Non-collinear Antiferromagnetic Order Research Articles

Observation of Thermally Induced Piezomagnetic Switching in Cu2OSeO3 Polymorph Synthesized under High‐Pressure

AbstractA polymorph of Cu2OSeO3 with the distorted kagome lattice is successfully obtained using the high‐pressure synthesis technique (Cu2OSeO3‐HP). The structural analysis using X‐ray and neutron powder diffraction suggests that the tetrahedral Cu2+ clusters [similar to those in Cu2OSeO3 ambient‐pressure phase (Cu2OSeO3‐AP)] exist in Cu2OSeO3‐HP but with three symmetry inequivalent sites. No structural change is observed between 1.5 K and the room temperature. The complex magnetic H‐T phase diagram is established based on the temperature‐ and field‐dependent magnetization data, indicating two distinct antiferromagnetic phases at low and intermediate temperatures, in addition to the higher‐temperature spin‐glass‐like phase. The low temperature phase is identified by neutron powder diffraction refinements as a canted noncollinear antiferromagnetic order with a weak ferromagnetic component along the b‐axis. Size of the refined ordered moment is ≈1.00(4) µB in Cu2OSeO3‐HP, indicating a large enhancement compared to that of Cu2OSeO3‐AP (≈0.61 µB). By applying a uniaxial stress, finite enhancement of weak ferromagnetic component in the noncollinear antiferromagnetic phase in Cu2OSeO3‐HP is observed, which is the clear evidence of the piezomagnetic effect. Interestingly, the sign of the induced magnetization changes on heating from the low‐temperature to the intermediate‐temperature phases, indicating a novel piezomagnetic switching effect in this compound.

Hidden non-collinear spin-order induced topological surface states

Rare-earth monopnictides are a family of materials simultaneously displaying complex magnetism, strong electronic correlation, and topological band structure. The recently discovered emergent arc-like surface states in these materials have been attributed to the multi-wave-vector antiferromagnetic order, yet the direct experimental evidence has been elusive. Here we report observation of non-collinear antiferromagnetic order with multiple modulations using spin-polarized scanning tunneling microscopy. Moreover, we discover a hidden spin-rotation transition of single-to-multiple modulations 2 K below the Néel temperature. The hidden transition coincides with the onset of the surface states splitting observed by our angle-resolved photoemission spectroscopy measurements. Single modulation gives rise to a band inversion with induced topological surface states in a local momentum region while the full Brillouin zone carries trivial topological indices, and multiple modulation further splits the surface bands via non-collinear spin tilting, as revealed by our calculations. The direct evidence of the non-collinear spin order in NdSb not only clarifies the mechanism of the emergent topological surface states, but also opens up a new paradigm of control and manipulation of band topology with magnetism.

Phase transitions associated with magnetic-field induced topological orbital momenta in a non-collinear antiferromagnet

Resistivity measurements are widely exploited to uncover electronic excitations and phase transitions in metallic solids. While single crystals are preferably studied to explore crystalline anisotropies, these usually cancel out in polycrystalline materials. Here we show that in polycrystalline Mn3Zn0.5Ge0.5N with non-collinear antiferromagnetic order, changes in the diagonal and, rather unexpected, off-diagonal components of the resistivity tensor occur at low temperatures indicating subtle transitions between magnetic phases of different symmetry. This is supported by neutron scattering and explained within a phenomenological model which suggests that the phase transitions in magnetic field are associated with field induced topological orbital momenta. The fact that we observe transitions between spin phases in a polycrystal, where effects of crystalline anisotropy are cancelled suggests that they are only controlled by exchange interactions. The observation of an off-diagonal resistivity extends the possibilities for realising antiferromagnetic spintronics with polycrystalline materials.

Crystalline and transport characteristics of ferrimagnetic and antiferromagnetic phases in Mn3Ga films

The Mn3Ga material is a promising candidate for memory and computing devices owing to its rich crystalline structures of tunable ferrimagnetic and collinear and non-collinear antiferromagnetic phases. In particular, Mn3Ga with non-collinear antiferromagnetic order exhibits giant anomalous and topological Hall conductivities and is a potential material platform for hosting spin-related quantum phenomena. In this study, we demonstrate Mn3Ga films grown on thermally oxidized Si substrates, with and without the Ta buffer, under different deposition temperatures (Ts). With increasing Ts, the dominant crystalline structure across all Mn3Ga films evolves from a cubic to hybrid tetragonal and hexagonal texture, wherein the crystalline orientation of spins endows the films with in-plane magnetic anisotropy. For Ta/Mn3Ga and Mn3Ga films grown under high Ts, the inhomogeneity in surface energy of the buffer layer results in a non-uniform granular film in the former. Notably, the Mn3Ga films of hexagonal texture exhibit topological Hall signatures. The density functional theory calculations on the hexagonal Mn3Ga phase corroborated with the experimental magnetic, structural, and transport properties. These findings establish an important platform for tailoring Mn3Ga films toward multifunctional applications.

Composite Spin Hall Conductivity from Non-Collinear Antiferromagnetic Order.

Non-collinear antiferromagnets (AFMs) are an exciting new platform for studying intrinsic spin Hall effects (SHEs), phenomena that arise from the materials' band structure, Berry phase curvature, and linear response to an external electric field. In contrast to conventional SHE materials, symmetry analysis of non-collinear antiferromagnets does not forbid non-zero longitudinal and out-of-plane spin currents with polarization and predicts an anisotropy with current orientation to the magnetic lattice. Here, multi-component out-of-plane spin Hall conductivities are reported in L12 -ordered antiferromagnetic PtMn3 thin films that are uniquely generated in the non-collinear state. The maximum spin torque efficiencies (ξ = JS /Je ≈ 0.3) are significantly larger than in Pt (ξ ≈ 0.1). Additionally, the spin Hall conductivities in the non-collinear state exhibit the predicted orientation-dependent anisotropy, opening the possibility for new devices with selectable spin polarization. This work demonstrates symmetry control through the magnetic lattice as a pathway to tailored functionality in magnetoelectronic systems.

Noncollinear antiferromagnetic textures driven high-harmonic generation from magnetic dynamics in the absence of spin-orbit coupling

We demonstrate the generation of high order harmonics in carrier pumping from precessing ferromagnetic or antiferromagnetic orders, excited via magnetic resonance, in the presence of topological antiferromagnetic textures. This results in an enhancement of the carrier dynamics by orders of magnitude, enabling for an emission deep in the THz frequency range. Interestingly, the generation process occurs in an intrinsic manner, and is solely governed by the interplay between the s-d exchange coupling underlying the noncollinear antiferromagnetic order and the dynamical s-d exchange constant of the magnetic drive. Therefore, the relativistic spin–orbit interaction is not required for the emergence of high harmonics in the pumped currents. Accordingly, the noncollinear topological antiferromagnetic order is presented as an alternative to spin–orbit interaction for the purpose of harnessing high harmonic emission in carrier pumping. Furthermore, we demonstrate the emergence of high harmonics from random magnetic impurities. This suggests the universality of the magnetically induced high harmonic emission in the presence of real and/or momentum space noncollinear textures. Our proposal initiates a tantalizing prospect for the utilization of topological textures in the context of the highly active domains of ultrafast spintronics and THz emission.

Anionic nickel and nitrogen effects in the chiral antiferromagnetic antiperovskite Mn3NiN.

Magnetic antiperovskites, having chiral noncollinear antiferromagnetic ordering, have shown remarkable properties that range from negative thermal expansion to anomalous Hall effects. Nevertheless, details on the electronic structure, related to the oxidation states and the octahedral center's site effects, are still scarce. Here, we show a theoretical study, based on first-principles calculations in the framework of density-functional theory (DFT), on the electronic properties associated with the nitrogen site effects on the structural, electronic, magnetic, and topological degrees of freedom. Thus, we show that the nitrogen vacancy increases the value of the anomalous Hall conductivity and retains the chiral Γ4g antiferromagnetic ordering. Moreover, we reveal, based on the Bader charges and the electronic structure analysis, the negative and positive oxidation states of the Ni- and Mn-sites, respectively. This is in agreement with the expected A3α+Bβ-Xδ- oxidation states to satisfy charge neutrality in antiperovskites, but the negative charge is rare for transition metals. Finally, we extrapolate our findings on the oxidation states to several Mn3BN compounds, showing that the antiperovskite structure is an ideal platform to encounter negative oxidation states for metals sitting at the corner B-sites.

Sign reversal of the anomalous Hall effect in antiperovskite (110)-oriented Mn3.19Ga0.81N1−δ film

Antiperovskite compounds with abundant magnetic phase transitions provide an ideal platform for exploring nontrivial magnetotransport responses. In this study, the anomalous Hall effect (AHE) in an antiperovskite (110)-oriented Mn3.19Ga0.81N1−δ film up to room temperature was observed, and an unusual sign reversal was detected in the Hall measurements. The AHE reversal suggests that the carrier reversal corresponds to a magnetic transition from a ferrimagnetic order to noncollinear antiferromagnetic order at about 240 K. Analysis of the scaling relation of AHE indicates that the sign reversal originates from the transition from the skew scattering dominated AHE to the intrinsic mechanism dominated AHE. These findings will inspire effective control of the magnetic configuration and innovative applications for manipulating carrier transport in spintronic memory devices based on antiperovskite films.

Ab initio electronic structure of metallized NiS2 in the noncollinear magnetic phase

We investigate the electronic structure of the archetypical Mott insulator ${\mathrm{NiS}}_{2}$ by means of density functional theory calculations in which we explicitly account for the noncollinear antiferromagnetic order, as recently established in the isoelectronic analog $\mathrm{Ni}{(\mathrm{S},\mathrm{Se})}_{2}$. For metallic ${\mathrm{NiS}}_{2}$ under high pressures, our calculations predict a Fermi surface topology and volume which are in excellent agreement with recent quantum oscillation studies. However, we find that density functional theory wrongly predicts a metallic ground state even at ambient pressures, similar to previous nonmagnetic or collinear antiferromagnetic models. By including a Hubbard interaction $U$ and an on-site exchange interaction $J$, the metallic phase is suppressed, but even such an extended model fails to describe the nature of the metal-to-insulating phase transition and describes the insulating phase itself incorrectly. These results highlight the importance of more sophisticated computational approaches even deep in the insulating phase, far away from the Mott insulating phase transition.

Strong correlation between structure and magnetic ordering in tetragonally distorted off-stoichiometric spinels Mn1.15Co1.85O4 and Mn1.17Co1.60Cu0.23O4

We report a systematic study on the structural and magnetic properties of off-stoichiometric polycrystalline bulk spinels ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$ using neutron and x-ray diffraction, ferromagnetic resonance, and magnetic measurements. Both compounds show a weak tetragonal distortion with $c/a<1$, where the crystal structure could be refined in the tetragonal space group $I{4}_{1}/amd$. Both ${\mathrm{Co}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ ions are located at the tetrahedral $A$ site, and ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Co}}^{3+}$ at the octahedral $B$ site. Ferrimagnetic (FI) ordering of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$ sets in below 184 and 164 K, respectively. Magnetic structure analysis revealed that the ferrimagnetically coupled ${A}^{2+}$- and ${B}^{3+}$-site moments are aligned parallel to the tetragonal $c$ axis. Additionally, a noncollinear antiferromagnetic order appears in the $ab$ plane, where the moments point along [110] and $[1\overline{1}0]$. The net magnetic moment [$2{\ensuremath{\mu}}_{\mathrm{FI}}({\mathrm{Mn}}_{B}$/${\mathrm{Co}}_{B})\ensuremath{-}{\ensuremath{\mu}}_{\mathrm{FI}}({\mathrm{Co}}_{A}$)] of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ obtained from neutron data varies between 0.88--1.08 ${\ensuremath{\mu}}_{B}$ which is in good agreement with $M=0.89\text{--}1.13$ ${\ensuremath{\mu}}_{B}$ as determined from magnetization measurements. However, for the Cu-containing compound a larger discrepancy in the magnetic moment was observed between the neutron data (1.89--1.92 ${\ensuremath{\mu}}_{B}$) and low-temperature ($T=1.9$ K) field-dependent ($H=90$ kOe) magnetization data ($0.97\text{--}1.21$ ${\ensuremath{\mu}}_{B}$). From the three-sublattice model we obtained canting angles ${28}^{\ensuremath{\circ}}$ and ${25}^{\ensuremath{\circ}}$ for ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$, respectively. Both the bulk systems exhibit high magnetocrystalline anisotropy (${K}_{\text{U}}\ensuremath{\sim}9\ifmmode\times\else\texttimes\fi{}{10}^{5}$ and $7.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ erg/${\mathrm{cm}}^{3}$) and a field-induced transition (${H}_{\text{D}}$) across 4.0 kOe due to the domain reorientation. Temperature ($1.9\text{--}350$ K) and field ($\ifmmode\pm\else\textpm\fi{}90$ kOe) dependence of magnetization data confirms the high-spin ($S=\frac{3}{2}$ and $S=2$) ground-state configuration for both the divalent Co and trivalent Mn.

Self-consistent analysis of doping effect for magnetic ordering in stacked-kagome Weyl system

We theoretically study the carrier doping effect for magnetism in a stacked-kagome system $\rm{{Co}_3{Sn}_2{S}_2}$ based on an effective model and the Hartree-Fock method. We show the electron filling and temperature dependences of the magnetic order parameter. The perpendicular ferromagnetic ordering is suppressed by hole doping, wheres undoped $\rm{{Co}_3{Sn}_2{S}_2}$ shows magnetic Weyl semimetal state. Additionally, in the electron-doped regime, we find a non-collinear antiferromagnetic ordering. Especially, in the non-collinear antiferromagnetic state, by considering a certain spin-orbit coupling, the finite orbital magnetization and the anomalous Hall conductivity are obtained.

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.

Epitaxial growth and electronic properties of an antiferromagnetic semiconducting VI2 monolayer.

The van der Waals materials down to the monolayer (ML) limit provide a fertile platform for exploring low-dimensional magnetism and developing the novel applications of spintronics. Among them, due to the absence of the net magnetic moment, antiferromagnetic (AFM) materials have received much less attention than ferromagnetic ones. Here, by combining scanning tunneling microscopy and state-of-the-art first-principles calculations, we investigate the preparation, and electronic and magnetic properties of a vanadium(II) iodide (VI2) ML. Single-layer VI2 has been grown by molecular beam epitaxy on Au(111) surfaces. A band gap of 2.8 eV is revealed, indicating the semiconducting nature of the VI2 ML. Vanadium and iodine vacancy defects give rise to additional feature states within the bandgap. A typical 120° AFM spin ordering is maintained in the ML limit of VI2, as revealed by the first-principles calculations. Besides, the AFM coupling is greatly enhanced by slightly decreasing lattice constants. Our work provides an ideal platform for further studying two-dimensional magnetism with non-collinear AFM ordering and for investigating the possibility of realizing the spin Hall effect in the ML limit.

Manipulation of crystalline structure, magnetic performance, and topological feature in Mn3Ge films

The Mn3X (where X = Ga, Ge, Sn, etc.) compounds have appealing prospects for spintronic applications due to their various crystal structures and magnetic properties for the design of reliable high-density memories. However, controlled growth of high-quality Mn3X thin films remains challenging in material science. Here, we reported the controlled film growth of Heusler alloy Mn3Ge, which could crystallize in respective tetragonal and hexagonal structures. The tetragonal D022-type Mn3Ge film exhibits strong perpendicular ferromagnetic anisotropy, while the hexagonal D019-type Mn3Ge film indicates non-collinear triangular antiferromagnetic order. From our experimental observations of structure characterizations, magnetic properties, anomalous Hall effect, and magnetoresistance measurements, we realized the manipulation of spin orientations and topological features. Majority/minority spin polarized Fermi surface and density of states of both tetragonal and hexagonal Mn3Ge structures were investigated by density functional theory calculations. Our work not only opens up technology routes toward the development of Mn3X-based devices for applications in topological spintronics and spin-torque memories but also leads to engineer the physical properties for fundamental study.

Magnetic structure of triangular lattice compound Tb2Ni0.90Si2.94

AlB2-type ternary intermetallic compound Tb2Ni0.90Si2.94 (space group P6/mmm,hP3, No. 191) was reported to exhibit spin freezing behaviour of the ferromagnetic clusters present in the system below Tf=9.9 K, along with the presence of spatially limited antiferromagnetic phase. In this work, on the basis of variable temperature zero-field neutron diffraction measurements, we have shown that the antiferromagnetic phase transition occurs for the compound below TN~13 K. Neutron diffraction study indicates ab-plane non-collinear sine-modulated antiferromagnetic ordering of the system with wave vectors of k1=[±1/6,±1/6,0] and k2=[±1/3,±1/3,0] down to 1.7 K. The weak and diffuse nature of the magnetic Bragg peaks along with limited coherence length further confirm the short-range nature of the antiferromagnetic phase in this compound.

Composite Spin Hall Conductivity from Non-collinear Antiferromagnetic Order in PtMn3

Composite Spin Hall Conductivity from Non-collinear Antiferromagnetic Order in PtMn3

Magnetic properties and magnetic structures of R2PdGe6 (R = Pr, Nd, Gd-Er) and R2PtGe6 (R = Tb, Ho, Er)

Polycrystalline samples of the intermetallic compounds R2PdGe6 (R = Pr, Nd, Gd-Er) and R2PtGe6 (R = Tb, Ho, Er) have been studied using X-ray diffraction as well as magnetometric and neutron diffraction measurements. All compounds have an orthorhombic crystal structure of the Yb2PdGe6-type (space group Cmca) and are antiferromagnetic with the Néel temperatures ranging from 4.9 K for Er2PtGe6 up to 48 K for Tb2PdGe6. The magnetic properties and specific heat data collected for Nd2PdGe6 show the presence of an additional phase transition below TN at T = 4 K. Based on the neutron diffraction data, the magnetic structures have been determined for R2PdGe6 (R = Pr, Nd, Tb, Dy, Ho) and R2PtGe6 (R = Tb and Er). Both the magnetic properties and neutron diffraction data indicate that the magnetic moment is localized on rare earth atoms. The magnetic unit cell is equal to the crystal one, however, individual compounds show different types of magnetic orderings. Magnetic moments in Pr2PdGe6 form a non-collinear antiferromagnetic structure at low temperatures with magnetic moments confined to the (001) plane. The low temperature magnetic structure in Nd2PdGe6 is a collinear antiferromagnetic one with moments parallel to the b-axis and coupled ferromagnetically within the (001) plane, while along the c-axis the moments follow the +−−+ sequence. With increasing temperature, atransition to a modulated magnetic structure is observed in Nd2PdGe6 at T = 4 K. The magnetic moments in R2PdGe6 (R = Tb, Dy, Ho) and Tb2PtGe6 assume a non-collinear antiferromagnetic order within the (001) plane with the +−+− sequence of signs of the moments in the neighboring planes along the c-axis. The Er moments in Er2PtGe6 form a collinear magnetic structure with magnetic moments oriented along the a-axis and coupled ferromagnetically within the (001) plane. Along the c-axis the moments follow the +−+− sequence. The magnetic structures determined here are discussed on the basis of the competition between the RKKY-type interactions and influence of Crystalline Electric Field.

Multiple helimagnetic phases in triclinic CuSeO3

Topologically nontrivial spin textures have attracted considerable attention not only because of rich physics related to the quantum phases, but also because of their potential for novel spintronic functions. We have successfully synthesized one of the rare triclinic phase of CuSeO3 via modified hydrothermal process. Magnetic studies on triclinic CuSeO3 implies helimagnetic spin structure that is well contrast with ground state ferromagnetic or antiferromagnetic states for monoclinic or orthorhombic phases of CuSeO3. The observed two helimagnetic phases can be further transformed into paramagnetic and induced ferromagnetic phases with tuning of temperature and applied magnetic field. Along with the experimental measurements, a first-principles calculation based on density functional theory reveals that triclinic CuSeO3 is a charge-transfer type Mott insulator with non-collinear antiferromagnetic order. A spiral magnetic spin texture has been found by the theoretical calculation.

Magnetic glassy behaviour coupled with long range non-collinear magnetic order in Tb3Co

Detailed and systematic study of rare earth rich intermetallic compound Tb3Co, using dc magnetisation, neutron powder diffraction, and linear and non-linear ac-susceptibilities, shows the presence of an unexpected magnetic glassy state along with complex non-collinear or modulated antiferromagnetic (AFM) order. Our neutron diffraction study shows that the magnetic structure remains more or less the same except for a decrease in moment values in the temperature range of 2 K–70 K and rules out any phase transition around 30 K. However, it reveals sharp changes in structural parameters around 30 K, which indicates strong spin-lattice coupling and change in strength. It appears to be mainly responsible for the observed increase in ZFC magnetisation on warming around 30 K. Another important unexpected result of this study is the strong frequency dispersion in linear and non-linear (higher order harmonics) ac-susceptibilities below K. The analysis in terms of various spin glass theoretical formulisms and even stronger frequency dispersion in non-linear susceptibilities provides evidence for the presence of a spin glass like state in Tb3Co. The final picture that emerges out of this study is that a spin glass like state coexists with the long range modulated AFM order below 72 K.

Anomalous Hall effect in the noncollinear antiferromagnetic antiperovskite Mn3Ni1−xCuxN

We report the anomalous Hall effect (AHE) in antiperovskite Mn$_{3}$NiN with substantial doping of Cu on the Ni site (i.e. Mn$_{3}$Ni$_{1-x}$Cu$_{x}$N), which stabilizes a noncollinear antiferromagnetic (AFM) order compatible with the AHE. Observed on both sintered polycrystalline pieces and single crystalline films, the AHE does not scale with the net magnetization, contrary to the conventional ferromagnetic case. The existence of the AHE is explained through symmetry analysis based on the $\Gamma_{\rm 4g}$ AFM order in Cu doped Mn$_{3}$NiN. DFT calculations of the intrinsic contribution to the AHE reveal the non-vanishing Berry curvature in momentum space due to the noncollinear magnetic order. Combined with other attractive properties, antiperovskite Mn$_{3}$AN system offers great potential in AFM spintronics.