Direction Of Magnetic Anisotropy Research Articles

Analysis of Magnetization Dynamics in NiFe Thin Films with Growth-Induced Magnetic Anisotropies

We have used angled magnetron sputter deposition with and without sample rotation to control the magnetic anisotropy in 20 nm NiFe films. Ferromagnetic resonance spectroscopy, with data analysis using a Bayesian approach, is used to extract material parameters relating to the magnetic anisotropy. When the sample is rotated during growth, only shape anisotropy is present, but when the sample is held fixed, a strong uniaxial anisotropy emerges with in-plane easy axis along the azimuthal direction of the incident atom flux. When the film is deposited in two steps, with an in-plane rotation of 90 degrees between steps, the two orthogonal induced in-plane easy-axes effectively cancel. The analysis approach enables precise and accurate determination of material parameters from ferromagnetic resonance measurements; this demonstrates the ability to precisely control both the direction and strength of uniaxial magnetic anisotropy, which is important in magnetic thin-film device applications.

60∘ and 120∘ domain walls in epitaxial BaTiO3 (111)/Co multiferroic heterostructures

We report on domain pattern transfer from a ferroelectric ${\mathrm{BaTiO}}_{3}$ substrate with a (111) orientation of the surface to an epitaxial Co film grown on a Pd buffer layer. Spatially modulated interfacial strain transfer from ferroelectric/ferroelastic domains and inverse magnetostriction in the ferromagnetic film induce stripe regions with a modulation of the in-plane uniaxial magnetic anisotropy direction. Using spin-polarized low-energy electron microscopy, we observe the formation of two distinct anisotropy configurations between stripe regions, leading to angles of ${60}^{\ensuremath{\circ}}$ or ${120}^{\ensuremath{\circ}}$ between the magnetizations of adjacent domains. Moreover, through application of a magnetic field parallel or perpendicular to these stripes, head-to-head or head-to-tail magnetization configurations are initialized. This results in four distinct magnetic domain-wall types associated with different energies and domain widths, which, in turn, affects whether domain pattern transfer can be achieved.

Magnetization dynamics in layered systems with coexisting bilinear and biquadratic interlayer exchange coupling

Important aspects of exchange-coupled magnetic layered structures are related to the noncollinear arrangement of sublayer magnetizations which can arise from competition between bilinear (BL) and biquadratic (BQ) interlayer exchange coupling (IEC). In this work, the influence of coexisting BL and BQ IEC of different strengths on magnetization precession in layered systems is investigated both experimentally and theoretically. Laser-induced magnetization precession has been studied in the $\text{Fe/Si}({d}_{\text{Si}}$) multilayers (MLS) as a function of the amplitude ($H$) and orientation angle (${\ensuremath{\theta}}_{H}$) of external magnetic field using time-resolved magneto-optical Kerr (TRMOKE) effect. Strongly changing characters of precession frequency dependencies $\ensuremath{\omega}(H,{\ensuremath{\theta}}_{H})$ for Fe sublayer thickness ${d}_{\text{Fe}}=3$ nm and Si spacer-layer thicknesses (${d}_{\text{Si}}$) varying in the range of 0.9--2.4 nm have been observed. Analytical formulas for acoustic and optic mode dispersion relations with coexisting BL and BQ IEC, scaled by ${J}_{1}$ and ${J}_{2}$ parameters, respectively, for the in-plane effective magnetic anisotropy and arbitrary magnetic field direction were derived, and very good agreement with the experimentally observed frequency dependencies has been obtained. It is shown that BQ coexisting with BL IEC significantly influences on the magnitude and form of dispersion relations. From analytical formula derived, it follows that zero-field optical mode frequency tends to zero as $|{J}_{1}|$ approaches $2{J}_{2}$. The acoustic and optic mode-crossing effect has been observed and it is found that values of crossing fields and frequency gaps strongly increase as ${\ensuremath{\theta}}_{H}$ angles decrease and depend on relative BL and BQ IEC strengths. The BL IEC is of ferromagnetic type with ${J}_{1}\ensuremath{\approx}1.6$ mJ/${\mathrm{m}}^{2}$ for the MLS with ${d}_{\text{Si}}=0.9$ nm, and changes to antiferromagnetic one with ${J}_{1}\ensuremath{\approx}\ensuremath{-}0.9$ mJ/${\mathrm{m}}^{2}$ for the MLS with ${d}_{\text{Si}}=1.4$ nm, while the ${J}_{2}$ parameter of BQ IEC decreases from 1.8 to 1.0 mJ/${\mathrm{m}}^{2}$. The coupling strengths decrease by one to two orders of magnitude for the sample with ${d}_{\text{Si}}=2.4$ nm, but both mode frequencies are still observed and well reproduced by the theory. It is shown that ${J}_{1}$ and ${J}_{2}$ parameters obtained in the TRMOKE experiment coincide within the estimated error bars with the determined from independent measurements of magnetization processes in the static magneto-optical Kerr effect and interpreted with the use of analytical formulas derived. Numerical solutions of coupled Landau-Lifshitz-Gilbert (LLG) equations for acoustic and optic modes, with inclusion of BL IEC, intrinsic Gilbert damping, and spin-pumping damping terms, and extended to include BQ IEC, were performed and fitted to experimental data. It is shown that determined effective damping coefficients on $H$ and ${\ensuremath{\theta}}_{H}$ dependencies for acoustic and optic modes are very well simulated with the use of LLG equation solutions with Gilbert damping, spin-pumping-damping--related effective spin-mixing conductance, and spin-diffusion length parameters included. The dependencies of the parameters on ${d}_{\text{Si}}$ spacer-layer thickness are discussed and compared with available data for other systems.

Study of ordering in 2D ferromagnetic nanoparticles arrays: Computer simulation

<abstract> <p>This article describes ordering in a 2D ferromagnetic nanoparticles array by computer simulation. The Heisenberg model simulates the behavior of spins in nanoparticles. Nanoparticles interact using dipole-dipole forces. Computer simulations use the Monte Carlo method and Metropolis algorithm. Two possible types of ordering for the nanoparticles' magnetic moments are detected in the system. The magnetic anisotropy direction for the nanoparticles determines the type of ordering. If the anisotropy direction is oriented perpendicular to the substrate plane, then a superantiferromagnetic phase with staggered magnetization is realized. If the magnetic anisotropy is oriented in the nanoparticle plane, the superantiferromagnetic phase has a different structure. The nanoparticle array is broken into chains parallel to the anisotropy orientations. In one chain of nanoparticles, magnetic moments are oriented in the same way. The magnetic moments of the nanoparticles are oriented oppositely in neighbor chains. The temperature of phase transitions is calculated based on finite dimensional scaling theory. Temperature depends linearly on the intensity of the dipole-dipole interaction for both types of superantiferromagnetic transition.</p> </abstract>

Controlling the Magnetoimpedance Property of Thin-Film Elements Using Joule Heating

Optimizing the performance of magnetoimpedance (MI) requires controlling the anisotropy of material which is often done with field annealing. However, field annealing needs large-scale equipment with a vacuum environment and consumes a large amount of electricity for heating and generating a large magnetic field. Conversely, Joule heating has advantages of simple, compact, and possible-to-use small current. Thus, we attempted controlling the magnetic anisotropy on thin-film MI elements using Joule heating and the applied field by magnets. Before annealing, the element has an easy axis with a direction parallel to the longitudinal direction of the element owing to shape anisotropy. After Joule heating, the MI behavior showed a double peak profile, which is a typical impedance profile when large impedance changes are obtained. The peak height in the impedance and inductance increases with the heating current up to 100 mA and then becomes constant or slightly increases. The field at which the impedance and inductance have a peak increases monotonically with increasing heat current. We confirm that the magnetic anisotropy direction changes with Joule heating and with a field applied by magnets based on observing the magnetic domain structure using a Kerr effect microscope.

A new method for obtaining the magnetic shape anisotropy directly from electron tomography images.

A new methodology to obtain magnetic information on magnetic nanoparticle (MNP) systems via electron tomography techniques is reported in this work. The new methodology is implemented in an under-development software package called Magn3t, written in Python and C++. A novel image-filtering technique that reduces the highly undesired diffraction effects in the tomography tilt-series has been also developed in order to increase the reliability of the correlations between morphology and magnetism. Using the Magn3t software, the magnetic shape anisotropy magnitude and direction of magnetite nanoparticles has been extracted for the first time directly from transmission electron tomography.

Observation of magnetic domain patterns with tilted uniaxial anisotropy using a single-spin magnetometer

Ferromagnets with uniaxial magnetocrystalline anisotropy have been a significant platform to explore applications in magnetic devices. Although it is commonly recognized that the magnetic anisotropy is crucial for the formation of noncollinear domain patterns, the effects of the anisotropy direction are usually neglected in recent studies. Here, we imaged stray fields generated by a polycrystalline MnNiGa sample with a nitrogen-vacancy color center in diamond and found that various stripe domains and magnetic bubbles exist in different regions. These domain patterns could be explained by the difference in anisotropy directions among crystallites via micromagnetic simulation. It can be inferred that the magnetic anisotropy direction plays a key role in energetically favorable domain structures based on our results. The magnetic anisotropy direction may be exploited as an ingredient to modulate domains and develop devices.

Upper Limit of Carbon Concentration in Ferromagnetic L1₀-Ordered FePt-C for Tb/in² Data Storage Density Heat-Assisted Magnetic Recording Media

In high-density magnetic recording media, magnetically isolated grains are required to increase the signal-to-noise ratio (SNR). Carbon can be used to isolate FePt grains enabling their grain size smaller than 4.3 nm. Carbon atoms segregate to the boundaries during growth and provide an exchange-breaking layer, however, some other carbon atoms remain dissolved in the magnetic alloy. To identify the upper limit of carbon concentration in $L 1_{0}$ -ordered (Fe0.5Pt0.5)100– x C x , first-principles calculations are performed based on the density functional theory (DFT). The Brillouin function and Callen–Callen empirical relation determine the temperature-dependent magnetization and magneto-crystalline anisotropy energy enabling the determination of magnetic properties and Curie temperature required by 4 Tb/in2 heat-assisted magnetic recording (HAMR) media and beyond. The calculated magnetization ( $M_{s}$ ) of $L 1_{0}$ -ordered (Fe0.5Pt0.5)100– x C x decreases to 770 emu/cm3 at $x =20$ from 1030 emu/cm3 at $x = 0$ at 300 K, and the magnetocrystalline anisotropy constant ( $K_{u}$ ) to 2.05 MJ/m3 at $x =20$ from 15.48 MJ/m3 at 300 K. It is striking to find that the Curie temperature ( $T_{C}$ ) increases to 728 K at $x =20$ from 719 K at $x =0$ . Regardless of carbon concentration, the magnetic anisotropy direction is the out-of-plane. Combining $M_{s}$ and $K_{u}$ at 300 K with $T_{C}$ , the $M_{s}$ – $K_{u}$ –C concentration relation is plotted to guide the design of $L 1_{0}$ -ordered Fe-Pt film for Tb/in2 recording media. It is found that the upper limit of carbon concentration is determined to be about 12 at.% to retain $M_{s} \ge800$ emu/cm3, $T_{C} \ge430$ K, and $K_{u} \ge 5$ MJ/m3, which are necessary to achieve areal densities of 4 Tb/in2 and beyond.

Investigating the magnetic and atomic interface configuration for a model Fe/CrN bilayer system

A bilayer of iron on chromium nitride (Fe/CrN) is an interesting system for exchange biasing and sensing applications as the Néel temperature of CrN is 280 K and the Curie temperature of Fe is 1043 K. In this paper, we study the crystal and magnetic structures of the Fe/CrN interface at the atomic level. High quality epitaxial Fe/CrN bilayers prepared by molecular beam epitaxy grow in 001 orientation on MgO(001) substrates with uniform layer thicknesses and sharp interfaces. Our data reveal the epitaxial correlation between Fe and CrN crystals and their magnetic structures at the interface. The magnetic anisotropy directions of Fe and CrN are found parallel to [110]MgO. We studied the electronic and magnetic properties of the interface by performing the first-principles total-energy calculations. We present a model that combines the crystal and magnetic structures of the Fe/CrN bilayer and fully explains all results.

Significant enhancement of magnetic anisotropy and conductivity in GaN/CrI3 van der Waals heterostructures via electrostatic doping

van der Waals magnetic heterostructures, consisting of a wide band-gap nitride semiconductor and an intrinsic ferromagnetic semiconductor, are potentially useful for low-dimensional spintronic field-effect transistors. However, there is a significant challenge. For instance, the integration often leads to a decreased Curie temperature, the magnetic anisotropy direction change, and low conductivities. Here, we employ the first-principles density-functional method to systematically investigate the electronic and magnetic properties of the $\mathrm{GaN}/{\mathrm{CrI}}_{3}$ van der Waals heterostructures under electrostatic doping. Though the easy magnetization axis of the monolayer ${\mathrm{CrI}}_{3}$ transforms from out-of-plane to in-plane direction over 0.2 electron doping per unit cell, the $\mathrm{GaN}/{\mathrm{CrI}}_{3}$ van der Waals heterostructure maintains perpendicular anisotropy under electron doping, crucial for high-density information storage. The Curie temperature of the $\mathrm{GaN}/{\mathrm{CrI}}_{3}$ van der Waals heterostructure can be enhanced under both electron and hole doping. The $\mathrm{GaN}/{\mathrm{CrI}}_{3}$ van der Waals heterostructure presents half-metallic properties and the spin-up conductivities are much larger than that of the monolayer ${\mathrm{CrI}}_{3}$ under the same electrostatic doping. Our results indicate that constructing heterostructures with ferromagnets and nonmagnetic semiconductors is an effective strategy for developing high-performance field-effect transistors.

Role of exchange splitting and ligand-field splitting in tuning the magnetic anisotropy of an individual iridium atom on TaS2 substrate

In this work, using first-principle calculation we investigate the magnetic anisotropy (MA) of single-atom iridium (Ir) on TaS2 substrate. We find that the strength and direction of MA in the Ir adatom can be tuned by strain. The MA arises from two sources, namely spin-conservation term and spin-flip term. The spin-conservation term is generated by spin-orbit coupling (SOC) interaction on dxy/dx2-y2 orbitals and is contributed to the out-of-plane MA. The spin-flip term is caused by SOC interaction on dxz/dyz and px/py orbitals and is responsible for the in-plane MA. We further find that strain-tuned MA is mainly determined by exchange splitting and ligand field splitting. Increase of strain will enhance the exchange splitting and reduce the ligand field splitting, resulting in the enhancement of the in-plane MA from dxy/dx2-y2 orbitals and the reduction of the out-of-plane MA from dxz/dyz and px/py orbitals, and hence leading to the change of the strength and direction of the total MA. Our study provides a way for tuning the MA of single-atoms magnet on 2D transition metal dichalcogenides substrate by control of the exchange splitting and the ligand field splitting.

Thickness induced magnetic anisotropic properties of Tb-Fe-Co thin films

The influence of Tb25Fe61Co14 thin film thicknesses varying from 2 to 300 nm on the structural and magnetic properties has been systematically investigated by using of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, magnetization, and magneto-optic Kerr effect microscopy measurements. Thin film growth mechanism is pursued and controlled by ex-situ X-ray refractometry measurements. X-ray diffraction studies reveal that the Tb25Fe61Co14 films are amorphous regardless of thin films thicknesses. The magnetic properties are found to be strongly related to thickness and preferred orientation. With an increase in film thickness, the easy axis of magnetization is reversed from in-plane to out-of-plane direction. The change in the easy axes direction also affects the remanence, coercivity and magnetic anisotropy values. The cause for the magnetic anisotropy direction change from in-plane to out-of-plane can be related to the preferred orientation of the thin film which depends on the large out-of-plane coercivity and plays an important role in deciding the easy axes direction of the films. According to our results, up to the 100 nm in-plane direction is dominated over the whole system under major Fe-Fe interaction region, after that point, the magnetic anisotropy direction change to the out-of-plane under major Tb-Fe/Tb-Co interaction region and preferred orientation dependent perpendicular magnetic anisotropic properties become more dominated with 2.7 kOe high coercive field values.

Orientation control of optical mode ferromagnetic resonance: From uniaxial to omni-directional

Both high ferromagnetic resonance frequency (fr) and homogeneous angular performance are important for soft magnetic films to be used in high-frequency integrated circuit devices. However, high fr are obtainable only along the easy axis direction of the magnetic anisotropic materials. In uniaxially anisotropic FeCoB/Ru/FeCoB films, we could obtain an ultrahigh optical mode ferromagnetic resonance frequency (frO) up to 19.16 GHz along the easy axis under a self-bias field due to the enhancement from strong interlayer exchange coupling. However, the uniaxial intensity distribution of optical mode resonance seriously hinders the practical application in microwave components. In order to obtain the desired homogeneous angular performance, soft magnetic films with widely distributed magnetic anisotropy directions in the films were prepared and a nearly omni-directional frO with uniform values around 13.5 GHz was achieved. This study demonstrates that controlling the magnetic anisotropy's angular distribution is an effective way to obtain isotropic, self-bias magnetic films with ultrahigh fr.

Carrier Concentration of CeFe<sub>2</sub>Al<sub>10</sub> as a Candidate of Thermoelectric Material

The study of Hall effect of Kondo semiconductor CeFe2Al10 is reported as a candidate of thermoelectric material used at low temperatures. Single crystals of CeFe2Al10 with orthorhombic crystal structure were grown by Al self-flux method. An anisotropy of the Hall effect is clarified by measuring Hall resistance by changing the direction of electrical current, magnetic field, and voltage respect to all the three crystal axes of orthorhombic crystal structure. The Hall effect of CeFe2Al10 has a strong anisotropy against the direction of magnetic field but weak anisotropy against the directions of current and voltage. The value of carrier concentration indicates that CeFe2Al10 is matallic, which causes a low performance as a thermoelectric material. In order to improve the value of dimensionless figure of merit, the electrons should be doped to CeFe2Al10.

Diffusion-regularized susceptibility tensor imaging (DRSTI) of tissue microstructures in the human brain.

Susceptibility tensor imaging (STI) has been proposed as an alternative to diffusion tensor imaging (DTI) for non-invasive in vivo characterization of brain tissue microstructure and white matter fiber architecture, potentially benefitting from its high spatial resolution. In spite of different biophysical mechanisms, animal studies have demonstrated white matter fiber directions measured using STI to be reasonably consistent with those from diffusion tensor imaging (DTI). However, human brain STI is hampered by its requirement of acquiring data at more than 10 head rotations and a complicated processing pipeline. In this paper, we propose a diffusion-regularized STI method (DRSTI) that employs a tensor spectral decomposition constraint to regularize the STI solution using the fiber directions estimated by DTI as a priori. We then explore the high-resolution DRSTI with MR phase images acquired at only 6 head orientations. Compared to other STI approaches, the DRSTI generated susceptibility tensor components, mean magnetic susceptibility (MMS), magnetic susceptibility anisotropy (MSA) and fiber direction maps with fewer artifacts, especially in regions with large susceptibility variations, and with less erroneous quantifications. In addition, the DRSTI method allows us to distinguish more structural features that could not be identified in DTI, especially in deep gray matters. DRSTI enables a more accurate susceptibility tensor estimation with a reduced number of sampling orientations, and achieves better tracking of fiber pathways than previous STI attempts on in vivo human brain.

Effective control of the magnetic anisotropy in ferromagnetic MnBi micro-islands

This work shows the possibility of tailoring the magnetic anisotropy direction in ferromagnetic LTP-MnBi thin films from out-of-plane to in-plane while also making possible for the first time the growth of quasi-hexagonal LTP-MnBi micro-islands. This has been managed with no need to apply any external magnetic field, but only by controlling the temperature growth. The rf sputtering of a Mn–Bi composite target produces highly crystalline <002> oriented micro-islands with a size distribution in the micrometer range (up to 80 μm) for samples grown at 300–425 K and <110> oriented elongated threads at 475 K. The <002> oriented films show perpendicular magnetic anisotropy while the <110> oriented ones show in-plane anisotropy. High values of coercivity have been obtained, reaching maximum values of 16.0 and 19.3 kOe at 300 and 400 K, respectively, for these LTP-MnBi thin films. Micromagnetic simulations have shown a clear correlation between the microstructure and the resulting magnetic properties. The route presented here represents a first step towards the possible fabrication of MnBi thin film micromagnets with customized magnetic anisotropy direction based on the deposition temperature.

Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing

Metastable face-centered-cubic Fe78Ni22 thin films grown on Cu(001) substrates are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxial magnetic in-plane anisotropy. The magnetocrystalline anisotropy in transformed areas is strong enough to stabilize the magnetization in transverse direction to the long axis of narrow waveguides. Therefore, it is possible to propagate spin waves in these waveguides without the presence of an external magnetic field in the favorable Demon-Eshbach geometry. Phase-resolved micro-focused Brillouin light scattering yields the dispersion relation of these waveguides in zero as well as in nonzero external magnetic fields.

Strain tuned electronic and magnetic anisotropy in SrTiO3 (110) surface

In this work, the density functional theory (DFT) calculations are employed to investigate the effect of in-plane strain on the electronic and magnetic anisotropy of SrTiO-terminated SrTiO3 (110) surface. The two-dimensional electron gas (2DEG) at the surface comes from the surface Ti 3d orbitals and exhibits anisotropic transport properties. The ordering of the lowest conduction band states can be tuned by in-plane strain. In addition, the in-plane strain can effectively modify and reduce the electron effective mass of the SrTiO3 (110) surface, and the effect of the in-plane strain on the two directions of [001] and [11¯0] is different. The magnetic moment of the unstrained surface Ti atom is 0.342 μB. The magnetic moment increases monotonically with the increasing of in-plane tensile strain and decreases quickly with increasing of the in-plane compressive strain. Importantly, the in-plane strain can change the magnetic moment of the system as well as its magnetic anisotropy energy and direction of easy magnetization axis. In-plane tensile strain causes a switch of the magnetic easy axis between parallel and perpendicular to the STO (110) surface.

Study of Peculiarities of the Microwave Absorption Spectrum of Nanocrystalline Thin Magnetic Films

Based on the micromagnetic model which takes into account the random distribution of the uniaxial magnetic anisotropy directions in crystallites of a nanocrystalline film, an effective method has been implemented for calculation of the magnetization dynamics in microwave fields. For a certain range of crystallite sizes, when the energy of the random magnetic anisotropy is comparable to the exchange energy, a significant change of the ferromagnetic resonance field, broadening of the resonance line, and the appearance of an asymmetry in the shape of the resonance curve were found. With an increase of the crystallite sizes, the resonance field first grows, then, it quickly decreases to its minimum, and then, it grows again to reach saturation. In this case, the steepness of the left slope of the broadening resonance curve first decreases faster than that of the right slope, leading to the symmetry breaking of the resonance curve shape, then, the curve becomes symmetrical again, and then, the steepness of the left slope becomes greater than that of the right slope.

Additive manufacturing of soft magnetic permalloy from Fe and Ni powders: Control of magnetic anisotropy

Abstract The influence of the process parameters in Laser Beam Melting (LBM) on the element distribution and magnetic properties of permalloy ( Ni 78.5 Fe 21.5 ) is studied. Iron and nickel powders are mixed in the respective proportions to build twenty-five permalloy samples. The process parameters for each sample are varied to achieve different volume energy densities. An increase of the saturation magnetization MS up to 14% of the samples with respect to the initial powder blend is found. For a volume energy density of 428 J mm 3 we detect a stripe-like segregation of iron and nickel in the uppermost layer. In the volume a homogeneous element distribution is found. The segregation at the surface leads to a sizable uniaxial magnetic anisotropy. When using parameter combinations resulting in similar volume energy densities, we observe different surface morphologies depending on scan speed and laser power. The implications for creating tailored magnetic anisotropy directions in Fe-Ni soft magnets are discussed.