Perpendicular Magnetic Anisotropy Research Articles

Large Field-like Spin-Orbit Torque and Enhanced Magnetization Switching Efficiency Utilizing Amorphous Mo.

Spin-orbit torque magnetic random access memory (SOT-MRAM) has great promise in high write speed and low power consumption. Mo can play a vital role in constructing a CoFeB/MgO-based MRAM cell because of its ability to enhance the perpendicular magnetic anisotropy (PMA), thermal tolerance, and tunneling magnetoresistance. However, Mo is often considered as a less favorable candidate among SOT materials because of its weak spin-orbit coupling. In this study, we experimentally investigate the SOT efficiencies in Mo/CoFeB/MgO heterostructures over a wide range of Mo thicknesses and temperature. Decent damping-like SOT efficiency |ξDL| = 0.015 ± 0.001 and field-like SOT efficiency |ξFL| = 0.050 ± 0.001 are found in amorphous Mo. The ξFL/ξDL ratio is greater than 3. Furthermore, efficient current-induced magnetization switching is demonstrated with the critical current density comparable with heavy metal Ir and W. Our work reveals new understanding and possibilities for Mo as both an SOT source component and PMA buffer layer in the implementation of SOT-MRAMs.

Giant magneto-optical effect in van der Waals room temperature ferromagnet Fe3GaTe2

Abstract The discovery of ferromagnetic two-dimensional (2D) van der Waals (vdWs) materials provides an opportunity to explore intriguing physics and develop innovative spin electronic devices. However, the main challenge for practical applications of vdWs ferromagnetic crystals lies in the weak intrinsic ferromagnetism and small perpendicular magnetic anisotropy (PMA) above room temperature. Here, we report the intrinsic vdWs ferromagnetic crystal Fe3GaTe2, synthesized by the self-flux method, exhibiting a Curie temperature (TC) of 370 K, a high saturation magnetization of 33.47 emu/g, and a large PMA energy density of approximately 4.17 × 105 J/m3. Furthermore, for the first time, the magneto-optical effect has been systematically investigated in Fe3GaTe2. The doubly degenerate E2g(Γ) mode reverses the helicity of incident photons, indicating the existence of pseudoangular momentum (PAM) and chirality. Meanwhile, the non-degenerate non-chiral A1g(Γ) phonon exhibits a significant magneto-Raman effect under an external out-of-plane magnetic field. These results lay the groundwork for studying phonon chirality and magneto-optical phenomena in 2D magnetic materials, providing the feasibility for further fundamental research and application in spintronic devices.

Single current control of magnetization in vertical high-aspect-ratio nanopillars on in-plane magnetization layers

Ferromagnetic pillars standing on a substrate hold promise for use in recording segments of multibit nonvolatile memories. These pillars exhibit high thermal stability in their magnetization owing to the influence of shape and perpendicular magnetic anisotropies. Recent micromagnetic simulations have demonstrated the feasibility of magnetization control in these pillars. Such control was achieved through the spin-transfer torque induced by the current flowing within the pillar and the spin-orbit torque generated by the current flowing through the heavy-metal lead at the bottom of the pillars. However, the presence of two current paths complicates circuit design, posing challenges in device integration. To solve this problem, we propose a new structure wherein a pillar is placed on a thin film with in-plane magnetization. When current flows through this structure, a torque is applied to the magnetization of the pillar, similar to that of the three-terminal structure. Magnetization reversal and control in the proposed structure were demonstrated using micromagnetic simulations. Specifically, magnetization reversal was achieved in a 100 nm-long permalloy pillar, whereas the magnetization corresponding to a three-bit sequence was generated in a 250 nm-long permalloy pillar. We propose two methods to control the magnetization of multibit memory. One method uses two different current intensities, whereas the other uses constant and pulsed currents of identical intensity. Notably, in the proposed structure, magnetization was controlled using only a unidirectional current. In particular, magnetization can be controlled with a pulsed current using a single current strength. This advancement will simplify the circuitry required to control magnetic memory, bringing the realization of magnetic memory devices closer to reality.

The role of non-uniform magnetization texture for magnon–magnon coupling in an antidot lattice

We numerically study the spin-wave dynamics in an antidot lattice based on a Co/Pd multilayer structure with reduced perpendicular magnetic anisotropy at the edges of the antidots. This structure forms a magnonic crystal with a periodic antidot pattern and a periodic magnetization configuration consisting of out-of-plane magnetized bulk and in-plane magnetized rims. Our results show a different behavior of spin waves in the bulk and in the rims under varying out-of-plane external magnetic field strength, revealing complex spin-wave spectra and hybridizations between the modes of these two subsystems. A particularly strong magnon–magnon coupling, due to exchange interactions, is found between the fundamental bulk spin-wave mode and the second-order radial rim modes. However, the dynamical coupling between the spin-wave modes at low frequencies, involving the first-order radial rim modes, is masked by the changes in the static magnetization at the bulk–rim interface with magnetic field changes. The study expands the horizons of magnonic-crystal research by combining periodic structural patterning and non-collinear magnetization texture to achieve strong magnon–magnon coupling, highlighting the significant role of exchange interactions in the hybridization.

Giant Spin-Orbit Torque in Antiferromagnetic-Coupled Pt/[Co/Gd]N Multilayers with Suppressed Spin Dephasing and Robust Thermal Stability.

Manipulating magnetization via power-efficient spin-orbit torque (SOT) has garnered significant attention in the field of spin-based memory and logic devices. However, the damping-like SOT efficiency (ξDL) in heavy metal (HM)/ferromagnetic metal (FM) bilayers is relatively small due to the strong spin dephasing accompanied by additional spin polarization decay. Furthermore, the perpendicular magnetic anisotropy (PMA) originating from the HM/FM interface is constrained by the thickness of FM, which is unfavorable for thermal stability in practical applications. Consequently, it is valuable to develop systems that not only exhibit large ξDL but also balance thermal stability. In this work, we designed antiferromagnetic-coupled [Co/Gd]N multilayers, where staggered Co and Gd magnetic moments effectively suppress the spin dephasing and additional spin polarization decay. The ordered Co-Gd arrangements along the out-of-plane direction provide bulk PMA, endowing Pt/[Co/Gd]N high thermal stability. The SOT of Pt/[Co/Gd]N was systematically studied with N, demonstrating a significantly large ξDL of up to 0.66. The ξDL of Pt/[Co/Gd]N is greater than those of Pt/Co and Pt/ferrimagnetic alloys. This significant enhancement relies on the effective suppression of spin dephasing in [Co/Gd]N. Our work highlights that the antiferromagnetic-coupled [Co/Gd]N multilayer is a promising candidate for low-consumption and high-density spintronic devices.

Comparison of Current Induced Domain Wall Motion Driven by Spin Transfer Torque and by Spin Orbit Torque in Ferrimagnetic GdFeCo Wires

Current-induced domain wall motion (CIDWM) in magnetic wires can be driven by spin transfer torque (STT) originating from transferring angular momentums of spin-polarized conducting electrons to the magnetic DW and can be driven by spin orbit torque (SOT) originating from the spin Hall effect (SHE) in a heavy metal layer and Dzyaloshinsky Moriya (DMI) generated at an interface between a heavy metal layer and a magnetic layer. In this work, we carried out a comparative study of CIDWM driven by STT and by SOT in ferrimagnetic GdFeCo wires with magnetic perpendicular anisotropy based on structures of SiN (10 nm)/GdFeCo (8 nm)/SiN (10 nm) and Pt (5 nm)/GdFeCo (8 nm)/SiN (10 nm). We found that CIDWM driven by SOT exhibited a much lower critical current density (JC), and much higher DW mobility (µDW). Our work might be useful for the realization and the development of low-power and high-speed memory devices.

Transport of skyrmions by surface acoustic waves

Magnetic skyrmions in thin films with perpendicular magnetic anisotropy are promising candidates for magnetic memory and logic devices, making the development of ways to transport skyrmions efficiently in a desired trajectory of significant interest. Here, we investigate the transport of skyrmions by surface acoustic waves (SAWs) via several modalities using micromagnetic simulations. We show skyrmion pinning sites created by standing SAWs at anti-nodes and skyrmion Hall-like motion without pinning driven by traveling SAWs. We also show how orthogonal SAWs formed by combining a longitudinal traveling SAW and a transverse standing SAW can be used for the 2D positioning of skyrmions. Our results also suggest SAWs offer a viable approach to the transport of multiple skyrmions along a multichannel racetrack.

Efficient magnetization switching induced by spin–orbit torque in perpendicularly magnetized Mn1+xCo2−xAl Heusler alloys

Perpendicularly magnetized Co-based Heusler alloys are promising candidates in high-performance spintronic devices. However, there is a contradiction between thermal stability and damping-like spin–orbit torque (SOT) efficiency in Co-based Heusler alloys with interface-induced perpendicular magnetic anisotropy (PMA). Here, we present epitaxially grown perpendicularly magnetized Mn1+xCo2−xAl (MCA) Heusler alloys through tetragonal distortion. Ferrimagnetism and distortion-induced PMA are obtained in MCA Heusler alloys. A large effective spin Hall angle (θeff) up to 0.33 is experimentally demonstrated in Pt/MCA bilayers, markedly surpassing that in conventional Pt/FM bilayers. Consequently, SOT-induced efficient magnetization switching is realized in Pt/MCA bilayers, with a critical switching current density (Jc) as low as 4 × 107 A/cm2. The large θeff and high SOT efficiency would be attributed to the staggered magnetic exchange torques in the ferrimagnetic MCA Heusler alloys. These findings demonstrate that perpendicularly magnetized ferrimagnetic MCA Heusler alloys are highly promising for high-density SOT-magnetic random-access memory with superior thermal stability and low power consumption.

Magnetic compensation in sputtered ferrimagnetic Mn4−xGaxN thin films

Abstract Fully compensated ferrimagnets are attracting increasing attention for spintronics. Here we report a series of Mn4−xGaxN ferrimagnetic thin films grown on (100)-oriented MgO by magnetron sputtering. A small tetragonal distortion with c/a = 0.99 leads to perpendicular magnetic anisotropy with a [001] easy axis. The magnetisation decreases with increasing Ga content up to x = 0.18, where the magnetic moment is close to zero at room temperature. It increases with further increase of x, indicating the magnetic compensation composition has been crossed and the dominant sublattice has switched from Mn on the corner sites, where it is progressively replaced by nonmagnetic Ga, to noncollinear Mn on the face-centered sites. Compensation found at 235K in a remanent scan of the x = 0.18 film is confirmed by measurements of the anomalous Hall effect and interpolation of the inverse coercivity. This work establishes the conditions for thin film growth of Mn4−xGaxN that could be used for room-temperature ferrimagnetic spintronics. The behaviour of thin films is compared with that of bulk material.

Multilayer Metamaterials with Ferromagnetic Domains Separated by Antiferromagnetic Domain Walls

AbstractMagnetic nano‐objects possess great potential for more efficient data processing, storage, and neuromorphic‐type applications. Using high perpendicular magnetic anisotropy synthetic antiferromagnets in the form of multilayer‐based metamaterials, the antiferromagnetic interlayer exchange energy is purposefully reduced below the out‐of‐plane demagnetization energy, which controls magnetic domain formation. In this unusual magnetic energy regime, as demonstrated via macroscopic magnetometry and microscopic Lorentz transmission electron microscopy, it becomes possible to stabilize nanometer‐scale stripe and bubble textures consisting of ferromagnetic out‐of‐plane domain cores separated by antiferromagnetic in‐plane Bloch‐type domain walls. This unique coexistence of mixed ferromagnetic/antiferromagnetic order on the nanometer scale opens so far unexplored perspectives in the architecture of magnetic domain landscapes as well as the design and functionality of individual magnetic textures, such as bubble domains with depth‐wise alternating chirality.

Boosting the Curie temperature of GaN monolayer through van der Waals heterostructures

The pursuit of van der Waals (vdW) heterostructures with high Curie temperature and strong perpendicular magnetic anisotropy (PMA) is vital to the advancement of next generation spintronic devices. First-principles calculations are used to study the electronic structures and magnetic characteristics of GaN/VS2 vdW heterostructure under biaxial strain and electrostatic doping. Our findings show that a ferromagnetic ground state with a remarkable Curie temperature (477 K), much above room temperature, exists in GaN/VS2 vdW heterostructure and 100% spin polarization efficiency. Additionally, GaN/VS2 vdW heterostructure still maintains PMA under biaxial strain, which is indispensable for high-density information storage. We further explore the electron, magnetic, and transport properties of VS2/GaN/VS2 vdW sandwich heterostructure, where the magnetoresistivity can reach as high as 40%. Our research indicates that the heterostructure constructed by combining the ferromagnet VS2 and the non-magnetic semiconductor GaN is a promising material for vdW spin valve devices at room temperature.

Half-metal Mn2GeI2 monolayer with high Curie temperature and large perpendicular magnetic anisotropy

Two-dimensional (2D) intrinsic ferromagnetic (FM) materials with high Curie temperatures (Tc), large perpendicular magnetic anisotropy (PMA), and large spin polarization are desirable for atomically thin spintronic devices. Herein, we propose a 2D intrinsic FM material Mn2GeI2 monolayer with thermodynamical stability and outstanding FM properties using first-principles calculations. Our calculations show that Mn2GeI2 monolayer is a half-metal with a spin gap of 1.76 eV, which ensures 100% spin polarization ratio at the Fermi level. Importantly, Mn2GeI2 monolayer has a large PMA energy of 2.90 meV and a high Tc of 648 K, ideal for practical applications at room temperature. Further in-depth investigation of microscopic coupling in the Mn2GeI2 monolayer reveals that the robust ferromagnetism mainly resulted from the synthetic effect of Ruderman–Kittel–Kasuya–Yosida exchange interaction between the Mn ion layers and superexchange interaction within the Mn ion layers. Our results give insights into the structure and electronic and magnetic properties of Mn2GeI2 monolayer and provide a promising candidate for 2D spintronic devices.

Magnetization switching driven by spin current in a T-type ferromagnetic trilayer

The T-type CoFeB/spacer/CoFeB structure is a promising candidate for the development of perpendicular spin–orbit torque (SOT) magnetic random-access memory and other SOT devices. It consists of an in-plane magnetized layer, a perpendicularly magnetized layer, and a non-magnetic metal spacer that induces interlayer exchange coupling. By engineering the W spacer, this system achieves field-free SOT switching with a nearly 100% switching ratio. Furthermore, it realizes a high exchange coupling field of 255 Oe using a relatively thinner spacer thickness, enhancing the reliability and energy efficiency of SOT switching. Measurement of current switching probability suggests that this perpendicular magnetic anisotropy system may enable the implementation of probability-adjustable true random number generators in future applications. The T-type structures with strong interlayer coupling exhibit great potential for spintronic device applications.

Layer-dependence study of two-dimensional ferromagnets: Fe3GeTe2 and Fe5Ge2Te2

We have investigated the electrical transport properties of nanodevices fabricated from exfoliated flakes of two-dimensional metallic ferromagnets Fe3GeTe2 (FGT) and Fe5Ge2Te2 (FG2T) down to below three layers in thickness. The per-layer anomalous Hall conductivity even in thick FGT and FG2T devices is found to be much smaller than ∼e2h, the approximate value calculated for thick undoped crystals. Moreover, we obtain a power-law scaling relation between the per-layer anomalous Hall and per-layer longitudinal conductivities with an exponent close to 1.6, which agrees with the universal value for poor ferromagnetic conductors. Both FGT and FG2T devices show clear layer-dependent Curie temperatures and layer-dependent perpendicular magnetic anisotropy, with FG2T dominating the former and FGT dominating the latter for all thicknesses. Despite their declining trend as the device thickness decreases, both Curie temperature and magnetic anisotropy retain a significant fraction of their bulk values (>60% and >80% of the bulk values, respectively, even in the thinnest FG2T device), indicating attractive potential for practical applications.

Study on Anomalous Hall Effect and Spin-Orbit Torque Effect of TbCo-Based Multilayer Films.

The anomalous Hall effect and spin-orbit torque of TbCo-based multilayer films have been methodically studied in recent years. Many properties of the films can be obtained by the anomalous Hall resistance loops of the samples. We report on the effects of a structure composed of two heavy metals as the buffer layers on the anomalous Hall resistance loops of TbCo-based multilayers at different temperatures. The results showed that the coercivity increases dramatically with decreasing temperature, and the samples without perpendicular magnetic anisotropy at room temperature showed perpendicular magnetic anisotropy at low temperatures. We quantified the spin-orbit torque efficiency and Dzyaloshinskii-Moriya interaction effective field size of the films W/Pt/TbCo/Pt at room temperature by measuring the loop shift of anomalous Hall resistance. The results showed that the study of anomalous Hall resistance loops plays an important role in the study of spintronics, which can not only show the basic properties of the sample, but can also obtain other information about the sample through the shift of the loops.

Perpendicular magnetic properties and field-free current-induced switching in W/CoFeB/MgO trilayers with a compensating oblique W underlayer

Zero-field magnetization switching (ZFS) driven by current-induced spin–orbit torque (SOT) holds significant importance in spintronic applications. The introduction of a lateral asymmetric structure (LAS) through oblique deposition proves to be an effective strategy for breaking inversion symmetry, thereby enabling SOT-driven ZFS. However, the coexistence of wedge thickness structure and slanted columnar microstructure in the obliquely deposited films poses challenges in distinguishing their respective effects. In this study, we conducted a comparative investigation of the perpendicular magnetic properties and current-induced switching in W/Co40Fe40B20/MgO films by oblique sputtering of the W underlayer at a fixed tilting angle and at two opposite tilting angles with its wedge thickness compensated. We have found that the perpendicular magnetic properties of the Co40Fe40B20 layer are significantly altered at large tilting angles, irrespective of whether the W wedge thickness is compensated. Notably, at a tilting angle of 50°, ZFS is realized for both the conventional oblique sample and the compensating oblique sample, with the switching polarity contingent on the final tilting direction of the W layer. We have identified a gradient in perpendicular magnetic anisotropy in these samples, attributed to the laterally varying roughness associated with the slanted columnar microstructure of the W underlayer. This study underscores the dominant role of microscopic LAS in obliquely deposited films in breaking SOT symmetry. Our research sheds light on the impact of the slanted columnar microstructure on the magnetic and magneto-transport properties of films, offering valuable insights for advancing spintronic device research.

Perpendicular magnetic anisotropy of Pd/Co2MnSi/AlN/Pd multilayer films

In this paper, the perpendicular magnetic anisotropy（PMA）of Pd/Co2MnSi (CMS)/AlN/Pd multilayer film is studied, which is designed based on the two-interface interaction, the orbital hybridization effect of Pd5d-Co3d electrons at the Pd/CMS interface and the orbital hybridization effect of Co3d-N2p electrons at the CMS/AlN interface. Compared with the Pd/CMS/Pd multilayer film, the PMA of the Pd/CMS/AlN/Pd structure shows significant enhancement with the insertion of the AlN layer. The Hall resistance (RHall) of the Pd/CMS/AlN/Pd structure reached 0.02 Ω (increased by 300 %) and the coercive force (Hc) is 388 Oe (increased by about 76 %). The magnetic anisotropic energy density (Keff) is 0.61Merg/cm3 (increased by about 135 %). The PMA was optimized with the film factor (layer thickness) of Pd(6 nm)/CMS(5 nm)/AlN(6 nm)/Pd(6 nm). The X-Ray Diffraction（XRD）and High Resolution Transmission Electron Microscope（HRTEM）analysis showed that the PMA is affected by the crystallinity and interface quality of multilayer films.

Segregation effects of doped VN and ZrN on magnetic and microstructural characteristics of FePt-based films

Segregants are essential materials for forming the complexation or granular microstructure in thin films for magnetic recording media. One such film, FePt-BN, has a metallic L10 FePt main phase as its matrix, in addition to having BN and C segregants at the grain boundaries. Amorphous BN has sufficient mechanical strength to support the columnar growth of FePt grains, however, it has a lower deposition rate resulting in columnar grains with a nonuniform aspect ratio. Moreover, some B diffuses in the interstitial site of FePt lattices. To overcome these limitations, this study added the transition nitride VN and ZrN with equal volume percentages to replace some BN segregants in FePt(BN, Ag, C) films: substantially different segregation behaviors were observed. Microstructural imaging mapping revealed that although VN segregated the FePt grains boundary, ZrN was diffused in the lattice. The FePt-BN-ZrN film thus had more disordered grains contributing to a higher in-plane magnetization and wider hard axis magnetic hysteresis loops than did FePt-BN-VN. The FePt-BN-VN film has the highest perpendicular magnetic anisotropy constant (Ku= 2.17 × 106 J/m3) and coercivity (Hc= 4 T); moreover, parallel remanence magnetization was minimal with a linear in-plane magnetization loop. The FePt-BN-VN film was sputtered directly on MgO(100) but depositing the FePt-BN-ZrN film required a 2-nm-thick FePt nucleation layer. The high atomic number of Zr and high sputter yield ratio (YN/Ymetal) of ZrN may increase the sites occupied by N, potentially affecting ZrN segregation.

Current-induced magnetization switching behaviors in perpendicular magnetized L10-MnAl/B2-CoGa bilayer

Abstract Rare-earth-free Mn-based binary alloy L10-MnAl with bulk perpendicular magnetic anisotropy (PMA) holds promise for high-performance magnetic random-access memory (MRAM) devices driven by spin-orbit torque (SOT). However, the lattice-mismatch issue makes it challenging to place the conventional spin current sources, such as heavy metals, between the L10-MnAl layer and substrate. In this work, we propose a solution by using the B2-CoGa alloy as the spin current source. The lattice-matching enables high-quality epitaxial growth of 2-nm-thick L10-MnAl on B2-CoGa, and the L10-MnAl exhibits a large PMA constant of 1.04×106 J/m3. Subsequently, the considerable spin Hall effect in B2-CoGa enables the achievement of SOT-induced deterministic magnetization switching. Moreover, we have quantitatively determined the SOT efficiency in the bilayer. Furthermore, we have designed an L10-MnAl/B2-CoGa/Co2MnGa structure to achieve field-free magnetic switching. Our results offer valuable insights for achieving high-performance SOT-MRAM devices based on L10-MnAl alloy

The Success of Fabrication of Pure SmFe2 Phase Film with Outstanding Perpendicular Magnetic Anisotropy.

This study used DC magnetron sputtering technology to fabricate Sm-Fe films and systematically investigated the phase transition behavior of Sm-Fe films with different Fe ratios. It was found that at higher Fe content, the films consisted of Sm2Fe17 or SmFe7 phases; as Fe content decreased, the films were mainly composed of SmFe3 or SmFe2 phases; at higher Sm content, the films primarily consisted of Sm phase. Sm is prone to volatilization at high temperatures, so Ta was used as a capping layer to effectively suppress Sm volatilization, successfully synthesizing pure SmFe2 phase films at a nearly 1:2 ratio. The magnetic properties and magnetostrictive behavior of the SmFe2 films were investigated, revealing that pure-phase SmFe2 films exhibit good perpendicular magnetic anisotropy and magnetostriction properties. The larger stress along the perpendicular-to-film direction, resulting from the absence of substrate-induced constraints, contributes to the excellent perpendicular magnetic anisotropy of the films. This study successfully synthesized pure-phase SmFe2 films and discovered a new method for fabricating perpendicularly anisotropic films. The research findings are of great significance for the efficient synthesis of desired films with high phase formation temperatures containing volatile elements.