Noncollinear Magnetic Configurations Research Articles

Magnon spin current from a non-collinear magnetic phase in a compensated rare earth ferrimagnet

The rare earth iron garnets have been commonly treated like ferromagnets due to the strong exchange coupling between the Fe3+ sublattices. However, the exchange coupling between the rare earth (RE) and transition metal (TM) in the RE-TM compounds is relatively weak and cannot hold the magnetic moments rigidly in a collinear state upon effective anisotropy energy. In this work, we show that a non-collinear magnetic phase is readily presented for a rare earth ferrimagnetic insulator, gadolinium iron garnet (GdIG), when a small magnetic field (H) is applied in the hard axis. We excite magnons from both the collinear and non-collinear magnetic configuration in GdIG via the spin Seebeck effect and detect these magnons in the adjacent normal metal. The comparison between the collinear and non-collinear magnetic configurations reveals a comparable magnon compensating temperature (TMM) for both cases. Moreover, as temperature decreases, the in-plane component of the magnon from the non-collinear case enhances and becomes comparable with that of the collinear case at TMM, representing the dominant role Gd3+ 4f spin plays at low T. Our study shows a wide magnetic tunability in the compensated ferrimagnetic insulator, which offers a myriad of opportunities for magnonic applications.

Magnetic tunability in tetragonal Mn–Rh–Ir–Sn inverse Heusler compounds

Gaining control over magnetic structure has been an ongoing challenge in materials that form complex, nanoscale, and non-collinear magnetic configurations. Recently, it was predicted that tuning the ratio of the Dzyaloshinskii–Moriya interaction to the uniaxial magnetic anisotropy in tetragonal inverse Heuslers through changes in composition could allow a range of interesting magnetic states to be accessed, from simple ferrimagnetism, to helical and antiskyrmionic phases. Here, we show tunability of the magnetic phase behavior in the Mn–Rh–Sn system through Ir substitution on the Rh substructure. Iridium substitution correlates to an increase in the strength of ferromagnetic exchange couplings, at the expense of antiferromagnetic exchange couplings. However, we do not observe the complex non-collinear magnetic phases proposed previously, likely due to the extremely narrow composition window where these phases are predicted to form in a bulk sample. This work highlights the sensitivity of complex magnetic structures to stoichiometry, which makes them difficult to discover empirically.

Coherent excitation of spin waves in synthetic antiferromagnets by subpicosecond spin-transfer-torque

The subpicosecond spin-transfer-torque (STT) related to the laser-induced spin current is an effective approach to exciting spin waves in a noncollinear magnetic configuration. However, its validity on the excitation of spin waves in a synthetic antiferromagnet is still far from conclusive. Here, we investigate the impact of subpicosecond STT on the coherent excitation of spin waves in CoFeB/Ru/CoFeB. After pulsed-laser exposure, we observe both the precessional signals of acoustic mode and optical mode and find that their precession amplitudes are magnetic configuration-dependent. Not as the common explanation in the previous reports that the spin waves are triggered by the transient modulation of effective field, we attribute the initial stimulation of magnetization precession mainly to the subpicosecond STT. This mechanism is further confirmed by the field-dependent precession amplitudes. Our results pave a way for the practice of subpicosecond STT in the potential application at terahertz range.

Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers.

The advent of twist engineering in two-dimensional crystals enables the design of van der Waals heterostructures with emergent properties. In the case of magnets, this approach can afford artificial antiferromagnets with tailored spin arrangements. Here we fabricate an orthogonally twisted bilayer by twisting two CrSBr ferromagnetic monolayers with an easy-axis in-plane spin anisotropy by 90°. The magnetotransport properties reveal multistep magnetization switching with a magnetic hysteresis opening, which is absent in the pristine case. By tuning the magnetic field, we modulate the remanent state and coercivity and select between hysteretic and non-hysteretic magnetoresistance scenarios. This complexity pinpoints spin anisotropy as a key aspect in twisted magnetic superlattices. Our results highlight control over the magnetic properties in van der Waals heterostructures, leading to a variety of field-induced phenomena and opening a fruitful playground for creating desired magnetic symmetries and manipulating non-collinear magnetic configurations.

Phase change caused by Jahn-Teller distortion in Ni doped MnCo2O4 spinel

Polycrystalline NixMnCo2−xO4 (x = 0, 0.25, 0.5, 0.75, and 1) powders were produced. The addition of nickel to the MnCo2O4 system modifies the magnetic ion concentration and regulates the Jahn-Teller distortion of the system. We have intended to replace octahedral Co with Ni. Beyond expectation, we found that the different types of Co were replaced by Ni with the doping concentration increasing. When the concentration of Ni doping is low, replacing the non-magnetic octahedral Co3+ (3d6, t2 g6eg0), Ni3+ (3d7, t2 g6eg1) enters octahedral sites. The bond angle between tetrahedral Co and octahedral Mn enlarges and the Neel temperature increases from 164 to 207 K. As Ni enters the octahedral site, the net magnetic moment at low temperature increases, which is believed that the sample exhibits a non-collinear magnetic configuration. As the doping Ni concentration is further increased, Ni2+ (3d8, eg4t2 g4) unexpectedly replaces the tetrahedral Co2+ (3d6, eg4t2 g4). The replacement makes the non-collinear frustrated transition temperature increase from 74 K to 87 K. The interaction between Co2+Tet-Ni3+Oct and Ni3+Oct-Mn3+Oct competes with each other to form a spin frustration at low temperature. An antiferromagnetic-dominated spin-glass phase occurs at low temperatures in the NiMnCoO4 system. The saturation magnetization in the NixMnCo2−xO4 system increases from 0.40 to 0.67 μB/formular (2 K).

Magnetic structures and correlated physical properties in antiperovskites

Compounds with perovskite structures have become one of the focuses in both materials science and condensed matter physics because of their fascinating physical properties and potential functionalities correlated to magnetic structures. However, the understanding of the intriguing physical properties is still at an exploratory stage. Herein, owing to the magnetic frustration prompted by Mn6N or Mn6C octahedra, the abounding magnetic structures of antiperovskites, including collinear antiferromagnetic, collinear ferromagnetic, collinear ferrimagnetic, non-collinear magnetic, and non-coplanar magnetic spin configurations, are systematically introduced through the updated coverage. In addition, owing to the “spin-lattice-charge” coupling of antiperovskites, a large number of physical properties, such as anomalous thermal expansion, giant magnetoresistance, anomalous Hall effect, piezomagnetic/baromagnetic effects, magnetocaloric effect, barocaloric effect, etc ., are summarized by combining the discussions of the determined magnetic structures. This review aims to clarify the current research progress in this field, focusing on the relationship between the magnetic structures and the correlated physical properties, and provides the conclusion and outlook on further performance optimization and mechanism exploration in antiperovskites.

A deep investigation of NiO and MnO through the first principle calculations and Monte Carlo simulations

In this study, we use Hubbard-corrected density functional theory (DFT+U) to derive spin model Hamiltonians consisting of Heisenberg exchange interactions up to the fourth nearest neighbors and bi-quadratic interactions. We map the DFT+U results of several magnetic configurations to the Heisenberg spin model Hamiltonian to estimate Heisenberg exchanges. We demonstrate that the number of magnetic configurations should be at least twice the number of exchange parameters to estimate exchange parameters correctly. To calculate biquadratic interaction, we propose specific non-collinear magnetic configurations that do not change the energy of the Heisenberg spin model. We use classical Monte Carlo (MC) simulations to evaluate DFT+U results. We obtain the temperature dependence of magnetic susceptibility and specific heat to determine the Curie–Weiss and Néel temperatures. The MC simulations reveal that although the biquadratic interaction can not change the Néel temperature, it modifies the order parameter. We indicate that for a fair comparison between classical MC simulations and experiments, we need to consider the quantum effect by applying correction in classical MC simulations.

Non-collinear magnetic configuration mediated exchange coupling at the interface of antiferromagnet and rare-earth nanolayers

Mn_{3}Ir/CoFe bilayer is a prototypical exchange-coupled antiferromagnet (AF)–ferromagnet (FM) system. Nevertheless, a strong exchange coupling between FM and rare-earth(RE) interfaces of Fe/Dy and Fe/Tb has been established earlier. Strong coupling at the FM–RE interface originates from the number of irreversible spins owing to the imbalance in the non-collinear configuration in RE. However, exchange coupling between AF–RE could not be established due to the minimal number of irreversible spins in AF and RE. A frustrated inter-domain magnetic interaction leads to the coexistence of spin-freezing-like ordering around the temperature range of helical spin modulation at the exchange-coupled interfaces of RE-based specimens. To overcome the lack of coupling between the AF–RE interface, we use a sandwich structure of AF–FM–RE layers (Mn_{3}Ir/CoFe/Dy) as we demonstrate establishing considerable exchange bias in the system. Changing the bias direction during field cooling introduces possible differences in non-collinear directions (helicities), which affects the number of irreversible spins and consequent exchange coupling differently for opposite directions. The non-collinear structures in RE are topologically stable; thus, their directions of orientation can be regarded as an additional degree of freedom, which can be manipulated in all-spin-based technology.

Efficient parameterisation of non-collinear energy landscapes in itinerant magnets

Magnetic exchange interactions determine the magnetic groundstate, as well as magnetic excitations of materials and are thus essential to the emerging and fast evolving fields of spintronics and magnonics. The magnetic force theorem has been used extensively for studying magnetic exchange interactions. However, short-ranged interactions in itinerant magnetic systems are poorly described by this method and numerous strategies have been developed over the years to overcome this deficiency. The present study supplies a fully self-consistent method for systematic investigations of exchange interactions beyond the standard Heisenberg model. In order to better describe finite deviations from the magnetic ground state, an extended Heisenberg model, including multi-spin interactions, is suggested. Using cross-validation analysis, we show that this extended Heisenberg model gives a superior description for non-collinear magnetic configurations. This parameterisation method allows us to describe many different itinerant magnetic systems and can be useful for high-throughput calculations.

Anomalous Hall conductivity control in Mn3NiN antiperovskite by epitaxial strain along the kagome plane

Antiferromagnetic manganese-based nitride antiperovskites, such as Mn$_3$NiN, hold a triangular frustrated magnetic ordering over their kagome lattice formed by the Mn atoms along the (111)-plane. As such, frustration imposes a non-trivial interplay between the symmetric and asymmetric magnetic interactions, which can only reach equilibrium in a noncollinear magnetic configuration. Consequently, the associated electronic interactions and their possible tuning by external constraints, such as applied epitaxial strain, play a crucial role in defining the microscopic and macroscopic properties of such topological condensed matter systems. Thus, in the present work, we explored and explained the effect of the epitaxial strain imposed within the (111)-plane, in which the magnetic and crystallographic symmetry operations are kept fixed, and only the magnitude of the ionic and electronic interactions are tuned. We found a linear shifting in the energy of the band structure and a linear increase/decrease of the available states near the Fermi level with the applied strain. Concretely, the compression strain reduces the Mn-Mn distances in the (111) kagome plane but linearly increases the separation between the stacked kagome lattices and the available states near the Fermi level. Despite the linear controlling of the available states across the Fermi energy, the anomalous Hall conductivity shows a non-linear behavior where the $\sigma_{111}$ conductivity nearly vanishes for tensile strain. On the other hand, $\sigma_{111}$ fetches a maximum increase of 26\% about the unstrained structure for a compression value close to $-$1.5\%.This behavior found an explanation in the non-divergent Berry curvature within the kagome plane, which is increased for constraining but significantly reduced for expansion strain values...

Density functional theory investigations into the magnetic ordering of U3O8

Density functional theory (DFT) has been highly successful in supporting experimental materials science; however, a correct electronic ground state is required to realize the full theoretical capacity of DFT. The uranium oxides, $\ensuremath{\alpha}\ensuremath{-}{\mathrm{U}}_{3}{\mathrm{O}}_{8}$ in particular, are simultaneously technologically important materials and theoretically challenging for DFT because the uranium magnetic ground state is not obvious. This is true for both experiment and theory---magnetic susceptibility measurements indicate an antiferromagnetic (AFM) ground state with transitions near 4.2 and 8.0 K, but the ordering itself is not known. Theoretical literature reports are in contradiction, with independent studies finding paramagnetic, ferromagnetic (FM), and AFM states as the lowest energy configuration. However, recent inelastic neutron scattering experiments suggested an uninvestigated magnetic structure with ordering along the [0.5 1 1] plane, motivating a theoretical reinvestigation. Using this insight, we calculated the relative energy of FM and AFM orderings along [0.5 1 1], [0.5 0 0], [0 1 0], and [0 0 1] using noncollinear DFT calculations with spin-orbital coupling. We found that the [0.5 1 1] AFM structure is lower in energy than FM or AFM orderings along the low Miller index directions. We also investigated polarization of the magnetic moment along each lattice vector and found that polarization along the out-of-plane direction is the energetically preferred orientation for the AFM structures. Additionally, we found in all calculations that moments initially pointing along the in-plane lattice vectors significantly relax until they point along the coordinate between the two crystallographically distinct uranium sites with complex noncollinear magnetic configurations. The new [0.5 1 1] AFM magnetic structure provides an additional path forward toward understanding the electronic structure of \ensuremath{\alpha}-${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ and lends theoretical credibility to recent neutron scattering results.

Spin Hall magnetoresistance of CoFe2O4/Pt heterostructures with interface non-collinear magnetic configurations

We have investigated the spin Hall magnetoresistance (SMR) in CoFe2O4/Pt heterostructures deposited on (001) MgO and (001) MgAl2O4 substrates, respectively. It is found that the interface non-collinear magnetic configurations should exist in both of the two films in the whole temperature range from 5 to 300 K but mainly dominate the spin transport properties at low temperature. Furthermore, the spin transport has featured epitaxial-strain-modified characteristics induced by different substrates, which has been investigated by magnetic curves, SMR, anisotropic magnetoresistance, and Hall effect measurements.

Adiabatic spin dynamics and effective exchange interactions from constrained tight-binding electronic structure theory: Beyond the Heisenberg regime

We consider an implementation of the adiabatic spin dynamics approach in a tight-binding description of the electronic structure. The adiabatic approximation for spin-degrees of freedom assumes that the faster electronic degrees of freedom are always in a quasi-equilibrium state, which significantly reduces the numerical complexity in comparison to the full electron dynamics. Non-collinear magnetic configurations are stabilized by a constraining field, which allows to directly obtain the effective magnetic field from the negative of the constraining field. While the dynamics are shown to conserve energy, we demonstrate that adiabatic spin dynamics does not conserve the total spin angular momentum when the lengths of the magnetic moments are allowed to change, which is confirmed by numerical simulations. Furthermore, we develop a method to extract an effective two-spin exchange interaction from the energy curvature tensor of non-collinear states, which we calculate at each time step of the numerical simulations. We demonstrate the effect of non-collinearity on this effective exchange and limitations due to multi-spin interactions in strongly non-collinear configurations beyond the regime where the Heisenberg model is valid. The relevance of the results are discussed with respect to experimental pump-probe experiments that follow the ultra-fast dynamics of magnetism.

Unified formulation of interfacial magnonic pumping from noncollinear magnets

We establish a general formulation for spin current pumped by magnons at the interface between a normal metal and a magnetic insulator, valid for any noncollinear magnetic configuration in the linear spin wave regime. This current is generated by driving the system in a nonequilibrium state, covering setups such as thermal spin injection (spin Seebeck effect) or spin voltage by irradiation of the insulator (spin pumping). We illustrate the formula in the special case of a honeycomb topological ferromagnet, for simplicity, and cover both the spin Seebeck and spin pumping setups. We show how the topological parameters influence the spin current and propose a way to obtain a contribution mainly from the topological edge magnons in the spin pumping case.

Ab initiocalculation of the magnetic Gibbs free energy of materials using magnetically constrained supercells

We present a first-principles approach for the computation of the magnetic Gibbs free energy of materials using magnetically constrained supercell calculations. Our approach is based on an adiabatic approximation of slowly varying local moment orientations, the so-called finite-temperature disordered local moment picture. It describes magnetic phase transitions and how electronic and/or magnetostructural mechanisms generate a discontinuous (first-order) character. We demonstrate that the statistical mechanics of the local moment orientations can be described by an affordable number of supercell calculations containing noncollinear magnetic configurations. The applicability of our approach is illustrated by firstly studying the ferromagnetic state in bcc Fe. We then investigate the temperature-dependent properties of a triangular antiferromagnetic state stabilizing in two antiperovskite systems Mn$_3$AN (A = Ga, Ni). Our calculations provide the negative volume expansion of these materials as well as the ab initio origin of the discontinuous character of the phase transitions, electronic and/or magnetostructural, in good agreement with experiment.

Controllable chiral Majorana modes by noncollinear magnetic configuration in a topological Josephson junction

We theoretically present controllable chiral Majorana modes (CMMs) by noncollinear magnetic configuration in a Josephson junction on a topological insulator, where two ferromagnetic insulators (FIs) are sandwiched in between two superconductors (SCs). It is shown that an additional phase shift is induced by the different chiralities of the CMMs at the two FI/SC interfaces. Its magnitude is determined by the misorientation angle θ, which can result in the 0 − π state transition and ϕ 0 supercurrent. Particularly, due to the selective equal spin Andreev reflection, the coexistence of fully spin-polarized spin-triplet and -singlet correlations is exhibited. However, for the two magnetizations only along the y-axis, there exist no additional phase shift and the 100% supercurrent magnetoresistance can be exhibited. The results can be employed to not only identify the topological SCs but also design a perfect topological supercurrent spin valve device.

Observation of structural distortion and topological Hall effect in noncollinear antiferromagnetic hexagonal Mn3Ga magnets

The structure, magnetism, and topological transport properties of noncollinear antiferromagnetic hexagonal Mn3Ga magnets have been investigated. Structural analyses reveal that as the temperature decreases, hexagonal Mn3Ga shows a Jahn–Teller-like distortion at T = 120 K. The distortion results in the anisotropy decreasing with decreasing temperature. Positive magnetoresistance is observed, and humps occur at low magnetic fields, which can be understood by breaking the ordered antiferromagnetic configuration by an external field and the spin-dependent scattering mechanism, respectively. In addition, a large topological Hall effect is observed in Mn3Ga, which originates from the noncollinear triangular magnetic configuration with slight distortion. The results are discussed in the context of coupling with noncollinear spin canting, lattice distortion, and topological transport.

MagneticTB: A package for tight-binding model of magnetic and non-magnetic materials

We present a Mathematica program package MagneticTB, which can generate the tight-binding model for arbitrary magnetic space group. The only input parameters in MagneticTB are the (magnetic) space group number and the orbital information in each Wyckoff position. Some useful functions including getting the matrix expression for symmetry operators, manipulating the energy band structure by parameters, and interfacing with other software are also developed. MagneticTB can help to investigate the physical properties in both magnetic and non-magnetic system, especially for topological properties. Program summaryProgram Title: MagneticTBCPC Library link to program files:https://doi.org/10.17632/kws9xbvz3y.1Developer's repository link:https://github.com/zhangzeyingvv/MagneticTBLicensing provisions: GNU General Public Licence 3.0Programming language: MathematicaExternal routines/libraries: ISOTROPY (iso.byu.edu)Nature of problem: Construct the symmetry adapted tight-binding model for the system with arbitrary magnetic space group.Solution method: The symmetry-adapted tight-binding model in this code is based on the Bloch theorem and group representation theory.Additional comments including restrictions and unusual features: This code supports the spinless system and spinful system with collinear or non-collinear magnetization configuration.

Benchmark for Ab Initio Prediction of Magnetic Structures Based on Cluster-Multipole Theory

The cluster multipole (CMP) expansion for magnetic structures provides a scheme to systematically generate candidate magnetic structures specifically including noncollinear magnetic configurations adapted to the crystal symmetry of a given material. A comparison with the experimental data collected on MAGNDATA shows that the most stable magnetic configurations in nature are linear combinations of only few CMPs. Furthermore, a high-throughput calculation for all candidate magnetic structures is performed in the framework of spin-density functional theory (SDFT). We benchmark the predictive power of CMP+SDFT with $2935$ calculations, which show that (i) the CMP expansion administers an exhaustive list of candidate magnetic structures, (ii) CMP+SDFT can narrow down the possible magnetic configurations to a handful of computed configurations, and (iii) SDFT reproduces the experimental magnetic configurations with an accuracy of $\pm0.5\,\mu_\text{B}$. For a subset the impact of on-site Coulomb repulsion $U$ is investigated by means of $1545$ CMP+SDFT+U calculations revealing no further improvement on the predictive power.

Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys

Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled $3d$ band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys.