Canting Of Moments Research Articles

The Structure, Magnetic and Magnetotransport Properties of Sr 1− x Y x CoO 3− δ Layered Cobaltites

The properties of Sr1−xYxCoO3−δ layered cobaltites are determined by synchrotron X-ray and neutron powder diffraction studies, by measurements of magnetic and magnetotransport properties. It is shown that the crystal structure changes from tetragonal P4/mmm (ap × ap × 2ap) to monoclinic A2/m (4√2ap × 2√2ap×4ap up to the magnetic ordering temperature and 2√2ap ×2√2ap × 4ap above) through the intermediate tetragonal I4/mmm (2ap × 2ap × 4ap) with alternating layers of CoO6 and CoO4+δ. The basic magnetic structure is G-type antiferromagnetic with Co3+ ions in a high spin/low spin (HS/LS) state mixture in both structural layers. The Co3+ magnetic moments in the CoO6 and CoO4+δ layers are 1.5 μB/Co and 2 μB/Co for x = 0.1 and 1.8 μB/Co and 2.7 μB/Co for x = 0.2, respectively. The antiferromagnetic–“ferromagnetic” transition in compounds with x > 0.2 is associated with the structural and orbital disorder. Magnetic transitions are accompanied by structural phase transitions. The presence of the ferromagnetic component is associated with the monoclinic distortions and canting of magnetic moments in anion-deficient layers due to orbital ordering. A sharp drop of the magnetization for x > 0.2 is caused by the LS Co3+ stabilization and orbital disordering. No spontaneous magnetization is found for compounds with x > 27%.

Magnon transport in the presence of antisymmetric exchange in a weak antiferromagnet

The Dzyaloshinskii-Moriya interaction (DMI) is at the heart of many modern developments in the research field of spintronics. DMI is known to generate noncollinear magnetic textures, and can take two forms in antiferromagnets: homogeneous or inter-sublattice, leading to small, canted moments and inhomogeneous or intra-sublattice, leading to formation of chiral structures. In this work, we first determine the strength of the effective field created by the DMI, using SQUID based magnetometry and transport measurements, in thin films of the antiferromagnetic iron oxide hematite, α-Fe2O3. We demonstrate that DMI additionally introduces reconfigurability in the long distance magnon transport in these films under different orientations of a magnetic field. This arises as a hysteresis centred around the easy-axis direction for an external field rotated in opposing directions whose width decreases with increasing magnetic field as the Zeeman energy competes with the effective field created by the DMI.

Magnetic Properties and Exchange Bias Behavior in Nanocrystalline (Ho1-xSmx)2CoMnO6 (x = 0 – 0.5) Double Perovskite

Single phase nanocrystalline samples of double perovskite (Ho1-xSmx)2CoMnO6 (x = 0 – 0.5) (HSCMO) were prepared by gel combustion method and their crystal structure and magnetic properties are studied. These samples crystallize into monoclinic structure with P21/n space group and their lattice parameters increase with Sm concentration. Temperature variation of magnetization measurements show that all the prepared samples exhibit ferromagnetic (FM) transition with transition temperature (TC) increasing from 83 K for x = 0.0 to 115 K for x = 0.5. A secondary rise in magnetization is observed for T < 50 K and is attributed to FM interaction between rare-earth (RE) and transition metal (TM) ions. The FM transition at higher temperature is attributed to strong super-exchange interaction in Co2+-O - Mn4+ networks. The M−H loops recorded at 5 K show a saturation magnetization (MS) in the range of 11.64 µB/f.u. for x = 0.0 to 7.86 µB/f.u. for x = 0.5 and it clearly indicates the contribution of RE ions to the net ferromagnetic moment. All the prepared samples show exchange bias behaviour with exchange bias field up to 237 Oe at 5 K and is attributed to competition between FM moment of TM ions and the spin canted magnetic moment of RE ions.

Coexistence of ferromagnetism, antiferromagnetism, and superconductivity in magnetically anisotropic (Eu,La)FeAs2

Materials with exceptional magnetism and superconductivity usually conceive emergent physical phenomena. Here, we investigate the physical properties of the (Eu,La)FeAs2 system with double magnetic sublattices. The parent EuFeAs2 shows anisotropy-associated magnetic behaviors, such as Eu-related moment canting and exchange bias. Through La doping, the magnetic anisotropy is enhanced with ferromagnetism of Eu2+ realized in the overdoped region, and a special exchange bias of the superposed ferromagnetic/superconducting loop revealed in Eu0.8La0.2FeAs2. Meanwhile, the Fe-related antiferromagnetism shows unusual robustness against La doping. Theoretical calculation and 57Fe Mössbauer spectroscopy investigation reveal a doping-tunable dual itinerant/localized nature of the Fe-related antiferromagnetism. The coexistence of the Eu-related ferromagnetism, Fe-related robust antiferromagnetism, and superconductivity is further revealed in Eu0.8La0.2FeAs2, providing a platform for further exploration of potential applications and emergent physics. Finally, an electronic phase diagram is established for (Eu,La)FeAs2 with the whole superconducting dome adjacent to the Fe-related antiferromagnetic phase, which is of benefit for seeking underlying clues to high-temperature superconductivity.

Single-ion anisotropy is necessary and appropriate to study the magnetic behavior of Tb3+ moments with Jeff=12 on the honeycomb lattice in Tb2Ir3Ga9

By developing models of increasing complexity, we show that a model without single-ion anisotropy (SIA) cannot explain the magnetic properties of ${J}_{\mathrm{eff}}=1/2 {\mathrm{Tb}}^{3+}$ moments in the orthorhombically distorted, honeycomb material ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$. In four different models for the magnetization of a single honeycomb layer, the only sources of anisotropy are symmetric exchange interactions ${J}_{n\ensuremath{\alpha}\ensuremath{\beta}}={J}_{n\ensuremath{\beta}\ensuremath{\alpha}}$ along three different bonds $n$, an anisotropic $\underline{g}$ tensor, and a Dzyalloshinskii-Moriya interaction (asymmetric exchange) that produces the observed canted moment along $\mathbf{b}$. With 21 parameters, the best such model yields ${\ensuremath{\chi}}^{2}=0.065$, which is substantially smaller than ${\ensuremath{\chi}}^{2}=0.112$ obtained using a Heisenberg model containing six parameters including easy-axis anisotropy. However, models without SIA fail to reproduce the linear dependence of the magnetization with a field perpendicular to the Ising axis while predicting a saturation magnetization that is far too low. Due to the complex crystal-field environments, we argue that SIA is necessary to study low-symmetry, three-dimensional ${J}_{\mathrm{eff}}=1/2$ materials containing ${\mathrm{Tb}}^{3+}$ ions.

Role of orbital off-diagonal spin and charge condensates in a three orbital model for Ca2RuO4 – Coulomb renormalized spin-orbit coupling, orbital moment, and tunable magnetic order

Strongly anisotropic spin-orbit coupling (SOC) renormalization and strongly enhanced orbital magnetic moments are obtained in the fully self consistent approach including the orbital off-diagonal spin and charge condensates. For moderate tetragonal distortion as in Ca2 RuO4, dominantly planar antiferromagnetic (AFM) order with small canting of moments in and about the crystal c axis are obtained. For reduced tetragonal distortion, we find a tunable regime wherein the magnetic order can be tuned (AFM or FM) by the bare SOC strength and octahedral tilting magnitude. In this regime, with decreasing tetragonal distortion, AFM order is maintained by progressively decreasing octahedral tilting, as observed in Ca2−xSrx RuO4. For purely planar order, the only self consistent solution is FM order along crystal b axis, which is relevant for the bilayer ruthenate compound Ca3 Ru2 O7.

Canted antiferromagnetic order and spin dynamics in the honeycomb-lattice compound Tb2Ir3Ga9

Single crystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements, and first-principles calculations are used to investigate the magnetic properties of the honeycomb lattice $\rm Tb_2Ir_3Ga_9$. While the $R\ln2$ magnetic contribution to the low-temperature entropy indicates a $\rm J_{eff}=1/2$ moment for the lowest-energy crystal-field doublet, the Tb$^{3+}$ ions form a canted antiferromagnetic structure below 12.5 K. Due to the Dzyalloshinskii-Moriya interactions, the Tb moments in the $ab$ plane are slightly canted towards $b$ by $6^\circ$ with a canted moment of 1.22 $\mu_{\rm B} $ per formula unit. A minimal $xxz$ spin Hamiltonian is used to simultaneously fit the spin-wave frequencies along the high symmetry directions and the field dependence of the magnetization along the three crystallographic axes. Long-range magnetic interactions for both in-plane and out-of-plane couplings up to the second nearest neighbors are needed to account for the observed static and dynamic properties. The $z$ component of the exchange interactions between Tb moments are larger than the $x$ and $y$ components. This compound also exhibits bond-dependent exchange with negligible nearest exchange coupling between moments parallel and perpendicular to the 4$f$ orbitals. Despite the $J_{{\rm eff}}=1/2$ moments, the spin Hamiltonian is denominated by a large in-plane anisotropy $K_z \sim -1$ meV. DFT calculations confirm the antiferromagnetic ground state and the substantial inter-plane coupling at larger Tb-Tb distances.

Room-Temperature Antiferromagnetic Resonance and Inverse Spin-Hall Voltage in Canted Antiferromagnets.

We study theoretically and experimentally the spin pumping signals induced by the resonance of canted antiferromagnets with Dzyaloshinskii-Moriya interaction and demonstrate that they can generate easily observable inverse spin-Hall voltages. Using a bilayer of hematite/heavy metal as a model system, we measure at room temperature the antiferromagnetic resonance and an associated inverse spin-Hall voltage, as large as in collinear antiferromagnets. As expected for coherent spin pumping, we observe that the sign of the inverse spin-Hall voltage provides direct information about the mode handedness as deduced by comparing hematite, chromium oxide and the ferrimagnet yttrium-iron garnet. Our results open new means to generate and detect spin currents at terahertz frequencies by functionalizing antiferromagnets with low damping and canted moments.

Epitaxial growth and transport properties of compressively-strained Ba2IrO4 films* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774153, 11861161004, 51772143, 11974163, and 51672125), the National Key Research and Development Program of China (Grant No. 2016YFA0201104), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 0213-14380167 and 0213-14380198), and the Hong Kong Research Grants Council

Ba2IrO4 is a sister compound of the widely investigated Sr2IrO4 and has no IrO6 octahedral rotation nor net canted antiferromagnetic moment, thus it acts as a system more similar to the high-T c cuprate. In this work, we synthesize the Ba2IrO4 epitaxial films by reactive molecular beam epitaxy and study their crystalline structure and transport properties under biaxial compressive strain. High resolution scanning transmission electron microscopy and x-ray diffraction confirm the high quality of films with partial strain relaxation. Under compressive epitaxial strain, the Ba2IrO4 exhibits the strain-driven enhancement of the conductivity, consistent with the band gap narrowing and the stronger hybridization of Ir-t2g and O-2p orbitals predicted in the first-principles calculations.

Pressure-tuned spin switching in compensated GdCrO3 ferrimagnet

The effect of hydrostatic pressure on antiferromagnetic ordering of Cr spins, magnetic compensation, and exotic spin switching in the single-crystal $\mathrm{GdCr}{\mathrm{O}}_{3}$ ferrimagnet is studied. The N\'eel temperature ${T}_{\mathrm{N}}=168\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ increases under pressure with the rate of 0.42 K/kbar, and the compensation temperature ${T}_{\mathrm{comp}}=144\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at which the canted ferromagnetic moment of Cr spins and antiparallel polarized moment of Gd spins cancel each other also increases by 0.3 K/kbar. It was found that the spin switching temperature ${T}_{\mathrm{sw}}$, at which the ferromagnetic moment is reversed, noticeably increases under pressure, and the spin switching energy required for magnetization reversal reduces significantly because of a decrease in magnetic anisotropy. Due to this mechanism, the spin switching line, described in the $T$\ensuremath{-}$H$ diagram by a certain ratio of switching energy to canted ferromagnetic moment, is shifted under pressure towards higher temperatures. Thus, switching between two opposite spin configurations in the $\mathrm{GdCr}{\mathrm{O}}_{3}$ ferrimagnet can be controlled by both applied magnetic field and external pressure.

Distinct magnetic ground states of R2ZnIrO6 (R=La,Nd) determined by neutron powder diffraction

Double perovskite iridates $A_2$ZnIrO$_6$ ($A$ = alkaline or lanthanide) show complex magnetic behaviors ranging from weak ferromagnetism to successive antiferromagnetic transitions. Here we report the static ($dc$) and dynamic ($ac$) magnetic susceptibility, and neutron powder diffraction measurements for $A$ = La and Nd compounds to elucidate the magnetic ground state. Below 10~K, the $A$ = La compound is best described as canted iridium moments in an antiferromagnet arrangement with a propagation vector \textbf{k} = 0 and a net ferromagnetic component along the $c$-axis. On the other hand, Nd$_2$ZnIrO$_6$ is described well as an antiferromagnet with a propagation vector \textbf{k} = (1/2~1/2~0) below $T_\mathrm{N} \sim$ 17 K. Scattering from both the Nd and Ir magnetic sublattices were required to describe the data and both were found to lie almost completely within the $ab$-plane. $Dc$ susceptibility revealed a bifurcation between the zero-field-cooled and field-cooled curves below $\sim$13 K in Nd$_2$ZnIrO$_6$. A glassy state was ruled out by $ac$ susceptibility but detailed magnetic isotherms revealed the opening of the loop below 13~K. These results suggest a delicate balance exists between the Dzyaloshinskii-Moriya, crystal field schemes, and $d$-$f$ interaction in this series of compounds.

Multiferroic properties of Mn-substituted BiFeO3

To study the multiferroic behavior of BiFeO3 at low concentrations of Mn as dopant, BiFe1−xMnxO3 (BFMO) at x = 0.01, 0.02, and 0.03 has been successfully synthesized via chemical combustion method. X-ray diffraction reveals distorted rhombohedral structure for BiFeO3 and respective variation in lattice parameters. The particle size is studied with FESEM images. Wide-range frequencies are used to carry out dielectric constant and tan δ (loss factor) measurements. The typical behavior of BFMO samples is attributed via charge-carrier hopping mechanisms due to structural inhomogeneity and formation of vacancies. The hysteresis loop measurements are used to study the ferroelectric and magnetic response. The canting of spin moments and cation–anion–cation exchange interactions leads to observed room-temperature ferromagnetism, while ferroelectric performance enhances with increasing Mn doping in BiFeO3. The altered ferroelectric and magnetic properties along with possible magnetoelectric coupling would make BFMO as suitable candidate for magnetoelectric devices. The maximum value obtained for $${\alpha }_{\text{ME}}$$ is 29.03 mV cm−1Oe−1 at Hdc = 1.4 kOe and could be attributed due to strong antiferromagnetic character of BFO with Mn doping.

Two-step magnetic ordering into a canted state in ferrimagnetic monoclinic Mn3As2

We report the magnetic structure of monoclinic Mn3As2 at 3 ​K and 250 ​K using neutron powder diffraction measurements. From magnetometry data, the Curie temperature of Mn3As2 was confirmed to be around 270 ​K. Calorimetry analysis showed the presence of another transition at 225 ​K. At 270 ​K, Mn3As2 undergoes a k=0 ferrimagnetic ordering in the magnetic space group C2/m (#12.58) with Mn moments pointing along b. Below 225 ​K, there is a canting of Mn moments in the ac plane which produces a multi-k non-collinear magnetic structure in space group C2/c (#15.85). The components of Mn moments along b follow k=0 ordering and the components along a and c have k=[0012] propagation vector. The change in the magnetic ground state with temperature provides a deeper insight into the factors that govern magnetic ordering in Mn–As compounds.

Evidence of Ba-substitution induced spin-canting in the magnetic Weyl semimetalEuCd2As2

Recently EuCd$_2$As$_2$ was predicted to be a magnetic Weyl semi-metal with a lone pair of Weyl nodes generated by A-type antiferromagnetism and protected by a rotational symmetry. However, it was soon discovered that the actual magnetic structure broke the rotational symmetry and internal pressure was later suggested as a route to stabilize the desired magnetic state. In this work we test this prediction by synthesizing a series of Eu$_{1-x}$Ba$_x$Cd$_2$As$_2$ single crystals and studying their structural, magnetic and transport properties via both experimental techniques and first-principles calculations. We find that small concentrations of Ba ($\sim 3-10\% $) lead to a small out-of-plane canting of the Eu moment. However, for higher concentrations this effect is suppressed and a nearly in-plane model is recovered. Studying the transport properties we find that all compositions show evidence of an Anomalous Hall Effect dominated by the intrinsic mechanism as well as large negative magnetoresistances in the longitudinal channel. A non-monotonic evolution of the transport properties is seen across the series which correlates to the proposed canting suggesting canting may enhance the topological effects. Careful density functional theory calculations using an all-electron approach revise prior predictions finding a purely ferromagnetic ground state with in-plane moments for both the EuCd$_2$As$_2$ and Eu$_{0.5}$Ba$_{0.5}$Cd$_2$As$_2$ compounds - corroborating our experimental findings. This work suggests that Ba substitution can tune the magnetic properties in unexpected ways which correlate to changes in measures of topological properties, encouraging future work to locate the ideal Ba concentration for Eu moment canting.

Anomalous Hall effect in κ -type organic antiferromagnets

We theoretically propose a mechanism for the anomalous Hall effect (AHE) in an antiferrromagnetic (AFM) state of $\kappa$-type organic conductors. We incorporate the spin-orbit coupling in the effective Hubbard model on the $\kappa$-type lattice structure taking into account the orientation of the molecules and their arrangement with dimerization. Treating this model by means of the Hartree-Fock approximation and the linear response theory, we find that an intrinsic contribution to the Hall conductivity becomes nonzero in the electron-doped AFM metallic phase with a small canted ferromagnetic moment. We show that, contrary to the conventional wisdom, the spin canting is irrelevant to the Hall response; the nonzero Hall conductivity originates from the collinear component of the AFM order in the presence of the spin-orbit coupling. These features are well explained analytically in the limit of strong dimerization on the anisotropic triangular lattice. Furthermore, we present an intuitive picture for the present AHE by considering the real-space configuration of emergent magnetic fluxes. We also find that the Hall response appears even in the undoped AFM insulating phase at nonzero frequency as the magneto-optical Kerr effect, which is enhanced around the charge transfer excitations. We discuss possible detections of the AHE in ET based compounds.

Antiferromagnetic CuMnAs: Ab initio description of finite temperature magnetism and resistivity

Noncollinear magnetic moments in antiferromagnets (AFM) lead to a complex behavior of electrical transport, even to a decreasing resistivity due to an increasing temperature. Proper treatment of such phenomena is required for understanding AFM systems at finite temperatures; however first-principles description of these effects is complicated. With ab initio techniques, we investigate three models of spin fluctuations (magnons) influencing the transport in AFM CuMnAs; the models are numerically feasible and easily implementable to other studies. We numerically justified a fully relativistic collinear disordered local moment approach and we present its uncompensated generalization. A saturation or a decrease of resistivity caused by magnons, phonons, and their combination (above approx. 400 K) was observed and explained by changes in electronic structure. Within the coherent potential approximation, our finite-temperature approaches may be applied also to systems with impurities, which are found to have a large impact not only on residual resistivity, but also on canting of magnetic moments from the AFM to the ferromagnetic (FM) state.

Magnetic structure of undistorted hexagonal ferrites, Lu0.2In0.8FeO3

We report on the crystal and magnetic structure of bulk hexagonal (Lu,In)FeO3. Neutron powder diffraction revealed that Lu0.2In0.8FeO3 has a single-phase P63/mmc structure down to T ≈ 3 K and shows an antiferromagnetic transition at TN ≈ 164 K. Unlike the distorted hexagonal LuFeO3 family with an A2(Γ2)-type spin configuration, undistorted hexagonal Lu0.2In0.8FeO3 shows either an A1(Γ1) or a B1(Γ3)-type spin configuration below TN, which does not produce the c-directional canted ferromagnetic moment. A significant reduction in the ordered magnetic moment was observed at 3 K without trimerization, and hints of a magnetic cluster state were observed in the paramagnetic phase near room temperature. Therefore, the system presents a rare example to study the geometrically frustrated magnetism in the undistorted hexagonal magnet that has a perfect triangular lattice below room temperature.

Emergent snake magnetic domains in canted kagome ice

We study the two-dimensional kagome-ice model derived from a pyrochlore lattice with second- and third-neighbor interactions. The canted moments align along the local $\langle 111 \rangle$ axes of the pyrochlore and respond to both in-plane and out-of-plane external fields. We find that the combination of further-neighbor interactions together with the external fields introduces a rich phase diagram with different spin textures. Close to the phase boundaries, metastable $\textit{"snake"}$ domains emerge with extremely long relaxation time. Our kinetic Monte Carlo analysis of the magnetic-field quench process from saturated state shows unusually slow dynamics. Despite that the interior spins are almost frozen in snake domains, the spins on the edge are free to fluctuate locally, leading to frequent creation and annihilation of monopole-anti-monopole bound states. Once the domains are formed, these excitations are localized and can hardly propagate due to the energy barrier of snakes. The emergence of such snake domains may shed light on the experimental observation of dipolar spin ice under tilted fields, and provide a new strategy to manipulate both spin and charge textures in artificial spin ice.

Ferrimagnetism and anisotropic phase tunability by magnetic fields inNa2Co2TeO6

${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ has recently been proposed to be a Kitaev-like honeycomb magnet. To assess how close it is to realizing Kitaev quantum spin liquids, we have measured magnetization and specific heat on high-quality single crystals in magnetic fields applied along high-symmetry directions. Small training fields reveal a weak but canonical ferrimagnetic behavior below 27 K, which suggests that the previously established zigzag antiferromagnetic order, with collinear moments pointing along the zigzag chains, must be supplemented by $\mathbf{q}=0$ (e.g., N\'eel type in the absence of structural modulations) moment canting. Moderate fields in the honeycomb plane suppress the thermal transition at 27 K, and seem to partly reverse the moment canting when applied perpendicular to the zigzag chains. In contrast, out-of-plane fields leave the transition largely unaffected, but promotes another transition below 10 K, possibly also related to canting reversal. The magnetism in ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ is highly anisotropic and close to tipping points between competing phases.

Engineering of octahedral rotations and electronic structure in ultrathin SrIrO3 films

Layered perovskite iridate ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ shares many similarities with high ${T}_{\text{c}}$ cuprates and is expected to host novel superconductivity but has never been realized experimentally. Despite the similarities, the prominent ${\mathrm{IrO}}_{6}$ octahedral rotations and sizable net canted antiferromagnetic moments lying in each ${\mathrm{IrO}}_{2}$ plane in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ are strikingly different from high ${T}_{\text{c}}$ cuprates where the octahedral rotations and net canted moment are much smaller or negligible. Here, using reactive molecular beam epitaxy, we demonstrate that the octahedral rotations around the in-plane and out-of-plane axes in epitaxial iridate films can be suppressed step-by-step via interfacial clamping imposed by cubic substrates as the films approach the two-dimensional limit. In situ angle-resolved photoemission spectroscopy and first-principles calculations show a gapped antiferromagnetic ground state with dispersive low-lying bands in 1- and 2-unit-cell-thick ${\mathrm{SrIrO}}_{3}$ films, providing ideal single- and bilayer analogies of high ${T}_{\text{c}}$ cuprates.