Noncollinear Arrangement Research Articles

The phenomenon of magnetic compensation in the multi-component compounds (Tb,Y,Sm)Fe2 and their hydrides

The series of ferrimagnetic Laves-phase compounds (Tb,Y)0.8Sm0.2Fe2 obtained by gradual substitution of yttrium for terbium as well as the hydrides (Tb,Y)0.8Sm0.2Fe2H3.7 (amount of hydrogen close to the maximum allowed by the structure type) are investigated. We find that the MgCu2-type crystal structure is preserved after hydrogenation while the unit cell volume increases by ∼28%. The Curie temperature (TC) of hydrides is almost half the TC’s of the parent compounds. For (Tb1−xYx)0.8Sm0.2Fe2 with Y concentration x=0.6, we observe a magnetic compensation, i.e. mutual cancellation of magnetic moments of the sublattices, which is fully consistent with the model of a three-sublattice collinear ferrimagnet. In the hydrides (Tb1-xYx)0.8Sm0.2Fe2H3.7 the magnetic compensation occurs at x=0.2. In this case, we argue that a non-collinear magnetic arrangement is placement in the compound. We demonstrate that hydrogenation is a simple tool of changing the composition for which the phenomenon of magnetic compensation occurs.

Magnetocrystalline anisotropy of small CoPt binary alloy metal clusters: interplay between structure, chemical composition, and spin-orbit coupling

The ground-state magnetic properties of ConPtm binary alloy clusters of size N = n + m ≤ 9 are studied systematically as a function of size, composition, and chemical order in the framework of the generalized gradient approximation (GGA) to density functional theory (DFT). Interestingly, strong cluster-size and chemical-composition dependence exhibiting substantial values (0.4–22 meV/atom) in the magnetic anisotropy energy (MAE) are revealed. Such behavior crucially depends on the Pt moments induced by the proximity to Co, and on the resulting spin-orbit interactions at the Pt atoms. Moreover, we show that the MAEs and direction of magnetization can be tuned to some extent by varying the Pt concentration showing spin reorientations and the easy axis of magnetization is cluster size dependent. A thorough cluster-geometry optimizations together with a collinear and noncollinear magnetic moment arrangement sampling including the spin-orbit coupling are carried out to identify the ground-state and the low-lying structures. The relative directions between Co and Pt magnetic moments induced by spin-orbit interaction show an early tendency to arrange the magnetic moments along with Co and Pt layers in a way resembling the L10 phase with tetragonal crystal lattice which can be appreciated in the clusters having N ≥ 5 atoms. Our results show that planar-like structures are preferred in small cluster sizes (N ≤ 4 atoms) while for N ≥ 5 tend to form three-dimensional core-shell compact structures with a composition around 50/50% ordering maximizing the number of Co-Pt bonds. The average magnetic moment per atom $\bar {\mu }_{T}$ increases approximately linearly with Co content and important enhancement of the local Co moments is observed as a result of Pt doping for n + m ≤ 4. This is mainly a consequence of the increase in the number of Co d holes due to Co to Pt charge transfer. Finally, our results show important spin and orbital moments induced at the Pt atoms as well as significant orbital moments at the Co atoms .

“Triangular Antiferromagnet” RbFe(MoO4)2 with the Replacement of Nonmagnetic Ions

The magnetization curves, as well as electron spin resonance and nuclear magnetic resonance spectra of 87Rb+ ions, have been measured in the quasi-two-dimensional antiferromagnet RbFe(MoO4)2 on a triangular lattice with the random modulation of the exchange bond network. Random modulation has been performed by means of the partial substitution of nonmagnetic Rb+ ions for K+ ions. It has been shown that random static disorder thus created at an impurity ion concentration of 15% drastically changes the spin structure. A noncollinear structure with significant sublattice magnetization components transverse to the magnetic field occurs in the doped compound. At the same time, the spin structure in the pure compound has the same magnetic moment, but transverse spin components disappear near a magnetic field of one third of the saturation field (three-sublattice structure with two upward sublattices and one downward sublattice). The revealed doping-induced drastic rearrangement of the spin structure of the triangular antiferromagnet has been explained by the interplay between the contribution of dynamic fluctuations, which makes maximally collinear states favorable in free energy, and the contribution from freeze-in disorder, which ensures the energy gain for the maximally noncollinear arrangement of sublattices.

Spin orbit coupling effects on the non-collinear magnetism of structurally relaxed Fe/Cu (001) thin films: First principles calculations

The effect of the spin–orbit coupling interaction and atomic relaxation, on the ground state non-collinear magnetism of deposited Fe thin films, on a Cu (001) surface, is studied by using first principles density functional theory calculations. The total energy values and spin and orbital magnetic local moments are calculated for a Fe film of N = 6 atomic layers deposited on a 12 layer thick Cu film playing the role of substrate. As a first step, we allow the thin film and surface layers to atomically relax. We found important contractions of about 20% between the first and second, the third and fourth, and firth and sixth Fe layers. Then, we obtain collinear and non-collinear magnetic arrangements which agree with the main experimental resultssatte that show a non-collinear configuration at low temperatures (around 40 K). The magnetic properties of the first four Fe layers are well reproduced. The agreement is better than the results obtained ignoring the spin–orbit coupling and the structure relaxation. Although both calculations predict non-collinear configurations with energy values slightly above that of a collinear ground state. In some cases, the energy differences between two collinear (or non-collinear) solutions with different spin orientation axis is only under the value of one meV. The calculated orbital local moment arrangements are clearly similar to the spin magnetic local moment ones and show a correct dependence on the dimensionality aspects of the system when changing the spin axis quantization direction.

Bulk magnetization, temperature - Dependent ac susceptibility and dc resistivity study on Mn0.7+xZn0.3SixFe2-2xO4 (x = 0.0–0.3) spinel ferrites

Polycrystalline samples of spinel ferrite series, Mn0.7+x Zn0.3SixFe2-2xO4 (x = 0.0, 0.1, 0.2, 0.3), were characterized by X-ray powder diffractometry, bulk magnetization at 300 K and 80 K, temperature dependence of low field ac susceptibility (χac (T)) (T = 300–525 K) and dc electrical resistivity (ρdc (T)) (T = 300–772 K) measurements. The variation of intrinsic and extrinsic magnetic parameters as a function of Mn–Si concentration (x) has been discussed. It is found that the system possesses a non-collinear spin arrangement and the canting angle increases with x. The Néel temperatures determined experimentally from susceptibility measurement adequately reflected the values calculated theoretically. The χac (T) curves revealed normal ferrimagnetic behavior while ρdc (T) depicts characteristic semiconducting behavior. The activation energy is found to increase with x that can be explained in light of the structural and microstructural parameters.

Breaking the ring: 53Cr-NMR on the Cr8Cd molecular nanomagnet

An accurate experimental characterization of finite antiferromagnetic (AF) spin chains is crucial for controlling and manipulating their magnetic properties and quantum states for potential applications in spintronics or quantum computation. In particular, finite AF chains are expected to show a different magnetic behaviour depending on their length and topology. Molecular AF rings are able to combine the quantum-magnetic behaviour of AF chains with a very remarkable tunability of their topological and geometrical properties. In this work we measure the 53Cr-NMR spectra of the Cr8Cd ring to study the local spin densities on the Cr sites. Cr8Cd can in fact be considered a model system of a finite AF open chain with an even number of spins. The NMR resonant frequencies are in good agreement with the theoretical local spin densities, by assuming a core polarization field AC = −12.7 T μB−1. Moreover, these NMR results confirm the theoretically predicted non-collinear spin arrangement along the Cr8Cd ring, which is typical of an even-open AF spin chain.

The crystallographic, magnetic, and electrical properties of Gd3+-substituted Ni–Cu–Zn mixed ferrites

Polycrystalline Ni0.48Cu0.12Zn0.40GdxFe2-xO4 (where x = 0.00, 0.02, 0.04, 0.08, and 0.10) was synthesized by the conventional solid-state reaction technique. The structure and surface morphology were studied by X-ray diffraction (XRD) and high-resolution optical microscopy, respectively. The XRD patterns indicated that the compositions formed a cubic spinel structure for a small amount of Gd3+ substitution. With increasing Gd3+ concentration, a gradual rise in the intensity of the extra peaks was observed, and those peaks were identified as corresponding to GdFeO3 (orthoferrite). Rietveld refinement was performed on the XRD data of Gd3+-substituted Ni0.48Cu0.12Zn0.40 ferrite samples for further verification, and the results were found to be in good agreement with results reported in the literature. Variation of the lattice constant obeyed Vegard's law. Microstructural studies showed that the average grain diameter increases as the Gd3+ content increases up to an optimum concentration and then it decreases. Magnetic properties were characterized by the complex initial permeability (100 Hz–100 MHz). The real part of the complex initial permeability (μi′) increases with Gd3+ concentration up to x = 0.04 and then decreases. Ni0.48Cu0.12Zn0.40Gd0.04Fe1.96O4 shows the highest value of μi′ (at the optimum sintering temperature), which is more than eight times that of the parent sample. Conversely, magnetic loss is reduced remarkably in this composition. The Néel temperature was determined from the temperature-dependent initial complex permeability, which showed a decreasing trend with increase in Gd3+ concentration owing to the weakening of the A-B interaction. DC magnetizations as a function of the applied magnetic field were evaluated with a vibrating-sample magnetometer at room temperature. The magnetization increased with increasing Gd3+ content up to x = 0.04 owing to the variation of cation distribution and then reduced because of the possibility of noncollinear spin arrangement with further increase of Gd3+ content. The number of Bohr magnetons (nB) and the Yaffet-Kittel angle (αY-K) were evaluated. The calculated αY-K values were a minimum for x = 0.04, while the nB variation showed the opposite trend. Because of the reduction of electron exchanges between Fe2+ and Fe3+ at the octahedral site, the room temperature dielectric constant and the dielectric loss were reduced with increasing Gd3+ content. There was an exceptional increase in the dielectric constant for x = 0.02 sample. The AC conductivity spectrum exhibited two regions, and the conductivity was found to decrease with the addition of Gd3+. Impedance spectroscopy studies indicated a non-Debye-type relaxation process. Ferroelectric transition temperatures, TC, were also determined from measurement of the temperature-dependent dielectric constant.

Study of magnetic behavior in co-precipitated Ni–Zn ferrite nanoparticles and their potential use for gas sensor applications

A series of NixZn1−xFe2O4 (x = 0.5, 0.6, 0.7) were obtained by co-precipitation technique and followed by sintering at 800 °C. The random variation of bond angles resembles possible cation redistribution in the present series of ferrite samples. The cation distributions estimated from Mössbauer analysis are used to find out the net magnetic moment. The magnetic moment n′B estimated from Neel’s sub-lattice model is not consistent with experimental magnetic momet nB. But the magnetic moment (nB′′) estimated from the Yafet–Kittel (Y–K) model is in good agreement with the experimental magnetic moment nB, supporting the possibility of non-collinear arrangement of spins at B–site. The non-collinear spin arrangement at octahedral (B) site is influencing the magnetization of present series ferrite samples. The increase in saturation magnetization with increase in Ni2+ ion concentration is expected due to the decrease in non-collinear spin arrangement at octahedral (B) site. Higher saturation magnetization of 61.32 emu/g was reported for the composition x = 0.7. The single domain structure of samples is evident from the hysteresis loops, but interestingly nature of hyperfine interactions can be found from the Mössbauer spectra under spectral line intensities. The increase in the blocking temperature (magnetic ordering) is due to the increase in magnetocrystalline anisotropy rather than an increase in particle sizes. The composition x = 0.7 is showing good sensor response for both LPG and acetone. Moreover, its response and recovery times are less comparing to the other two compositions and therefore it is useful for the sensor fabrication.

Metamagnetic Transitions versus Magnetocrystalline Anisotropy in Two Cobalt Arsenates with 1D Co2+ Chains.

We have investigated two original hydrated cobalt arsenates based on Co2+ octahedral edge-sharing chains. Their different magnetocrystalline anisotropies induce different types of metamagnetic transitions: spin-flop versus spin-flip. In both compounds, a strong local anisotropy (Ising spins) is favored by the spin-orbit coupling present in the CoO6 octahedra, while ferromagnetic (FM) exchanges predominate in the chains. Co2(As2O7)·2H2O (1) orders antiferromagnetically below TN = 6.7 K. The magnetic structure is a noncollinear antiferromagnetic spin arrangement along the zigzag chains with DFT calculations implying frustrated chains and weakened anisotropy. A metamagnetic transition suggests a spin-flop process above μ0H = 3.2 T. In contrast, in BaCo2As2O8·2H2O (2) linear chains are arranged in disconnected layers, with only interchain ferromagnetic exchanges, therefore increasing its magnetocrystalline anisotropy. The magnetic structure is collinear with a magnetic easy axis that allows a spin-flop to a sharp spin-flip transition below TN = 15.1 K and above μ0H = 6.2 T.

Field-responsive colloidal assemblies defined by magnetic anisotropy.

Particle dispersions provide a promising tool for the engineering of functional materials that exploit self-assembly of complex structures. Dispersion made from magnetic colloidal particles is a great choice; they are biocompatible and remotely controllable among many other advantages. However, their dominating dipolar interaction typically limits structural complexity to linear arrangements. This paper shows how a magnetostatic equilibrium state with noncollinear arrangement of the magnetic moments, as reported for ferromagnetic Janus particles, enables the controlled self-organization of diverse structures in two dimensions via constant and low-frequency external magnetic fields. Branched clusters of staggered chains, compact clusters, linear chains, and dispersed single particles can be formed and interconverted reversibly in a controlled way. The structural diversity is a consequence of both the inhomogeneity and the spatial extension of the magnetization distribution inside the particles. We draw this conclusion from calculations based on a model of spheres with multiple shifted dipoles. The results demonstrate that fundamentally new possibilities for responsive magnetic materials can arise from interactions between particles with a spatially extended, anisotropic magnetization distribution.

Magnetization reversal, magnetic anisotropy and ordering studies on quenched copper-ferri-aluminates

The magnetic properties of thermally quenched spinel ferrite system, CuAlxFe2-xO4 (x = 0.0, 0.2, 0.4 and 0.6), have been investigated by means of DC magnetization, hysteresis loop characteristics and thermal variation of low field ac susceptibility measurements. The system possesses a tetragonally distorted spinel structure and exhibits a non-collinear spin arrangement. The magnetic characteristics suggest that the ferrites are low anisotropy compounds, the negative magnetization observed for x = 0.6 composition at the compensation temperature T = 22 K is due to the copper ion moment with a net effect antiparallel to that of the Fe3+ ions with different temperature dependence giving rise to a compensation effect. The low-temperature anomalies on the zero field cooled curves are due to the coexistence and interplay between the ferrimagnetic long-range ordering and short-range ordering in the system.

Magnetic phase diagram, magnetotransport and inverse magnetocaloric effect in the noncollinear antiferromagnet Mn5Si3

The antiferromagnet Mn$_5$Si$_3$ has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn$_5$Si$_3$ using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases which has ferromagnetic character. The latter magnetic phase might be caused by strain produced by Cu inclusions that lead to quenched fluctuations of the mixed character magnetic ordering in this compound.

First principles study of electronic structure, magnetism and ferroelectric properties of rhombohedral AgFeO2

Using first principles calculation in the framework of density functional theory we have investigated the electronic structure, magnetism and ferroelectric polarization in the triangular lattice antiferromagnet AgFeO2, and compared it to the isostructural system CuFeO2. Our calculations reveal that spin-orbit interaction plays an important role in determining the magnetic property of AgFeO2 and is possibly responsible for its different magnetic ground state in comparison to CuFeO2. Calculations of ferroelectric polarization of AgFeO2 suggest that the spontaneous polarization arises from noncollinear spin arrangement via the spin-orbit coupling. Our calculations also indicate that in addition to electronic contribution, the lattice mediated contribution to the polarization is also important for AgFeO2.

Na2Mn3Se4: Strongly Frustrated Antiferromagnetic Semiconductor with Complex Magnetic Structure

A new ternary selenide, Na2Mn3Se4, was prepared by a stoichiometric reaction between Na2Se4 and metallic Mn at 923 K. Crystal structure determination revealed a new structure type, built of alternating layers of Na+ ions and [Mn3Se4]2- anionic slabs. Band structure calculations indicate that Na2Mn3Se4 is an indirect band gap semiconductor with Eg = 1.59 eV, although a direct band gap is only marginally larger, at 1.64 eV. The material shows antiferromagnetic (AFM) ordering at 27 K, while the Weiss constant of ∼-400 K suggests much stronger nearest-neighbor AFM exchange between the Mn sites. This discrepancy is attributed to the strong spin frustration caused by a triangulated arrangement of the Mn sites in the [Mn3Se4]2- layer. The magnetic frustration leads to the stabilization of a complex AFM ordered structure with non-collinear arrangement of the Mn magnetic moments, as established from neutron diffraction data.

Spin decoupling under a staggered field in the Gd2Ir2O7 pyrochlore

The influence of a staggered molecular field in frustrated rare-earth pyrochlores, produced via the magnetic iridium occupying the transition metal site, can generate exotic ground states, such as the fragmentation of the magnetization in the Ho compound. At variance with the Ising Ho$^{3+}$ moment, we focus on the behavior of the quasi isotropic magnetic moment of the Gd$^{3+}$ ion at the rare-earth site. By means of macroscopic measurements and neutron scattering, we find a complex situation where different components of the magnetic moment contribute to two antiferromagnetic non-collinear arrangements: a high temperature all in - all out order induced by the Ir molecular field, and Palmer and Chalker correlations that tend to order at much lower temperatures. This is enabled by the anisotropic nature of the Gd-Gd interactions and requires a weak easy-plane anisotropy of the Gd$^{3+}$ moment due to the mixing of the ground state with multiplets of higher spectral terms.

A Novel Design of N-Fiducial Phantom for Automatic Ultrasound Calibration.

Background:Freehand ultrasound (US) is a technique used to acquire three-dimensional (3D) US images using a tracked 2D US probe. Calibrating the probe with a proper calibration phantom improves the precision of the technique and allows several applications in computer-assisted surgery. N-fiducial phantom is widely used due to the robustness of precise fabrication and convenience of use. In principle, the design supports single-frame calibration by providing at least three noncollinear points in 3D space at once. Due to this requirement, most designs contain multiple N-fiducials in unpatterned and noncollinear arrangements. The unpatterned multiple N-fiducials appearing as scattered dots in the US image are difficult to extract, and the extracted data are usually contaminated with noise. In practice, the extraction mostly relied on manual interventions, and calibration with N-fiducial phantom has not yet achieved high accuracy with single or few frame calibrations due to noise contamination.Aims:In this article, we propose a novel design of the N-fiducial US calibration phantom to enable automatic feature extraction with comparable accuracy to multiple frame calibration.Materials and Methods:Along with the design, the Random Sample Consensus (RANSAC) algorithm was used for feature extraction with both 2D and 3D models estimation. The RANSAC feature extraction algorithm was equipped with a closed-form calibration method to achieve automatic calibration.Results:The accuracy, precision, and shape reconstruction errors of the calibration acquired from the experiment were significantly matched with the previous literature reports.Conclusions:The results showed that our proposed method has a high efficiency to perform automatic feature extraction compared to conventional extraction performed by humans.

Structural and magnetic properties of bulk Mn2PtSn

Interplay between structural and magnetic order parameters is one of the key mechanisms of tuning properties of materials intended for device applications in spintronics. Here, using density functional calculations, we study combined effects of tetragonal distortion and non-collinear magnetic order in Mn2PtSn. We show that this material has two energetically close energy minimums corresponding to tetragonal lattice. In one of these phases, Mn2PtSn exhibits ferrimagnetic order with nearly fully compensated total magnetic moment, while in the other phase that corresponds to the lowest energy, a non-collinear magnetic arrangement emerges, with very large canting angle of the Mn local magnetic moments. The non-collinear alignment is explained through the interplay of exchange couplings between nearest and next nearest neighbor Mn atoms. Results are compared with those reported in recent literature, both experimental and theoretical.

Nature of magnetoelectric coupling in corundum antiferromagnet Co4Ta2O9

Abstract We study magnetocapacitance (MC) effect and magnetoelectric (ME) coupling in spin-flop driven antiferromagnet Co4Ta2O9. Powder neutron diffraction data reveal that the magnetic structure corresponds to a non-collinear arrangement along with a non-centrosymmetric crystal structure below Neel Temperature. Electric polarization is achieved below Neel temperature only when the sample is cooled in the presence of external magnetic field. The magnetocapacitance data at high magnetic fields are analyzed by phenomenological Ginzburg-landau theory of ferro-electromagnets and it is found that change in dielectric constant is proportional to the square of magnetization. The saturation polarization and magnetoelectric coupling are estimated to be 52 µC/m2 and γ = 1.4 × 10−3 (emu/g)−2 respectively at 6 Tesla. Strong magnetoelectric coupling and ferroelectric phase in the anti-ferromagnetic Co4Ta2O9 are correlated to magnetic structure as derived from neutron diffraction data.

Effective damping enhancement in noncollinear spin structures

Damping mechanisms in magnetic systems determine the lifetime, diffusion and transport properties of magnons, domain walls, magnetic vortices, and skyrmions. Based on the phenomenological Landau-Lifshitz-Gilbert equation, here the effective damping parameter in noncollinear magnetic systems is determined describing the linewidth in resonance experiments or the decay parameter in time-resolved measurements. It is shown how the effective damping can be calculated from the elliptic polarization of magnons, arising due to the noncollinear spin arrangement. It is concluded that the effective damping is larger than the Gilbert damping, and it may significantly differ between excitation modes. Numerical results for the effective damping are presented for the localized magnons in isolated skyrmions, with parameters based on the Pd/Fe/Ir(111) model-type system.

Magnetic structure of the magnetoelectric material Ca2CoSi2O7

Detailed investigation of ${\mathrm{Ca}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. The crystal and magnetic structures well below the antiferromagnetic transition temperature of ${T}_{\text{N}}\ensuremath{\approx}5.7$ K were determined using neutron diffraction. Neutron diffraction data imply no structural phase transition from 10 K down to 2.5 K and are well described within the orthorhombic space group $P{2}_{1}{2}_{1}2$ with a $3\ifmmode\times\else\texttimes\fi{}3\ifmmode\times\else\texttimes\fi{}1$ supercell compared with the high-temperature unmodulated state (tetragonal space group $P\overline{4}{2}_{1}m$). We found that in zero magnetic field the magnetic space group is $P{2}_{1}{2}_{1}^{\ensuremath{'}}{2}^{\ensuremath{'}}$ with antiferromagnetic order along the [100] or [010] axes for two types of ${90}^{\ensuremath{\circ}}$ twin domains, while neighboring spins along the [001] axis are ordered ferromagnetically. A noncollinear spin arrangement due to small canting within the $ab$ plane is allowed by symmetry and leads to the existence of the tiny spontaneous magnetization below ${T}_{\text{N}}$. The ordered moment with a magnitude of about 2.8 ${\ensuremath{\mu}}_{\text{B}}/{\mathrm{Co}}^{2+}$ at 2.5 K lies in the $ab$ plane. Distinct differences between the magnetic structure of ${\mathrm{Ca}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ as compared to those of ${\mathrm{Ba}}_{2}{\mathrm{CoGe}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Sr}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ are discussed.