Perpendicular Anisotropy Research Articles

A Spin‐Orbit Torque Ratchet at Ferromagnet/Antiferromagnet Interface via Exchange Spring

AbstractThe antiferromagnet (AFM) and ferromagnet (FM) interface is a unique branch of magnetics of broad scientific interest. AFMs play an important role in spin‐orbit torque devices based on their ability to generate spin‐polarized current and exchange bias when combined with FMs. In this study, an interesting spin‐orbit torque (SOT) ratchet involving the exchange spring effect in an IrMn/CoFeB bilayer device with perpendicular anisotropy and exchange bias is developed. The combined use of electrical and spectroscopic analysis reveals that the exchange spring in IrMn/CoFeB bilayer yields unidirectional anisotropy, resulting in a collinear/orthogonal AFM/FM spin configuration at the interface upon switching CoFeB magnetization upward/downward. The ratcheting characteristics resulting from unidirectional anisotropy manifest in SOT switching. In this process, magnetization against the exchange spring features digital‐like switching with a sharp transition, whereas the reverse function is characteristic of analog switching with a gradual transition tail. The dual digital‐analog characteristics of the IrMn/CoFeB bilayer may be of benefit in neuromorphic and memory applications.

Size-dependent enhancement of passive microwave rectification in magnetic tunnel junctions with perpendicular magnetic anisotropy

Spintronic rf detectors are efficient nanoscale counterparts to conventional semiconductor-based components for energy harvesting and wireless communication at low input power. Here, we report on the optimization of the rectified output dc voltage using magnetic tunnel junctions (MTJs) with strong perpendicular anisotropy of both the polarizing and free layers. The magnetization of the polarizing layer is fixed out-of-plane, while the free layer thickness is adjusted so that its magnetization orientation changes from in-plane to out-of-plane. Rectification dc output voltages in the mV range are obtained for moderate rf source powers with a signal-to-noise ratio of 26–39 dB for Prf = −25 dBm and a sensitivity ε of 300 mV/mW. The rectified signal shows a strong dependence on MTJ dimensions: it increases by a factor of 5–6 when reducing the diameter from 180 to 50 nm. Furthermore, this enhancement can be doubled when reducing the free layer thickness from 1.8 to 1.6 nm. This size-related enhancement is attributed to several jointly acting effects: the amplitude of the spin transfer torque that depends inversely on the diameter, the effective anisotropy that depends on the thickness of the excited layer, and the tunneling magneto-resistance ratio that for the devices studied here depends on diameter. The obtained results indicate that the geometry of the MTJ can be used to design spintronic based rf detectors with optimized sensitivity.

Precession Frequency and Switching Time of the Magnetization Vector in the Spin-Valve Active Layer With Perpendicular Anisotropy

Precession Frequency and Switching Time of the Magnetization Vector in the Spin-Valve Active Layer With Perpendicular Anisotropy

Two-dimensional heterotriangulene-based manganese organic frameworks: bipolar magnetic and half semiconductors with perpendicular magnetocrystalline anisotropy

Two-dimensional (2D) organic intrinsic magnetic semiconductors have potential applications in low-dimensional organic spintronic devices due to their remarkable physical properties. However, 2D metal-organic frameworks with magnetic and semiconducting properties are rare. In this work, the electronic and magnetic properties of 2D heterotriangulene-based manganese organic frameworks including triphenylamine (TPA) and triphenylborane (TPB) organic ligands with methylene (M), carbonyl (C) or oxygen (O) coordination groups were studied by first-principles calculations. XTPA-Mn (X = M and O) is a bipolar magnetic semiconductor with a large spin-flip band gap. CTPA-Mn and XTPB-Mn (X = M, C and O) are half semiconductors with perpendicular magnetocrystalline anisotropy. The electronic properties of materials ranging from half semiconductors to bipolar magnetic semiconductors appear in CTPA-Mn and XTPB-Mn (X = M and C) at biaxial strains. XTPA-Mn and XTPB-Mn with a frustrated antiferromagnetic configuration are semiconductors with good ductility and stability. These results enrich the diversity of 2D organic intrinsic magnetic semiconductors, which have potential applications in spintronic devices.

Whistler waves generated by nongyrotropic and gyrotropic electron beams during asymmetric guide field reconnection

Using a two-dimensional particle-in-cell simulation of asymmetric reconnection with a guide field whose strength is 0.3 times the reconnecting magnetic field, we study electron distribution functions and wave intensities in the diffusion region, focusing on the electron diffusion region (EDR). Wave activities with frequencies below the electron cyclotron frequency are observed, and these are whistler waves propagating almost anti-parallel to the magnetic field. The waves are concentrated near the magnetospheric separatrix away from the X line, but the wave activity also spreads through the EDR near the X line. The reconnection outflows are asymmetric in the outflow direction in the magnetospheric side, and the wave intensity is stronger in the side of the faster electron outflow. We study the whistler waves using the fast Fourier transform, analyses of electron velocity distribution functions, and the dispersion solver calculation. Along the magnetospheric separatrix in the stronger outflow side, highly anisotropic electron beams exist with super-Alfvénic drift speeds. The dispersion analysis shows that there are two modes: a temperature anisotropy mode and a beam mode. Outside the EDR, the whistler wave intensity is highest near the separatrix, but the wave intensity decreases if we move away from the separatrix toward the magnetic neutral line because of the increase in the electron population near zero parallel velocity. In the EDR, in the velocity plane perpendicular to the magnetic field, ring/crescent electron distribution functions are observed. Near the X-line, the wave power is enhanced where nongyrotropic electrons contribute to increase the perpendicular temperature anisotropy.

First-principle calculations of the magnetic anisotropy of Fe and Co films separated by an interlayer of non-magnetic metals

This paper presents the results of numerical calculations of the magnetic characteristics of Co and Fe monolayer films on Cu and Pt surfaces using the VASP software package. The values of the difference between the total energies of the antiferromagnetic and ferromagnetic configurations are calculated depending on the convergence parameters and the thickness of the nonmagnetic material. The values of the magnetic anisotropy and magnetic moment of atoms in the structures of Co/Cu/Co, Fe/Pt/Fe, Co/Pt/Co, Pt/Co/Cu/Co/Pt are determined depending on the orientation of the surface face. For the (110) and (111) faces, the phenomenon of reorientation in the Co/Cu/Co structure is confirmed, when the anisotropy of the cobalt films parallel to the surface plane is replaced by the perpendicular anisotropy due to the introduction of an ultrathin platinum film into the structure. Keywords: Magnetic anisotropy, ultrathin magnetic films.

STDP implementation using multi-state spin−orbit torque synapse

The abundance of data to be processed calls for new computing paradigms, which could accommodate, and directly map artificial neural network architectures at the hardware level. Neuromorphic computing has emerged as a potential solution, proposing the implementation of artificial neurons and synapses on physical substrates. Conventionally, neuromorphic platforms are deployed in complementary metal-oxide-semiconductor technology. However, such implementations still cannot compete with the highly energy-efficient performance of the brain. This calls for novel ultra-low-power nano-scale devices with the possibility of upscaling for the implementation of complex networks. In this paper, a multi-state spin−orbit torque (SOT) synapse based on the three-terminal perpendicular anisotropy magnetic tunnel junction (P-MTJ) is proposed. In this implementation, P-MTJs use common heavy metals but with different cross-section areas, thereby creating multiple states that can be harnessed to implement synapses. The proposed multi-state SOT synapse can solve the state-limited issue of spin-based synapses. Moreover, it is shown that the proposed multi-state SOT synapse can be programmed to reproduce the spike-timing-dependent plasticity learning algorithm.

Effect of substrate temperature on plasma-enhanced self-assembling formation of high-density FePt nanodots

We formed FePt magnetic nanodots (NDs) by exposing an ultrathin bilayer metal stack on ∼3.0 nm SiO2/Si(100) substrates to a remote H2 plasma (H2-RP) and studied the effect of external heating during the exposure to H2-RP on the formation and magnetic properties of NDs. The ultrathin bilayer with a uniform surface coverage drastically changed to NDs with an areal density as high as ∼3.5 × 1011 cm−2 by exposing to H2-RP with external heating. We also found that NDs formed by the exposure to H2-RP at 400 °C exhibited a perpendicular anisotropy with a perpendicular coercivity of ∼1.5 kOe, reflecting the magneto-crystalline anisotropy of (001)-oriented L10 phase FePt.

Easy-cone state in spin-torque diode under combined action of magnetostatics and perpendicular anisotropy

Spin-torque diodes (STDs) with interfacial perpendicular magnetic anisotropy (IPMA) in the free layer (FL) demonstrate outstanding microwave signal rectification performances. Large sensitivity values in such systems are usually associated with an easy-cone (EC) magnetic state, when the magnetization in the FL is tilted from the normal to the plane of the film. Here, we theoretically investigate the phase diagram for the EC state in an infinite FL of the magnetic tunnel junction (MTJ) considering both IPMA (of the first and of the second order) and magnetostatic interaction. We show that the increase of the magnetostatic field leads to the EC state phase expansion. For elliptical MTJ nanopillars we investigate the influence of the orientation of the nanopillar ellipticity on the obtained phase diagrams. And finally, we consider the dynamic properties and rectification efficiency of the STD under microwave current injection. Our results clarify the role of magnetostatic interaction for microwave rectification with the IPMA-based STDs and suggest approaches to the EC state effective rectification phase extension through the parameters optimization.

Scaling of Dzyaloshinskii-Moriya interaction with magnetization in Pt/Co(Fe)B/Ir multilayers

Magnetic multilayers with perpendicular anisotropy and an interfacial Dzyaloshinskii-Moriya interaction (DMI) contain chiral domain walls and skyrmions that are promising for applications. Here, we measure the temperature dependence of the DMI in Pt/CoFeB/Ir and Pt/CoB/Ir multilayers by means of static domain imaging. First, the temperature dependences of saturation magnetization $({M}_{\mathrm{S}})$, exchange stiffness $(A)$, and intrinsic perpendicular anisotropy $({K}_{\mathrm{u}})$ are determined. Then the demagnetized domain pattern in each multilayer is imaged by wide-field Kerr microscopy in the temperature range 9--290 K, and the characteristic domain period at each temperature is determined. We calculate the DMI constant $D$ from an analytical expression for the domain wall energy density that treats the multilayer as a uniform medium. Scaling laws for ${K}_{\mathrm{u}}$ and $D$ with the magnetization are established from the experiments. While the scaling of ${K}_{\mathrm{u}}$ is consistent with Callen-Callen theory, we find that the scaling of $D$ is like that of $A$ predicted theoretically $(\ensuremath{\sim}1.8)$.

Effect of Fe/Fe3O4 Nanoparticles Stray Field on the Microwave Magnetoresistance of a CoFeB/Ta/CoFeB Synthetic Ferrimagnet.

The effect of the stray field of Fe/Fe3O4 nanoparticles on the angular dependence of the microwave absorption derivative in CoFeB/Ta/CoFeB synthetic ferrimagnetic structures and CoFeB films with perpendicular anisotropy is analyzed, and its application for sensor technology is proposed. The effective field of the "platform-particles" system controlled by the magnetic dipole interaction of the CoFeB-Fe/Fe3O4 system decreased to zero in areas where the platform was magnetostatically coupled with nanoparticles. Micromagnetic modeling demonstrated the distribution of magnetization and resistance in local areas of CoFeB/Ta/CoFeB structures under the nanoparticles. The microwave absorption derivative can be used as an indicator of local magnetization switching of the giant magnetoresistance (GMR) structure under scattering fields of NPs or magnetically labeled cells. The limiting sensitivity of the detection method was 2.4 × 107 nanoparticles, which covered the spin-valve surface. We have proposed to combine the advantages of a GMR sensor with wireless technology of microwave reading of magnetoresistance for the detection of magnetically labeled cells.

Magnetic properties of 3d, 4d, and 5d transition-metal atomic monolayers in Fe/TM/Fe sandwiches: Systematic first-principles study

Previous studies have accurately determined the effect of transition metal point defects on the properties of bcc iron. The magnetic properties of transition metal monolayers on the iron surfaces have been studied equally intensively. In this work, we investigated the magnetic properties of the 3d, 4d, and 5d transition-metal (TM) atomic monolayers in Fe/TM/Fe sandwiches using the full-potential local-orbital (FPLO) scheme of density functional theory. We prepared models of Fe/TM/Fe structures using the supercell method. We selected the total thickness of our system so that the Fe atomic layers furthest from the TM layer exhibit bulk iron-bcc properties. Along the direction perpendicular to the TM layer, we observe oscillations of spin and charge density. For Pt and W we obtained the largest values of perpendicular magnetocrystalline anisotropy and for Lu and Ir the largest values of in-plane magnetocrystalline anisotropy. All TM layers, except Co and Ni, reduce the total spin magnetic moment in the generated models, which is in good agreement with the Slater-Pauling curve. Density of states calculations showed that for Ag, Pd, Ir, and Au monolayers, a distinct van Hove singularity associated with TM/Fe interface can be observed at the Fermi level.

Theoretical estimation of surface magnetic anisotropy on [formula omitted]-FePt thin films: Case of perfect and defect Surfaces.

We carried out first-principle calculations of magnetocrystalline anisotropy of L10-FePt thin films terminated by either perfect Fe (Pt) layers or defect surfaces, including vacancies or substituted surface atoms. Inspired by the experimental strategy for determining magnetic anisotropy, we present here a theoretical model for analyzing volume and surface magnetic anisotropies for L10-FePt thin films with different surface terminations. We demonstrate that surface anisotropy does not depend solely on the film thickness, but also on the atomic environment of the surface layer. Perpendicular surface anisotropy of L10-FePt thin films reaches its maximum with full Pt termination, which decreases with increasing Pt vacancies and becomes parallel to the surface for a full Fe surface. In the case of mixed surfaces, surface anisotropy of (3Pt,1Fe) is enhanced compared to platinum-rich surface with one vacancy, and tends to be more in-plane for an iron-rich surface (3Fe,1Pt). Our results could be used to study surface anisotropy effects on L10-FePt surfaces by mean of classical magnetic models, and they are very promising for the development of high density magnetic data storage such as magnetic memory.

Highly efficient spin-current generation from Pt/Ru multilayers

Increasing spin Hall angle of heavy metals has attracted considerable attention due to their potential applications in spintronic technology. Here, we demonstrate that the spin Hall angle could be enhanced dramatically in the Ta/[Pt/Ru]n/Pt/Co/Ta multilayers. The spin Hall angle reaches a maximum value of 0.28 when n = 5 from the loop-shift method with a relative low resistivity of approximately 48 μΩ cm. Meanwhile a minimum critical switching current density approximately 3.2 × 106 A/cm2 under an in-plane bias magnetic field around 5% of the perpendicular anisotropy field was obtained for the sample with n = 5 in current-induced magnetization switching experiment. We find that the extrinsic spin Hall mechanism changes from the spin skew scattering when n < 2 to spin side-jump mechanism when n > 2. This work provides an effective way in increasing spin Hall angle for low-power consumption spintronics devices.

Proton cyclotron and mirror instabilities in marginally stable solar wind plasma

ABSTRACT This paper formulates a velocity moment-based quasi-linear theory that combines the impacts of weakly unstable proton–cyclotron- (or, equivalently, electromagnetic ion cyclotron) and proton-mirror instabilities on the solar wind plasma initially characterized by an excessive perpendicular proton temperature anisotropy. The present formalism is an alternative to the existing model in that the weakly unstable modes are characterized by analytical formalism that involves the assumption of weak growth rate and/or fluid-theoretical dispersion relation, in place of numerical root-finding method based on the transcendental plasma dispersion function. This results in an efficient numerical platform for analyzing the quasi-linear development of the said instabilities. Such a formalism may be useful in the larger context of global solar wind modelling effort where an efficient calculation of self-consistent wave–particle interaction process is called for. A direct comparison with spacecraft observations of solar wind proton data distribution shows that the present weak growth rate formalism of quasi-linear calculation produces results that are consistent with the observation.

Electron mirror and cyclotron instabilities for solar wind plasma

ABSTRACT The solar wind plasma is characterized by unequal effective kinetic temperatures defined in perpendicular and parallel directions with respect to the ambient magnetic field. For electrons, the excessive perpendicular temperature anisotropy leads to quasi-parallel electromagnetic electron cyclotron (or whistler) instability and aperiodic electron-mirror instability with oblique wave vectors. The present paper carries out a direct side-by-side comparison of quasi-linear (QL) theory and particle-in-cell (PIC) simulation of combined mirror and cyclotron instabilities acting upon the initially anisotropic electron temperatures, and find that the QL theory satisfactorily encapsulates the non-linear aspect of the combined instability effects. However, a discrepancy between the present study and a previous PIC simulation result is also found, which points to the need for further investigation to resolve such an issue.

Unconventional Seedless Multilayers with Large Perpendicular Anisotropy for Back-End-of-Line Compatible Spintronic Devices

An unconventional class of hard and out-of-plane magnetized seedless multilayers (SL-MLs) of the form [Co/insertion layer/Pt]n, grown as a top reference layer in magnetic tunnel junctions (MTJ), was investigated. Among different non-ferrous insertion layers (Ta, Cu, Mg, W, Ru, and Al), Ta produces the highest perpendicular magnetic anisotropy. A back-end-of-line (BEOL) compatible hard top reference layer using Ta-inserted SL-MLs was achieved, exhibiting more than twice effective perpendicular anisotropy compared to a conventional top reference layer. Using this top reference layer, we developed three types of spintronic memory stacks: (a) top-pinned MTJ stacks that can withstand annealing up to 425 °C, (b) BEOL compatible double-MTJ memory cells with read/write mode control layer enabling a faster readout and low-voltage writing, and (c) 2-bit MTJ stacks combining spin-transfer torque and spin-orbit torque writing. The magnetic properties of these three types of memory stacks are discussed.

Manipulation of crystalline structure, magnetic performance, and topological feature in Mn3Ge films

The Mn3X (where X = Ga, Ge, Sn, etc.) compounds have appealing prospects for spintronic applications due to their various crystal structures and magnetic properties for the design of reliable high-density memories. However, controlled growth of high-quality Mn3X thin films remains challenging in material science. Here, we reported the controlled film growth of Heusler alloy Mn3Ge, which could crystallize in respective tetragonal and hexagonal structures. The tetragonal D022-type Mn3Ge film exhibits strong perpendicular ferromagnetic anisotropy, while the hexagonal D019-type Mn3Ge film indicates non-collinear triangular antiferromagnetic order. From our experimental observations of structure characterizations, magnetic properties, anomalous Hall effect, and magnetoresistance measurements, we realized the manipulation of spin orientations and topological features. Majority/minority spin polarized Fermi surface and density of states of both tetragonal and hexagonal Mn3Ge structures were investigated by density functional theory calculations. Our work not only opens up technology routes toward the development of Mn3X-based devices for applications in topological spintronics and spin-torque memories but also leads to engineer the physical properties for fundamental study.

Large 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 readout 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 huge DMI can be obtained in Ir/CoFe structures with an Fe-terminated configuration. Moreover, Brillouin light-scattering measurements show that indeed Ta/Ir/Co 20 Fe 60 B 20 /MgO thin films with perpendicular magnetic anisotropy exhibit a large DMI value (1.13 mJ/m 2 ) when the thickness of the CoFeB layer is 1.1 nm, 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 the 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. Large DMI in Ir/CoFeB/MgO thin films with perpendicular magnetic anisotropy First-principles calculations combined with Brillouin light-scattering methods Significant role of interfacial configuration on the regulation of DMI Sub-100 nm room-temperature magnetic skyrmions in Ir/CoFeB/MgO multilayers Chen et al. observe a large Dzyaloshinskii-Moriya interaction, combined with the evidence of sub-100-nm room-temperature skyrmions in Ir/CoFeB/MgO structures. This material stack may provide a platform to realize skyrmionics, especially the nucleation and electrical detection of skyrmions in perpendicular anisotropy-based magnetic tunnel junctions.

Dzyaloshinskii-Moriya interaction probed by magnetization reversal in bilayer Pt/Co/Ir/Co/Pt synthetic ferrimagnets

Here, we study the effect of the Dzyaloshinskii-Moriya interaction (DMI) on magnetic nuclei expansion in Pt/Co/Ir/Co/Pt perpendicular synthetic ferrimagnets. Magnetization reversal is explored as a response to the DMI, affecting the expansion of the magnetic nuclei. We report a nontrivial inverted dependence of the velocity of the double domain wall on the in-plane (IP) magnetic field. This effect is due to competition of the DMI contributions, resulting in effective fields of different signs in the two Co layers. We report a macroscopic manifestation of the DMI as magnetization reversal accelerated by the applied IP magnetic field. Accelerated magnetic relaxation is provided by the DMI-induced ellipticity of the magnetic nuclei, the spreading of which is accompanied by an enhanced number of nuclei contacts in comparison with round nuclei in the absence of an IP field. The obtained results clarify how the DMI contributes to macroscopic magnetic relaxation in the presence of the interlayer Heisenberg exchange interaction in synthetic ferrimagnets with perpendicular anisotropy.