magnet. We have accordingly identified 2 key properties to be achieved by developing new magnetic materials for future spintronic devices: (1) Half-metallicity at room temperature (RT); (2) Perpendicular anisotropy in nano-scale devices at RT. For the first property, 5 major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: Heusler alloys, ferrites, rutiles, perovskites and dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level EF only for its minority spins, achieving 100% spin polarisation at EF. We have also evaluated L10-alloys and D022-Mn-alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarisation. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations and interdisciplinary applications within 35 years time scale.

Towards a table-top microscope for nanoscale magnetic imaging using picosecond thermal gradients.

Research advancement in magnetoelectronics is challenged by the lack of a table-top magnetic measurement technique with the simultaneous temporal and spatial resolution necessary for characterizing magnetization dynamics in devices of interest, such as magnetic memory and spin torque oscillators. Although magneto-optical microscopy provides superb temporal resolution, its spatial resolution is fundamentally limited by optical diffraction. To address this challenge, we study heat rather than light as a vehicle to stroboscopically transduce a local magnetic moment into an electrical signal while retaining picosecond temporal resolution. Using this concept, we demonstrate spatiotemporal magnetic microscopy using the time-resolved anomalous Nernst effect (TRANE). Experimentally and with supporting numerical calculations, we find that TRANE microscopy has temporal resolution below 30 ps and spatial resolution determined by the area of thermal excitation. Based on these findings, we suggest a route to exceed the limits imposed by far-field optical diffraction.