Magnetic Anisotropy Research Articles

Occupation distribution analysis of Co in La-Co substituted M-type hexaferrite and its significant effect on magnetic properties

In recent years, La-Co co-substituted M-type strontium hexaferrite (La-Co SrM) has gained significant attention due to its improved permanent magnet performance and elevated operating temperature. However, the occupation of Co ions within the lattice of La-Co SrM has sparked considerable debate over the past two decades. To address this, neutron powder diffraction and Raman spectroscopy were employed in this study to analyze Co occupancy in La-Co SrM single crystal and polycrystalline samples. Results consistently reveal a preference for most Co ions to occupy the spin-down tetrahedral 4f1 site, which enhances the net magnetic moment and uniaxial magnetic anisotropy. Differences in Co ion distribution between single crystals (octahedral 12k and 2a sites) and polycrystalline powders (bipyramidal 2b sites) are attributed to sample preparation conditions, often overlooked in previous discussions. Therefore, precise regulation of Co occupancy, particularly maximizing the occupation of the 4f1 site, proves pivotal in enhancing magnetic properties.

Spin waves in antiferromagnetically coupled bilayers of transition-metal dichalcogenides with Dzyaloshinskii–Moriya interaction

In this paper we analyse quantized spin waves (also referred to as magnons) in bilayers of two-dimensional van der Waals materials, like Vanadium-based dichalcogenides, VX2 (X = S, Se, Te) and other materials of similar symmetry. We assume that the materials exhibit Dzyaloshinskii–Moriya interaction and in-plane easy-axis magnetic anisotropy due to symmetry breaking induced externally (e.g. by strain, gate voltage, proximity effects to an appropriate substrate/oberlayer, etc.). The considerations are limited to a collinear spin ground state, stabilized by a sufficiently strong in-plane magnetic anisotropy. The theoretical analysis is performed within the general spin wave theory based on the Holstein–Primakoff–Bogoliubov transformation. Accordingly, the description takes into account quantum antiferromagnetic fluctuations. However, it is limited to linear spinwave modes. The Dzyaloshinskii–Moriya interaction is shown to modify the spin wave spectrum of the bilayers, making its low energy part qualitatively similar to the electronic spectrum of the Rashba spin–orbit model.

Spin and lattice dynamics of the two-dimensional van der Waals ferromagnet CrI3

Chromium tri-iodide (CrI3) is a prototypical ferromagnetic van der Waals insulator with its genuinely two-dimensional (2D) long-range magnetic order below 45 K demonstrated recently. The underlying magnetic anisotropy has not been completely understood while both the Dzyaloshinskii-Moriya (DM) interaction and the Kitaev−Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Gamma$$\\end{document} type interaction have been considered as the relevant magnetic Hamiltonian. In addition, the relation between the crystal structure and the magnetic order needs to be further elucidated concerning their possible coupling in few-layer samples and in the topmost surface layers of bulk samples. Here, we investigate these issues via temperature- and magnetic field-dependent terahertz spectroscopy on bulk CrI3 single crystals, focusing on the dynamics of ferromagnetic resonances (FMRs) and optical phonons in the terahertz (THz) region from 4 to 120 cm−1 (from 0.5 to 15 meV). We narrow down the possible ranges of the interaction parameters such as the off-diagonal symmetric terms and the single-ion anisotropy. The accurate values of these parameters significantly constrain the magnitude of possible Kitaev−Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Gamma$$\\end{document} exchange interaction and the topological magnon gap. Moreover, the structure-magnetism relationship was critically analyzed based on the temperature- and field-dependences of two Eu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\rm{E}}}_{{\\rm{u}}}$$\\end{document} in-plane optical phonon modes, which shows that the commonly believed structural phase diagram of CrI3, derived from surface-preferential data, has to be seriously modified.

Microstructural study with enhanced dielectric and magnetic properties of Cu1-xCdxFe2O4 nanoparticles prepared at low-temperature green-synthesized technique

The green elaboration tool has taken much consideration due to its consistent and economical ways of producing pure and regular dispersed NPs. The preparation tool is a simple, environmentally friendly, and cost-effective method that does not use toxic chemicals. Therefore, the biological mechanisms themselves act as a reducing agent that prevents, on the surface of nanoparticles, the adsorption of chemical agents. Therefore, we focus in this work on the bio-synthesizes and the characterization (crystallographic, magnetic, transport, and dielectric study) of Cu1-xCdxFe2O4(x = 0.0, 0.3 and x = 0.5) spinel ferrites. X-ray diffraction, transmission electron microscopy (TEM), and photoelectron spectroscopy (XRD and XPS) have been employed to study crystallographic characteristics and chemical bond formation. Surface parameters analysis was conducted using BET isotherms and validated the decreases in specific surface area in the substitution sample. The electrical investigation proves the semiconductor behavior, and the thermal activation are dominated by the small polaron hopping mechanism in the nano-ferrites Cu1-xCdxFe2O4. In addition, the prepared NPS exhibits low dielectric losses. Using the ZFC-FC and M(H) measurement techniques, important modifications in the magnetic behaviors have been detected in the elaborated nanoparticles. Accordingly, the substitution of Cu1-xCdxFe2O4 leads to transitions from ferromagnetic to superparamagnetic behaviors, the increase of the magnetic saturation, and decreased magnetic anisotropy has been shown in Cu0·7Cd0·3Fe2O4 spinel structure.

Observation of pressure-induced variation in magnetization and magnetic anisotropy for half-doped La0.5Ca0.5MnO3 nanoparticles

The effects of pressure on the magnetic properties of half-doped La0.5Ca0.5MnO3(LCMO) were investigated. Pressure was found to decrease the transition temperature (Tc) from the paramagnetic (PM) phase to the ferromagnetic (FM) phase, from 226 K to 223 K. Moreover, the pressure was also involved in decreasing the transition temperature (TN) for the FM-antiferromagnetic (AFM) phase from 180 K to 175 K. Below TN, the inhomogeneous AFM phase, where both the FM and AFM phases coexist, is observed in LCMO. Pressure induces changes in the zero-field-cooled magnetization (ΔMZFC) exhibiting ΔMZFC ≈ 0 in the PM region above 285 K.

Imaging nanomagnetism and magnetic phase transitions in atomically thin CrSBr

Since their first observation in 2017, atomically thin van der Waals (vdW) magnets have attracted significant fundamental, and application-driven attention. However, their low ordering temperatures, Tc, sensitivity to atmospheric conditions and difficulties in preparing clean large-area samples still present major limitations to further progress, especially amongst van der Waals magnetic semiconductors. The remarkably stable, high-Tc vdW magnet CrSBr has the potential to overcome these key shortcomings, but its nanoscale properties and rich magnetic phase diagram remain poorly understood. Here we use single spin magnetometry to quantitatively characterise saturation magnetization, magnetic anisotropy constants, and magnetic phase transitions in few-layer CrSBr by direct magnetic imaging. We show pristine magnetic phases, devoid of defects on micron length-scales, and demonstrate remarkable air-stability down the monolayer limit. We furthermore address the spin-flip transition in bilayer CrSBr by imaging the phase-coexistence of regions of antiferromagnetically (AFM) ordered and fully aligned spins. Our work will enable the engineering of exotic electronic and magnetic phases in CrSBr and the realization of novel nanomagnetic devices based on this highly promising vdW magnet.

Energy Barriers for the Spontaneous Magnetization Reversal of the Atomic Co Chains on Pt(664) Surface in the Model with Dzyaloshinskii–Moriya Interaction

The analytical approach has been developed in the framework of the continuous XY-model. This approach allows calculating the spontaneous magnetization reversal time of finite-length atomic chains on the metallic surface. The interaction of the magnetic moments of atoms is described by the classical Hamiltonian, which includes the Heisenberg exchange interaction, the Dzyaloshinskii–Moriya interaction, and the magnetic anisotropy energy. Using the Co/Pt(664) system as an example, it has been shown that the proposed method is in a good agreement with the results of the numerical simulation in the limit of short and long atomic chains. And for atomic chains of intermediate length, it can be used to estimate an upper bound on the spontaneous magnetization reversal time. We obtained the dependences of the spontaneous magnetization reversal time of finite-length Co chains the value of the exchange integral, parameters of the magnetic anisotropy, and also on the value of the projection of the Dzyaloshinskii vector onto the axis perpendicular to the plane containing the magnetic moments of the atoms. It is shown that the proposed method has a wide range of applicability both in terms of temperature and the values of the physical parameters characterizing the magnetic properties of the atomic chains.

Skyrmion crystal formation and temperature–magnetic-field phase diagram of the frustrated triangular lattice Heisenberg magnet with easy-axis magnetic anisotropy

Skyrmion crystal formation and temperature–magnetic-field phase diagram of the frustrated triangular lattice Heisenberg magnet with easy-axis magnetic anisotropy

Thermal Effects on Domain Wall Stability at Magnetic Stepped Nanowire for Nanodevices Storage.

In the future, DW memory will replace conventional storage memories with high storage capacity and fast read/write speeds. The only failure in DW memory arises from DW thermal fluctuations at pinning sites. This work examines, through calculations, the parameters that might help control DW thermal stability at the pinning sites. It is proposed to design a new scheme using a stepped area of a certain depth (d) and length (λ). The study reveals that DW thermal stability is highly dependent on the geometry of the pinning area (d and λ), magnetic properties such as saturation magnetization (Ms) and magnetic anisotropy energy (Ku), and the dimensions of the nanowires. For certain values of d and λ, DWs remain stable at temperatures over 500 K, which is beneficial for memory applications. Higher DW thermal stability is also achieved by decreasing nanowire thickness to less than 10 nm, making DW memories stable below 800 K. Finally, our results help to construct DW memory nanodevices with nanodimensions less than a 40 nm width and less than a 10 nm thickness with high DW thermal stability.

Interface Structures on Mechanically Polished Surface of Spinel Ferrite and Its Effect on the Magnetic Domains.

Soft magnetic spinel ferrites are indispensable parts in devices such as transformers and inductors. Mechanical surface processing is a necessary step to realize certain shapes and surface roughness in producing the ferrite but also has a negative effect on the magnetic properties of the ferrite. In the past few years, a new surface layer was always believed to form during the mechanical surface processing, but the change of atomic structure on the surface and its effect on the magnetic structure remain unclear. Herein, an interface structure consisting of a rock-salt sublayer, distorted NiFe2O4 sublayer, and pristine NiFe2O4 was found to form on mechanically polished single-crystal NiFe2O4 ferrite. Such an interface structure is produced by phase transformation and lattice distortion induced by the mechanical processing. The magnetic domain observation and electrical property measurement also indicate that the magnetic and electrical anisotropy are both enhanced by the interface structure. This work provides deep insight into the surface structure evolution of spinel ferrite by mechanical processing.

Spin–wave dynamics in perpendicularly magnetized antidot multilayers

Using all-optical time-resolved magneto-optical Kerr effect measurements we demonstrate an efficient modulation of the spin–wave (SW) dynamics via the bias magnetic field orientation around nanoscale diamond shaped antidots that are arranged on a square lattice within a [Co(0.75 nm)/Pd(0.9 nm)]8 multilayer with perpendicular magnetic anisotropy (PMA). Micromagnetic modeling of the experimental results reveals that the SW modes in the lower frequency regime are related to narrow shell regions around the antidots, where in-plane (IP) domain structures are formed due to the reduced PMA, caused by Ga+ ion irradiation during the focused ion beam milling process of antidot fabrication. The IP direction of the shell magnetization undergoes a striking change with magnetic field orientation, leading to the sharp variation of the edge localized (shell) SW modes. Nevertheless, the coupling between such edge localized and bulk SWs for different orientations of bias field in PMA systems gives rise to interesting Physics and attests to new prospects for developing energy efficient and hybrid-system-based next-generation nanoscale magnonic devices.

Control of Anisotropy and Magnetic Hyperthermia Effect by Addition of Cobalt on Magnetite Nanoparticles.

Magnetic hyperthermia (MH) has emerged as a promising technology with diverse applications in medical and technological fields, leveraging the remote induction of temperature elevation through an alternating magnetic field. While Fe3O4 nanoparticles with an average size around 12-25 nm are commonly employed in MH systems, this study introduces a strategy to produce smaller particles (less than or equal to 10 nm) with enhanced heating efficiency, as measured by specific power absorption (SPA). We conducted an exhaustive and detailed investigation into the morphological and magnetic properties of CoxFe3-xO4 nanoparticles, aiming to optimize their MH response. By varying the Co content, we successfully tuned the effective magnetic anisotropy while maintaining saturation magnetization nearly constant. The MH analysis indicates that these nanoparticles predominantly heat through the Néel mechanism, demonstrating robust reproducibility across different concentrations, viscosity mediums, and ac field conditions. Notably, we identified an optimal anisotropy or Co concentration that maximizes SPA, crucial for developing magnetic systems requiring particles with specific sizes. This work contributes to advancing the understanding and application of MH, particularly in tailoring nanoparticle properties for targeted and efficient heat generation in various contexts.

Experimental and computational Insights Into the magnetic anisotropy and magnetic behaviour of layered room-temperature ferromagnet Cr1.38Te2

We investigate the structural, magnetocrystalline anisotropy, critical behaviour, and magnetocaloric effect in the layered room-temperature monoclinic ferromagnet Cr1.38Te2. The critical behavior is studied by employing various techniques such as the modified Arrott plot (MAP), the Kouvel-Fisher method (KF), and the critical isothermal analysis (CI) around the Curie temperature (T C ) of 316 K. The derived critical exponents are self-consistent and obey the rescaling analysis. The Monte-Carlo simulations reproduce the experimentally obtained critical exponents. However, the derived critical exponents do not suggest any single universality class of the magnetic interactions. On the other hand, the renormalization group (RG) theory suggests 3D-Ising type long-range exchange interactions [J(r)], decaying with distance (r) as J(r) = r −(d+σ) = r −4.73. Further, magnetocrystalline anisotropy energy density (K u ) is found to be temperature dependent. The ground state magnetic easy-axis (b-axis) is identified by analyzing the magnetocrystalline anisotropy energy (MAE) using the density functional theory calculations. Maximum entropy change - ΔSmmax ≈2.51 J/kg-K is found near the T C .

Giant Room-Temperature Topological Hall Effect in a Square-Net Ferromagnet LaMn2Ge2.

A topological magnetic material showcases a multitude of intriguing properties resulting from the compelling interplay between topology and magnetism. These include notable phenomena such as a large anomalous Nernst effect (ANE), an anomalous Hall effect (AHE), and a topological Hall effect (THE). In most cases, topological transport phenomena are prevalent at temperatures considerably lower than room temperature, presenting a challenge for practical applications. However, the noncollinear ferromagnetic (FM) LaMn2Ge2, characterized by a Mn square-net lattice and a notably high Curie temperature (TC) of approximately 325 K, defies this trend as a topological semimetal. This work observes a giant topological Hall resistivity, , of ≈4.5 µΩ cm at room temperature when the angle between the applied field and the c-axis is 75°, which is significantly higher than state-of-the-art materials with noncoplanar spin structures. The single crystal neutron diffraction measurements agree with an incommensurate conical magnetic structure as the ground state. This observation suggests the enhanced spin chirality resulting from the noncoplanar spin configuration when the applied field is away from the magnetic easy axis as the origin of a large contribution to the observed THE. The findings unequivocally demonstrate that the FM LaMn2Ge2 holds great promise as a potential topological semimetal for spintronic applications even at room temperature.

First-principles study of the valley-polarized quantum anomalous Hall effect in TiBrTe monolayers

Valley-polarized quantum anomalous Hall effect is significant for quantum information processing based on the valley degree of freedom, but its implementation in two-dimensional materials is currently challenging. Through first-principle calculations, we investigate the electronic structure, magnetic and topological properties of the TiBrTe monolayer. TiBrTe monolayer is an intrinsic ferrovalley semiconductor with an in-plane easy axis of magnetization and a high Curie temperature of 569 K. The valley polarization in the TiBrTe monolayer can be adjusted or flipped by manipulating the magnetization orientation of the magnetic atoms. The electronic correlation effect can be used to manipulate the different energy gaps of K and -K valleys. A topological phase transition occurs with increasing electronic correlation effect, and valley-polarized quantum anomalous Hall effect can be induced, which originates from the sign-reversible Berry curvature and band inversion between dx2−y2/dxy and dz2 orbitals. These results provide an avenue for the applications of valleytronic quantum devices.

Strain-tunable magnetic property of ferromagnetic square CoSe monolayer

Two-dimensional (2D) intrinsic ferromagnetic materials with large perpendicular magnetic anisotropy (PMA) and high Curie temperature have attracted extensive attention for their potential applications. However, 2D intrinsic ferromagnets prepared experimentally are still very few and most of them have low PMA and Curie temperature. Here, using first-principles calculations, we investigate the structural stability, electronic and magnetic properties of square CoSe monolayer. The calculations indicate that CoSe monolayer is ferromagnetic metal with high Curie temperature of 251.9 K and PMA of 0.825 meV/unit cell. Additionally, the effect of biaxial strain on the magnetic and electronic properties of CoSe monolayer is explored. The CoSe monolayer has strong ferromagnetism over a biaxial strain range of −5 % to 5 %. Tensile strain can induce a transition from metal to half-metal. The Curie temperature of CoSe monolayer can be enhanced under compressive strain. Tensile strain increases the magnetic anisotropy energy by 92.2 % compared with that of the unstrained system. The robust ferromagnetism as well as the regulated magnetic anisotropy and Curie temperature of CoSe monolayer provide a new direction for the development of spintronics.

Direct Probe of Conical Intersection Photochemistry by Time-Resolved X-ray Magnetic Circular Dichroism.

The direct probing of photochemical dynamics by detecting the electronic coherence generated during passage through conical intersections is an intriguing challenge. The weak coherence signal and the difficulty in preparing purely excited wave packets that exclude coherence from other sources make it experimentally challenging. We propose to use time-resolved X-ray magnetic circular dichroism to probe the wave packet dynamics around the conical intersection. The magnetic field amplifies the relative strength of the electronic coherence signal compared to populations through the magnetic field response anisotropy. More importantly, since the excited state relaxation through conical intersections involves a change of parity, the magnetic coupling matches the symmetry of the response function with the electronic coherence, making the coherence signal only sensitive to the conical intersection induced coherence and excludes the pump pulse induced coherence between the ground state and excited state. In this theoretical study, we apply this technique to the photodissociation dynamics of a pyrrole molecule and demonstrate its capability of probing electronic coherence at a conical intersection as well as population transfer. We demonstrate that a magnetic field can be effectively used to extract novel information about electron and nuclear molecular dynamics.

Realizing Room-Temperature Ferromagnetism in Molecular-Intercalated Antiferromagnet VOCl.

2D van der Waals (vdW) magnets are gaining attention in fundamental physics and advanced spintronics, due to their unique dimension-dependent magnetism and potential for ultra-compact integration. However, achieving intrinsic ferromagnetism with high Curie temperature (TC) remains a technical challenge, including preparation and stability issues. Herein, an applicable electrochemical intercalation strategy to decouple interlayer interaction and guide charge doping in antiferromagnet VOCl, thereby inducing robust room-temperature ferromagnetism, is developed. The expanded vdW gap isolates the neighboring layers and shrinks the distance between the V-V bond, favoring the generation of ferromagnetic (FM) coupling with perpendicular magnetic anisotropy. Element-specific X-ray magnetic circular dichroism (XMCD) directly proves the source of the ferromagnetism. Detailed experimental results and density functional theory (DFT) calculations indicate that the charge doping enhances the FM interaction by promoting the orbital hybridization between t2 g and eg. This work sheds new light on a promising way to achieve room-temperature ferromagnetism in antiferromagnets, thus addressing the critical materials demand for designing spintronic devices.

Magnetostrictive strain-sensitivity synergy for laser-beam powder bed fusion processed Fe81Ga19 alloys by magnetic field annealing

Magnetostrictive Fe–Ga alloys have been demonstrated potentialities for numerous applications, whereas, suffering a tradeoff between large magnetostrictive strain and high sensitivity. Herein, bulk polycrystalline Fe81Ga19 alloys were prepared by laser-beam powder bed fusion (LPBF) and then annealed in magnetic field for manipulating the comprehensive magnetostrictive properties. Results indicate that <001> oriented grains are developed in the LPBF-prepared Fe81Ga19 alloys due to high temperature gradient. After magnetic field annealing (MFA), the magnetic domains within the alloys gradually transformed into well-arranged stripe domains, especially, flat and smooth 90° domains were established in the alloys annealed at 2600 ​Oe. As a result, the induced <001> orientation grains and 90° domains contributed to an improved effective magnetic anisotropy constant (57.053 ​kJ/m3), leading to an enhanced magnetostrictive strain of 92 ​ppm. Moreover, the MFA-treated alloys also displayed enhanced magnetostrictive sensitivity (0.097 ​ppm/Oe) owing to the smooth domain structures and low dislocation densities, demonstrating a fruitful strain-sensitivity synergy. In addition, good magnetostrictive dynamic response and enhanced compressive yield strength were also observed for the prepared alloys. This work demonstrates that LPBF and MFA might be an attractive strategy to resolve the tradeoff between strain and sensitivity, providing a basis for the preparation of high-performance magnetostrictive materials.

Spin reorientation in premartensite and austenite Ni–Mn–Ga

The premartensite state of Ni–Mn–Ga magnetic shape memory alloy, sometimes called the martensite precursor state, was studied by careful and detailed measurement of the evolution of magnetization curves of magnetically closed samples to evidence local symmetry breaking. During the heating cycle after the martensite transformation, the magnetization loop slowly transforms from a typical sigmoidal shape, corresponding to the magnetization along the easy axis, to a constricted loop indicative of magnetization along a harder magnetic axis. These changes are explained by a switching of the macroscopic magnetic easy axis from [100] to [110]. Above the premartensite transformation temperature, the magnetic easy axis slowly changes back to [100]. After cooling the sample, starting at the Curie temperature, the process reverses.