Pseudodipolar Interaction Research Articles

Chirality-selective topological magnon phase transition induced by interplay of anisotropic exchange interactions in honeycomb ferromagnet

A variety of distinct anisotropic exchange interactions commonly exist in one magnetic material due to complex crystal, magnetic and orbital symmetries. Here we investigate the effects of multiple anisotropic exchange interactions on topological magnon in a honeycomb ferromagnet, and find a chirality-selective topological magnon phase transition induced by a complicated interplay of Dzyaloshinsky–Moriya interaction and pseudo-dipolar interaction, accompanied by the bulk gap close and reopen with chiral inversion. Moreover, this novel topological phase transition involves band inversion at high symmetry points K and K′, which can be regarded as a pseudo-orbital reversal, i.e. magnon valley degree of freedom, implying a new manipulation corresponding to a sign change of the magnon thermal Hall conductivity. Indeed, it can be realized in 4d or 5d correlated materials with both spin–orbit coupling and orbital localized states, such as iridates and ruthenates, etc. This novel regulation may have potential applications on magnon devices and topological magnonics.

Topological magnons in one-dimensional ferromagnetic Su–Schrieffer–Heeger model with anisotropic interaction

Topological magnons in a one-dimensional (1D) ferromagnetic Su–Schrieffer–Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the intercellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii–Moriya interaction and pseudo-dipolar interaction, also can induce the emergence of the non-trivial phase with two degenerate in-gap edge states separately localized at the two ends of the 1D chain, while the intracellular interactions instead unfavors the topological phase. The interplay among them has synergistic effects on the topological phase transition, very different from that in the two-dimensional (2D) ferromagnet. These results demonstrate that the 1D magnons possess rich topological phase diagrams distinctly different from the electronic version of the SSH model and even the 2D magnons. Due to the low dimensional structural characteristics of this 1D topological magnonic system, the magnonic crystals can be constructed from bottom to top, which has important potential applications in the design of novel magnonic devices.

Emergent interlayer magnetic order via strain-induced orthorhombic distortion in the 5d Mott insulator Sr2IrO4

We report a La2CuO4-like interlayer antiferromagnetic order in Sr2IrO4 films with large orthorhombic distortion (> 1.5%). The biaxial lattice strain in epitaxial heterostructures of Sr2IrO4/Ca3Ru2O7 lowers the crystal symmetry of Sr2IrO4 from tetragonal (C4) to orthorhombic (C2), guiding the Ir 5d Jeff = 1/2 pseudospin moment parallel to the elongated b-axis via magnetic anisotropy. From resonant X-ray scattering experiments, we observed an antiferromagnetic order in the orthorhombic Sr2IrO4 film whose interlayer stacking pattern is inverted from that of the tetragonal Sr2IrO4 crystal. This interlayer stacking is similar to that of the orthorhombic La2CuO4, implying that the asymmetric interlayer exchange interaction along a and b-directions exceeds the anisotropic interlayer pseudo-dipolar interaction. Our result suggests that strain-induced distortion can provide a delicate knob for tuning the long-range magnetic order in quasi-two-dimensional systems by evoking the competition between the interlayer exchange coupling and the pseudo-dipolar interaction.

Phases and quantum phase transitions in the anisotropic antiferromagnetic Kitaev-Heisenberg- Γ magnet

We study the Kitaev-Heisenberg-$\Gamma$ model with antiferromagnetic Kitaev exchanges in the strong anisotropic (toric code) limit to understand the phases and the intervening phase transitions between the gapped $Z_2$ quantum spin liquid and the spin-ordered (in the Heisenberg limit) as well as paramagnetic phases (in the pseudo-dipolar, $\Gamma$, limit). We find that the paramagnetic phase obtained in the large $\Gamma$ limit has no topological entanglement entropy and is proximate to a gapless critical point of a system described by an equal superposition of differently oriented stacked one-dimensional $Z_2\times Z_2$ symmetry protected topological phases. Using a combination of exact diagonalization calculations and field-theoretic analysis we map out the phases and phase transitions to reveal the complete phase diagram as a function of the Heisenberg, the Kitaev, and the pseudo-dipolar interactions. Our work shows a rich plethora of unconventional phases and phase transitions and provides a comprehensive understanding of the physics of anisotropic Kitaev-Heisenberg-$\Gamma$ systems along with our recent paper [Phys. Rev. B 102, 235124 (2020)] where the ferromagnetic Kitaev exchange was studied.

Non-covalent interactions governing the supramolecular assembly of copper(II) complexes with hydrazone-type ligand: Experimental and quantum chemical study

A series of two new mono- and one binuclear µ-nitrato bridged copper(II) complexes [Cu(L)(HL)]ClO4 (1), [Cu(HL)(NO3)(H2O)]2NO3∙H2O (2) and [Cu2(L2)(µ-NO3)2] (3), with an unsymmetrical NNO donor Schiff base (HL) have been synthesized and characterized by elemental analysis, FTIR, CV, UV–vis and EPR spectroscopy. Their molecular structures were also determined by single crystal X-ray crystallography. In the binuclear complex 3, the Cu⋯ Cu distance is 3.494 Å. In 1, 2 and 3, the Cu(II) centers have distorted square pyramidal geometry (τ5 = 0.05–0.17). Evidence of weak π⋯π stacking intermolecular interactions along with other non-covalent interactions (hydrogen bonding) was observed by analyzing the respective crystal structures of the complexes. Thus, these hydrogen bonds, π⋯π stacking interactions and other weak intermolecular interactions establish in the form of supramolecular architectures a crystalline “network” environment. The non-covalent interactions were also investigated by employing Hirshfeld Analysis. The room temperature magnetic moments of the mononuclear complexes are less than the spin only values which are indicative of small interactions. Also, significant magnetic interactions were not exhibited by binuclear copper(II) complex 3 in the variable temperature magnetic measurements. The X-band EPR spectra of all three complexes exhibit copper(II) hyperfine structures as well as zero-field splitting which are appropriate for the triplet states of dimers. In complexes 1 and 2, the presence of pseudo dipolar interactions is proposed. Quantum chemical calculations (DFT) were carried out on complexes 1–3 to explore the electronic and spectral properties of these newly synthesized complexes. These complexes show significant antiproliferative and SOD activity. The SOD activity measured in terms of kMcCF is in the range 4.94–12.31 (mol L)−1s−1)× 104.

Theory of the low-temperature longitudinal spin Seebeck effect

Using a simplified microscopic model of coupled spin and lattice excitations in a ferromagnetic insulator we evaluate the magnetic-field dependence of the spin Seebeck effect at low temperatures. The model includes Heisenberg exchange coupling, a harmonic lattice potential, and a pseudo-dipolar exchange interaction. Our approach goes beyond previous work [Phys. Rev. B 98, 134421 (2018)] in that it does not rely on the a priori assumption of a fast equilibration of the magnon and phonon distributions. Our theory shows that singular features in the magnetic-field dependence of the spin Seebeck effect at low temperatures observed by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)] are independent of the relative strength of magnon-impurity and phonon-impurity scattering.

Phases and quantum phase transitions in an anisotropic ferromagnetic Kitaev-Heisenberg- Γ magnet

We study the spin-$1/2$ ferromagnetic Heisenberg-Kitaev-$\Gamma$ model in the anisotropic (Toric code) limit to reveal the nature of the quantum phase transition between the gapped $Z_2$ quantum spin liquid and a spin ordered phase (driven by Heisenberg interactions) as well as a trivial paramagnet (driven by pseudo-dipolar interactions, $\Gamma$). The transitions are obtained by a simultaneous condensation of the Ising electric and magnetic charges-- the fractionalized excitations of the $Z_2$ quantum spin liquid. Both these transitions can be continuous and are examples of deconfined quantum critical points. Crucial to our calculations are the symmetry implementations on the soft electric and magnetic modes that become critical. In particular, we find strong constraints on the structure of the critical theory arising from time reversal and lattice translation symmetries with the latter acting as an anyon permutation symmetry that endows the critical theory with a manifestly self-dual structure. We find that the transition between the quantum spin liquid and the spin-ordered phase belongs to a self-dual modified Abelian Higgs field theory while that between the spin liquid and the trivial paramagnet belongs to a self-dual $Z_2$ gauge theory. We also study the effect of an external Zeeman field to show an interesting similarity between the polarised paramagnet obtained due to the Zeeman field and the trivial paramagnet driven the pseudo-dipolar interactions. Interestingly, both the spin liquid and the spin ordered phases have easily identifiable counterparts in the isotropic limit and the present calculations may shed insights into the corresponding transitions in the material relevant isotropic limit.

Unidirectional anisotropy in bent ferromagnetic microwires

New measurements are described on hysteresis loops of bent amorphous Fe-based microscopic wires. In our setup, a wire consists of a straight part along which the longitudinal field is applied, and other straight parts, one or two, which are perpendicular to the applied field. The magnetization of these other parts does not change during the hysteresis experiment. The observed switching field depends on the spatial configuration of the wire. In particular, unidirectional anisotropy is detected, which is visible as a reduction of the switching field for ascending or descending direction of changes of the applied field. The results are explained in terms of pseudodipolar interaction of the wire parts. We also demonstrate that anisotropy of the same sign is produced by the Dzyaloshinskii-Moriya interaction.

Nematic phase in the spin one two dimensional anisotropic antiferromagnet

We study quantum phase transitions in the anisotropic Heisenberg model with a pseudodipolar interaction on the square lattice, at zero temperature, using the SU(3) Schwinger boson technique in a mean field approximation. We present the dispersion relation for several values of the parameters. When we take into account the effect of the next near neighbor interaction, the phase diagram, at zero temperature shows the existence of a region in the intermediate frustrated regime, even for zero anisotropy parameter, where the system is magnetically disordered. A prediction of our model is the existence of a finite gap in this region.

Dynamics of a quantum spin liquid beyond integrability: The Kitaev-Heisenberg- Γ model in an augmented parton mean-field theory

We present an augmented parton mean-field theory which (i) reproduces the $exact$ ground state, spectrum, and dynamics of the quantum spin liquid phase of Kitaev's honeycomb model; and (ii) is amenable to the inclusion of integrability breaking terms, allowing a perturbation theory from a controlled starting point. Thus, we exemplarily study dynamical spin correlations of the honeycomb Kitaev quantum spin liquid within the $K-J-\Gamma$ model which includes Heisenberg and symmetric-anisotropic (pseudo-dipolar) interactions. This allows us to trace changes of the correlations in the regime of slowly moving fluxes, where the theory captures the dominant deviations when integrability is lost. These include an asymmetric shift together with a broadening of the dominant peak in the response as a function of frequency; the generation of further-neighbour correlations and their structure in real- and spin-space; and a resulting loss of an approximate rotational symmetry of the structure factor in reciprocal space. We discuss the limitations of this approach, and also view the neutron scattering experiments on the putative proximate quantum spin liquid material, $\alpha$-RuCl$_3$, in the light of the results from this extended parton theory.

Anomalous magnon Nernst effect of topological magnonic materials

The magnon transport driven by a thermal gradient in a perpendicularly magnetized honeycomb lattice is studied. The system with the nearest-neighbor pseudodipolar interaction and the next-nearest-neighbor Dzyaloshinskii–Moriya interaction has various topologically nontrivial phases. When an in-plane thermal gradient is applied, a transverse in-plane magnon current is generated. This phenomenon is termed as the anomalous magnon Nernst effect that closely resembles the anomalous Nernst effect for an electronic system. The anomalous magnon Nernst coefficient and its sign are determined by the magnon Berry curvature distributions in the momentum space and magnon populations in the magnon bands. We predict a temperature-induced sign reversal in anomalous magnon Nernst effect under certain conditions.

Ruderman-Kittel interaction between Si in URu2Si2

29Si nuclear magnetic resonance (NMR) has been studied in a 29Si–enriched single crystal sample of URu2Si2. The spin-echo decay for applied field H || [110] and [001] directions has been measured at 50 K. A clear spin-echo decay oscillation is observed for both cases, reflecting the Ruderman-Kittel interaction between Si nuclei. Since the observed oscillation frequency depends on the direction of applied magnetic field, the Ruderman-Kittel and anisotropic pseudo-dipolar interactions may not be negligible in this compound. The origin of spin-echo decay oscillations is discussed.

Disorder-Induced Mimicry of a Spin Liquid in YbMgGaO_{4}.

We suggest that a randomization of the pseudodipolar interaction in the spin-orbit-generated low-energy Hamiltonian of YbMgGaO_{4} due to an inhomogeneous charge environment from a natural mixing of Mg^{2+} and Ga^{3+} can give rise to orientational spin disorder and mimic a spin-liquid-like state. In the absence of such quenched disorder, 1/S and density matrix renormalization group calculations both show robust ordered states for the physically relevant phases of the model. Our scenario is consistent with the available experimental data, and further experiments are proposed to support it.

T dependence of nuclear spin-echo decay at low temperatures in YbRh2Si2

The authors report $^{29}\mathrm{Si}$ nuclear spin-echo oscillations and decay measurements on a single crystal of ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$. These quantities are found to be $T$ independent from 300 K down to $\ensuremath{\sim}20$ K, showing, however, a strong $T$ dependence below 20 K. These results indicate that electronic states near the Fermi level are modified at low temperatures. The observed spin-echo oscillations can be interpreted with the Ruderman-Kittel and pseudodipolar interactions between nearest-neighbor Si nuclei driven by conduction electron scattering at the Fermi surface. Possible modifications to the Fermi surface at low temperatures are discussed.

Chiral magnetism of magnetic adatoms generated by Rashba electrons

We investigate long-range chiral magnetic interactions among adatoms mediated by surface states spin-splitted by spin–orbit coupling. Using the Rashba model, the tensor of exchange interactions is extracted wherein a thepseudo-dipolar interaction is found, in addition to the usual isotropic exchange interaction and the Dzyaloshinskii–Moriya interaction. We find that, despite the latter interaction, collinear magnetic states can still be stabilized by the pseudo-dipolar interaction. The interadatom distance controls the strength of these terms, which we exploit to design chiral magnetism in Fe nanostructures deposited on a Au(111) surface. We demonstrate that these magnetic interactions are related to superpositions of the out-of-plane and in-plane components of the skyrmionic magnetic waves induced by the adatoms in the surrounding electron gas. We show that, even if the interatomic distance is large, the size and shape of the nanostructures dramatically impacts on the strength of the magnetic interactions, thereby affecting the magnetic ground state. We also derive an appealing connection between the isotropic exchange interaction and the Dzyaloshinskii–Moriya interaction, which relates the latter to the first-order change of the former with respect to spin–orbit coupling. This implies that the chirality defined by the direction of the Dzyaloshinskii–Moriya vector is driven by the variation of the isotropic exchange interaction due to the spin–orbit interaction.

29Si NMR spin-echo decay in YbRh2Si2

29Si nuclear magnetic resonance (NMR) has been measured in a 29Si-enriched single crystal sample of YbRh2Si2. The spin-echo decay for applied field H ∥, ⊥ the c-axes has been measured at 100 K. A clear spin-echo decay oscillation is observed for both cases, possibly reflecting the Ruderman-Kittel (RK) interaction. Since the observed oscillation frequency depends on the direction of applied magnetic field, anisotropic RK coupling and pseudo-dipolar (PD) interactions may not be negligible in this compound. The origin of spin-echo decay oscillations is discussed.

Multiple orbital effects and magnetic ordering in a neutral radical.

The alternating ABABAB π-stacked architecture of the EtCN solvate of the iodo-substituted, oxobenzene-bridged bisdithiazolyl radical IBBO (space group Pnma) gives rise to strong ferromagnetic exchange along the π-stacks, and the material orders as a spin-canted antiferromagnet with T(N) = 35 K, with a spontaneous (canted) moment M(spont) = 1.4 × 10(-3) μB and a coercive field H(c) = 1060 Oe (at 2 K). The observation of spin-canting can only be understood in terms of multiorbital contributions to both isotropic and anisotropic exchange interactions, the magnitude of which are enhanced by spin-orbit effects arising from the heavy-atom iodine substituent. Pseudodipolar interactions lead to a net canted moment along the c-axis, while the sublattice magnetization is predicted to possess an easy a-axis.

Order-by-disorder in theXYpyrochlore antiferromagnet

We investigate the properties of the XY pyrochlore antiferromagnet with local <111> planar anisotropy. We find the ground states and show that the configurational ground state entropy is subextensive. By computing the free energy due to harmonic fluctuations and by carrying out Monte Carlo simulations, we confirm earlier work indicating that the model exhibits thermal order-by-disorder leading to low temperature long-range order consisting of discrete magnetic domains. We compute the spin wave spectrum and show that thermal and quantum fluctuations select the same magnetic structure. Using Monte Carlo simulations, we find that the state selected by thermal fluctuations in this XY pyrochlore antiferromagnet can survive the addition of sufficiently weak nearest-neighbor pseudo-dipolar interactions to the spin Hamiltonian. We discuss our results in relation to the Er2Ti2O7 pyrochlore antiferromagnet.

Large Spin-Wave Energy Gap in the Bilayer IridateSr3Ir2O7: Evidence for Enhanced Dipolar Interactions Near the Mott Metal-Insulator Transition

Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudodipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudodipolar interactions, may emerge in small charge-gap iridates.

Dimensionality Driven Spin-Flop Transition in Layered Iridates

Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO2 layers is due to strong competition among intra- and interlayer bond-directional pseudodipolar interactions of the spin-orbit entangled J(eff)=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.