Magnetic Multilayers Research Articles

Artificial Magnetic Pattern Arrays Probed by Polarized Neutron Reflectivity.

Traditionally, neutron scattering is an essential method for the analysis of spin structures and spin excitations in bulk materials. Over the last 30 years, polarized neutron scattering in terms of reflectometry has also contributed largely to the analysis of magnetic thin films and magnetic multilayers. More recently it has been shown that polarized neutron reflectivity is, in addition, a suitable tool for the study of thin films laterally patterned with magnetic stripes or islands. We provide a brief overview of the fundamental properties of polarized neutron reflectivity, considering different domain states, domain fluctuations, and different domain sizes with respect to the neutron coherence volume. The discussion is exemplified by a set of simulated reflectivities assuming either complete polarization and polarization analysis, or a reduced form of polarized neutron reflectivity without polarization analysis. Furthermore, we emphasize the importance of the neutron coherence volume for the interpretation of specular and off-specular intensity maps, in particular when studying laterally non-homogeneous magnetic films. Finally, experimental results, fits, and simulations are shown for specular and off-specular scattering from a magnetic film that has been lithographically patterned into a periodic stripe array. These experiments demonstrate the different and mutually complementary information that can be gained when orienting the stripe array parallel or perpendicular to the scattering plane.

Evidence of magnetization switching by anomalous spin Hall torque in NiFe

Recently the anomalous Hall effect was predicted as a possible mechanism to produce magnetization-dependent spin current. Here we have applied NiFe/Ru/perpendicular magnetic multilayers and a specific geometry to demonstrate magnetization switching driven by anomalous spin-orbit torque resulting from the bulk anomalous Hall effect. The anomalous spin Hall torque of NiFe is strong enough not only to switch the magnetization of perpendicular Co but also leads to opposite switching chirality compared with the ordinary spin Hall torque. This work confirmed the existence of a strong anomalous spin Hall torque due to anomalous Hall effect, which may lay a foundation to develop spin-orbit torque devices free of heavy metals.

Influence of the anisotropy on the magneto-acoustic response of magnetic surface acoustic wave resonators**Project supported by the National Natural Science Foundation of China (Grant No. 61871081) and the National Key Scientific Instrument and Equipment Development Project of China (Grant No. 51827802).

One-port magnetic surface acoustic wave (MSAW) resonators are fabricated by stacking multilayered (FeCoSiB/SiO2)n films directly on top of interdigital electrodes. It is shown that the magneto-acoustic response of the MSAW resonators critically depends the hysteresis of ΔE effect. For the magnetic multilayer without induced magnetic anisotropy, the resonance frequency (fR) exhibits a butterfly-like dependence on the external field, therefore, enabling bipolar detection of magnetic field smaller than its coercive field. However, for the magnetic multilayers with induced magnetic anisotropy, butterfly-like or loop-like fR–H curves are measured along the interdigtial electrode fingers or the SAW propagation direction, which can be attributed to the competition between the magnetic field-induced anisotropy and the stress-induced or shape anisotropy.

Gilbert Damping in Annealed Perpendicular Exchange Biased CoFeB/Pt/MnIr Multilayers

Exchange bias and intrinsic Gilbert damping [Formula: see text] are two important parameters in magnetic multilayers. To explore the correlation between exchange coupling and damping, the samples Ta (2.6[Formula: see text]nm)/Pt (2.5[Formula: see text]nm)/Co[Formula: see text]Fe[Formula: see text]B[Formula: see text] (0.63[Formula: see text]nm)/Pt (0.5[Formula: see text]nm)/Mn[Formula: see text]Ir[Formula: see text] ([Formula: see text])/Ta (4[Formula: see text]nm) are deposited by using pulsed-DC magnetron sputtering. The static and dynamic magnetic responses with different thickness of antiferromagnetic MnIr are investigated using vibrating sample magnetometer (VSM) and the pump-probe time-resolved magneto-optical Kerr effect (TR-MOKE). The exchange bias initially increases and then decreases with increasing thickness of MnIr layer, which is associated with the interfacial exchange coupling. The dynamic results demonstrate that the Gilbert damping of CoFeB/Pt/MnIr can increase by up to 40% compared with CoFeB/Pt. Based on the fact that the spin pumping has been excluded because of the Pt interlayer, we attribute the enhancement in Gilbert damping to the exchange coupling between AFM and FM.

Creating zero-field skyrmions in exchange-biased multilayers through X-ray illumination

Skyrmions, magnetic textures with topological stability, hold promises for high-density and energy-efficient information storage devices owing to their small size and low driving-current density. Precise creation of a single nanoscale skyrmion is a prerequisite to further understand the skyrmion physics and tailor skyrmion-based applications. Here, we demonstrate the creation of individual skyrmions at zero-field in an exchange-biased magnetic multilayer with exposure to soft X-rays. In particular, a single skyrmion with 100-nm size can be created at the desired position using a focused X-ray spot of sub-50-nm size. This single skyrmion creation is driven by the X-ray-induced modification of the antiferromagnetic order and the corresponding exchange bias. Furthermore, artificial skyrmion lattices with various arrangements can be patterned using X-ray. These results demonstrate the potential of accurate optical control of single skyrmion at sub-100 nm scale. We envision that X-ray could serve as a versatile tool for local manipulation of magnetic orders.

Effect of the oxide layer on the interfacial Dyzaloshinskii-Moriya interaction in perpendicularly magnetized Pt/Co/SmOx and Pt/Co/AlOx heterostructures

The interfacial Dyzaloshinskii-Moriya interaction (DMI) plays a crucial role in stabilizing a chiral Néel-type magnetic domain wall in perpendicularly magnetized multilayers. In this work, we investigate the effect of oxide layers on interfacial DMI in ultrathin Pt/Co/SmOx and Pt/Co/AlOx stacks with the perpendicular magnetic anisotropy. The DMI effective field was characterized using the field driven domain wall creep and current-induced hysteresis loop shift methods. The experimental results show that an enhanced interfacial DMI is obtained in Pt/Co/SmOx stacks, which may be related to the oxidation degrees of the Co layer in view of the difference of Co/metallic oxide interfaces. Moreover, the X-ray photoelectron spectroscopy with the argon ion etching technique was used to further verify the weaker oxidation of the Co layer existed in Pt/Co/SmOx than that in Pt/Co/AlOx in the same annealing process due to the smaller electronegativity of the Sm compared to the Al element. Our results provide a promising route to effectively manipulate the interfacial DMI in spin-orbit-torque-based spintronic devices through the element electronegativity.

Effect of random anisotropy on magnetization reversal in dipolarly coupled layered thin films

Magnetic thin films and multilayers offer many novel physical phenomena with tremendous potential for applications. Various growth processes and methods are in place to make homogeneous films. However, dispersion in local easy axes is an inherent issue which may give rise to an effective random anisotropy in thin film. It is important to investigate the effect of random anisotropy on the magnetic properties of the thin films. In this report we address the aspect of local dispersion induced random anisotropy on magnetization reversal in ferromagnetic (FM)/non-magnetic(NM)/ferromagnetic(FM) stacks. We show that the magnetization reversal is not only governed by the inter-layer interaction but the intrinsic anisotropies also play a crucial role. In this context we have studied the magnetization reversal of Co/AlOX/Co trilayers with various thicknesses of AlOX which acts as a spacer between the two Co layers. Presence of random anisotropy in addition to growth induced uniaxial anisotropy was observed in all the samples. Magneto-optic Kerr effect (MOKE) based microscopy revealed dipolar-coupled layer-by-layer magnetization reversal of the Co layers which was corroborated by polarized neutron reflectometry (PNR) experiments. Micromagnetic simulations confirmed that the presence of random anisotropy in addition to uniaxial anisotropy leads to layer-by-layer magnetization reversal.

Electric dipole moment as descriptor for interfacial Dzyaloshinskii-Moriya interaction

Chiral magnets are of fundamental interest and have important technological ramifications. The origin of chiral magnets lies in the Dzyaloshinskii-Moriya interaction (DMI), an interaction whose experimental and theoretical determination is laborious. We derive an expression that identifies the electric dipole moment as descriptor for the systematic design of chiral magnetic multilayers. Using density functional theory calculations, we determine the DMI of (111)-oriented metallic ferromagnetic $Z$/Co/Pt multilayers of ultrathin films. The non-magnetic layer $Z$ determines the DMI at the Co-Pt interface. The results validate the electric and magnetic dipole moments as excellent descriptors. We found a linear relation between the electric dipole moment of Pt, the Allen electronegativity of $Z$, and the contribution of Pt to the total DMI.

The formation process and structure of the skyrmion bubble lattice in magnetic multilayers

Magnetic skyrmions are attracting considerable interest in scientific research and practical applications. Recently, a novel phenomenon was observed in which a skyrmion bubble lattice could be induced by a slightly tilted in-plane magnetic field in a multilayer film. In this article, the formation process of a skyrmion bubble lattice was investigated by micromagnetic simulation considering the Dzyaloshinskii–Moriya (DM) interaction and distribution of magnetic anisotropy. The exchange field that prevents the reversal of moments was minimized when the magnetic moments were aligned by a slightly tilted in-plane magnetic field. Many small reversed domains were formed in the regions where the demagnetization field conquers the anisotropy field after the removal of the external field and developed into a stable skyrmion bubble array. The DM interaction has an obvious influence on the size and precise moment structure of the stable skyrmion bubbles but has little effect on the nucleation of reversed domains.

Magnetization switching driven by current-induced torque from weakly spin-orbit coupled Zr

The authors presents results that show sizable current-induced torque and a robust current-induced magnetization switching in weakly spin-orbital coupled Zr based perpendicular magnetized multilayers. Current induced effective torque field changes its sign as the Zr layer thickness varies, indicating the competition of multiple effects. Possible roles of the orbital Hall effect, which generates sizable torque as predicted by theoretical calculation, are discussed.

Enhanced spin-triplet pairing in magnetic junctions with s -wave superconductors

A common path to superconducting spintronics, Majorana fermions, and topologically-protected quantum computing relies on spin-triplet superconductivity. While naturally occurring spin-triplet pairing is elusive and even common spin-triplet candidates, such as Sr$_2$RuO$_4$, support alternative explanations, proximity effects in heterostructures can overcome these limitations. It is expected that robust spin-triplet superconductivity in magnetic junctions should rely on highly spin-polarized magnets or complex magnetic multilayers. Instead, we predict that the interplay of interfacial spin-orbit coupling and the barrier strength in simple magnetic junctions, with only a small spin polarization and s-wave superconductors, can lead to nearly complete spin-triplet superconducting proximity effects. This peculiar behavior arises from an effective perfect transparency: interfacial spin-orbit coupling counteracts the native potential barrier for states of a given spin and wave vector. We show that the enhanced spin-triplet regime is characterized by a huge increase in conductance magnetoanisotropy, orders of magnitude larger than in the normal state.

Tailored Flux Pinning in Superconductor-Ferromagnet Multilayers with Engineered Magnetic Domain Morphology From Stripes to Skyrmions

Superconductor/Ferromagnet (S/F) hybrid systems show interesting magneto-transport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of applications. Here we show that engineering the F magnetization reversal allows tuning the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This is done by using magnetic multilayers in which Co thin films are combined with different heavy metals (Ru, Ir, Pt). By choosing the materials, thicknesses, and stacking order of the layers, we can design the characteristic domain size and morphology, from out-of-plane magnetized stripe domains to much smaller magnetic skyrmions. These changes strongly affect the magneto-transport properties. The underlying mechanisms are identified by comparing the experimental results to a magnetic pinning model.

Engineering the dynamics of topological spin textures by anisotropic spin-orbit torques

Integrating topologically stabilized magnetic textures such as skyrmions as nanoscale information carriers into future technologies requires the reliable control by electric currents. Here, we uncover that the relevant skyrmion Hall effect, which describes the deflection of moving skyrmions from the current flow direction, acquires important corrections owing to anisotropic spin-orbit torques that alter the dynamics of topological spin structures. Thereby, we propose a viable means for manipulating the current-induced motion of skyrmions and antiskyrmions. Based on these insights, we demonstrate by first-principles calculations and symmetry arguments that the motion of spin textures can be tailored by materials design in magnetic multilayers of Ir/Co/Pt and Au/Co/Pt. Our work advances the understanding of the current-induced dynamics of these magnetic textures, which underlies a plethora of memory and logic applications.

Dodecanacci extrinsic magnetized plasma multilayer

An extrinsic magnetized plasma quasi-multilayer is demonstrated using the transfer-matrix method with an extrinsic bulk plasma inspired structure. The transmittance of the extrinsic bulk plasma quasi-system has been shown using the Dodecanacci quasi-sequence. For the periodic structure, two significant gaps emerge and only one of them, the bandgap above the plasma frequency, appears in any of the proposed Dodecanacci systems and it is magnetically tunable and robust against the position of the layers. The other bandgap, observed below the plasma frequency is also magnetically tunable but it vanishes when the layers are quasiperiodically disposed because the emergence of several resonant peaks. The count and width of the transmission window in such vanished bandgaps decrease as the Dodecanacci generation number increases. The results also show that the electron density can control the gap location and gap width for all Dodecanacci structures. These listed results ensure that the approach of Dodecanacci extrinsic magnetized plasma multilayers can be applied to the design of future architectures for the control and the manipulation of the electromagnetic wave-matter interaction in microwave frequencies.

Long-range coupling interaction between a non-magnetic transition metal capping layer and a neighboring magnetic layer

In nanoscale magnetic multilayers, capping layers are often used to protect the underlying magnetic layers from oxidation. However, little research has investigated possible long-range coupling interactions between nonmagnetic transition metal (TM) capping layers and neighboring magnetic layers. In this paper, the temperature (T) dependence of the magnetic moment of different thicknesses of cobalt (Co) was studied in a tantalum (Ta)/Co/TM trilayer structure with four TM capping layers, where the TMs were Ta, Chromium (Cr), titanium (Ti), and zirconium (Zr), respectively. It was found that the capping layer had a large effect on the phase-transition behavior and thermal stability of the Co layer. In the Ta and Cr layers, the T-dependence of Co magnetic moment showed nonmonotonic behavior, and in the Ti and Zr layers, the Co M-T curve exhibited very few effects of the capping layer. We attribute this phenomenon to the long-range coupling between the Co and TM layers. Furthermore, the coupling mechanism was linked to the indirect magnetic exchange coupling in Co/TM multilayers, similar to the Ruderman-Kittel-Kasuya-Yoshida coupling. The results of this work will support further development of the understanding of the coupling between the 3d ferromagnetic (FM) metal and nonmagnetic TM at nanoscales. Relative to potential applications, it will inspire us to rediscover the role of both the TM capping layer and buffer nonmagnetic layer in FM/TM multilayers, especially for nanoscale magnetic multilayers with spin-dependent effects, such as spin valves, spin halls, spin transfer torque, and spin–orbit coupling, which are in widespread use in the manufacture of various spintronics devices.

Spin accumulation dynamics in spin valves in the terahertz regime

The notion of spin accumulation and spin relaxation in the diffusive regime was introduced by Valet and Fert in 1993 to describe the current-perpendicular-to-plane (CPP) diffusive transport in metallic magnetic multilayers. This theory has been quite successful in explaining the giant magnetoresistance of magnetic multilayers in CPP geometry in the frequency range from DC to a few gigahertz. In this paper, we investigate the dynamic aspect of spin accumulation from the theoretical point of view when reaching the terahertz (THz) frequency range. The characteristic relaxation time of electron elastic scattering is typically in the femtosecond range. However, since spin accumulation results from a diffusion process involving a very large number of individual scattering events, the characteristic time of spin accumulation variation when the current and/or the magnetic configuration are varied can be significantly longer than that of the input signal, eventually reaching the picosecond range. In spintronic devices operating in the THz range such as those based on ferrimagnetic or antiferromagnetic materials, the spin accumulation amplitude and, correlatively, the device magnetoresistance can therefore depend on the actual device operating frequency. We investigate this question by extending the Valet and Fert theory in the time domain.

Study on Magnet Eddy Current Losses in Interior Permanent Magnet Machines for Aircraft Electric Green Taxiing Systems

The magnet eddy current of multi-layered interior permanent magnet (IPM) machines is studied in the paper for the aircraft electric green taxiing systems. The armature reaction flux density distribution, eddy current density and harmonic spectrum at the end of permanent magnet are analysed. A method to reduce the magnet eddy current of multi-layered IPM machines is presented. That is to increase the distance between outer permanent magnet and rotor surface and the distance between two layers of permanent magnet so as to provide a larger core path for armature reactive flux. This method can effectively reduce the magnet eddy-current loss of the multi-layered IPM machines and improve the operation efficiency.

Current induced domain wall motion with a Ta/Gd-Fe/Si-N magnetic nanowire for a magneto-optical light modulator

We investigated a magnetic nanowire device for a light modulator which is very important for 3D holography display application. It has a Si-N/Ta/Gd-Fe/Si-N nanowire with 0.5 × 17 μm2 dimension and a Co/Pd multilayer hard magnet with its size of 0.5 × 3 μm2 which is attached underneath the nanowire for initial domain nucleation. Both the nanowire and the hard magnet have perpendicular anisotropy. An initial domain was successfully nucleated by pulse current and found spin-orbit torque from Ta and local magnetic field from the nanomagnet was responsible for the domain nucleation. Current induced domain wall (DW) motion was observed and the DW moved along current direction without magnetic field. DW velocity was significantly enhanced with in-plane magnetic field (Hinp) of 25mT and velocity more than 200 m/s was obtained, which was more than 25 times faster than that without magnetic field. We also show that the fast DW velocity is attributed to chiral Neel domain wall induced by external in-plane magnetic field. This fast DW motion is very useful for large scale light modulator which refreshes large number of pixels, such as 10K × 10K pixels, over a limited time scale of 8 ∼ 16.7 ms which corresponds to TV frame rate of 120 or 60 Hz.

Determination of fine magnetic structure of magnetic multilayer with quasi antiferromagnetic layer by using polarized neutron reflectivity analysis

We carried out polarized neutron reflectivity (PNR) analysis to determine the fine magnetic structure of magnetic multilayers with quasi-antiferromagnetic (quasi-AFM) layers realized by 90-deg coupling using two Co90Fe10 layers, and quantitatively evaluated the magnetization of quasi-AFM layers. Two types of samples with different buffer layers, Ru buffer and a NiFeCr buffer, were investigated and the average angles between the respective magnetization of the two Co90Fe10 layers were estimated to be +/− 39 degrees and +/− 53 degrees. In addition, less roughness was found in the NiFeCr buffer sample resulting stronger 90-deg coupling. A perfect quasi-AFM is expected to be realized by a flat interface of the magnetic multilayer.

Intensity enhancement of ferromagnetic resonance modes in exchange coupled magnetic multilayers

In this work, the ferromagnetic resonance characteristics of a NiFeCu/Non-magnetic(NM)/NiFe/NM/CoFe/NM/Co multilayer is studied from a theoretical point of view, and comparisons with the ferromagnetic resonance of a NiFe/NM/CoFe magnetic bilayer are presented. It is found that the resonance modes of the multilayer tend to be more intense than those of the bilayer for several combinations of applied field and interlayer exchange coupling. Furthermore, rules governing the individual layer contributions to the resonance modes of a exchanged coupled magnetic multilayer are presented, which would apply to any number of layers. These results open the possibility to tailor the resonance frequencies of the multilayer structure by either engineering the interlayer exchange coupling or by applying a perpendicular magnetic field for multiband high frequency magnetic devices.