Uniaxial Magnetic Anisotropy Research Articles

Tailoring Magnetic and Transport Anisotropies in Co100−x–Cux Thin Films through Obliquely Grown Nano-Sheets

The magnetic and transport properties of pulsed laser-deposited Co100−x–Cux thin films were tailored through their nano-morphology and composition by controlling for the deposition geometry, namely normal or oblique deposition, and their Cu content. All films were composed of an amorphous Co matrix and a textured growth of Cu nanocrystals, whose presence and size d increased as x increased. For x = 50, all films were superparamagnetic, regardless of deposition geometry. The normally deposited films showed no in-plane magnetic anisotropy. On the contrary, controllable in-plane uniaxial magnetic anisotropy in both direction and magnitude was generated in the obliquely deposited films. The magnetic anisotropy field Hk remained constant for x = 0, 5 and 10, Hk ≈ 35 kAm−1, and decreased to 28 and 26 kAm−1 for x = 20 and 30, respectively. This anisotropy had a magnetostatic origin due to a tilted nano-sheet morphology. In the normally deposited films, the coercive field Hc increased when x increased, from 200 (x = 0) to 1100 Am−1 (x = 30). In contrast, in obliquely deposited films, Hc decreased from 1500 (x = 0) to 100 Am−1 (x = 30) as x increased. Activation energy spectra corresponding to structural relaxation phenomena in obliquely deposited films were obtained from transport property measurements. They revealed two peaks, which also depended on their nano-morphology and composition.

Large magnetic anisotropy in Co–Fe–Ni–N ordered structures: a first-principles study

Material design of promising rare-earth free permanent magnet requires tailoring and controlling the intrinsic magnetic properties namely large saturation magnetization μ 0 M s, giant uniaxial magnetic anisotropy K u, and high Curie temperature T C. Based on first-principles electronic structure calculations, we present a detailed analysis for the intrinsic magnetic properties of Co x Fe1−x Ni and Co x Fe1−x NiN0.25 ordered structures. We predict an enhanced structural stability with improved K u ranging from 1.53–2.29 MJ m−3 for Co x Fe1−x NiN0.25 ordered structures, with the exception of CoNiN0.25 having planar anisotropy. Detailed analysis of the predicted large K u, based on perturbation theory and electronic structure calculations, is attributed to the cumulative effect of contribution from the increased tetragonal distortion and induced orbital distortion from the simultaneous Co substitution and interstitial N-doping. By tailoring the K u, we may create efficient and affordable PMs, bridging the gap between commonly used ferrite and high-performance Nd–Fe–B magnets.

Angular dependence of saturation magnetization in single crystals with uniaxial magnetic anisotropy

A remarkable feature of some high anisotropy intermetallic compounds based on R-T systems, where R stands for a rare earth atom and T is a 3d transition metal element, is a prominent difference of saturation magnetization along easy and hard magnetization axes: MEA and MHA. This anisotropy of saturation magnetization needs to be considered when magnetization curves are evaluated to derive the magnetic anisotropy constants. We investigated the angular dependence of saturation magnetization to verify experimentally the assumption of Ms(θ)=MEAcos2θ+MHAsin2θ using a single crystal of Y2Co7, which has an uniaxial magnetic anisotropy and saturation magnetization difference of 7.9% at 300 K. Our proposed verification approach is universal and would be relevant for any intermetallic compound with a prominent anisotropy of saturation magnetization.

Large stress-induced anisotropy in soft magnetic films for synthetic spin valves

We obtain a large in-plane magnetic uniaxial anisotropy in the soft magnetic Fe60Co20B20 (FeCoB) thin films prepared by rotational sputtering. The anisotropy field (Ha) of 75 to 175 Oe was found in the films with wide ranged thickness from 2.5 to 100 nm, which was attributed to the magnetoelastic energy in association with anisotropic tensile stress. This stress-induced anisotropy has outstanding thermal stability that survives up to 350 °C in the annealing process. The similar large uniaxial anisotropy can be realized in other soft magnetic thin films, such as Fe, Co, Ni, FeCo, and NiFe, with the same synthesis technique. The anisotropic FeCoB film was used as a free layer in a synthetic spin valve. A linear resistance change against external field with a range wider than ±100 Oe together with a significantly reduced coercivity of ∼1.1 Oe (∼8.5 Oe in the case with isotropic free layer) was observed in the transfer curve. The results of this work not only confirm the feasibility of films with large stress-induced magnetic anisotropy as a functional layer in spin-valve devices but also demonstrate a simple synthesis route to induce the magnetic anisotropy, which provides an additional control parameter for the spintronic device design.

Thickness gradient related magnetic anisotropy of wedged Co nanocluster assemblies deposited on glass plates

We prove the existence of uniaxial magnetic anisotropy (UMA) directly related to thickness gradients in a material system comprised of planar assemblies of Co nanoclusters (diameter∼2.5 nm), grown by pulsed laser deposition (PLD) on bare glass substrates. PLD provides a widespread coating that takes place with a decreasing thickness along the radial directions of a glass plate faced perpendicular to the ablation jet, yielding a wedge-like shape of the film thickness profile. Reflection and transmission magneto-optical effects enabled us to study magnetization vector as a function of the film thickness and location, revealing in-plane magnetization with a well-defined UMA along the in-plane direction parallel to the thickness gradient only in high-gradient films. The dependence of anisotropy energy on film thickness has been analyzed within a phenomenological model, accounting for the surface and bulk contributions to the effective magnetic anisotropy energy (MAE). According to that, below a threshold thickness, the volume contribution is low and Nèel-like interface contribution to MAE is negligible, as well as the null magneto-crystalline contributions of such a disorder cluster system, leaving only potential surface terms left. MAE is thus tentatively explained with the extra cluster terraces related to high-gradient thickness. Additionally, the magnetization along the in-plane hard axis or thickness gradient direction (TGD) includes a minor out-of-plane component noticed as a Faraday rotation in transmission geometry. We speculate it may arise from geometrical frustration of moments occurred in the hard axis magnetization of the network of interconnected assemblies.

DyOCl: A rare-earth based two-dimensional van der Waals material with strong magnetic anisotropy

Comparing with the widely known transitional metal based van der Waals (vdW) materials, rare-earth based ones are rarely explored in the research of intrinsic two-dimensional (2D) magnetism. In this work, we report the physical properties of DyOCl, a rare-earth based vdW magnetic insulator with a direct band gap of $\ensuremath{\sim}5.72\phantom{\rule{3.33333pt}{0ex}}\text{eV}$. The magnetic order of bulk DyOCl is determined by neutron scattering as the $A$-type antiferromagnetic structure below the N\'eel temperature ${T}_{N}=10\phantom{\rule{3.33333pt}{0ex}}\mathrm{K}$. The large magnetic moment near 10.1 ${\ensuremath{\mu}}_{B}$/Dy lies parallel to the $a$-axis with strong uniaxial magnetic anisotropy. At $2\phantom{\rule{3.33333pt}{0ex}}\text{K}$, a moderate magnetic field ($\ensuremath{\sim}2\phantom{\rule{3.33333pt}{0ex}}T$) applied along the easy axis generates spin-flip transitions and polarizes DyOCl to a ferromagnetic state. Density functional theory calculations reveal an extremely large magnetic anisotropy energy ($\ensuremath{-}5850\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}\text{eV}/Dy$) for DyOCl, indicating the great potential to realize magnetism in the 2D limit. Furthermore, the mechanical exfoliation of bulk DyOCl single crystals down to seven layers is demonstrated. Our findings suggest DyOCl is a promising material playground to investigate 2D $f$-electron magnetism and spintronic applications at the nanoscale.

Polarized Neutron Diffraction: An Excellent Tool to Evidence the Magnetic Anisotropy—Structural Relationships in Molecules

This publication reviews recent advances in polarized neutron diffraction (PND) studies of magnetic anisotropy in coordination compounds comprising d or f elements and having different nuclearities. All these studies illustrate the extent to which PND can provide precise and direct information on the relationship between molecular structure and the shape and axes of magnetic anisotropy of the individual metal sites. It makes this experimental technique (PND) an excellent tool to help in the design of molecular-based magnets and especially single-molecule magnets for which strong uniaxial magnetic anisotropy is required.

Quasi-two-dimensional ferromagnetism and anisotropic interlayer couplings in the magnetic topological insulator MnBi2Te4

MnBi2Te4(MBT) is a promising van der Waals layered antiferromagnetic (AF) topological insulator that combines a topologically non-trivial inverted Bi-Te band gap with ferromagnetic (FM) layers of Mn ions. We perform inelastic neutron scattering (INS) on co-aligned single crystals to study the magnetic interactions in MBT. Consistent with previous work, we find that the AF interlayer exchange coupling and uniaxial magnetic anisotropy have comparable strength, which supports metamagnetic transitions that allow access to different magnetic symmetries in applied fields. Modelling of the two-dimensional intralayer FM spin waves requires the introduction of long-range and competing Heisenberg FM and AF interactions, up to at least the seventh nearest-neighbor, and possess anomalous damping, especially near the Brillouin zone boundary. First-principles calculations of insulating MBT find that both interlayer and intralayer magnetic interactions are long-ranged. We discuss the potential roles that bulk $n$-type charger carriers and chemical disorder play in the magnetism of MBT.

A Novel Thin Film Cascade Matrix Coupled Inductor for Integrated Voltage Regulators

This letter demonstrates an on-silicon integrated thin film coupled inductor using a novel cascade matrix concept to suit multiphase integrated voltage regulator (IVR) applications. The novel cascade matrix coupled (CMC) stripline inductor facilitates all-phase-coupled configuration to reduce the phase ripple current in IVRs. It also shows excellent scalability and neat floor planning. Moreover, the proposed CMC inductor uses double layer metal scheme for windings and interconnects, which significantly reduces the ac winding resistance. A four-phase CMC inductor has been successfully fabricated on-silicon substrate and characterized, which uses conventional thin film magnetic core deposition process to achieve in-plane magnetic uniaxial anisotropy. The device occupies 3.2 × 1.7 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> floor area and provides 19.5 nH phase inductance with 120 mΩ dc resistance. The saturation current for each phase is up to 0.4 A when the phase inductance drops by approximately 20%. The coupling coefficient between any two phases of the proposed CMC inductor is 0.27, much higher and more controllable than that of toroid core-based coupled inductors.

Crystalline orientation-related magnetic anisotropy in transition metal dichalcogenides 1T′-MoTe2/Co heterostructures

The semimetal transition metal dichalcogenides (TMD) MoTe2 have attracted extensive research interests in Spintronics. The understanding of the magnetic properties of MoTe2/Ferromagnetic (FM) heterostructures is critical to the design of novel Spintronics-based devices. However, there is lack of research on the magnetic anisotropy of the semimetal TMD/FM. Here, we demonstrate the in-plane crystalline orientation-related magnetic properties of MoTe2/Co to clarify the relationship between the crystalline orientation and the magnetic anisotropy. We carried out the studies of crystalline orientation-related magnetic anisotropy in 1T′-MoTe2/Co heterostructures from both experiments and first-principles calculations. Our results show that for Co with different thicknesses, the easy axis of Co film is along the b-axis of MoTe2 with uniaxial magnetic anisotropy. It is also consistent with the results of our theoretical results. When Co film thickness increases, the magnetocrystalline anisotropy (MCA) oscillates around a constant value with a lower MCA along the out-of-plane direction. Meanwhile, the demagnetization energy which increases linearly as a function of Co thickness, which would make the easy axis in-plane. Our work may give a fundamental inspiration for Spintronics based devices with semi-metallic TMDs/ferromagnetic materials.

Observation of uniaxial magnetic anisotropy and out-of-plane coercivity in W/Co20Fe60B20/W structures with high thermal stability

In this study, the development of uniaxial magnetic anisotropy (UMA) and out-of-plane coercivity (Hc┴) in W/Co20Fe60B20(CoFeB)/W structures by tuning the annealing temperature (TA) such as 300°C, 400°C and 500°C were achieved. For 400°C film, W and CoFeB interfaces play a major role in achieving the large Hc┴. Furthermore, the observation of large Hc┴ was found to be significantly induced by the interface on the two sides of CoFeB layer. The two-fold UMA strongly is stabilized in W/CoFeB/W trilayers up to 500°C. A maximum in-plane uniaxial anisotropy energy density (Ku) was observed for the 500°C annealed films, which is accompanied by the dominance of Hc┴ owing to the over-oxidation and/or occurrence of inter-diffusion across the interfaces. The oxidation and relieving of boron atoms in the CoFeB at higher annealing temperatures were confirmed by X-ray photoelectron spectroscopy (XPS). The control over the interface shared between the W and CoFeB is concluded to be essential in order to alter the strength of magnetic anisotropy. Hence, an investigation on W/CoFeB/W structures could be very beneficial for the modern Spin-Orbit Torque (SOT) based data storage applications and this inspires relevant research work to widen magnetic systems.

Tailorable exchange bias and memory of frozen antiferromagnetic spins in epitaxial CoO(1 1 1)/Fe(1 1 0) bilayers

We report on exchange bias and interfacial antiferromagnetic spin orientation in epitaxial CoO(111)/Fe(110) bilayers. Our X-ray magnetic linear dichroism and magneto-optic Kerr effect results show that exchange bias sets on in the zero-field-cooled system and is governed by in-plane magnetic state Fe(110) sublayer above Néel temperature of CoO. Uniaxial magnetic anisotropy of Fe layer determines the direction of interfacial frozen antiferromagnetic CoO spins within CoO(111)//Fe(110) sample plane. Choice of the particular magnetic state of Fe sublayer, when passing Néel temperature of CoO, determines both the axis and direction of interfacial antiferromagnetic spins after the sample is cooled and allows for imprinting their +/- 90° and 0/180° alignment within the sample plane.

Strain-controlled domain wall injection into nanowires for sensor applications

We investigate experimentally the effects of strain on the injection of 180° domain walls (DWs) from a nucleation pad into magnetic nanowires, as typically used for DW-based sensors. In our study, the strain, generated by substrate bending, induces in the material a uniaxial anisotropy due to magnetoelastic coupling. To compare the strain effects, Co40Fe40B20, Ni, and Ni82Fe18 samples with in-plane magnetization and different magnetoelastic coupling are deposited. In these samples, we measure the magnetic field required for the injection of a DW, by imaging using differential contrast in a magneto-optical Kerr microscope. We find that strain increases the DW injection field and that the switching mechanism depends strongly on the strain direction. We observe that low magnetic anisotropy facilitates the creation of a domain wall at the junction between the pad and the wire, whereas a strain-induced magnetic easy axis significantly increases the coercive field of the nucleation pad. Moreover, we find that these effects of strain-induced anisotropy can be counteracted by an additional magnetic uniaxial anisotropy perpendicular to the strain-induced easy axis. We perform micromagnetic simulations to support the interpretation of our experimental findings showing that the above described observations can be explained by the effective anisotropy in the device. The anisotropy influences the switching mechanism in the nucleation pad as well as the pinning of the DW at the wire entrance. As the DW injection is a key operation for sensor performances, the observations show that strain is imposing a lower limit for the sensor field operating window.

Elliptical skyrmion moving along a track without transverse speed

It was recently demonstrated that introducing the shape anisotropy of a skyrmion is an alternative strategy for controlling the skyrmion Hall effect. For example, an elliptical skyrmion shows anisotropic motion under the applied driving force. Here, we study the fundamental properties of an elliptical skyrmion induced by uniaxial in-plane magnetic anisotropy. The dynamics of a skyrmion under various parameters, such as uniaxial in-plane magnetic anisotropy, interfacial Dzyaloshinskii-Moriya interaction, and the damping constant, are investigated, and we employ Thiele's theory and derive expressions for the velocity of the skyrmion that match very well with micromagnetic simulation results. We find that the skyrmion Hall angle can reach ${90}^{\ensuremath{\circ}}$ with the optimized angle between the in-plane easy axis and the current. Our results offer a potential application of racetrack memory based on skyrmions, and a type of racetrack configuration was designed.

Nuclear and Electron Spin Resonance Studies on Skyrmion‐Hosting Lacunar Spinels

Magnetic resonance techniques at nuclei and electrons are applied to characterize the electronic structure and collective magnetic excitations throughout the magnetic phase diagrams of the lacunar spinels GaV4S8 and GaV4Se8 showing cycloidal, Néel‐type skyrmion lattice and ferromagnetically polarized phases. 71Ga nuclear magnetic resonance (NMR) provides a local probe of the rhombohedral distortion and the resulting uniaxial magnetic anisotropy via the detection of electric field gradients (EFGs) and hyperfine coupling at the gallium sites of these lacunar spinels. Broadband electron spin resonance (ESR) allows identifying clockwise, counterclockwise, and breathing modes of the skyrmion‐lattice phase supported by theoretical simulations.

Effect of V addition on the body-centered cubic to body-centered tetragonal to face-centered cubic structural transformation of N-containing Fe and FeCo films

In this study, the effect of adding V to N-containing Fe and FeCo on the stability of the body-centered tetragonal (bct) structure was investigated. The crystal structures of FeN, FeVN, FeCoN, and FeCoVN films with a thickness of 20 nm were evaluated, and the body-centered cubic to bct to face-centered cubic structural transformation was observed based on the N content. Results indicate that V addition stabilizes the bct structure and the Co content effectively promotes the formation of the bct structure. The uniaxial magnetic anisotropy of approximately 1.0 MJ/m3 was obtained in the FeCoVN films with the bct structure.

High frequency dynamics and magnetic anisotropy of bcc Co films grown on Si (0 0 1) substrate

Dynamic magnetization and magnetic anisotropy of Cu/bcc-Co/CoSi2 /Si(001) system were investigated by Magneto-Optical Kerr Effect (MOKE) analysis and vector network analyzer ferromagnetic resonance (VNA-FMR). Typical fourfold magnetocrystalline anisotropy of bcc Co film and superposed uniaxial magnetic anisotropy were observed. The uniaxial magnetic anisotropy originated from shape anisotropy as confirmed by micromagnetic simulation. The large magnetocrystalline anisotropy results in high resonance frequency even in the zero applied field. The Gilbert damping parameter α obtained by analysis of the dynamic process is close to the theoretically estimated value of bcc phase Co. The Gilbert damping parameter of the bcc-Co film is larger than those of fcc-Co and hcp-Co films. This work indicates the Co damping parameter can be enhanced by using bcc structure, which will help create high-speed magnetoelectronic devices.

Robust formation of skyrmion and skyrmionium in magnetic hemispherical shells and their dynamic switching

We explored the variations of the topological magnetic textures of vortices, skyrmions, and skyrmioniums in magnetic elements of hemispherical-shell shape with respect to surface-normal uniaxial magnetic anisotropy constant ${K}_{u}$, Dzyaloshinskii-Moriya interaction (DMI) constant ${D}_{\mathrm{int}}$, and shell diameter $2R$. For given values of $2R$, the combination of ${K}_{u}$ and ${D}_{\mathrm{int}}$ plays a crucial role in the stabilization of those different spin textures. With decreasing $2R$, the geometrical confinement of hemispherical shells more significantly affects the stabilization of skyrmions owing to curvature-induced DM-like interaction. This effect is contrastingly dependent on the sign of ${D}_{\mathrm{int}}$: skyrmion formation is more favorable for positive ${D}_{\mathrm{int}}$ values, whereas it is less favorable for negative ones. A quite promising feature is that skyrmions can be stabilized even in the absence of intrinsic DMI for $2R&lt;25\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. We also explored characteristic dynamic properties of skyrmions excited by in-plane and out-of-plane oscillating magnetic fields. Similarly to the fundamental dynamic modes found in planar dots, in-plane gyration and azimuthal spin-wave modes as well as out-of-plane breathing modes were found, but additional higher-frequency hybrid modes also appeared due to coupling between radially quantized and azimuthal spin-wave modes. Finally, we found a switching behavior of skyrmion polarity through a transient skyrmionium state using very-low-strength AC magnetic fields. This work provides further physical insight into the static and dynamic properties of skyrmions in curved-geometry nanodots and suggests potential applications to low-power-consumption and ultra-high-density information-storage devices.

Magnetic anisotropy in permalloy antidot square lattice

Magnetic anisotropy of Permalloy (Py) antidot square lattice was investigated by torquemetry method using Rotation Magneto-Optic Kerr Effect (ROTMOKE). We find that there exists a field-dependent 4-fold magnetic anisotropy with the easy magnetization axis along the [11] axis of the antidot square lattice. In addition, there also exists an artifact of a uniaxial magnetic anisotropy in ROTMOKE result. We show that both results are due to the period wiggling of the magnetization in space which was confirmed by magnetic imaging using magnetic transmission soft x-ray microscopy (MTXM). Micromagnetic simulation from MuMax3 supports the wiggling structure of the magnetization, as well as reproduces ROTMOKE result. A simplified model was developed based on the periodic wiggling of the magnetization and successfully explored the physical origin of the field-dependent 4-fold anisotropy and the artifact of the uniaxial anisotropy.

Growth and in-situ characterization of magnetic anisotropy of epitaxial Fe thin film on ion-sculpted Ag (001) substrate

Growth and magnetic characterization of Fe film on ion-sculpted Ag(001) substrate have been studied in-situ using reflection high energy electron diffraction and magneto-optical Kerr effect. Fe film is found to grow epitaxially on the Ag surface and exhibits epitaxial island formation up to ∼ 1 nm film thickness. Ion beam sculpting of the substrate surface, prior to the deposition of the Fe film, induces an uniaxial magnetic anisotropy in Fe film, which couples with the cubic four-fold anisotropy of bcc-Fe, lead to the multiple-step jump in magnetic hysteresis loops. Magnetic switching and its correlation with the domain dynamics of the film, as studied by the Kerr microscopy technique, revealed a striking difference in domain structure depending upon the direction of the applied magnetic field with respect to crystallographic direction. It is found that the magnetization reversal takes place via a “single-step jump” mechanism through sweeping of 180° domain walls, whereas the combination of coherent rotation and the sweeping of 90° domain walls is responsible for a “double step jump” magnetization reversal.