Magnon thermal Hall effect induced by symmetric exchange interaction

We investigate a microscopic origin of nonreciprocal magnon that is distinct from the Dzyaloshinskii-Moriya interaction in a honeycomb antiferromagnet. The key ingredient is a symmetric anisotropic exchange interaction depending on the bond direction, which results in a valley-type nonreciprocal magnon excitations under the staggered antiferromagnetic ordering. Furthermore, we find this type of nonreciprocal magnon exhibits a peculiar magnetic-field response; the nonreciprocal direction can be manipulated by the in-plane rotating magnetic field. The obtained results can be accounted for the emergence of the magnetic toroidal multipoles.

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

We report the direct observation of a commensurate-ordered antiferromagnetic (AFM) state but incommensurate helical spin dynamics in the natural mineral brochantite ${\mathrm{Cu}}_{4}{\mathrm{SO}}_{4}{(\mathrm{OH})}_{6}$ through neutron diffraction and neutron spectroscopy measurements. Inelastic neutron scattering measurements reveal magnonlike excitations with considerable dispersion along the $c$ axis and almost flat branches in other principal directions, indicating the strong one-dimensional character of the magnetic correlations. We experimentally observe the effect of the uniform Dzyaloshinskii-Moriya (DM) interaction, which elevates the degeneracy of the spin-wave modes, shifting them in opposite directions in reciprocal space. The system has a commensurate AFM ground state, stabilized by the anisotropic symmetric Heisenberg exchange interactions, and quasi-one-dimensional chiral spin dynamics due to the antisymmetric DM interaction. Employing linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian. We quantify both the symmetric exchange parameters and the DM vector components in ${\mathrm{Cu}}_{4}{\mathrm{SO}}_{4}{(\mathrm{OH})}_{6}$ and determine the mechanism of the magnetic frustration. Our work provides detailed insights into the complex dynamics of the spin chain in the presence of uniform DM interaction.

The ground-state properties of the pentameric Co(II) cluster [Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)](12-) were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na(12)[Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)].46D(2)O. The encapsulated magnetic Co(5) unit consists of three octahedral and two tetrahedral oxo-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co(5) spin cluster is anisotropic and is expressed as H = -2 summation operator(i= x,y,z)J(1)(i)[S(1)(i)S(2)(i) + S(2)(i)S(3)(i)] + J(2)(i)[S(1)(i)S(5)(i) + S(2)(i)S(5)(i) + S(2)(i)S(6)(i) + S(3)(i)S(6)(i)], where J(1)(i) are the components of the exchange interaction between the octahedral Co(II) ions and J(2)(i) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure 1d). The study of the exchange interactions in the two structurally related polyoxoanions [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10)(-) and [Co(3)W(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) allowed an independent determination of the ferromagnetic exchange parameters J(1)(x) = 0.70 meV, J(1)(y) = 0.43 meV, and J(1)(z) = 1.51 meV (set a) and J(1)(x) = 1.16 meV, J(1)(y) = 1.16 meV and J(1)(z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchange interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2)(x) = -1.24 meV, J(2)(y) = -0.53 meV, and J(2)(z) = -1.44 meV (set a) or J(1)(x) = -1.19 meV, J(1)(y) = -0.53 meV, and J(1)(z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties of the title compound.

We investigate the magnetic properties of the multiferroic quantum-spin\nsystem LiCu$_2$O$_2$ by electron spin resonance (ESR) measurements at $X$- and\n$Q$-band frequencies in a wide temperature range $(T_{\\rm N1} \\leq T \\leq\n300$\\,K). The observed anisotropies of the $g$ tensor and the ESR linewidth in\nuntwinned single crystals result from the crystal-electric field and from local\nexchange geometries acting on the magnetic Cu$^{2+}$ ions in the zigzag-ladder\nlike structure of LiCu$_2$O$_2$. Supported by a microscopic analysis of the\nexchange paths involved, we show that both the symmetric anisotropic exchange\ninteraction and the antisymmetric Dzyaloshinskii-Moriya interaction provide the\ndominant spin-spin relaxation channels in this material.\n

Nonreciprocal magnon by symmetric anisotropic exchange interaction in honeycomb antiferromagnet

Ensembles of magnetic skyrmions in confined geometries are shown to exhibit thermally driven motion on two different time scales. The intrinsic fluctuating dynamics (t ∼ 1 ps) are governed by short-range symmetric and antisymmetric exchange interactions, whereas the long-time limit (t ≳ 10 ns) is determined by the coaction of skyrmion–skyrmion-repulsion and the system’s geometry. Micromagnetic simulations for realistic island shapes and sizes are performed and analyzed, indicating the special importance of skyrmion dynamics at finite temperatures. We demonstrate how the competition between skyrmion mobility and observation time directly affects the addressability of skyrmionic bits, which is a key challenge on the path of developing skyrmion-based room-temperature applications. The presented quasiparticle Monte Carlo approach offers a computationally efficient description of the diffusive motion of skyrmion ensembles in confined geometries, like racetrack memory setups.

Periodical equilibrium states of magnetization exist in chiral ferromagnetic films, if the constant of antisymmetric exchange (Dzyaloshinskii-Moriya interaction) exceeds some critical value. Here, we demonstrate that this critical value can be significantly modified in curved film. The competition between symmetric and antisymmetric exchange interactions in a curved film can lead to a new type of domain wall which is inclined with respect to the cylinder axis. The wall structure is intermediate between Bloch and N\'eel ones. The exact analytical solutions for phase boundary curves and the new domain wall are obtained.

We report the magnetic structure and anisotropy of the quasi-one-dimensional $S=\frac{1}{2}$ antiferromagnet ${\mathrm{Na}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Cl}}_{2}$ obtained by single-crystal neutron scattering, electron spin resonance (ESR), and magnetization measurements, following an earlier study of its dynamics [M. Fujihala et al., Phys. Rev. B 101, 024410 (2020)]. A N\'eel-type spin structure is formed within the chain of this compound, where the spins point along the $b$ axis, and ESR data indicate an antisymmetric exchange with a uniform Dzyaloshinskii-Moriya (DM) vector pointing along the $b$ axis. The anisotropy $g$ factor and magnetic structure are strong indicators of magnetic anisotropy originating from a symmetric anisotropic exchange interaction and/or a magnetic dipole interaction. These results suggest that these terms of the anisotropic spin Hamiltonian counteract the effect of the DM interaction and stabilize the N\'eel-type structure in ${\mathrm{Na}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Cl}}_{2}$.

Dephasing dynamics of exciton fine structure (EFS) caused by long and short range exchange interactions in GaN was investigated by using a four‐wave‐mixing (FWM) spectroscopy. The type of exchange interactions was selected by samples (uniaxially strained c ‐plane GaN films and a thick bulk GaN) and their geometries in optical excitation. Since each component of EFS is highly polarized, the polarization‐dependent FWM spectra successfully resolve the dephasing dynamics in the individual EFS resonances. The results show a difference in dephasing time, suggesting a contribution of anisotropic exchange interaction (AEI). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The phase diagrams of four graphene-like nanoparticles described by the Ising model are investigated by the use of the effective-field theory with correlations. The particle is represented by a two-dimensional array of spin-1/2 atoms with two types of exchange interactions, J and JS. The effects of surface exchange interaction JS on the phase diagrams are examined. We have found some characteristic behaviours.

The anti-symmetric and anisotropic symmetric exchange interactions between two magnetic dipole moments – responsible for intriguing magnetic textures (e.g., magnetic skyrmions) – have been discovered since last century, while their electric analogues were either hidden for a long time or still not known. It is only recently that the anti-symmetric exchange interactions between electric dipoles was proved to exist (with materials hosting such an interaction being still rare) and the existence of anisotropic symmetric exchange interaction between electric dipoles remains ambiguous. Here, by symmetry analysis and first-principles calculations, we identify hafnia as a candidate material hosting the non-collinear dipole alignments, the analysis of which reveals the anti-symmetric and anisotropic symmetric exchange interactions between electric dipoles in this material. Our findings can hopefully deepen the current knowledge of electromagnetism in condensed matter, and imply the possibility of discovering novel states of matter (e.g., electric skyrmions) in hafnia-related materials.

Exchange-induced in-plane magnetic anisotropies in a single-crystalline Fe/CoO/MgO(001) system were quantitatively investigated using the planar Hall effect as a function of temperature. Field cooling can induce a strong uniaxial anisotropy in Fe film with the easy axis along the CoO〈110〉 directions close to the cooling field direction. The exchange coupling also induces a 4-fold anisotropy with the easy axis along the CoO〈100〉 directions. Our results prove that the strong magneto-crystalline anisotropy of CoO antiferromagnetic spin plays a significant role in exchange-induced anisotropy.

We consider a crystalline solid with two types of exchange interactions: a direct exchange coupling of localized spins and $s\ensuremath{-}d$ or $s\ensuremath{-}f$ exchange between localized spins and conduction electrons. The model developed here is a generalization of the theory proposed by Kasaya and Hihara to explain the weak ferromagnetism of Mn${\mathrm{B}}_{2}$. In contrast to the latter theory, here the finite spatial extension of the $d$ or $f$ wave functions and the distortion of the electronic energy bands induced in the ordered state by the $s\ensuremath{-}d$ or $s\ensuremath{-}f$ interactions are taken into account and significantly affect the criteria for weak ferromagnetism. Comparisons are made to the Zener and Rudermann-Kittel-Kasuya-Yosida theories, which are limiting cases of the theory presented here. In particular, the nonlinear response of the conduction electrons to the exchange field is demonstrated, with important implications for first-order magnetic phase transitions.

Cu2OSeO3 is a chiral magnet with a rich phase diagram. The positions of the phase transitions are sensitive to chemical substitution; however, the exact mechanism by which dopants influence magnetic behaviour remains elusive. In this work, Cu2OSe1−xTexO3 with (0 ≤ x ≤ 0.1) is synthesised with the lattice expanding upon Te inclusion. Our SANS and magnetometry data indicate that doping lowers the paramagnetic to helical ordering temperature and the critical field required for the conical to field-polarised phase transition. We present a universality class analysis on the materials using the Heatmap Modified Iteration Method to evaluate the critical behaviour and show that the Heisenberg to Ising transition is robust to chemical doping. However, we demonstrate that the inclusion of Te decreases both the strengths of the Dzyaloshinskii-Moriya and Symmetric Exchange Interactions. This led to an expansion of the skyrmion region and decreased both the critical temperature and field.