Perpendicular Anisotropy Research Articles

Strain tuning of the Curie temperature and valley polarization in two dimensional ferromagnetic WSe2/CrSnSe3 heterostructure

Magnetic proximity effect can be used to tailor the magnetic ground state and valley polarization in the monolayer of transition metal dichalcogenides. Thus, we explore the effect of biaxial tensile and compressive strain on valley polarization in the WSe2/CrSnSe3 heterostructures with different stacking orders systematically. The indirect band gaps in the two most stable stackings; hollow (0.27 eV) and top (0.33 eV) were further enhanced to 0.35 eV under tensile strain while suppressed to almost 0.13 eV under compressive strain. The heterostructures had a FM ground state with a total magnetic moment per unit cell of 6 μ B in pristine as well as strained structures. In hollow stacking and compressively strained structures, we obtained a perpendicular magnetocrystalline anisotropy, while the top stacking and tensile strain structures had small in-plane anisotropy. An enhancement was found in Curie temperature from 73 K in pristine to 128 K in a 6% tensile strained structure. The valley splittings found in pristine hollow (4 meV) and top (9 meV) stacked heterostructures were further enhanced to 29 meV and 22 meV at 5% compressive strain respectively. This enhancement was attributed to the increased spatial dependence of the charge density along K+ and K− directions of the Brillouin zone, which give rise to the different local dipolar fields at these valleys. Our results suggest that strain could be an effective way to control or tune the valley splitting in WSe2/CrSnSe3 heterostructures.

Multiple Magnetoionic Regimes in Ta/Co20Fe60B20/HfO2

In Ta/CoFeB/HfO2 stacks a gate voltage drives, in a nonvolatile way, the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state with IPA. The IPA$\,\to\,$PMA regime is found to be significantly faster than the PMA$\,\to\,$IPA regime, while only the latter shows full reversibility under the same gate voltages. The effective damping parameter also shows a marked dependence with gate voltage in the IPA$\,\to\,$PMA regime, going from 0.029 to 0.012, and only a modest increase to 0.014 in the PMA$\,\to\,$IPA regime. The existence of two magneto-ionic regimes has been linked to a difference in the chemical environment of the anchoring points of oxygen species added to underoxidized or overoxidized layers. Our results show that multiple magneto-ionic regimes can exist in a single device and that their characterization is of great importance for the design of high performance spintronics devices.

Spin-torque induced wall motion in perpendicularly magnetized discs: Ballistic versus oscillatory behavior

We use time-resolved measurement and modeling to study the spin-torque induced motion of a domain wall in perpendicular anisotropy magnets. We show that the most important factor governing domain wall dynamics is the energy difference between a wall at the center of the disk with either a Bloch-type configuration or the N\'eel-type configuration; this energy difference strongly depends on the disk diameter. When between 70 and 100 nm, the wall drifts across the disk with pronounced back-and-forth oscillations that arise because the wall moves in the Walker regime. Several switching paths occur stochastically and lead to distinct switching durations. The wall can cross the disk center either in a ballistic manner or with variably marked oscillations before and after the crossing. The crossing of the center can even occur multiple times if a vertical Bloch line nucleates within the wall. The wall motion is analyzed using a collective coordinate model parametrized by the wall position $q$ and the tilt $\ensuremath{\phi}$ of its in-plane magnetization projection. The dynamics results from the stretch field, which describes the affinity of the wall to reduce its length and the wall stiffness field describing the wall tendency to reduce dipolar energy by rotating its tilt. The wall oscillations result from the continuous exchange of energy between to the two degrees of freedom $q$ and $\ensuremath{\phi}$. The stochasticity of the wall dynamics can be understood from the concept of the retention pond: a region in the $q\text{\ensuremath{-}}\ensuremath{\phi}$ space in which walls are transiently bound to the disk center. Walls having trajectories close to the pond must circumvent it and therefore have longer propagation times. The retention pond disappears for a disk diameter of typically 40 nm: the wall then moves in a ballistic manner irrespective of the dynamics of its tilt. The propagation time is then robust against fluctuations hence reproducible.

Alignment-Free Sensing Module for Absolute and Incremental Lines in Linear Positioning System Based on Tunneling-Magnetoresistance Sensors.

An alignment-free sensing module for the positioning system based on tunneling magnetoresistive (TMR) sensors with an absolute-incremental-integrated scale is demonstrated. The sensors of the proposed system for both lines consist of identical layer stacks; therefore, all sensors can be fabricated in identical processes from thin film deposition to device patterning on a single substrate. Consequently, the relative position of the sensors can be predefined at the lithography stage and the alignment error between sensors caused by the manual installation is completely eliminated. Different from the existing sensing scheme for incremental lines, we proposed to utilize the magnetic tunnel junctions with a perpendicular anisotropy reference layer and an in-plane anisotropy sensing layer. The sensors are placed parallel to the scale plane with magnetization of the sensing layer in the plane, which show the capability of polarity detection for the absolute line and reveal sinusoidal output signal for the incremental line. Furthermore, due to the large signal of TMR, the working distance can be further improved compared with conventional sensors. In addition, the cost of the positioning system is expected to be lowered, since all the sensors are fabricated in the same process without extra installation. Our design may pave a new avenue for the positioning system based on a magnetic detection scheme.

Surface acoustic wave assisted depinning of magnetic domain walls

We investigate the effects of high frequency strain on the depinning of magnetic domain walls in perpendicular anisotropy materials. Micron wide stripes of [Co(0.3 nm)/Pt(0.6 nm)]5 are patterned between a pair of identical inter-digital transducers that generate high frequency (114.8 MHz) standing surface acoustic waves. We use magneto-optical Kerr effect microscopy to characterize the thermally-assisted depinning of domain walls at defect sites within the strips. Our results show that the excitation of the domain walls with surface acoustic waves results in an increase in their depinning probabilities by approximately a factor of 10. Our data are consistent with a model in which the magnetoelastic anisotropies induced by the acoustic waves modulate the energy barriers that pin the domain walls. These results suggest an alternative route to domain wall depinning in thin films and nanostructures and are relevant to the development of racetrack memories, where domain wall pinning can result in reduced velocities and non-deterministic motion.

Giant Dzyaloshinskii-Moriya Interaction and Room-Temperature Nanoscale Skyrmions in CoFeB/MgO Heterostructures

Magnetic skyrmions in heavy metal (HM)/CoFeB/MgO structures are of particular interest for skyrmion-based magnetic tunnel junction (MTJ) devices because of their reliable generation, stability and read-out through purely electrical methods. To optimize the properties, such as stability, a strong Dzyaloshinskii-Moriya interaction (DMI) is required at room temperature. Here, using first-principles calculations, we demonstrate that giant DMI can be obtained in Ir/CoFe structures with an Fe-terminated configuration. Moreover, Brillouin light scattering measurements show that indeed Ta/Ir/Co20Fe60B20/MgO thin films with perpendicular magnetic anisotropy exhibit a large DMI value (1.13 mJ/m2), which can be attributed to the smooth and Fe-rich interface between Ir and CoFeB layers. Furthermore, we observe stable sub-100 nm magnetic skyrmions at room temperature in Ir/CoFeB/MgO systems by magnetic force microscope. This work paves the way for promoting the application of skyrmions in CoFeB/MgO-based perpendicular anisotropy MTJ structures.

Spin wave modes of skyrmioniums in the presence of Dzyaloshinskii-Moriya interaction

A skyrmionium or 2π skyrmion is an exotic magnetic configuration that can be stabilized in ferromagnetic nanostructures, e.g., nanodisks. Differently from the standard (or π) skyrmion, it can move without suffering deflections due to the skyrmion Hall effect (SkHE), a property that is desirable for potential applications in spintronics. In this work, using micromagnetic simulations, it is investigated the influence of the Dzyaloshinskii-Moriya interaction on the azimuthal and radial spin wave spectra of the skyrmionium. First, the ground state of the ferromagnetic disk is obtained for several values of Dzyaloshinskii-Moriya exchange constant Dint and a fixed value of perpendicular anisotropy constant Kz. The spin wave modes of the skyrmionium were obtained using in-plane and out-of-plane magnetic pulses. The results show that the values of the frequencies of the spin wave modes depend on the values of Dint. Additionally, it is shown that it is possible to create a skyrmionium using in-plane rotating magnetic fields, by tuning parameters such as the intensity of the applied perpendicular magnetic fields, and their duration.

Spin torque nano-oscillators based on isolated edge skyrmions in nano-pillars with perpendicular anisotropy

The magnetic skyrmions have received a lot of attention because they have potential applications in spin torque nano-oscillators. In this paper, we numerically and analytically study the dynamics of edge skyrmions in circular nano-pillar oscillators with perpendicular magnetic anisotropy (PMA) free layer and spin polarizer. Numerical simulations demonstrate that a steady edge skyrmion oscillation mode with GHz frequency can be driven by spin currents and magnetic fields. The oscillation frequencies in circular nano-pillars with different radius are analytically derived, which agrees well with the numerical results. In addition, the steady oscillations of edge skyrmions are also observed in elliptical nano-pillars, in which the oscillation frequencies strongly depend on the eccentricity and perimeter of the elliptical nanopillar. It is found that the oscillation frequency of this edge skyrmion can range from 1 GHz to 6.7 GHz. The line width is about 55 MHz.

Magnetization Switching by Spin–Orbit Torque in L10-FePt and Ta/FePt Films With Large Perpendicular Magnetic Anisotropy

The current induced perpendicular magnetization switching in L10-FePt single layer and Ta/FePt bilayer thin films was investigated. Here, L10-FePt (6 nm) single-layer film and Ta (1-5 nm)/FePt (6 nm) bilayer films were deposited on MgO substrate at 700°C, showing good perpendicular anisotropy with the out-of-plane coercivity of 7.5 kOe for FePt and around 5 kOe for Ta/FePt bilayers. It is found that current-induced partial magnetization-based switching was realized with the SOT switching ratio of about 3% in all films. For adding the Ta capping layer, the critical current density is reduced and the stability of the switching loop is relatively better as compared with the single FePt layer. Furthermore, harmonic Hall voltage results demonstrate that the SOT efficiency (βDL) of FePt single-layer is approximately 50 Oe/ (107A cm-2), significantly higher than that of Ta/FePt bilayers, which is several times higher than that of the traditional Ta/CoFeB/MgO structure reported before. More importantly, Ta/FePt bilayer structure exhibits very good memristive switching behavior, which is expected to be applied to the spin-synapse of neural networks in the future.

L-shaped electrode design for high-density spin–orbit torque magnetic random access memory with perpendicular shape anisotropy

Magnetic tunneling junctions with strong perpendicular shape anisotropy attract attention due to their high-density magnetic random access memory. As thermal stability increases, the power consumption also increases. To solve this problem, devices are made to be driven by spin–orbit torque (SOT) instead of spin-transfer torque. However, the assisting field needed for deterministic switching is a major obstacle for SOT devices. In this work, we demonstrate an L-shaped electrode structure attached to the magnetic recording layer to induce a composite SOT, achieving high-speed and field-free magnetization switching. Meanwhile, a comparative study between L-shaped and sidewall electrode structure demonstrates that the L-shaped structure leads to fast and low-power switching. Finally, the switching characteristic at various current densities and spin Hall angles is studied and it turns out that to achieve high-speed reversal, the current density and the spin Hall angle need to be optimized, which might be attributed to strong in-plane effective field component disturbance. The novel L-shaped structure is feasible for high-speed, low-power and deterministic switching and has great potential in spintronic applications.

Magnetic skyrmions generation and control in FePt nanoparticles through shape and magnetocrystalline anisotropy variation: a finite elements method micromagnetic simulation study

Magnetic skyrmions created during magnetization reversal in cylindrical, reuleaux and polygon-based magnetic nanoparticles with perpendicular magnetocrystalline anisotropy (MCA) similar to that of partially chemically ordered FePt were studied using finite elements method micromagnetic simulations. Néel chiral stripes, horseshoe, labyrinth skyrmionic textures along with multiple skyrmions were unveiled in different systems generated by the variation of the MCA magnitude and the nanoparticles geometrical shape. These skyrmionic textures under certain conditions can be stable in a range of external magnetic fields and for different MCA values. Simulations revealed the inherent relation of skyrmionic states with nanoparticle geometry and the energy differences between successive external field values observed during the magnetization reversal process. Energetical transitions from non-skyrmionic to skyrmionic and from skyrmionic to different skyrmionic states were quantified and associated with the individual anisotropy, exchange and demagnetization energy contributions for the nanoparticles studied. Finally, the diameters of Néel type skyrmions created through the nanoparticle shape variation were reported for different MCA and external magnetic field values.

Influence of interface anisotropy on ferromagnetic resonance in CoP/(NiP)am/CoP films

The paper presents the results of ferromagnetic resonance studies of three-layer magnetic films, where the magnetic layers are amorphous magnetically soft CoP layers, and the intermediate non-magnetic spacer is amorphous NiP layer. We found that, regardless of the nonmagnetic layer thickness, the interlayer interaction is ferromagnetic. Perpendicular anisotropy forms at the interface between the CoP and NiP layers. It is reasonable to suppose that the interlayer interaction is long-range, what is not typical for systems with metallic non-magnetic spacers.

High-frequency GMI hysteresis effect analysis by first-order reversal curve (FORC) method

In this work, we investigate the giant magnetoimpedance (GMI) hysteresis at high-frequency (10 MHz – 1 GHz) of Co-based amorphous magnetic ribbons. The first-order reversal curve (FORC) method was used to analyze their hysteretic behavior present at a low magnetic field (below 7 Oe). Results show that the GMI hysteresis is related to both the system magnetic anisotropy and the current frequency (through the skin depth). The GMI hysteresis gradually decreases upon frequency increase before disappearing, while a clockwise rotation of the FORC distributions is observed. These results are attributed to a heterogeneous magnetic structure in the magnetic ribbon volume. On the other side, while the cut-off frequency depends on the magnetic anisotropy strength, an inverted GMI hysteresis revives for the higher anisotropy ribbon. Above this frequency inversion, FORC results indicate a hardening of the effective switching field as a thicker region is probed. We propose that in this case, the stronger perpendicular anisotropy creates important differences between the superficial and volumetric anisotropy, enough to induce interaction between the two regions of different magnetization directions. This sensitive FORC protocol can be applied for investigating the GMI hysteresis, and thus the complex magnetic structure of several soft magnetic systems.

Design parameters for field-free spin–orbit torque switching of perpendicular synthetic antiferromagnets

We report micromagnetic simulations of spin–orbit torque (SOT) induced magnetization switching of a ferromagnetic layer with perpendicular anisotropy in the absence of an external magnetic field. Field-free switching is achieved by antiferromagnetic interlayer exchange coupling (IEC) between two perpendicular ferromagnetic layers. At appropriate IEC values and an SOT current density exceeding the critical value (Jc), magnetization reversal can be achieved within sub-ns. The complete magnetization reversal of the synthetic antiferromagnetic free layer occurs upon removing the current pulse. Higher damping is preferred for the proposed switching scheme, as Jc decreases with the increase of damping. Remarkably, we also found that Jc has a parabolic dependence on the nanomagnet's diameter, with the vertex occurring at 90 nm.

Skyrmion battery effect via inhomogeneous magnetic anisotropy

Magnetic skyrmions are considered a promising candidate for the next-generation information processing technology. Being topologically robust, magnetic skyrmions are swirling spin textures that can be used in a broad range of applications from memory devices and logic circuits to neuromorphic computing. In a magnetic medium lacking inversion symmetry, magnetic skyrmion arises as a result of the interplay among magnetic exchange interaction, Dzyaloshinskii-Moriya interaction, and magnetic anisotropy. Instrumental to the integrated skyrmion-based applications are the creation and manipulation of magnetic skyrmions at a designated location, absent any need of a magnetic field. In this paper, we propose a generic design strategy to achieve that goal and a model system to demonstrate its feasibility. By implementing a disk-shaped thin film heterostructure with an inhomogeneous perpendicular magnetic anisotropy, stable sub-100-nm size skyrmions can be generated without magnetic field. This structure can be etched out via, for example, focused ion beam microscope. Using micromagnetic simulation, we show that such heterostructure not only stabilizes the edge spins of the skyrmion but also protects its rotation symmetry. Furthermore, we may switch the spin texture between skyrmionic and vortex-like ones by tuning the slope of perpendicular anisotropy using a bias voltage. When embedded into a magnetic conductor and under a spin polarized current, such heterostructure emits skyrmions continuously and may function as a skyrmion source. This unique phenomenon is dubbed a skyrmion battery effect. Our proposal may open a novel venue for the realization of all-electric skyrmion-based devices.

Perpendicular Magnetic Anisotropy Electric Field Modulation in Magnetron-Sputtered Pt/Co/X/MgO Ultrathin Structures With Chemically Tailored Top Interface

Ab initio calculations of voltage-controlled perpendicular magnetic anisotropy predict that the adjunction of Pt at the top of Co/MgO interface in Pt/Co/MgO ultrathin structures would enhance the perpendicular anisotropy and its electric field ( E-field) variation rate. Following these theoretical expectations, using the magnetron-sputtering technique, we first designed and elaborated magnetic multilayered samples which have subsequently been patterned by UV lithography in magnetotransport devices. The as-deposited samples show perpendicular magnetization configuration whose E-field modulation has been studied by anomalous Hall effect magnetometry experiments under applied E-field in patterned devices. The measured voltage anisotropy modulation effect is found to be asymmetric with respect to the E-field polarity, in qualitative agreement with theoretical predictions.

Analysis of influences of pressure anisotropies on the 3D MHD equilibrium in LHD

3D equilibria with an anisotropic pressure component in the large helical device are analyzed with respect to their magnetic axis locations. The anisotropic extension of the 3D equilibrium solver variational moments equilibrium code, anisotropic neumann inverse moments equilibrium code, is used to compute fixed-boundary plasma equilibria based on a bi-Maxwellian distribution function describing the anisotropic particles. Different heating scenarios are assessed by means of parallel and perpendicular pressure anisotropies with different radial anisotropic pressure profiles imposed. A theoretical predicted scaling of the magnetic axis location with the auxiliary parameter βeq as predicted for classical stellarators and heliotrons by Hitchon [Nucl. Fusion 23, 383 (1983)] is found to be applicable to the large helical device in the case of a flat hot-particle profile for parallel or weak perpendicular dominated anisotropies with β⊥/β∥≤2. For strong perpendicular or non-flat hot-particle profiles, a deviation from the predicted scaling of the magnetic axis location is found. Whereas center-peaked profiles show a stronger shift of the magnetic axis, edge-peaked profiles show no significant change of its radial location. High critical magnetic fields are identified as a necessary condition for strong perpendicular anisotropies. The observed deviations are ascribed to the magnetic field structure and negative pressure gradients. The invalidity of the theoretical predictions in the case of certain configurations is found to be caused by higher-order terms in the pressure components, which are not accounted for by the ordering on which the theory is based.

Nanosecond Random Telegraph Noise in In-Plane Magnetic Tunnel Junctions.

We study the timescale of random telegraph noise (RTN) of nanomagnets in stochastic magnetic tunnel junctions (MTJs). From analytical and numerical calculations based on the Landau-Lifshitz-Gilbert and the Fokker-Planck equations, we reveal mechanisms governing the relaxation time of perpendicular easy-axis MTJs (p-MTJs) and in-plane easy-axis MTJs (i-MTJs), showing that i-MTJs can be made to have faster RTN. Superparamagnetic i-MTJs with small in-plane anisotropy and sizable perpendicular effective anisotropy show relaxation times down to 8ns at negligible bias current, which is more than 5 orders of magnitude shorter than that of typical stochastic p-MTJs and about 100 times faster than the shortest time of i-MTJs reported so far. The findings give a new insight and foundation in developing stochastic MTJs for high-performance probabilistic computers.

Theory of relaxation time of stochastic nanomagnets

Reducing the relaxation time between binary spin states is crucial to enhance the speed and accuracy of a spintronics-based approach for a probabilistic computer, as envisioned by R.P. Feynman. Contrary to previous perception that the reduction of the energy barrier between the states is the most efficient way, the authors discover here that the relaxation time in in-plane easy-axis nanomagnets is widely controllable by changing the effective perpendicular anisotropy field, even though it has nothing to do with the energy barrier.

Irradiation Effects on Perpendicular Anisotropy Spin–Orbit Torque Magnetic Tunnel Junctions

We study the impact of irradiation on magnetic tunnel junction (MTJ) films with perpendicular magnetic anisotropy (PMA) and spin-orbit torque (SOT) switching using magneto-optical Kerr effect and transmission electron microscopy. Our results show that the thin-film stack is robust to gamma ionizing dose up to 1 Mrad(Si) and Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ion irradiation fluences up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , showing SOT PMA MTJs are radiation-hard. But, at very high Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ion irradiation between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , reduced coercivity and eventually greatly reduced PMA are observed, corresponding with an increase in intermixing of the CoFeB-MgO layers, particularly at the lower CoFeB-MgO interface. These results agree with displacement damage modeling that predicts higher damage in the layers closer to the bottom heavy metal and substrate. Compared to spin transfer torque and in-plane anisotropy MTJs, needing a top-pinned stack, a thicker heavy metal layer, and perpendicular anisotropy that is pinned out of plane by interfaces, all could make SOT PMA MTJs more susceptible to damage at high doses.