Finite-temperature Magnetic Properties Research Articles

Tunable magnetism in nitride MXenes: consequences of atomic layer stacking.

We have performed density functional theory (DFT) based calculations to investigate the effects of stacking patterns on the electronic and magnetic properties of several nitride MXenes. MXenes, a relatively new addition to the family of two-dimensional materials, have exhibited fascinating properties in several occasions, primarily due to their compositional flexibility. However, compared to carbide MXenes, nitride MXenes are much less explored. Moreover, the structural aspects of MXenes and the tunability they may offer have not been explored until recently. In this work, we have combined these two less-explored aspects to examine the structure-property relationships in the field of magnetism. We find that in the family of M2NT2 (M = Sc, Ti, V, Cr, Mn; T = O, F) MXenes, the stacking of transition metal planes has a substantial effect on the ground state and finite temperature magnetic properties. We also find that the electronic ground states can be tuned by changing the stacking pattern in these compounds, making the materials appropriate for applications as magnetic devices. Through a detailed analysis, we have connected the unconventional stacking pattern-driven tunability of these compounds with regard to electronic and magnetic properties to the local symmetry, inhomogeneity (or lack of it) of structural parameters, and electronic structures.

High Curie temperature and high magnetization potential Fe2CoS alloy soft magnet

Developing soft magnets is attracting extensive research efforts for several device applications and other issues. Here, we investigate the finite temperature magnetic properties of the Fe2CoS alloy using first-principles density functional theory calculations and Monte Carlo simulations. Without a demagnetization factor, it exhibits semi-hard-magnetic properties at room temperature. For instance, it exhibits a coercivity of 290 mT, a magnetic anisotropy constant of 218 kJ/m3, and a magnetic hardness parameter of 0.3. By considering the demagnetization factor at 300 K, the coercivity, magnetic anisotropy constant, and hardness parameter are reduced to 1.5 mT, 0.7 kJ/m3, and 0.035. Besides, compared to ferrites soft magnets, the Fe2CoS alloy shows a high relative permeability of 10,000 and low energy loss of 0.16 Watt/kg. The curie temperature (940 K) and high saturation magnetization MS (1.49 T) of the Fe2CoS alloy are significantly higher than those of soft ferrites and Permalloy. Therefore, we propose that the Fe2CoS alloy can be used as a potential Fe-based soft magnet.

Robust ferromagnetism in two-dimensional GeC/CrN heterobilayers.

We have investigated the electronic and finite temperature magnetic properties of germanium carbide (GeC) and ferromagnetic chromium nitride (CrN) heterobilayers by using first-principles calculations based on density functional theory with Hubbard U correction and an effective anisotropic Heisenberg spin model. The dynamical stability of different stacking formations of heterobilayers is ensured by considering the phonon spectra. All the stacking patterns show half-metallicity with an out-of-plane easy-axis ferromagnetic ground state. We find a high Curie temperature for GeC/CrN heterobilayers within the random phase approximation (RPA). In addition to the symmetric stackings, i.e., AA and AB, the electronic properties of non-symmetric stackings at three different twist angles are also analyzed. The electronic structure analysis of twisted structures demonstrates that the half-metallicity of the GeC/CrN heterobilayer is stack independent. Furthermore, we have investigated the electronic properties, magnetic anisotropy energy, Curie temperature, and spin wave spectrum in the presence of biaxial strain. It is shown that the compressive strain dramatically reduces the magnetic anisotropy energy of the GeC/CrN heterobilayer and Curie temperature, but the Curie temperature still remains well above room temperature for all strain values. The increasing values of tensile strain reduce the magnetic exchange while it increases the magnetic anisotropy energy of the heterobilayer system which enhances the Curie temperature of the structures. The monolayer CrN on the GeC with a wide band gap and commensurate lattice together with a high Tc value can be a feasible candidate for future spintronic applications.

Dynamical correlations in single-layer CrI3

Chromium triiodide is an intrinsically magnetic van der Waals material down to the single-layer limit. Here, we provide a first-principles description of finite-temperature magnetic and spectral properties of monolayer (ML) ${\mathrm{CrI}}_{3}$ based on fully charge self-consistent density functional theory (DFT) combined with dynamical mean-field theory, revealing a formation of local moments on Cr from strong local Coulomb interactions. We show that the presence of local dynamical correlations leads to a modification of the electronic structure of ferromagnetically ordered ${\mathrm{CrI}}_{3}$. In contrast to conventional $\mathrm{DFT}+U$ calculations, we find that the top of the valence band in ML ${\mathrm{CrI}}_{3}$ demonstrates essentially different orbital character for minority and majority spin states, which is closer to the standard DFT results. This leads to a strong spin polarization of the optical conductivity upon hole doping, which could be verified experimentally.

First-principles Calculations of Magnetic Properties for Analysis of Magnetization Processes in Rare-earth Permanent Magnets

It has been empirically known that the coercivity of rare-earth permanent magnets depend on the size and shape of fine particles of the main phase in the system. Also, recent experimental observations have suggested that the atomic scale structures around the grain-boundaries of the fine particles play a crucial role to determine their switching fields. In this article, we review a theoretical attempt to describe the finite temperature magnetic properties and to evaluate the reduction of the switching fields of fine particles of several rare-earth permanent magnet materials based on an atomistic spin model that is constructed using first-principles calculations. It is shown that, over a wide temperature range, the spin model gives a good description of the magnetization curves of rare-earth intermetallic compounds such as R2Fe14B (R = Dy, Ho, Pr, Nd, Sm) and SmFe12. The atomistic spin model approach is also used to describe the local magnetic anisotropy around the surfaces of the fine particles, and predicts that the rare-earth ions may exhibit planar magnetic anisotropy when they are on the crystalline-structure surfaces of the particles. The dynamical simulation of the atomistic spin model and the corresponding micromagnetic simulation show that the planar surface magnetic anisotropy causes a reduction in the switching field of fine particles by approximately 20-30%, which may be relevant to the atomic scale surface effects found in the experimental studies.

Effect of element doping and substitution on the electronic structure and macroscopic magnetic properties of SmFe12 -based compounds

The mechanisms underlying the enhancement of magnetic anisotropies (MAs) of Sm ions, owing to valence electrons at the Sm site and the screened nuclear charges of ligands, are clarified using a detailed analysis of crystal fields (CF). In order to investigate the finite-temperature magnetic properties, we developed an effective spin model for SmFe$_{12}X$ ($X$=H, B, C, and N) and SmFe$_{11}M$ ($M$=Ti, V, and Co), where the magnetic moments, CF parameters, and exchange fields were determined by first-principle calculations. Using this model, the MA constants and magnetization curves at finite temperatures were investigated using a recently introduced analytical method [T. Yoshioka, H. Tsuchiura, and P. Nov\'ak, Phys. Rev. B {\bf 102}, 184410 (2020)]. In SmFe$_{12}X$, the doped light elements $X$ are assumed to be at the $2b$ site, and in SmFe$_{11}M$, the substitution site of Fe is systematically investigated for all inequivalent $8f$, $8i$, and $8j$ sites. We found that the first-order MA constant $K_1$ is increased by a factor of about two when hydrogen is doped to the $2b$ site and when Fe is replaced by Ti or V at the $8j$ site, owing to the attraction of the prolate $4f$ electron cloud to the screened positive charges of the surrounding ligand ions. We found that when Fe is replaced by Co, the MA increases at all temperatures regardless of the substitution site. The substituted Co attracts electrons, which reduces the electron density in the region from the Sm site to the empty $2b$ site. This causes the $4f$ electron cloud at the Sm site to be fixed along the $c$-axis direction, which improves the MA. The calculated temperature dependence of $K_1(T)$ and $K_2(T)$ in SmFe$_{11}$Co qualitatively reproduces the experimental results in the case of Sm(Co$_x$Fe$_{1-x}$)$_{12}$ for $x$=0.1 and 0.07.

Random iron-nickel alloys: From first principles to dynamic spin fluctuation theory

We provide a systematic analysis of finite-temperature magnetic properties of random alloys ${\mathrm{Fe}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}$ with the face-centered-cubic structure over a broad concentration range $x$. By means of the spin-polarized relativistic Korringa-Kohn-Rostoker method we calculate the electronic structure of disordered iron-nickel alloys and discuss how a composition change affects magnetic moments of Fe and Ni and the density of states. We investigate how the Curie temperature depends on Fe concentration using conventional approaches, such as mean-field approximation or Monte Carlo simulations, and dynamic spin-fluctuation theory. Being devised to account for spin fluctuations explicitly, the latter method shows the best fit to experimental results.

First-principles calculations of magnetic properties for analysis of magnetization processes in rare-earth permanent magnets

ABSTRACT It has been empirically known that the coercivity of rare-earth permanent magnets depend on the size and shape of fine particles of the main phase in the system. Also, recent experimental observations have suggested that the atomic scale structures around the grain-boundaries of the fine particles play a crucial role to determine their switching fields. In this article, we review a theoretical attempt to describe the finite temperature magnetic properties and to evaluate the reduction of the switching fields of fine particles of several rare-earth permanent magnet materials based on an atomistic spin model that is constructed using first-principles calculations. It is shown that, over a wide temperature range, the spin model gives a good description of the the magnetization curves of rare-earth intermetallic compounds such as FeB (=Dy, Ho, Pr, Nd, Sm) and SmFe. The atomistic spin model approach is also used to describe the local magnetic anisotropy around the surfaces of the fine particles, and predicts that the rare-earth ions may exhibit planar magnetic anisotropy when they are on the crystalline-structure surfaces of the particles. The dynamical simulation of the atomistic spin model and the corresponding micromagnetic simulation show that the planar surface magnetic anisotropy causes a reduction in the switching field of fine particles by approximately 20-30%, which may be relevant to the atomic scale surface effects found in the experimental studies.

Micromagnetics of Nd–Fe–B magnets at finite temperature

A three-dimensional micromagnetic model for Nd–Fe–B magnets, where the polycrystalline grain size and grain boundary width are adjustable, is built to calculate the finite temperature magnetic properties by the Hybrid Monte Carlo method. At 300 K, it is found that the increase of the energy product (BH)max of Nd–Fe–B is mainly contributed by the higher saturation magnetization M s. M–H loops are simulated from 300 to 450 K, and the measured temperature-dependent coercivity H c(T) and remanence M r(T) are explained quantitatively and fit well with experiments.

Finite-temperature magnetic properties of Sm2Fe17Nx using an ab initio effective spin model

In this study, we investigate the finite-temperature magnetic properties of Sm2Fe17Nx (x = 0,3) using an effective spin model constructed based on the information obtained by first-principles calculations. We find that assuming the plausible trivalent Sm3+ configuration results in a model that can satisfactorily describe the magnetization curves of Sm2Fe17N3. By contrast, the model based on the divalent Sm2+ configuration is suitable to reproduce the magnetization curves of Sm2Fe17. These results expand the understanding of how electronic structure affects the magnetic properties of these compounds.

Mn1/4NbS2 : Magnetic and magnetotransport properties at ambient pressure and ferro- to antiferromagnetic transition under pressure

Transition-metal dichalcogenides (TMDCs) stand out with their high chemical stability and the possibility to incorporate a wide range of atoms and molecules between the layers. The behavior of conduction electrons in such $3d$-metal-inserted materials is closely related to their magnetic properties and can be sensitively controlled by external magnetic fields. Here, we study the magnetotransport properties of Mn-inserted $\mathrm{Nb}{\mathrm{S}}_{2}, {\mathrm{Mn}}_{1/4}\mathrm{Nb}{\mathrm{S}}_{2}$, demonstrating a complex behavior of the magnetoresistance and of the ordinary and anomalous Hall resistivity. Application of high pressure as tuning parameter leads to the drastic changes of the magnetotransport properties of ${\mathrm{Mn}}_{1/4}\mathrm{Nb}{\mathrm{S}}_{2}$ exhibiting large negative magnetoresistance up to $\ensuremath{-}65%$ at 7.1 GPa. First-principles electronic structure calculations indicate a pressure-induced transition from a ferromagnetic to antiferromagnetic state. Theoretical calculations accounting for the finite temperature magnetic properties suggest a field-induced metamagnetic ferromagnetic-antiferromagnetic transition as an origin of the large negative magnetoresistance. These results inspire the development of materials for spintronic applications based on $3d$-element-inserted TMDCs with a well controllable metamagnetic transition.

Statistical and analytical approaches to finite-temperature magnetic properties of the compound SmFe12

To investigate the magnetic properties of $\mathrm{Sm}{\mathrm{Fe}}_{12}$, we construct an effective spin model, where magnetic moments, crystal-field (CF) parameters, and exchange fields at 0 K are determined by first-principles calculations. Finite-temperature magnetic properties are investigated by using this model. We further develop an analytical method with strong mixing of states with a different quantum number of angular momentum $J$ ($J$-mixing), which is caused by a strong exchange field acting on the spin component of $4f$ electrons. Comparing our analytical results with those calculated by Boltzmann statistics, we clarify that the previous analytical studies for Sm transition-metal compounds overestimate the $J$-mixing effects. The present method enables us to perform a quantitative analysis of the temperature dependence of magnetic anisotropy (MA) with high reliability. The analytical method with model approximations reveals that the $J$-mixing caused by the exchange field increases the spin angular momentum, which enhances the absolute value of the orbital angular momentum and MA constants via spin-orbit interaction. It is also clarified that these $J$-mixing effects remain even above room temperature. Magnetization of $\mathrm{Sm}{\mathrm{Fe}}_{12}$ shows a peculiar field dependence known as the first-order magnetization process (FOMP), where the magnetization shows an abrupt change at a certain magnetic field. The result of the analysis shows that the origin of FOMP is attributed to competitive MA constants between positive ${K}_{1}$ and negative ${K}_{2}$. The sign of ${K}_{1(2)}$ appears due to an increase in the CF potential denoted by the parameter ${A}_{2}^{0}\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}$ (${A}_{4}^{0}\ensuremath{\langle}{r}^{4}\ensuremath{\rangle}$) caused by hybridization between $3d$-electrons of Fe on the $8i$ ($8j$) site and $5d$ and $6p$ valence electrons on the Sm site. It is verified that the requirement for the appearance of FOMP is given as $\ensuremath{-}{K}_{2}<{K}_{1}<\ensuremath{-}6{K}_{2}$.

Ferromagnetism in Layered Metallic Fe1/4TaS2 in the Presence of Conventional and Dirac Carriers**Supported by the National Natural Science Foundation of China under Grant Nos 11774350, 11534010 and 11574315.

We present the microscopic origin of the ferromagnetism of Fe0.25TaS2 and its finite-temperature magnetic properties. The band structures of Fe0.25TaS2 are first obtained by the first-principles calculations and it is found that both conventional and Dirac carriers coexist in metallic Fe0.25TaS2. Accordingly, considering the spin-orbit coupling of Fe 3d ion, we derive an effective Ruderman–Kittle–Kasuya–Yosida-type Hamiltonian between Fe spins in the presence of both the conventional parabolic-dispersion and the Dirac linear-dispersion carriers, which contains a Heisenberg-like, an Ising-like and an XY-like term. In addition, we obtain the ferromagnetic Curie temperature Tc by using the cluster self-consistent field method. Our results could address not only the high ferromagnetic Curie temperature but also the large magnetic anisotropy in FexTaS2.

A simulation approach for the finite-temperature magnetic properties, stochastic dynamics and heating properties of magnetic nanoparticles composed of FM core/AFM shell

Using the Monte Carlo simulation method based on the Metropolis algorithm, we present some results regarding the magnetic and heating properties of three-dimensional nanoparticles composed of a ferromagnetic core surrounded by an antiferromagnetic shell. We investigate the variation of the exchange bias effect and its dependence on the particle shell thickness, as well as on the temperature. In terms of the stochastic dynamics, we applied a time-dependent alternating magnetic field on the system, and calculated the dynamic-order parameters, response functions and the hysteresis loop area from the AC hysteresis curves from which we have also quantified the specific absorption rate (SAR) of the particle over a wide range of field frequencies. A number of interesting results have been found regarding the variation of the heating properties of the system as a function of the antiferromagnetic shell thickness and the period of the alternating field.

Site dependent substitution and half-metallic behaviour in Heusler compounds: A case study for Mn2RhSi, Co2RhSi and CoRhMnSi

Using Density Functional Theory, we have investigated the emergence of half-metallicity in a quaternary Heusler compound with both 3d and 4d electrons as constituents, by chemical substitutions at different sites in the lattice. Mn2RhSi, Co2RhSi and CoRhMnSi, the parents-daughter combination where one of the ternary parents is a normal metal is chosen as an example. We find that the substituting site, and the electronic structures of the constituent magnetic components are responsible for sustenance of half-metallicity or absence of it while making quaternary daughter from ternary parents. We have studied the effects of site-dependent substitutions on the finite temperature magnetic properties and predicted that in this family, half-metals with high Curie temperatures can be obtained in substitutionally disordered quaternary compounds, apart from the ordered ones. We have also proposed that similar phenomena can be observed in other parent-daughter combinations having both 3d and 4d elements in the same compound and having same combination of electronic properties in the context of half-metallic behaviour.

Finite temperature magnetic properties of CrxCoyNi100−x−y medium entropy alloys from first principles

The magnetic structure of polymorphic Cr-Co-Ni medium entropy alloys is investigated as a function of temperature and chemical composition by ab initio calculations. Besides the thermal lattice expansion, the longitudinal spin fluctuations (LSFs) are accounted for in determining the magnetic state at finite temperature. We show that sizable local magnetic moments persist on all alloy components in the paramagnetic state for both face-centered cubic and hexagonal close-packed structures, and each alloy species exhibits particular temperature and concentration dependencies. The crucial role of LSFs for the finite temperature magnetic state and its impact on the temperature dependent elastic parameters are demonstrated.

The full magnon spectrum of yttrium iron garnet

The magnetic insulator yttrium iron garnet can be grown with exceptional quality, has a ferrimagnetic transition temperature of nearly 600 K, and is used in microwave and spintronic devices that can operate at room temperature. The most accurate prior measurements of the magnon spectrum date back nearly 40 years, but cover only 3 of the lowest energy modes out of 20 distinct magnon branches. Here we have used time-of-flight inelastic neutron scattering to measure the full magnon spectrum throughout the Brillouin zone. We find that the existing models of the excitation spectrum fail to describe the optical magnon modes. Using a very general spin Hamiltonian, we show that the magnetic interactions are both longer-ranged and more complex than was previously understood. The results provide the basis for accurate microscopic models of the finite temperature magnetic properties of yttrium iron garnet, necessary for next-generation electronic devices.

Nonmagnetic impurities and roughness effects on the finite temperature magnetic properties of core–shell spherical nanoparticles with antiferromagnetic interface coupling

Being inspired by a recent study (Dimitriadis et al., 2015), we study the finite temperature magnetic properties of the spherical nanoparticles with antiferromagnetic interface coupling including quenched (i) surface and (ii) interface nonmagnetic impurities (static holes) as well as (iii) roughened interface effects. The particle core is composed of ferromagnetic spins, and it is surrounded by a ferromagnetic shell. By means of Monte Carlo simulation based on an improved Metropolis algorithm, we implement the nanoparticles using classical Heisenberg Hamiltonians. Particular attention has also been devoted to elucidate the effects of the particle size on the thermal and magnetic phase transition features of these systems. For nanoparticles with imperfect surface layers, it is found that bigger particles exhibit lower compensation point which decreases gradually with increasing amount of vacancies, and vanishes at a critical value. In view of nanoparticles with diluted interface, our Monte Carlo simulation results suggest that there exists a region in the disorder spectrum where compensation temperature linearly decreases with decreasing dilution parameter. For nanoparticles with roughened interface, it is observed that the degree of roughness does not play any significant role on the variation of both the compensation point and critical temperature. However, the low temperature saturation magnetizations of the core and shell interface regions sensitively depend on the roughness parameter.

Extended spin model in atomistic simulations of alloys

An extended atomistic spin model allowing for studies of the finite temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parameterization from a first principles basis, interpolating from both the low temperature ferromagnetic and the high temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material Permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.

Anomalous spin disordered properties of strongly correlated honeycomb compound In3Cu2VO9

We study the ground-state and finite-temperature magnetic properties of an interlayer frustrated J1 − J2 − Jc Heisenberg model on three-dimensional honeycomb lattice by employing the Schwinger boson mean-field theory, focusing on the low-energy physics in In3Cu2VO9. We find that with the increase of interlayer coupling Jc from 0 to 3.6 meV, the interlayer frustrated system transits from an antiferromagnetic (AFM) phase to a state with intralayer AFM order and interlayer disorder. This spin disordered phase explains not only the intralayer phase transition at TN = 38 K, but also the qualitative behaviors of the intermediate-temperature specific heat and magnetic susceptibility of In3Cu2VO9.