Noncollinear Magnetism Research Articles

Stability and magnetism of FeN high-pressure phases.

Most of the experimentally discovered compounds in the iron-nitrogen system belong to the low concentration part of the Fe-N phase diagram. In our paper, which is based on ab initio calculations, we have studied the formation and stability of high-pressure iron mono-nitride phases, and in particular a new magnetic phase with a NiAs-type structure. We have investigated the role of dynamic, thermodynamic and electronic properties, such as electronic correlations and pressure-induced phase stabilisation. Additionally, we have demonstrated that the new hexagonal FeN phase is stable over a wide range of external pressures and can persist at zero pressure as a metastable phase. Further, we have shown that this phase has a relatively low Curie temperature and may possess non-collinear magnetism.

Competitive exchange interaction and noncollinear magnetic structure in Mn-based Ni2In type alloys

Novel magnetic skyrmion state has recently been identified in the centrosymmetric hexagonal MnNiGa alloy, rendering it interesting for the exploration of the origin of noncollinear magnetism in MnNiGa and related Ni2In-type MnMX alloys. In this work, we investigate the magnetic interaction and noncollinear magnetic structure in MnNiGa, MnFeGe and MnCoSn alloys, mainly by the method of first principles studies. It is revealed that in these systems, the shortest Mn-Mn separation, denoted by the parameter d1, plays the dominant role in determining the magnetic state. The calculation of the exchange parameters Jij shows that they exhibit a competing ferromagnetic and antiferromagnetic interaction along the c axis. A noncollinear calculation that considers a canting Mn moment on the neighbour sites reveals a stable state at a specific tilting angle. Preliminary experimental results for MnFeGe1-xSnx (x = 0, 0.05 and 0.1) alloys demonstrate a reduced saturated magnetization with a growing value of d1, proving the validity of our calculation. The calculation of the magnetocrystalline anisotropy and quality factor further rationalize the possibility of hosting magnetic skyrmions or bubbles in Mn-based Ni2In alloys.

Helical magnetism in Sr-doped CaMn7O12 films

Noncollinear magnetism can play an important role in multiferroic materials but is relatively understudied in oxide heterostructures compared to their bulk counterparts. Using variable temperature magnetometry and neutron diffraction, we demonstrate the presence of helical magnetic ordering in $\mathrm{CaM}{\mathrm{n}}_{7}{\mathrm{O}}_{12}$ and $\mathrm{C}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{M}{\mathrm{n}}_{7}{\mathrm{O}}_{12}$ (for $x$ up to 0.51) thin films. Consistent with bulk $\mathrm{C}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{M}{\mathrm{n}}_{7}{\mathrm{O}}_{12}$, the net magnetization increases with Sr doping. Neutron diffraction confirms that the helical magnetic structure remains incommensurate at all values of $x$, while the fundamental magnetic wavevector increases upon Sr substitution. This result demonstrates a chemical-based approach for tuning helical magnetism in quadruple perovskite films and enables future studies of strain and interfacial effects on helimagnetism in oxide heterostructures.

Noncollinear spin density of an adatom on a magnetic surface

We show that an individual adatom on a magnetic surface can exhibit a noncollinear spin density. Using density functional theory we study Co and Ir adatoms on a Mn monolayer on the W(110) surface which possesses a noncollinear canted spin structure. Due to hybridization with the nearest and next-nearest Mn atoms of the monolayer the spin direction of the underlying substrate is encoded into the orbitals of the adatom. This explains recent scanning tunneling microscopy experiments showing a spin sensitive shape asymmetry of adatoms [Serrate \emph{et al.}, Phys. Rev. B \textbf{93}, 125424 (2016)] which confirms the intra-atomic noncollinear magnetism of the adatoms.

Magnetism induced by nonlocal spin-entangled electrons in a superconducting spin-valve

In the traditional view, the magnetic moment appearing in the superconducting region is induced by equal-spin triplet superconducting correlations in superconductor (S) ferromagnet (F) heterostructure with noncollinear magnetization. In this paper, we represent that in (N-normal-metal) spin-valve structure the induced magnetic moment emerging in both the S and N regions can also be generated by Cooper pair splitting: one electron coherently tunnels from the S layer into the F1 layer, and the other one stays in the S layer or tunnels into the N layer. Two electrons are spatially separated from each other but their total spin ground state is entangled in this process. In contrast, the magnetic moment induced by the equal-spin triplet correlations hardly penetrates from the S layer into the N layer. In particular, by tuning the size of the exchange field and the thickness of the F1 layer, one may control the direction of the induced magnetic moment in the N layer. This interesting phenomenon can be attributed to the phase-shift obtained by the spin-entangled electrons. Our theoretical proposal will offer an effective way to control the entanglement of the nonlocal electrons, and also may provide possible explanations for previous and recent experimental observations (Stamopoulos et al 2005 Phys. Rev. B 72 212514; Ovsyannikov et al 2016 J. Exp. Theor. Phys. 122 738; Flokstra et al 2016 Nat. Phys. 12 57).

First-Principle Calculation of Thermoelectric Coefficients for the Nanocluster Lattices of Noble Metals

The results of calculating the thermal electromotive force for a crystalline state and a model lattice of nanoclusters of noble transition metals Ag, Au, and Pd are presented. The electron structure was calculated by the method of density functional theory (DFT) with the plane-wave pseudopotential and the generalized gradient approximation of the density functional with consideration for relativistic effects and noncollinear magnetism. The electron transport coefficients were found by analyzing the electron structure with the use of the semiclassic solution of the kinetic Boltzmann equation in the constant relaxation time approximation. The obtained results on the temperature dependence of the thermal electromotive force demonstrated quantitative coincidence with the tabular values within a temperature range of 200–500 K for crystalline Au and 200–400 K for Ag and Pd. At 300 K, the precision of calculated results attained 4% for Au. For the simplified model of the nanostructured material representing lattices of Me13 nanoclusters, the thermal electromotive force was revealed to grow by an order of magnitude in comparison with the crystalline state.

Gilbert damping in FeCo alloy: From weak to strong spin disorder

We study systematically the Gilbert damping of FeCo alloy from first-principles scattering calculations. For collinear magnetization, the nonmonotonic concentration dependence of the reported Gilbert damping is reproduced. At Co-rich concentration, the Gilbert damping in the faced-centred-cubic phase is found to be a half to a third of that in the body-centred-cubic phase for the same alloy concentration. Spin-wave increases the Gilbert damping in the form consistent with the previously discovered spin pumping in noncollinear magnetization. We also consider the Gilbert damping with strong spin disorder that significantly reduces the total magnetization from its saturation magnetization. In such cases, the dimensionless Gilbert damping parameter tends to diverge near the ferromagnetic-paramagnetic phase transition point while the damping frequency saturates and therefore is more appropriate to describe the damping in magnetization dynamics with strong spin disorder. The spin-mixing conductance that gives rise to the interface damping enhancement is almost unaffected by spin waves or strong spin fluctuation even near the transition point.

Real-space pseudopotential method for calculating magnetocrystalline anisotropy

We present a real-space pseudopotential method for calculating magnetocrystalline anisotropy within relativistic density-functional theory. The spin-orbit interaction, a fundamental origin of magnetocrystalline anisotropy, is incorporated with norm-conserving pseudopotentials expressed on a real-space grid. We demonstrate the utility of our method by calculating the magnetocrystalline anisotropy constant, ${K}_{1}$, of ${\mathrm{YCo}}_{5}$, which is a prototype compound with large ${K}_{1}$. Our calculated ${K}_{1}$ agrees with previous theoretical work. We show that our formalism also works for describing magnetocrystalline anisotropy in other transition-metal compounds, such as ${\mathrm{Mn}}_{2}\mathrm{Ga}$ and FeNi, which have modest values for ${K}_{1}$. Our real-space pseudopotential method is well suited for a parallel computing environment and is an efficacious approach to solving the relativistic Kohn-Sham equations, which include spin-orbit effects as well as noncollinear magnetism.

Inverse magnetostrictive stress sensors based on crossed pinned CoFeB/MgO/CoFeB tunnel junctions

The usage of double pinned magnetic CoFeB/MgO/CoFeB tunnel junctions with noncollinear magnetizations as sensors of mechanical stress is shown. For this purpose, tunnel magnetoresistive (TMR) stacks with MnIr based pinning of both electrodes have been prepared. By using different MnIr thicknesses, we were able to realize noncollinear magnetizations. A series of annealing steps with a field perpendicular to the initial exchange bias direction have been performed in order to find the optimum thermal treatment for the best performance of the sensors. On these systems, bending experiments were completed which, show the suitability of our double-pinned TMR elements as stress sensors and their performance in differentiating tensile and compressive stress.

Noncollinear magnetism in nanosized cobalt chromite

Using a combination of neutron diffraction with XYZ polarization analysis and magnetization measurements, the noncollinear magnetism in nanosized cobalt chromite, a potential multiferroic material, is revealed. For noninteracting 26.9(1) nm nanoparticles, a bulklike behavior is identified, including a ferrimagnetic Curie temperature of 99 K and a transition to the spin spiral magnetic phase at 27 K with a temperature-dependent, incommensurate propagation vector. A lock-in transition towards a commensurate propagation vector is not observed. Much smaller, 3.1(2) nm, nanoparticles reveal a strong cluster glass behavior, characterized by ferrimagnetic behavior below the Curie temperature of 43 K and a transition to asperomagnetic behavior at 18 K, with the absence of any magnetic reflections at a base temperature of 5 K.

Firefly Algorithm Applied to Noncollinear Magnetic Phase Materials Prediction.

In most noncollinear crystal magnets, the number of metastable states is quite large and any calculation that tries to predict the ground state can fall into one of the possible metastable phases. In this work, we generalize the population based meta-heuristic firefly algorithm to the problem of the noncollinear magnetic phase ground state prediction within density functional theory (DFT). We extend the different steps in the firefly algorithm to this specific problem by using polarized constrained DFT calculations, whereby using Lagrange multipliers the directions of the atom magnetic moments remain fixed. By locking the directions of the magnetic moments at each search iteration, the method allows one to explore the entire Born-Oppenheimer energy surface of existing and physically plausible noncollinear configurations present in a crystal. We demonstrate that the number of minima can be large, which restrains the use of exhaustive searches.

Antiferromagnetic order in CaK(Fe1−xNix)4As4 and its interplay with superconductivity

The magnetic order in CaK(Fe[1-x]Ni[x])4As4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated to the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and non-collinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.

Spin-triplet supercurrents in Josephson junctions containing strong ferromagnetic materials.

The proximity effect between a superconducting material and a non-superconducting normal metal can extend over distances of the order of micrometres at sufficiently low temperatures. If the normal metal is replaced by a ferromagnetic material, the spatial extent of the proximity effect drops precipitously due to the exchange splitting between the majority and minority spin bands in the ferromagnet. In 2001, several theorists predicted that spin-triplet pair correlations could be induced in proximity systems involving multiple ferromagnetic materials (or multiple domains in one material) with non-collinear magnetizations. Such spin-triplet pair correlations should extend deep into the ferromagnet, producing a long-range proximity effect. In this paper, we review our experimental work in this area, which has focused primarily on Josephson junctions containing strong ferromagnetic materials. We show that Josephson junctions containing particular combinations of strong ferromagnetic materials can carry spin-triplet supercurrent over distances of at least several tens of nanometres, whereas spin-singlet supercurrent in similar samples decays over a length scale of about 1 nm. We also mention important work by other groups; however, this article is not intended to be a review of the whole field.This article is part of the theme issue 'Andreev bound states'.

Comparative study of DFT+U functionals for non-collinear magnetism

We performed comparative analysis for DFT+U functionals to better understand their applicability to non-collinear magnetism. Taking LiNiPO4 and Sr2IrO4 as examples, we investigated the results out of two formalisms based on charge-only density and spin density functional plus U calculations. Our results show that the ground state spin order in terms of tilting angle is strongly dependent on Hund J. In particular, the opposite behavior of canting angles as a function of J is found for LiNiPO4. The dependence on the other physical parameters such as Hubbard U and Slater parameterization is investigated. We also discuss the formal aspects of these functional dependences as well as parameter dependences. The current study provides useful information and important intuition for the first-principles calculation of non-collinear magnetic materials.

Structural and magneto-transport properties of Mn1+xCo1−xSn (x = 0.0–1.0) alloys

Hexagonal MnMX alloys present interesting properties like non-collinear magnetism, reentrant spin glass, novel skyrmion-domain formation, etc. In this work, we synthesized Mn1+xCo1−xSn (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) alloys using arc melting and melt spinning techniques. A pure Ni2In-type structure was obtained in alloys of x = 0.4–0.8. The lattice parameters increase systematically with an increase in the Mn content. ZFC-FC measurements show reentrant spin-glass-like and non-collinear magnetic transition, reflecting the competition of several exchange interactions. The magnetic properties also show strong dependence on the anti-site disorder. First-principles calculations are applied to explain the evolution of the magnetic state. The magnetoresistance values for all alloy ribbons are negative and linearly changed, dominated by the spin-dependent scattering. Both magnetoresistance and anomalous Hall effect show hysteresis at low temperatures, consistent with the magnetization measurements.

Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling

Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been studied via inelastic neutron scattering. A simple Heisenberg model with a single-ion anisotropy describes most features of the spin-wave dispersion curves. However, there is shown to be a renormalization of the magnon energies located at around 11 meV. Since both the magnon-magnon interaction and magnon-phonon coupling can affect the renormalization in a noncollinear magnet, we have accounted for both of these couplings by using a Heisenberg XXZ model with 1=S expansions [1] and the Einstein site phonon model [13], respectively. This quantitative analysis leads to the conclusion that the renormalization effect primarily originates from the magnon-phonon coupling, while the spontaneous magnon decay due to the magnon-magnon interaction is suppressed by strong two-ion anisotropy.

Crystallography Based on Synchrotron Radiation: Experiments of Russian Users of the ESRF BM01 Diffraction Beam Line

The review deals with studies carried out at the BM01 diffraction beam line of the European Synchrotron Radiation Facility. X-ray diffraction analysis of single-crystal proton conductors demonstrates the possibilities of a precise diffraction experiment in which phase transitions associated with the release of crystallization water and transformation of the network of hydrogen contacts are investigated. Scanning of reciprocal space with the help of a 2D detector enables us to determine the new phase symmetry in a thin-film multiferroic sample based on bismuth ferrite (the given phase is stable only under thin-film conditions). A combination of Bragg and diffusion scattering processes is employed to investigate the interdependence between the structure and dynamics of a crystal lattice and the physical properties of a relaxor material with a perovskite-like structure. The complementarity and synergy of neutron and synchrotron experiments are demonstrated using the combined study of materials from the manganese-silicide group, which has revealed a nontrivial relationship between magnetic and crystallographic chiralities in noncollinear magnets. Although the given review is limited to only a few experiments carried out by Russian scientists at the BM01 beam line, they still illustrate a variety of problems that can be solved using a modern diffraction station where the bending magnet of a third-generation synchrotron is employed as the synchrotron radiation source.

Current phase relation from graphs and diagrams and application to thick ferromagnetic Josephson junctions

In this work we present a method of representing terms in the current-phase-relation of a ballistic Josephson junction by combinations of diagrams, used in previous work to represent an equivalent of the matching condition determinant of the junction. This is accomplished by the expansion of the logarithm of this determinant in Taylor series and keeping track of surviving terms, i.e. terms that do not annihilate each other. The types of the surviving terms are represented by connected graphs, whose points represent diagrammatic terms of the determinant expansion. Then the theory is applied to obtain approximations of the current-phase relation of relatively thick ballistic ferromagnetic Josephson junctions with non-collinear magnetizations. This demonstrates the versatility of the method in developing approximations schemes and providing physical insight into the nature of contributions to the supercurrent from the available particle excitations in the junction. We also discuss the strong second harmonic contribution to the supercurrent in junctions with three mutually orthogonal magnetization vectors and a weak intermediate ferromagnet.

Half-metallic superconducting triplet spin multivalves

We study spin switching effects in finite-size superconducting multivalve structures. We examine $\rm F_1F_2SF_3$ and $\rm F_1F_2SF_3F_4$ hybrids where a singlet superconductor ($\rm S$) layer is sandwiched among ferromagnet ($\rm F$) layers with differing thicknesses and magnetization orientations. Our results reveal a considerable number of experimentally viable spin valve configurations that lead to on-off switching of the superconducting state. For $\rm S$ widths on the order of the superconducting coherence length $\xi_0$, non-collinear magnetization orientations in adjacent $\rm F$ layers with multiple spin-axes leads to a rich variety of triplet spin-valve effects. Motivated by recent experiments, we focus on samples where magnetizations in the $\rm F_1$ and $\rm F_4$ layers exist in a fully spin-polarized half metallic phase, and calculate the superconducting transition temperature, spatially and energy resolved density of states, and the spin-singlet and spin-triplet superconducting correlations. Our findings demonstrate that superconductivity in these devices can be completely switched on or off over a wide range of magnetization misalignment angles due to the generation of equal-spin and opposite-spin triplet pairings.

Transport theory for femtosecond laser-induced spin-transfer torques

Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser pulse is accompanied by a nonthermal spin-polarized current of hot electrons. These spin currents are studied here theoretically in a spin valve with noncollinear magnetizations. To this end, we introduce an extended model of superdiffusive spin transport that enables the treatment of noncollinear magnetic configurations, and apply it to the perpendicular spin valve geometry. We show how spin-transfer torques arise due to this mechanism and calculate their action on the magnetization present, as well as how the latter depends on the thicknesses of the layers and other transport parameters. We demonstrate that there exists a certain optimum thickness of the out-of-plane magnetized spin-current polarizer such that the torque acting on the second magnetic layer is maximal. Moreover, we study the magnetization dynamics excited by the superdiffusive spin-transfer torque due to the flow of hot electrons employing the Landau–Lifshitz–Gilbert equation. Thereby we show that a femtosecond laser pulse applied to one magnetic layer can excite small-angle precessions of the magnetization in the second magnetic layer. We compare our calculations with recent experimental results.