Crystalline Anisotropy Energy Research Articles

Re/Ir@Os-doping induced insulator-to-metal transition in Mott-insulator Ca2FeOsO6: octahedral distortion effects.

Regardless of several investigations to elucidate the ground state of the strongly correlated electron systems in doped Mott-insulators (MIs), the origin of the doping-induced insulator-to-metal transition (IMT) remains a crucial and debatable subject in solid-state physics. Herein, we explore the consequences of Re/Ir-doping at the Os-site (Re/Ir@Os), on the physical properties of the MI ferrimagnetic (FiM) Ca2FeOsO6 double perovskite oxides using density functional theory calculations. The doped structures' solidity is analyzed by computing the defect formation energies in terms of the dopant-rich situation, which confirms their growth credibility at ambient conditions along with mechanical and dynamical stabilities. Various FiM spin-ordering is taken into account in the doped structures to analyze the magnetic ground state, which is FiM-I/FiM-II in the Re/Ir@Os-doped system. Remarkably, an IMT is predicted in the Re/Ir@Os-doped structures, which is due to the admixture of the partially occupied 5d orbitals of these ions. The calculated partial spin magnetic moments (ms) of +4.12, -1.58, -0.75 and +0.88μB on the Fe, Os, Re, and Ir ions, endorse the +3, +5, +5, and +4 states having electronic configurations of t32g↑t02g↓e2g↑e0g↓, t32g↑t02g↓e0g↑e0g↓, t22g↑t02g↓e0g↑e0g↓, t32g↑t22g↓e0g↑e0g↓, respectively. The "+" and "-" signs on the ions ms values, lead the systems into various FiM magnetic ordering. Moreover, the estimated Curie temperature (TC) using the Heisenberg model in the pristine structure is 334 K, which is close to the experimentally observed value of 320 K along with a colossal uniaxial magneto crystalline anisotropy energy constant (K) of 2.95 × 107 erg cm-3 having the easy magnetic axis of the ac-plane ([101]). It is established that TC/K reduces and enhances to 298 K/1.33 × 107 erg cm-3 and 365 K/4.71 × 107 erg cm-3 for the Re@Os and Ir@Os-doped motif due to an increase and decrease in the octahedral distortions compared to that of the pristine system, respectively.

Computational Discovery of High-Temperature Ferromagnetic Semiconductor Monolayer H-MnN2.

In the past few years, two-dimensional (2D) high-temperature ferromagnetic semiconductor (FMS) materials with novelty and excellent properties have attracted much attention due to their potential in spintronics applications. In this work, using first-principles calculations, we predict that the H-MnN2 monolayer with the H-MoS2-type structure is a stable intrinsic FMS with an indirect band gap of 0.79 eV and a high Curie temperature (Tc) of 380 K. The monolayer also has a considerable in-plane magnetic anisotropy energy (IMAE) of 1005.70 μeV/atom, including a magnetic shape anisotropy energy induced by the dipole-dipole interaction (shape-MAE) of 168.37 μeV/atom and a magnetic crystalline anisotropy energy resulting from spin-orbit coupling (SOC-MAE) of 837.33 μeV/atom. Further, based on the second-order perturbation theory, its in-plane SOC-MAE of 837.33 μeV/atom is revealed to mainly derive from the couplings of Mn-dxz,dyz and Mn-dx2-y2,dxy orbitals through Lz in the same spin channel. In addition, the biaxial strain and carrier doping can effectively tune the monolayer's magnetic and electronic properties. Such as, under the hole and few electrons doping, the transition from semiconductor to half-metal can be realized, and its Tc can go up to 520 and 620 K under 5% tensile strain and 0.3 hole doping, respectively. Therefore, our research will provide a new, promising 2D FMS for spintronics devices.

First-principles calculations to investigate structural, electronic and magnetic anisotropy energy of ZnFe[formula omitted]O[formula omitted] spinel ferrite for spintronics applications

Structural, electronic and magnetic investigations of ZnFe2O4 have been made by using density functional theory with Perdew–Burke–Ernzerhof Generalized-Gradient Approximation as exchange and correlation functional. Additionally, the effect of Hubbard, U, and spin orbit coupling, SOC on electronic and magnetic properties has also been observed. The U-value for Zinc and Iron are used, 6.0 eV (0.44 Ry) and 4.3 eV (0.316 Ry) respectively. The optimized lattice parameter matches well with experimental with mere percentage difference of 0.039%. This optimized lattice constant is used for SCF calculations to calculate the above mentioned properties of zinc ferrite. It is found that zinc ferrite have high value of magnetic moments, 19.721μB, 20.003μB, and 19.580μB for PBE, PBE with Hubbard, and with spin orbit coupling respectively. The magneto crystalline anisotropy energy calculations confirm the utility of this material for spintronics device fabrication.

Systematic exploration of magnetic mechanism of La3Co29Si4B10 by first principles calculation

Rare earth transition metal compounds with intrinsic and high-performance magnetic behaviors have been attractively research topic as a promising permanent magnets material. In particularly, La3Co29Si4B10 is one of typically rare earth ferromagnetism materials with large magnetic moment and high transition temperature. In order to investigate the magnetic origin of sublattice and guide the design of permanent magnet materials with high magnetic moment and high magnetic crystalline anisotropy, we adopt first principles calculations method to investigate the magnetic mechanism of La3Co29Si4B10 combine local coordination environment with density of states. The results show that the magnetic moment mainly comes from the Co atom. According to the different local symmetry and atom hybridization, the Co atoms at different crystal positions can be divided into high spin state with maximum local magnetic moment at 2c position, low spin state with minimum magnetic moment at 8i1 position and medium spin state corresponding moderate magnetic moment size at other positions. Furthermore, the strain-stress of La3Co29Si4B10 will increase the total magnetic moment. Spin orbital coupling calculation indicate that the easy magnetization direction of La3Co29Si4B10 is along in plane with magnetic crystalline anisotropy energy of 0.68 meV/unit cell, which agreement with experimental. Our calculations lay a theoretical foundation for further experimental design of high-performance permanent magnet materials.

Effect of exchange-correlation spin\u2013torque on spin dynamics

A recently proposed exchange-correlation functional within density functional theory, which ensures that the exchange-correlation magnetic field is source-free, is shown to give non-zero internal spin–torque. This spin–torque is identically zero for all conventional local and semi-local functionals. Extension of this source-free functional to the time domain is used to study the effect of the internal spin–torque on the laser induced spin-dynamics in bulk Co, Ni and interfaces of these metals with Pt. It is shown that the internal spin–torque contributes significantly to spin-dynamics only when the magneto crystalline anisotropy energy is small, as in the case of cubic bulk materials. For surfaces or interfaces, where the anisotropy energy is large, these torques are too small to cause any significant precession of spins in early times (<100 fs). Furthermore, it is shown that the spin-dynamics caused by the internal spin–torque is slow compared to the inter-site spin transfer and spin–orbit mediated spin-flips.

Magnetization of Extraterrestrial Allende material may relate to terrestrial descend

The origin of magnetization in Allende may have significant implications for our understanding of core formation/differentiation/dynamo processes in chondrite parent bodies. The magnetic Allende data may contain information that could constrain the magnetic history of Allende. The measurements on Allende chondrules reveal an existence of magnetization component that was likely acquired during the meteorite transit to terrestrial conditions. Both the pyrrhotite carrying magnetic remanence intensity and direction of the chondrules change erratically when subjecting the Allende meteorite's chondrules to temperatures near 77 K and back to room temperature. Chondrules with more intense original magnetization are denser and contain larger inverse thermoremanent magnetization (ITRM). Temperature dependent monitoring of ITRM revealed that magnetization was acquired at temperature near 270 K. Such temperature is consistent with the condition when, in addition to temperature increase, the atmospheric uniaxial pressure applied during the meteorite entry on the porous material was responsible for meteorite break up in the atmosphere. During this process, collapse of the pore space in the matrix and some chondrules would generate crystalline anisotropy energy accumulation within pyrrhotite grains in form of parasitic magnetic transition.

First principles study of magnetic properties of Co1ML/Ni2ML/Cu(111) multilayer structure

We carry out first principles calculations to study the magnetic properties of a multilayer structure consisting of one monolayer (ML) of Co (Co1ML) on top of two ML of Ni (Ni2ML) deposited on top of Cu(111), i.e., Co1ML/Ni2ML/Cu(111). We found that the magnetic anisotropy of the Co1ML/Ni2ML multilayer does not change much with variation in the in-plane and inter-layer structures of the Co/Ni multilayer. On the other hand, interaction with the Cu(111) plays an important role. As a result of the electron transfer from the Cu(111) substrate to the first deposited Ni-layer, i.e., Ni(1), there is a reduction in the magnetic moment of Ni(1) and an increase in the magnetic crystalline anisotropy energy of Co1ML/Ni2ML/Cu(111) multilayer structure.

Critical Dzyaloshinskii–Moriya interaction energy density for the skyrmion states formation in ultrathin ferromagnetic layer

We investigate the condition for the skyrmion state formation in ultrathin ferromagnetic layers with the Dzyaloshinskii–Moriya interaction (DMI). By using micromagnetic simulations with DMI term, we found skyrmion state is a ground state instead of a collinear single domain when DMI is larger than a critical value. When the skyrmion arrays are formed, a specific length scale of skyrmion is governed the spin configurations. We found that the critical DMI energy density for the skyrmion state is related with not only crystalline anisotropy energy, but also the dipole–dipole interaction.

The effects of surface spin on magnetic properties of weak magnetic ZnLa0.02Fe1.98O4 nanoparticles.

In order to prominently investigate the effects of the surface spin on the magnetic properties, the weak magnetic ZnLa0.02Fe1.98O4 nanoparticles were chosen as studying objects which benefit to reduce as possibly the effects of interparticle dipolar interaction and crystalline anisotropy energies. By annealing the undiluted and diluted ZnLa0.02Fe1.98O4 nanoparticles at different temperatures, we observed the rich variations of magnetic ordering states (superparamagnetism, weak ferromagnetism, and paramagnetism). The magnetic properties can be well understood by considering the effects of the surface spin of the magnetic nanoparticles. Our results indicate that in the nano-sized magnets with weak magnetism, the surface spin plays a crucial rule in the magnetic properties.

Polarization rotation theory for field-induced-phase transitions in BaTiO3 single crystal

We have proposed the polarization rotation theory to understand the underlying physics of the large piezoelectric effect in ferroelectric crystals by referring to the coherent rotation model in ferromagnetism. When both the electric field energy and crystalline anisotropy energy are taken into account, the polarization in BaTiO3 crystal can be rotated toward the field direction. The numerical results are in agreement with the experimental observations and have indicated the highly anisotropic polarization rotations and the field-induced-phases with different symmetries in the crystal. This theory is helpful for understanding the polarization reversal and the electromechanical effect in ferroelectric materials.

Magnetic and electrical properties of the half-metallic ferromagnets Co2CrAl

This paper presents the results of measurements of the magnetic and electrical properties of the ferromagnetic alloy Co2CrAl in two structural states: (i) after severe plastic deformation and (ii) after shortterm high-temperature annealing of the deformed specimens. The experiments have been performed at temperatures in the range from 2 to 900 K in magnetic fields H ≤ 50 kOe. The ferromagnetic Curie temperature T C and the paramagnetic Curie temperature Θ have been determined (T C = 305 K and Θ = 326 K), as well as the spontaneous magnetic moment μ S and the effective magnetic moment μeff per molecule of the alloy (μ S = 1.62 μB and μ eff 2 = 8.2 μ B 2 ). It has been shown that the magnetic crystalline anisotropy energy of the alloy is on the order of ∼5 × 105 erg/g. The specific features of the electrical properties are associated with the presence of an energy gap in the electronic spectrum near the Fermi level E F and with the change in the parameters of the energy gap as a function of the temperature.

High-Resolution Terahertz Optical Absorption Study of the Antiferromagnetic Resonance Transition in Hematite (α-Fe2O3)

We report high-resolution optical absorption measurements between 1.5 and 10 cm–1 of the antiferromagnetic (AFM) resonance transition in α-Fe2O3. The AFM transition is measured over a range of temperatures from 4 to 325 K that includes the low- and high-temperature branches separated by a magnetic phase transition near 260 K. The high-resolution measurements devoid of optical interference were made possible by improving the frequency stability of the near-infrared lasers used to generate terahertz radiation (±0.0003 cm–1) and by taking advantage of the large temperature dependence of the AFM transition frequency. The temperature dependence of both branches could be well-fit to a magnon model. A slight difference in temperature dependence between the hematite samples with different annealing treatments was observed in the low-temperature branch. The difference is qualitatively explained by changes in the higher order crystalline anisotropy energy resulting from slightly altered magnetic interactions due to the annealing treatment. The sensitivity achieved using these methods permits a detailed characterization of the optically excited AFM magnon in the absence of magnetic fields over a wide temperature range, thus making the terahertz-based optical platform suitable for nondestructive examination applications for many AFM materials and devices.

Through process texture evolution and magnetic properties of high Si non-oriented electrical steels

Abstract A detailed understanding of microstructural changes in a sequence of thermomechanical processing allows the improvement of magnetic properties in FeSi strips. The current contribution considers the texture evolution in non-oriented electrical steels of high Si content. Hot band strips of various textures were subjected to cold rolling and recrystallization annealing. The findings suggest that the crystallographic orientations observed after cold rolling are correlated with the hot band texture. In contrast, the evolution of recrystallization textures was more likely affected both by the hot and cold rolling microstructural features. The evolution of recrystallization textures is discussed on the basis of crystal plasticity calculations while the magnetic properties are correlated with the crystalline anisotropy energy density.

Numerical Analyses of Effectiveness of Magnetic Field on Variant Selection in FePd by Phase Field Modeling

The effect of magnetic field on variant selection in FePd has been investigated by phase field modeling. In this study, multi phase field modeling [Physica D, 94 (1996), 135] was used for calculations. Chemical free energy, interface energy and magnetic energy were incorporated in our calculation. As for magnetic energy, magnetic crystalline anisotropy energy was taken into account. Disordered FePd at 50 K below the transition temperature is selected as the initial state. Then, calculations have been performed with the presence or absence of the external magnetic field as a variable. First of all, calculations under no magnetic field showed that the volume fractions of three variants were almost equal to each other. Secondly, calculations under magnetic field of 5 T showed that the volume fraction of magnetically favorable variant was much more than other variants. Finally, the timing to apply magnetic field was intentionally changed with fixing the length of time for application, in computational experiments to examine at what stage the magnetic field is most effective. It is found that applying magnetic field from the beginning of order–disorder transition results in the slightly more volume fraction of the favorable variant, and, in turn, the less interface energy per unit volume, than others, which leads to the dominance of the favorable variant at the end after variant coarsening driven by interface energy, resulting in successful grain alignment, although applying the magnetic field at later stage shows little effect.

Thermal behavior for a nanoscale two ferromagnetic phase system based on random anisotropy model

Advances in theory that explain the magnetic behavior as function of temperature for two phase nanocrystalline soft magnetic materials are presented. The theory developed is based on the well known random anisotropy model, which includes the crystalline exchange stiffness and anisotropy energies in both amorphous and crystalline phases. The phenomenological behavior of the coercivity was obtained in the temperature range between the amorphous phase Curie temperature and the crystalline phase one.

Canted magnetization texture in ferromagnetic tunnel junctions

We study the formation of inhomogeneous magnetization texture in the vicinity of a tunnel junction between two metallic ferromagnets nominally in the antiparallel configuration and its influence on the magnetoresistance of such a device. Such a texture, dependent on the magnetization rigidity and crystalline anisotropy energy in the ferromagnet, develops when the ferromagnetic coupling across the tunnel barrier increases above a critical value.

Separation of exchange anisotropy and magnetocrystalline anisotropy inCo∕CoObilayers by means of ac susceptibility measurements

In $\mathrm{Co}∕\mathrm{Co}\mathrm{O}$ bilayers separation of the unidirectional anisotropy energy from crystalline anisotropy is accomplished with the temperature variation of the ac susceptibility from room temperature to $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Measurements above the blocking temperature of the CoO layer give a clear measure of crystalline anisotropy in the Co. At $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 85% of the total anisotropy is from ${K}_{\mathit{ud}}$ while the anisotropy of the Co layer only contributes about 15%. The small crystalline anisotropy energy does not account for the large difference between the exchange anisotropy determined by ac susceptibility and hysteresis loops.

Design aspects in processing of Ni-Zn ferrites for high frequency applications

Manufacture of a high quality and low cost ferrite material for power transformer cores in switched mode power supplies is an important aspect for investigation. These cores require high saturation flux densities, low hysteresis and low eddy current losses at high frequencies (upto a few megahertz). Eddy current losses can be minimized by, besides optimizing the microstructure, increasing the resistivity of a ferrite material. Hysteresis loss can be lowered by a decrease in anisotropy constant and magnetostriction which have inverse relations with the permeability [1, 2]. Snoek [3] demonstrated that an increase in permeability would increase the operating frequency of a ferrite only when there is a simultaneous increase in saturation magnetization. Therefore, it necessitates careful planning even at the compositional stage in the development of these materials. The aim of this paper is to examine and discuss the possibilities of producing materials with optimum characteristics required for high frequency power applications using low-cost raw materials and by inexpensive preparation methodology. Since Ni-Zn ferrites normally exhibit high resistivities and can be easily sintered in air atmospheres, a nickel-zinc ferrite composition with high room temperature saturation magnetisation, Ni0.65Zn0.35Fe2O4, was considered to be suitable for the current study. However, this composition has some disadvantages since its anisotropy constant, |K1| and the associated losses are high [4]. Therefore, compositional modifications are necessary and before making such substitutions two aspects have to be considered; the first is to optimize the sintering conditions as well as the microstructure for the substituted samples, and the other is that the substitutions are to be made in such a way that the resultant magnetization of the final product should remain at its peak or close to peak. It should mention here that in this work the selection of the sintering conditions for the substituted samples has to be made mainly with an emphasis to improve upon their magnetic properties. With these considerations, compositional modifications have been aimed at to the basic nickel-zinc ferrite with varying concentrations of Zn and Sc. The concept of simultaneous substitutions [5] has been tried in an attempt to obtain as high as possible magnetization out of a given system. Zn2+ ions are selected for substitution in the expectation that they will improve magnetization and reduce the anisotropy constant [6] as well as magnetostriction [7]; thus an increase in permeability and a decrease in hysteresis loss may be realized. On the other hand, the choice of scandium ions, expected to be in octahedral sites, is made to increase the resistivity and thereby decreasing the eddy current losses. Polycrystalline Ni-Zn-Sc ferrites with the formula Ni0.65−x Zn0.35+x Fe2−x Scx O4, where x values range from 0.000 to 0.125 in steps of 0.025, have been prepared by conventional ceramic technique using the procedure described elsewhere [8]. Sintering of the samples has been carried out at 1300 ◦C for 4 h in air atmosphere followed by natural cooling. X-ray studies confirm single phase spinel structure in all the samples. Regarding characterisation of the samples, lattice constant and Curie temperature of the basic composition were measured and these were found to be in good agreement with the respective parameters of the same composition reported earlier [9, 10]. DC resistivity measurements have been made on the samples by standard two probe method using spring loaded copper electrodes connected to a Keithley model 614 digital electrometer. Permeability and loss factor (tan δ) measurements were made in the range from 10 kHz to 13 MHz by using HP4192A LF Impedance Analyzer. Saturation magnetization studies were made by the Ponderometer method. Bulk density and grain size estimations have been made by using Archimedes principle and Jeol T330A scanning electron microscope respectively. Table I gives the values of saturation magnetization (4IIMs), initial permeability (μi), tan δ/μi, dc resistivity (ρ), grain size and bulk density of various Ni-Zn-Sc ferrites. Density and grain sizes are found to increase for initial concentrations of scandium and for subsequent concentrations they remained either almost unaltered or decreased slightly. These results indicate that the presence of scandium enhances sinterability due to an increase in bulk diffusion [11] thus promoting densification as well as grain growth processes for smaller concentrations. Increased zinc content is known to decrease crystalline anisotropy and exchange energy [6] thus causing a decrease in the grain size as is the case with higher concentrations in the study. The substitutions have resulted in higher DC resistivities with the increase in x . This increase in resistivity is accounted for by a larger ionic radii and specific site preferences of scandium ions in different lattice sites [8] and will contribute to lower eddy current losses. The simultaneous substitution of zinc and scandium ions aiming at dilution of both the sublattices because

Generalization of a two-dimensional micromagnetic model to nonuniform thickness

A two-dimensional micromagnetic model is extended to support simulation of films with nonuniform thickness. Zeeman and crystalline anisotropy energies of each cell scale with the cell thickness, while the exchange energy of a pair of neighbor cells scales by a weight dependent on the thicknesses of both cells. The self-magnetostatic energy is computed by scaling the moment of each cell by its thickness, and adding a local correction to the out-of-plane field. The calculation of the magnetostatic field for a 10×10×1 oblate spheroid is shown to be more accurate by the nonuniform thickness model than by a uniform thickness model. With the extended model a 530×130×10 nm film in the shape of a truncated pyramid with tapering over the 15 nm nearest the edges is shown to have smaller switching field and different reversal mechanism compared with uniform thickness films of similar size and shape.

Annealing effects on structural and magnetic properties of α-Fe 2O 3 nanoparticles

The effect of transformations induced by annealing on the structure and magnetic properties of α-Fe 2O 3 antiferromagnetic nanoparticles synthesized by chemical route has been investigated by transmission electron microscopy, magnetization measurements and Mössbauer spectroscopy. Annealing of these samples re-crystallize the nanoparticles without changing its mean size modifying the crystalline anisotropy energy of the nanoparticles leading to a change in the reversible–irreversible regime and in the Morin transition behaviour.