Noncollinear Spin Structure Research Articles

Giant Non-Saturating Exchange Striction in a Noncollinear Antiferromagnet.

Magnetostriction, discovered by Joule in 1842, refers to the mechanical strain that a material undergoes in the presence of a magnetic field. Conventionally, it originates from the spin-orbit coupling and has been predominantly explored in ferromagnets. In this work, a giant magnetostriction effect is reported in the high-quality single crystal of a noncollinear antiferromagnet Mn3Sn. Non-saturating magnetostriction exceeding 400ppm is obtained, which is even larger than the saturation values of the well-known Fe-based giant magnetostriction ferromagnetic materials such as FeGa. Theoretical calculations reveal that the large non-saturating magnetostriction results from a sophisticated exchange striction effect of the noncollinear antiferromagnetic spin structure, leading to a nearly linear dependence of the strain output on the applied magnetic field. This work provides an unprecedented strategy to design next-generation magnetoelastic materials with noncollinear compensated spin structures.

Giant Domain Wall Anomalous Hall Effect in a Layered Antiferromagnet EuAl_{2}Si_{2}.

Generally, the dissipationless Hall effect in solids requires time-reversal symmetry breaking (TRSB), where TRSB induced by external magnetic field results in the ordinary Hall effect, while TRSB caused by spontaneous magnetization gives rise to the anomalous Hall effect (AHE) which scales with the net magnetization. The AHE is therefore not expected in antiferromagnets with vanishing small magnetization. However, large AHE was recently observed in certain antiferromagnets with noncollinear spin structure and nonvanishing Berry curvature. Here, we report another origin of AHE in a layered antiferromagnet EuAl_{2}Si_{2}, namely, the domain wall (DW) skew scattering with Weyl points near the Fermi level, in experiments for the first time. Interestingly, the DWs form a unique periodic stripe structure with controllable periodicity by external magnetic field, which decreases nearly monotonically from 975nm at 0T to 232nm at 4T. Electrons incident on DW with topological bound states experience strong asymmetric scattering, leading to a giant AHE, with the DW Hall conductivity (DWHC) at 2K and 1.2T reaching a record value of ∼1.51×10^{4} Scm^{-1} among bulk systems and being 2 orders of magnitude larger than the intrinsic anomalous Hall conductivity. The observation not only sets a new paradigm for exploration of large anomalous Hall effect, but also provides potential applications in spintronic devices.

Field-free switching of perpendicular magnetization in a noncollinear antiferromagnetic Mn3Sn/[Pt/Co]4 heterostructure

Field-free switching of perpendicular magnetization in a noncollinear antiferromagnetic Mn3Sn/[Pt/Co]4 heterostructure

Emergence of field-induced memory effect in spin ices

Out-of-equilibrium investigation of strongly correlated materials deciphers the hidden equilibrium properties. Herein, we have investigated the out-of-equilibrium magnetic properties of polycrystalline Dy2Ti2O7 and Ho2Ti2O7 spin ices. Our experimental findings reveal the emergence of magnetic field-induced anomalous hysteresis observed solely in temperature-and magnetic field-dependent AC susceptibility measurements. The observed memory effect (anomalous thermomagnetic hysteresis) exhibits a strong dependence on both thermal and non-thermal driving variables. Owing to the non-collinear spin structure, the applied DC bias magnetic field produces quenched disorder sites in the cooperative Ising spin matrix and suppresses the spin–phonon coupling. These quench disorders create a dynamic spin correlation, having slow spin relaxation and quick decay time, which additionally contribute to AC susceptibility. The initial conditions and measurement protocol decide the magnitude and sign of this dynamical term contributing to AC susceptibility. It is being suggested that such out-of-equilibrium properties arise from the combined influences of geometric frustration, disorder, and the cooperative nature of spin dynamics exhibited by these materials.

Vector spin Seebeck effect and spin swapping effect in antiferromagnetic insulators with non-collinear spin structure

Antiferromagnets (AFs) are prospective for next-generation high-density and high-speed spintronic applications due to their negligible stray field and ultrafast spin dynamics, notwithstanding the challenges in detecting and manipulating AF order with no magnetization (M = 0). Among the AFs, non-collinear AFs are of particular interest because of their unique properties arising from the non-collinear spin structure and the small magnetization M. In this work, we describe the recently observed vector spin Seebeck effect in non-collinear LuFeO3, where the magneto-thermovoltage under an in-plane temperature gradient, not previously observed, is consistent with the predicted spin swapping effect. Our results shed light on the importance of the non-collinear spin structure in the emerging spin phenomena in non-collinear AFs and offer a new class of materials for AF spintronics and spin caloritronics.

First-Principles Studies on Spin-Dependent Transport of Magnetic Junctions with Half-Metallic Heusler Compounds

Two types of magnetoresistance (MR), i.e., tunnel magnetoresistance (TMR) and current perpendicular to plane giant magnetoresistance (CPP-GMR), are theoretically discussed for systems with half-metallic (HFM) Co-based full Heusler compounds. In TMR, the non-collinear spin structure at the Co2MnSi(CMS)/MgO(001) interface that results from thermal spin fluctuations significantly reduced the TMR ratio at room temperature (RT). Enhancement of the exchange stiffness of CMS/MgO was essential to suppress the reduction in TMR at RT. Furthermore, it is proposed that inserting of a B2-CoFe layer between CMS and MgO is a promising way to enhance the interface exchange stiffness constant. In CPP-GMR, the interface spin asymmetry γ in the Valet-Fert model was essential to enhance GMR at RT, because the temperature dependence of γ in experiments was much larger than that of the bulk spin asymmetry β. I showed that the experimental CPP-GMR ratio increases with the decrease in the theoretical resistance-area product RPA, which is roughly consistent with the RP dependence of γ. This means that matching of the conductive channel between HMF and a non-magnetic metallic spacer is a key parameter to enhance γ. These findings will be very important for room temperature spintronics devices applied in future artificial intelligence hardware.

Field-free magnetization switching in CoPt induced by noncollinear antiferromagnetic Mn3Ga

The magnetic spin Hall effect (MSHE) may produce the out-of-plane spin current with $z$-direction polarization $({\mathbit{\ensuremath{\sigma}}}_{\mathbf{z}})$, thus allowing the field-free switching of perpendicular magnetization in ferromagnetic films. Here, we fabricate the epitaxial $L{1}_{1}\text{\ensuremath{-}}\mathrm{CoPt}/D{0}_{19}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ bilayers on MgO(111) substrates and characterize their magnetic and electrical transport properties. In the $L{1}_{1}\text{\ensuremath{-}}\mathrm{CoPt}/D{0}_{19}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ bilayers, we observe the spin-orbit torque switching of perpendicular magnetization under zero external field. The magnetization switching ratio and polarity are specifically related to the N\'eel vector direction of the antiferromagnetic ${\mathrm{Mn}}_{3}\mathrm{Ga}$ as well as the crystalline orientations which the pulse current is applied along. The field-free switching in the bilayers is attributed to the ${\mathbit{\ensuremath{\sigma}}}_{\mathbf{z}}$ spin current generated by the MSHE, which is driven by the reduced symmetry of ${\mathrm{Mn}}_{3}\mathrm{Ga}$ film with noncollinear spin structure.

Thermal stability of non-collinear antiferromagnetic Mn3Sn nanodot

D019-Mn3Sn, an antiferromagnet having a non-collinear spin structure in a kagome lattice, has attracted great attention owing to various intriguing properties such as large anomalous Hall effect. Stability of a magnetic state against thermal fluctuation, characterized in general by the thermal stability factor Δ, has been well studied in ferromagnetic systems but not for antiferromagnets. Here, we study Δ of the antiferromagnetic Mn3Sn nanodots as a function of their diameter D. To quantify Δ, we measure the switching probability as a function of the pulse-field amplitude and analyze the results based on a model taking account of two and sixfold magnetic anisotropies in the kagome plane. We observe no significant change in Δ down to D = 300 nm below which it decreases with D. The obtained D dependence is well explained by a single-domain and nucleation-mediated reversal models. These findings provide a basis to understand the thermal fluctuation and reversal mechanism of antiferromagnets for device applications.

Spin currents with unusual spin orientations in noncollinear Weyl antiferromagnetic Mn3Sn

There are intensive efforts to search for mechanisms that lead to spin-orbit torque with unusual spin orientation, particularly out-of-plane spin orientation which can efficiently switch perpendicular magnetizations. Such a phenomenon has been observed in materials with low structural symmetry, ferromagnetic materials, and antiferromagnets with noncollinear spin structures. Here, we demonstrate the observation of, in addition to out-of-plane spin orientation, spin orientation along the charge current direction in ${\mathrm{Mn}}_{3}\mathrm{Sn}$, a noncollinear antiferromagnet and Weyl semimetal. The mechanism arises from noncollinear spin structure with spin-orbit coupling and it can be viewed as spin rotation around the octupole moment, the lowest order of cluster multipole moment pertaining to the ${\mathrm{Mn}}_{3}\mathrm{Sn}$ crystal group.

Evolution of anisotropic magnetic properties through helix-to-fan transition in helical antiferromagnetic EuCo2As2

A helimagnet comprises a noncollinear spin structure formed by competing exchange interactions. Recent advances in antiferromagnet-based functionalities have broadened the scope of target materials to include noncollinear antiferromagnets. However, a microscopic understanding of the magnetic anisotropy associated with the intricate evolution of noncollinear spin states has not yet been accomplished. Here, we have explored the anisotropic magnetic aspects in a layered helimagnet of EuCo2As2 by measuring the magnetic field and angle dependence of the magnetic torque. By adopting an easy-plane anisotropic spin model, we can visualize the detailed spin configurations that evolve in the presence of rotating magnetic fields. This is directly related to the two distinctive magnetic phases characterized by the reversal of the magnetic torque variation across the helix-to-fan transition. Our advanced approach provides an in-depth understanding of the anisotropic properties of noncollinear-type antiferromagnets and a useful guidance for potential applications in spin-processing functionalities.

Effect of Fe doping on the magnetic property of perovskite TmCr1-xFexO3

Effect of Fe doping on the magnetic property of perovskite TmCr1-xFexO3

Magnon drag induced by magnon-magnon interactions characteristic of noncollinear magnets

A noncollinear magnet consists of the magnetic moments forming a noncollinear spin structure. Because of this structure, the Hamiltonian of magnons acquires the cubic terms. Although the cubic terms are the magnon-magnon interactions characteristic of noncollinear magnets, their effects on magnon transport have not been clarified yet. Here we show that in a canted antiferromagnet the cubic terms cause a magnon drag that magnons drag magnon spin current and heat current, which can be used to enhance these currents by tuning a magnetic field. For a strong magnetic field, we find that the cubic terms induce low-temperature peaks of a spin-Seebeck coefficient, a magnon conductivity, and a magnon thermal conductivity, and that each value is one order of magnitude larger than the noninteracting value. This enhancement is mainly due to the magnetic field dependence of the coupling constant of the cubic terms through the magnetic-field dependent canting angle. Our magnon drag offers a way for controlling the magnon currents of noncollinear magnets via the many-body effect.

Noncollinear ferrimagnetism and anomalous Hall effects in Mn4N thin films

The magnetic structure of thin films of ferrimagnetic metallic Mn${}_{4}$N differs from the triangular ferrimagnetism of the bulk. Based on distance-dependence of Mn-Mn exchange and magnetic structures of other Mn${}_{3}$$X$N perovskites, a topological noncollinear spin structure is proposed for [001] Mn${}_{4}$N films with perpendicular anisotropy, which is confirmed by temperature-independent anomalous Hall conductivity coexisting with a normal temperature-dependent contribution. The study shows how a small symmetry change in the spin structure can influence magnetotransport properties of frustrated ferrimagnetic films.

Switching the sign of magnetic anisotropy field in YBCO/NiFe/IrMn heterostructure induced by superconducting transition

We studied the influence of the superconducting state on the magnetic properties in a thick IrMn(100 nm)/NiFe(100 nm) bilayer deposited by magnetron sputtering onto an yttrium–barium–copper–oxide (YBCO) substrate that was previously synthesized by the acetate method. The results from magnetization experiments showed that the sign of the effective exchange fields switched from positive, in the as-prepared heterostructure, to negative values when the sample goes below the superconducting temperature of the YBCO substrate. We, thus, demonstrated that the YBCO substrate, in its superconducting state, strongly influences the magnetic anisotropy of the thick ferromagnetic NiFe layer due to the proximity effect that occurs at the YBCO–NiFe interface, where a non-collinear magnetic spin structure is formed during the in-field sample deposition.

Symmetry-correct bonding in density functional theory calculations for delta phase Pu

Symmetry-correct bonding in density functional theory calculations for delta phase Pu

Rare-earth-free noncollinear metallic ferrimagnets Mn4-xZxN with compensation at room temperature

Compensated ferrimagnets show no net magnetization like antiferromagnets, but their transport and magneto-optic properties resemble those of ferromagnets, thereby creating opportunities for applications in high-frequency spintronics and low energy loss communications. Here we study the modification of the noncollinear triangular ferrimagnetic spin structure of Mn4N by a variety of metallic substitutions Z (Z = Cu — Ge and Ag — Sn) to achieve compensation at room temperature. The noncollinear frustrated 2.35µB moments of Mn on 3c sites of the (111) kagome planes tilt about 20° out-of-plane in Mn4N and are easily influenced by the substitutions on 1a sites, leading to an efficiency of compensation in Mn4-xZxN that increases gradually from group 11 (Cu, Ag) to group 14 (Ge, Sn) with increasing number of valence electrons. Elements from the 5th period are more efficient for compensation than those from the 4th period due to lattice expansion. The manganese site moments analyzed by constrained density functional theory are determined by Z, orbital hybridization, charge transfer and the tilt angle. The Ga compound with compensation at room temperature for x ≈ 0.26 is recommended for high-frequency spintronic applications.

Impact of B-doping on topological Hall resistivity in (111)- and (110)-oriented Mn4N single layers with the non-collinear spin structure

Controllability of the topological Hall resistivity (ρxyTHE) via the doping effect of light elements was investigated for the sputter-deposited (111)-oriented Mn4N single layer. The component of ρxyTHE relative to the anomalous Hall resistivity (ρxyAHE) for host Mn4N was found to increase with decreasing temperature. Boron (B), one of the 2p light elements acting as an interstitial impurity, was doped to the (111)-oriented Mn4N single layer. The microstrain, grain diameter, and surface roughness were found to decrease, resulting in the reduction of ρxyTHE for all temperatures without a change in the antiperovskite bone structure of Mn4N. These results show a dilution effect in the spin frustration state with topological spin texture by B-doping. The effect of B on ρxyTHE for a different orientation of (110) was similar to that of (111), while the enhancement of ρxyTHE was observed by a higher amount of B. B-doping could, thus, be a promising approach to realize tailor-made spintronic devices based on the topological spin state owing to its material versatility.

Noncollinear antiferromagnetic order and effect of spin-orbit coupling in spin-1 honeycomb lattice

Motivated by the recently synthesized insulating nickelate ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, which has been reported to have an unusual noncollinear magnetic order of ${\mathrm{Ni}}^{2+}\phantom{\rule{4pt}{0ex}}S=1$ moments with a nontrivial angle between adjacent spins, we construct an effective spin-1 model on the honeycomb lattice, with the exchange parameters determined with the help of first-principles electronic-structure calculations. The resulting bilinear-biquadratic model, supplemented with the realistic crystal-field induced anisotropy, favors the collinear N\'eel state. We find that the crucial key to explaining the observed noncollinear spin structure is the inclusion of the Dzyaloshinskii--Moriya (DM) interaction between the neighboring spins. By performing variational mean-field and linear spin-wave theory (LSWT) calculations, we determine that a realistic value of the DM interaction $D\ensuremath{\approx}2.78$ meV is sufficient to quantitatively explain the observed angle between the neighboring spins. We furthermore compute the spectrum of magnetic excitations within the LSWT and random-phase approximation, which should be compared to future inelastic neutron measurements.

Effect of Co and Mg doping at Cu site on structural, magnetic and dielectric properties of α–Cu2V2O7

We have studied the effect of doping of both magnetic (Co) and nonmagnetic (Mg) ions at the Cu site on phase transition in polycrystalline α–Cu2V2O7 through structural, magnetic, and electrical measurements. X-ray diffraction reveals that Mg doping triggers an onset of α- to β-phase structural transition in Cu2−x Mg x V2O7 above a critical Mg concentration x c = 0.15, and both the phases coexist up to x = 0.25. Cu2V2O7 possesses a non-centrosymmetric crystal structure and antiferromagnetic ordering along with a non-collinear spin structure in the α phase, originated from the microscopic Dzyaloshinskii–Moriya interaction between the neighboring Cu spins. Accordingly, a weak ferromagnetic (FM) behavior has been observed up to x = 0.25. However, beyond this concentration, Cu2−x Mg x V2O7 exhibits complex magnetic properties. A clear dielectric anomaly is observed in α–Cu2−x Mg x V2O7 around the magnetic transition temperature, which loses its prominence with the increase in Mg doping. The analysis of experimental data shows that the magnetoelectric coupling is nonlinear, which is in agreement with the Landau theory of continuous phase transitions. Co doping, on the other hand, initiates a sharp α to β phase transition around the same critical concentration x c = 0.15 in Cu2−x Co x V2O7 but the FM behavior is very weak and can be detected only up to x = 0.10. We have drawn the magnetic phase diagram which indicates that the rate of suppression in transition temperature is the same for both types of doping, magnetic (Co) and nonmagnetic (Zn/Mg).

Cluster magnetic octupole induced out-of-plane spin polarization in antiperovskite antiferromagnet

Out-of-plane spin polarization σz has attracted increasing interests of researchers recently, due to its potential in high-density and low-power spintronic devices. Noncollinear antiferromagnet (AFM), which has unique 120° triangular spin configuration, has been discovered to possess σz. However, the physical origin of σz in noncollinear AFM is still not clear, and the external magnetic field-free switching of perpendicular magnetic layer using the corresponding σz has not been reported yet. Here, we use the cluster magnetic octupole in antiperovskite AFM Mn3SnN to demonstrate the generation of σz. σz is induced by the precession of carrier spins when currents flow through the cluster magnetic octupole, which also relies on the direction of the cluster magnetic octupole in conjunction with the applied current. With the aid of σz, current induced spin-orbit torque (SOT) switching of adjacent perpendicular ferromagnet is realized without external magnetic field. Our findings present a new perspective to the generation of out-of-plane spin polarizations via noncollinear AFM spin structure, and provide a potential path to realize ultrafast high-density applications.