Biaxial Anisotropy Research Articles

Spin-orbit torque switching of an antiferromagnetic metallic heterostructure

The ability to represent information using an antiferromagnetic material is attractive for future antiferromagnetic spintronic devices. Previous studies have focussed on the utilization of antiferromagnetic materials with biaxial magnetic anisotropy for electrical manipulation. A practical realization of these antiferromagnetic devices is limited by the requirement of material-specific constraints. Here, we demonstrate current-induced switching in a polycrystalline PtMn/Pt metallic heterostructure. A comparison of electrical transport measurements in PtMn with and without the Pt layer, corroborated by x-ray imaging, reveals reversible switching of the thermally-stable antiferromagnetic Néel vector by spin-orbit torques. The presented results demonstrate the potential of polycrystalline metals for antiferromagnetic spintronics.

Anisotropy of magnetoelectric effects in an amorphous ferromagnet-piezoelectric heterostructure

In-plane anisotropy of linear and nonlinear magnetoelectric (ME) effects in a heterostructure composed of mechanically coupled layers of amorphous iron-based ferromagnet and piezoelectric lead zirconate-titanate ceramics has been investigated. The measurements were performed on a disk-shaped sample subjected to a low-frequency excitation magnetic field h oriented either parallel or perpendicular to a bias magnetic field H0. The first harmonic of the voltage response was recorded in the case of the linear ME effect, which was excited at frequencies of acoustic resonances of the sample, and the second harmonic in the case of the nonlinear ME effect excited in the non-resonance regime. Strong uniaxial or biaxial anisotropy for different directions of the bias field within the sample plane was observed. The uniaxial anisotropy of the ME effect occurs due to the uniaxial magnetic and magnetostrictive anisotropy of the amorphous ferromagnetic layer that is a result of its fabrication by the ultrafast cooling method. The biaxial anisotropy at resonant frequencies is the manifestation of the spatial structure of the excited acoustic modes. The anisotropy of ME effects should be taken into account when developing magnetoelectric devices based on composite heterostructures.

Comparison of high temperature growth versus post-deposition in situ annealing in attaining very low Gilbert damping in sputtered Co2FeAl Heusler alloy films

In this work, a detail study on the effectiveness of high substrate temperature (TS) during deposition versus in situ annealing of the films at high temperature (TA) after sputtering at room-temperature is compared with an objective to obtain Heusler alloy thin films of Co2FeAl (CFA) having optimally low αint (with an upper bound of 1.75 × 10-3) and 12–17% smaller MS by employing ion beam sputtering technique and Si(100) as substrates for current induced magnetization switching application. In each of the two series of CFA films, prepared with different choice of TS and TA (lying in 300–773 K range) were explored for optimally tailoring their structural, static and dynamic magnetization properties. Structural study revealed that although all the CFA films were polycrystalline but the post-deposition annealed films were structurally optimal with higher density (6.36 ± 0.09 g/cm3) and lower interface roughness (0.48 ± 0.03 nm) compared to the CFA films grown at high TS (6.23 ± 0.06 g/cm3, 0.61 ± 0.01 nm). In plane field-angle dependent longitudinal magneto optical Kerr effect (L-MOKE) study revealed the existence of a coupling between the weak uniaxial and biaxial magnetic anisotropies present in these films, attributed to the employed deposition geometry. Ferromagnetic resonance (FMR) spectroscopy measurements, performed in both the geometries - in-plane as well as out of plane, revealed a lowest value of 1.19 (±0.08) × 10-3 of αint in the film post-annealed at 773 K. In addition, the observed non-linear relation between the αint and the dynamical ‘g’-factor suggests that the contribution of spin orbit interaction to the damping is far less compared to the damping contribution from the DOS present near the Fermi level. It is remarkable to note that the as-grown CFA films sputtered at room temperature exhibited a record lowest value of 1.73 (±0.09) × 10-3 of αint. Attainment of such small value of Gilbert damping and having moderate magnetization in high spin polarized Co2FeAl Heusler films sputtered on the Si (100) substrate opens up their great application potential in future spintronics devices.

Electrical control of the optical dielectric properties of PEDOT:PSS thin films

Poly (3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) exhibits attractive optical tunability via electrochemical redox doping under applied voltages. Here, the influence of applied voltage on the optical parameters of PEDOT:PSS thin films was systemically investigated. The PEDOT:PSS film was prepared by the spin-coating method. A well-defined reduction degree from 0 to 100% was set in a Na+ electrolyte by using electrochemical analysis. The biaxial anisotropy optical complex refractive index of PEDOT:PSS thin films at different reduction degrees were determined in the entire visible spectrum by using the variable angle ellipsometry spectrometer combined with the Drude-Lorentz model. The results show that both the refractive index and the extinction coefficient increase with the increase of the reduction degree. This implies potential applications for PEDOT:PSS films on electrically controlling devices.

Robust Monolayer Exchange-Bias Effect in Molecular Crane-Pulley Response at Magnetic Surface

Exchange-bias effect is a widely investigated topic for spintronic applications in magnetic data storage read heads and spin-valve sensors. This effect is observed in bilayers of magnetic materials comprising of a soft ferromagnet (FM) and an antiferromagnet (AFM). In the last few years, unexpected observations of such an effect in ferromagnet-molecule heterostructures has raised curiosity in probing their underlying mechanism for potential technological applications. Here, we report our interface study between thin films of $\mathrm{Fe}$ and monolayer metal-phthalocyanine (MPc) molecules providing evidence of a robust exchange bias in zero-field cooled devices that is stable even after multiple training cycles. We ascribe the underlying mechanism to a combination of surface magnetic hardening and weakening of interlayer magnetic exchange that decouples the surface MPc-$\mathrm{Fe}$ hard magnetic layer from the soft switchable $\mathrm{Fe}$ layer underneath. We refer to this combined response to a molecular crane-pulley effect arising from the strong $\ensuremath{\pi}\ensuremath{-}d$ hybridization induced changes in the magnetization properties of surface $\mathrm{Fe}$ relative to the underlying film. Furthermore, analysis of our in-plane angle-dependent anisotropy magnetoresistance measurements reveal a surprisingly large biaxial anisotropy of the hard-surface layer with switching field exceeding 7 T. These studies provide us the opportunity to engineer the form of magnetic anisotropy at these spin interfaces.

Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction

The orientation of a chiral magnetic domain wall in a racetrack determines its dynamical properties. In equilibrium, magnetic domain walls are expected to be oriented perpendicular to the stripe axis. We demonstrate the appearance of a unidirectional domain wall tilt in out-of-plane magnetized stripes with biaxial anisotropy and Dzyaloshinskii–Moriya interaction (DMI). The tilt is a result of the interplay between the in-plane easy-axis anisotropy and DMI. We show that the additional anisotropy and DMI prefer different domain wall structure: anisotropy links the magnetization azimuthal angle inside the domain wall with the anisotropy direction in contrast to DMI, which prefers the magnetization perpendicular to the domain wall. Their balance with the energy gain due to domain wall extension defines the equilibrium magnetization the domain wall tilting. We demonstrate that the Walker field and the corresponding Walker velocity of the domain wall can be enhanced in the system supporting tilted walls.

Stabilization of exponential number of discrete remanent states with localized spin–orbit torques

Using bilayer films of β-Ta/Ni0.8Fe0.2, we fabricate structures consisting of two, three, and four crossing ellipses, which exhibit shape-induced bi-axial, tri-axial, and quadro-axial magnetic anisotropy in the crossing area, respectively. Structures consisting of N crossing ellipses can be stabilized in 2N remanent states by applying (and removing) an external magnetic field. However, we show that with field-free spin–orbit torques induced by flowing currents in individual ellipses, the number of remanent states grows to 2N. Furthermore, when the current flows between the edges of different ellipses, the number of remanent states jumps to 22N, including states that exhibit a π-Néel domain wall in the overlap area. The very large number of accessible remanent magnetic states that are exhibited by the relatively simple magnetic structures paves the way for intriguing spintronics applications including memory devices.

Efficient Control of Stochastic Switching via Spin Pumping in Antiferromagnetic Structures

Electrically controlled switching in an antiferromagnet (AFM), utilizing a currentless mechanism, is theoretically examined at finite temperatures. The structure consists of a metallic AFM with biaxial magnetic anisotropy sandwiched between a ferromagnetic spin filter and a semiconductor Schottky junction in a two-terminal pillar configuration. The calculations show that the torque necessary for the desired ${90}^{\ensuremath{\circ}}$ rotation of the N\'eel vector between two easy axes can be provided efficiently by pumping spin-polarized electrons into and out of the AFM through the metallic ferromagnetic layer. Consideration of thermal fluctuations illustrates the stochastic nature of the switching, whose probability distribution can be tailored by the electrical signal pulse as well as by the device dimensions. Detection of the N\'eel-vector state following this rotation may also be achieved straightforwardly via the large anisotropic magnetoresistance of the biaxial antiferromagnetic material. These properties, along with an ultrafast switching speed and a low energy requirement, are expected to be well suited for applications in nonvolatile memory and probabilistic computing.

Magnetic properties of a spin-2 antiferromagnet with metal-radical hybrid spins

We present an $S=2$ antiferromagnet composed of a Mn-verdazyl complex $[\mathrm{Mn}{(\mathrm{hfac})}_{2}]\phantom{\rule{0.16em}{0ex}}[o\text{\ensuremath{-}}\mathrm{Py}\text{\ensuremath{-}}\mathrm{V}\text{\ensuremath{-}}{(4\text{\ensuremath{-}}\mathrm{F})}_{2}]$. An analysis of the magnetic behavior reveals that the strong antiferromagnetic intramolecular interaction between spins on ${\mathrm{Mn}}^{2+}$ and the verdazyl radical forms an $S=2$ hybrid spin below approximately 100 K. In the low-temperature regions, the magnetic susceptibility and the specific heat indicate a phase transition to the three-dimensional order of the $S$ = 2 system. Below the transition temperature, the magnetization curve exhibits a metamagnetic phase transition. From the analysis of electron spin resonance modes, we evaluate the on-site biaxial anisotropy responsible for the anisotropic magnetic behavior.

Observation of unexpected uniaxial magnetic anisotropy in La2/3Sr1/3MnO3 films by a BaTiO3 overlayer in an artificial multiferroic bilayer.

We studied in detail the in-plane magnetic properties of heterostructures based on a ferroelectric BaTiO3 overlayer deposited on a ferromagnetic La2/3Sr1/3MnO3 film grown epitaxially on pseudocubic (001)-oriented SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 and LaAlO3 substrates. In this configuration, the combination of both functional perovskites constitutes an artificial multiferroic system with potential applications in spintronic devices based on the magnetoelectric effect. La2/3Sr1/3MnO3 single layers and BaTiO3/La2/3Sr1/3MnO3 bilayers using the pulsed-laser deposition technique. We analyzed the films structurally through X-ray reciprocal space maps and high-angle annular dark field microscopy, and magnetically via thermal demagnetization curves and in-plane magnetization versus applied magnetic field loops at room temperature. Our results indicate that the BaTiO3 layer induces an additional strain in the La2/3Sr1/3MnO3 layers close to their common interface. The presence of BaTiO3 on the surface of tensile-strained La2/3Sr1/3MnO3 films transforms the in-plane biaxial magnetic anisotropy present in the single layer into an in-plane uniaxial magnetic anisotropy. Our experimental evidence suggests that this change in the magnetic anisotropy only occurs in tensile-strained La2/3Sr1/3MnO3 film and is favored by an additional strain on the La2/3Sr1/3MnO3 layer promoted by the BaTiO3 film. These findings reveal an additional mechanism that alters the magnetic behavior of the ferromagnetic layer, and consequently, deserves further in-depth research to determine how it can modify the magnetoelectric coupling of this hybrid multiferroic system.

Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

Recent research works have shown that the magnetic order in some antiferromagnetic materials can be manipulated and detected electrically, due to two physical mechanisms: Neel-order spin-orbit torques and anisotropic magnetoresistance. While these observations open up opportunities to use antiferromagnets for magnetic memory devices, different physical characterization methods are required for a better understanding of those mechanisms. Here we report a magnetic field induced rotation of the antiferromagnetic Neel vector in epitaxial tetragonal CuMnAs thin films. Using soft x-ray magnetic linear dichroism spectroscopy, x-ray photoemission electron microscopy, integral magnetometry and magneto-transport methods, we demonstrate spin-flop switching and continuous spin reorientation in antiferromagnetic films with uniaxial and biaxial magnetic anisotropies, respectively. From field-dependent measurements of the magnetization and magnetoresistance, we obtain key material parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, spin-flop and exchange fields.

Steady one-dimensional domain wall motion in biaxial ferromagnets: Mapping of the Landau-Lifshitz equation to the sine-Gordon equation

Motivated by the difference between the dynamics of magnetization textures in ferromagnets and antiferromagnets, the Landau-Lifshitz equation of motion is explored. A typical one-dimensional domain wall in a bulk ferromagnet with biaxial magnetic anisotropy is considered. In the framework of Walker-type of solutions of steady-state ferromagnetic domain wall motion, the reduction of the non-linear Landau-Lifshitz equation to a Lorentz-invariant sine-Gordon equation typical for antiferromagnets is formally possible for velocities lower than a critical velocity of the topological soliton. The velocity dependence of the domain wall energy and the domain wall width are expressed in the relativistic-like form in the limit of large ratio of the easy-plane/easy-axis anisotropy constants. It is shown that the mapping of the Landau-Lifshitz equation of motion to the sine-Gordon equation can be performed only by going beyond the steady-motion Walker-type of solutions.

Magnetic Anisotropy of a Quasi Two-Dimensional Canted Antiferromagnet.

We report the control of the interplane magnetic exchange coupling in CaIrO3 perovskite thin films and superlattices with SrTiO3. By analyzing the anisotropic magneto-transport data, we demonstrate that a semimetallic paramagnetic CaIrO3 turns into a canted antiferromagnetic Mott insulator at reduced dimensions. The emergence of a biaxial magneto-crystalline anisotropy indicates the canted moment responding to the cubic symmetry. Extending to superlattices and probing oxygen octahedral rotation by half-integer X-ray Braggs diffraction, a more complete picture about the canted moment evolution with interplane coupling can be understood. Remarkably, a rotation of the canted moments' easy axes by 45° is also observed by a sign reversal of the in-plane strain. These results demonstrate the robustness of anisotropic magnetoresistance in revealing quasi two-dimensional canted antiferromagnets, as well as valuable insights about quadrupolar magnetoelastic coupling, relevant for designing future antiferromagnetic spintronic devices.

Incident angle dependence-reduced polarization grating performance by using optically biaxial polymer liquid crystal.

We have succeeded in forming a polarization grating whose polarization diffraction properties are extremely independent of the incident angle by using polymer liquid crystal exhibiting biaxial optical anisotropy. It is considered that the extension of the optical path length and the decrease in the effective amplitude of optical anisotropy due to oblique incidences are offset by the biaxial optical anisotropy and, as a result, the retardation is compensated. The properties of this developed device have been experimentally demonstrated and theoretically explained.

Switching of multi-state magnetic structures via domain wall propagation triggered by spin-orbit torques

Spin-orbit torques emerge as a promising method for manipulating magnetic configurations of spintronic devices. Here, we show that these torques can induce a magnetization reversal via domain wall propagation which may open new ways in developing novel spintronic devices and in particular in realizing high-density multi-level magnetic memory. Our devices are bi-layer heterostructures of Ni0.8Fe0.2 on top of β-Ta patterned in the form of two or three crossing ellipses which exhibit in the crossing area shape-induced biaxial and triaxial magnetic anisotropy, respectively. We demonstrate field-free switching between discrete remanent magnetic states of the structures by spin-orbit torques induced by flowing electrical current through one of the ellipses. We note switchings induced by the coupling between the ellipses where current flowing in one ellipse triggers a reversal in a neighboring ellipse which propagates from the center outwards. Numerical tools successfully simulate the observed coupling-induced switching using experimentally extracted parameters.

Electron spin resonance in spiral antiferromagnet linarite: Theory and experiment

We present combined experimental and theoretical investigation of the low-frequency ESR dynamics in the ordered phases of magnetic mineral linarite. This material consists of weakly coupled spin-1/2 chains of copper ions with frustrated ferro- and antiferromagnetic interactions. In zero magnetic field, linarite orders into a spiral structure and exhibits a peculiar magnetic phase diagram sensitive to the field orientation. The resonance frequencies and their field dependence are analyzed combining microscopic and macroscopic theoretical approaches and precise values of magnetic anisotropy constants are obtained. We conclude that possible realization of exotic multipolar quantum states in this material is greatly influenced by the biaxial anisotropy.

Magnetization reversal in NiFe2O4/SrTiO3 nanoheterostructures grown by laser molecular beam epitaxy

NiFe2O4/SrTiO3(001) nanoheterostructures have been fabricated by laser molecular beam epitaxy method. Surface morphology and crystal structure of Ni-ferrite films were analysed by atomic force microscopy (AFM), reflection of high energy electron diffraction (RHEED), X-ray diffraction (XRD). X-ray methods prove the presence of inverse spinel crystal structure of films that was confirmed by measurements of spectral dependence of optical polar Kerr (PMOKE) effect. Study of magnetization reversal for different orientations of magnetic field carried out by vibration sample magnetometry (VSM) and longitudinal magneto-optical Kerr effect (LMOKE) are presented. In-plane magnetization loops exhibit 90° period indicating presence of biaxial magnetic anisotropy. Asymmetry of LMOKE hysteresis loops is related to manifestation of quadratic in magnetization effects in reflection of light.

Oblique angle deposition of nickel thin films by high-power impulse magnetron sputtering.

Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy.Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS.Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.

Formation of spiral ordering by magnetic field in frustrated anisotropic antiferromagnets

We discuss theoretically phase transitions in frustrated antiferromagnets with biaxial anisotropy or dipolar forces in magnetic field applied along the easy axis at $T=0$. There are well-known sequences of phase transitions upon the field increasing: the conventional spin-flop transition and the flop of the spiral plane at strong and weak easy-axis anisotropy, respectively. We argue that much less studied scenarios can appear at moderate anisotropy in which the magnetic field induces transitions of the first order from the collinear state to phases with spiral orderings. Critical fields of these transitions are derived in the mean-field approximation and the necessary conditions are found for the realization of these scenarios. We show that one of the considered sequences of phase transitions was found in multiferroic MnWO$_4$ both experimentally and numerically (in a relevant model) and our theory reproduces quantitatively the numerical findings.

Spontaneous spin-nematic ordering in a spin-chain system

The spontaneous onset of the spin-nematic order in spin-chain systems is studied. The ordering persists due to distortions of either magnetic ions, or ligands taking part in the superexchange between magnetic ions. The spin-nematic order yields a nonzero biaxial magnetic anisotropy for the spin chain, violating the U(1) symmetry. The influence of the external magnetic field and nonzero temperature is considered. We predict how the spin-nematic ordering can manifest itself in the temperature and magnetic field behavior of magnetoacoustic characteristics.