arXiv subjects

O. Tchernyshyov

Publications and source records attributed to O. Tchernyshyov.

At least 19 recordsLinked to original sources

arXiv2022

Piezomagnetic switching of anomalous Hall effect in an antiferromagnet at room temperature

Piezomagnetism couples strain linearly to magnetic order producing magnetization. Thus, unlike magnetostriction, it enables bidirectional control of a net magnetic moment. If this effect becomes large at room temperature, it may be technologically relevant, similar to its electric analogue, piezoelectricity. To date, however, the studies of the piezomagnetic effect have been primarily restricted to antiferromagnetic (AF) insulators at cryogenic temperatures. Here we report the discovery of a large piezomagnetism in a metal at room temperature. Strikingly, by using the AF Weyl semimetal Mn$_3$Sn, known for its nearly magnetization-free anomalous Hall effect (AHE), we find that an application of small uniaxial strain of the order of 0.1 % can control both the sign and size of the AHE. Our experiment and theory show that the piezomagnetism can control the AHE distinctly from the magnetization, which will be useful for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2020

Non-collinear ground state from a four-spin chiral exchange in a tetrahedral magnet

We propose a quartic chiral term $m_x m_y m_z \nabla \cdot \mathbf{m}$ for the energy density of a cubic ferromagnet with broken parity symmetry (point group $T_d$). We demonstrate that this interaction causes a phase transition from a collinear ferromagnetic state to a non-collinear magnetic cone ground state provided its strength exceeds the geometric mean of magnetic exchange and cubic anisotropy. The corresponding non-collinear ground state may also be additionally stabilized by an external magnetic field pointing along certain crystallographic directions. The four-spin chiral exchange does also manifest itself in peculiar magnon spectra and favors spin waves with the wave vector that is perpendicular to the average magnetization direction.

cond-mat.str-el↗

arXiv2020

Antichiral spin order its Goldstone modes and their hybridization with phonons in the topological semimetal Mn3Ge

Quantum materials with strong transport responses to disparate physical quantities are of great fundamental significance and may hold technological potentials. The interplay between interactions and topology drive such responses through the effects of spontaneous symmetry breaking and the associated domain configurations on quantum transport. Here we provide a comprehensive description of the magnetism of Mn3Ge, an antiferromagnetic kagomebased semimetal with room temperature transport anomalies associated with topologically protected Weyl nodes. Using polarized neutron diffraction, we show the all-important magnetic structure is anti-chiral and coplanar carrying the symmetry of a ferromagnet without appreciable magnetization. We probe and classify the long wavelength excitations that determine its macroscopic responses including a set of collective magneto-elastic modes. We develop a phenomenological spin Hamiltonian with exchange, Dzyaloshinskii-Moriya, and crystal field interactions to describe its collective magnetism. The itinerant character of the magnetism that drives quantum transport is apparent in spin wave damping and extended magnetic interactions. Our work provides the scientific basis for manipulation of the chiral antiferromagnetic texture of Mn3Ge to control its topological quantum transport.

cond-mat.str-el↗

arXiv2020

Low energy magnons in the chiral ferrimagnet $\text{Cu}_2\text{OSeO}_3$: a coarse-grained approach

We report a comprehensive neutron scattering study of low energy magnetic excitations in the breathing pyrochlore helimagnetic $\text{Cu}_2\text{OSeO}_3$. Fully documenting the four lowest energy magnetic modes that leave the ferrimagnetic configuration of the "strong tetrahedra" intact ($|\hbarω|<13$ meV), we find gapless quadratic dispersion at the $Γ$ point for energies above 0.2 meV, two doublets separated by 1.6(2) meV at the $R$ point, and a bounded continuum at the $X$ point. Our constrained rigid spin cluster model relates these features to Dzyaloshinskii-Moriya (DM) interactions and the incommensurate helical ground state. Combining conventional spin wave theory with a spin cluster form-factor accurately reproduces the measured equal time structure factor through multiple Brillouin zones. An effective spin Hamiltonian describing the complex anisotropic inter-cluster interactions is obtained.

cond-mat.str-el↗

arXiv2019

Orientation Dependence of the Magnetic Phase Diagram of Yb$_2$Ti$_2$O$_7$

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb$_2$Ti$_2$O$_7$ as a putative U(1) QSL but a new class of ultra-pure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $\vec{B}\,\parallel\,\langle 100\rangle$, while field parallel to $\langle 110 \rangle$ and $\langle 111 \rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb$_2$Ti$_2$O$_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{\mathrm{C}}$ and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.

cond-mat.str-el↗

arXiv2017

Asymmetric splitting of an antiferromagnetic resonance via quartic exchange interactions in multiferroic hexagonal HoMnO$_3$

The symmetric splitting of two spin-wave branches in an antiferromagnetic resonance (AFR) experiment has been an essential measurement of antiferromagnets for over half a century. In this work, circularly polarized time-domain THz spectroscopy experiments performed on the low symmetry multiferroic h-HoMnO$_3$ reveal an AFR of the Mn sublattice to split asymmetrically in applied magnetic field, with an $\approx$ 50\% difference in $g$-factors between the high and low energy branches of this excitation. The temperature dependence of the $g$-factors, including a drastic renormalization at the Ho spin ordering temperature, reveals this asymmetry to unambiguously stem from Ho-Mn interactions. Theoretical calculations demonstrate the AFR asymmetry is not explained by conventional Ho-Mn exchange mechanisms alone and are only reproduced if quartic spin interactions are also included in the spin Hamiltonian. Our results provide a paradigm for the optical study of such novel interactions in hexagonal manganites and low symmetry antiferromagnets in general.

cond-mat.str-el↗

arXiv2017

Reentrant Phase Diagram of $\rm{Yb_2Ti_2O_7}$ in $\langle 111 \rangle$ Magnetic Field

We present a magnetic phase diagram of rare-earth pyrochlore $\rm{Yb_2Ti_2O_7}$ in a $\langle 111 \rangle$ magnetic field. Using heat capacity, magnetization, and neutron scattering data, we show an unusual field-dependence of a first-order phase boundary, wherein a small applied field increases the ordering temperature. The zero-field ground state has ferromagnetic domains, while the spins polarize along $\langle 111 \rangle$ above 0.65T. A classical Monte Carlo analysis of published Hamiltonians does account for the critical field in the low T limit. However, this analysis fails to account for the large bulge in the reentrant phase diagram, suggesting that either long-range interactions or quantum fluctuations govern low field properties.

cond-mat.str-el↗

arXiv2016

Magnetoelectric domain wall dynamics and its implications for magnetoelectric memory

Domain wall dynamics in a magnetoelectric antiferromagnet is analyzed, and its implications for magnetoelectric memory applications are discussed. Cr$_2$O$_3$ is used in the estimates of the materials parameters. It is found that the domain wall mobility has a maximum as a function of the electric field due to the gyrotropic coupling induced by it. In Cr$_2$O$_3$ the maximal mobility of 0.1 m/(s$\times$Oe) is reached at $E\approx0.06$ V/nm. Fields of this order may be too weak to overcome the intrinsic depinning field, which is estimated for B-doped Cr$_2$O$_3$. These major drawbacks for device implementation can be overcome by applying a small in-plane shear strain, which blocks the domain wall precession. Domain wall mobility of about 0.7 m/(s$\times$Oe) can then be achieved at $E=0.2$ V/nm. A split-gate scheme is proposed for the domain-wall controlled bit element; its extension to multiple-gate linear arrays can offer advantages in memory density, programmability, and logic functionality.

cond-mat.mtrl-sci↗

arXiv2014

A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNb$_2$O$_6$ investigated by high resolution time-domain terahertz spectroscopy

Kink bound states in the one dimensional ferromagnetic Ising chain compound CoNb$_2$O$_6$ have been studied using high resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schrödinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near two times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states -- two pairs of kinks on neighboring chains.

cond-mat.str-el↗

arXiv2011

Destruction of valence-bond order in a $S=1/2$ sawtooth chain with a Dzyaloshinskii-Moriya term

A small value of the spin gap in quantum antiferromagnets with strong frustration makes them susceptible to nominally small deviations from the ideal Heisenberg model. One of such perturbations, the anisotropic Dzyaloshinskii-Moriya interaction, is an important perturbation for the $S=1/2$ kagome antiferromagnet, one of the current candidates for a quantum-disordered ground state. We study the influence of the DM term in a related one-dimensional system, the sawtooth chain that has valence-bond order in its ground state. Through a combination of analytical and numerical methods, we show that a relatively weak DM coupling, $0.115J$, is sufficient to destroy the valence-bond order, close the spin gap, and turn the system into a Luttinger liquid with algebraic spin correlations. A similar mechanism may be at work in the kagome antiferromagnet.

cond-mat.str-el↗

arXiv2011

Two-stage ordering of spins in a dipolar spin ice on the kagome lattice

Spin ice, a peculiar thermal state of a frustrated ferromagnet on the pyrochlore lattice, has a finite entropy density and excitations carrying magnetic charge. By combining analytical arguments and Monte Carlo simulations, we show that spin ice on the two-dimensional kagome lattice orders in two stages. The intermediate phase has ordered magnetic charges and is separated from the paramagnetic phase by an Ising transition. The transition to the low-temperature phase is of the three-state Potts or Kosterlitz-Thouless type, depending on the presence of defects in charge order.

cond-mat.str-el↗

arXiv2011

The Sensitivity of the Magnetic Properties of the ZnCr2O4 and MgCr2O4 Spinels to Non-stoichiometry

We report that small amounts of metal atom non-stoichiometry are possible in the ZnCr2O4 and MgCr2O4 spinels. The non-stoichiometry, though less than 2%, significantly impacts TN and the nature of the magnetic correlations above TN. The Zn1+xCr2-xO4 spinel is particularly sensitive. While stoichiometric ZnCr2O4 displays antiferromagnetic short range correlations in the susceptibility above TN, ferromagnetic correlations are observed in non-stoichiometric, hole doped Zn1+xCr2-xO4. The Mg1+xCr2-xO4 spinels are less profoundly affected by non-stoichiometry, though significant changes are also observed. We contrast the magnetic properties of Zn1+xCr2-xO4 and Mg1+xCr2-xO4 (x=0, 0.02, 0.04) with those of materials with the equivalent amounts of isovalent non-magnetic Ga3+ substituted on the Cr3+ site to separate the effects of static site disorder and hole doping.

cond-mat.mtrl-sci↗

arXiv2010

Halfvortices in flat nanomagnets

We discuss a new type of topological defect in XY systems where the O(2) symmetry is broken in the presence of a boundary. Of particular interest is the appearance of such defects in nanomagnets with a planar geometry. They are manifested as kinks of magnetization along the edge and can be viewed as halfvortices with winding numbers \pm 1/2. We argue that halfvortices play a role equally important to that of ordinary vortices in the statics and dynamics of flat nanomagnets. Domain walls found in experiments and numerical simulations are composite objects containing two or more of these elementary defects. We also discuss a closely related system: the two-dimensional smectic liquid crystal films with planar boundary condition.

cond-mat.mtrl-sci↗

arXiv2009

Spin-lattice coupling in frustrated antiferromagnets

We review the mechanism of spin-lattice coupling in relieving the geometrical frustration of pyrochlore antiferromagnets, in particular spinel oxides. The tetrahedral unit, which is the building block of the pyrochlore lattice, undergoes a spin-driven Jahn-Teller instability when lattice degrees of freedom are coupled to the antiferromagnetism. By restricting our considerations to distortions which preserve the translational symmetries of the lattice, we present a general theory of the collective spin-Jahn-Teller effect in the pyrochlore lattice. One of the predicted lattice distortions breaks the inversion symmetry and gives rise to a chiral pyrochlore lattice, in which frustrated bonds form helices with a definite handedness. The chirality is transferred to the spin system through spin-orbit coupling, resulting in a long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit models of spin-lattice coupling using local phonon modes, and their applications in other frustrated magnets.

cond-mat.str-el↗

arXiv2008

Partial order from disorder in a classical pyrochlore antiferromagnet

We investigate theoretically the phase diagram of a classical Heisenberg antiferromagnet on the pyrochlore lattice perturbed by a weak second-neighbor interaction J_2. The huge ground state degeneracy of the nearest-neighbor Heisenberg spins is lifted by J_2 and a magnetically ordered ground state sets in upon approaching zero temperature. We have found a new, partially ordered phase with collinear spins at finite temperatures for a ferromagnetic J_2. In addition to a large nematic order parameter, this intermediate phase also exhibits a layered structure and a bond order that breaks the sublattice symmetry. Thermodynamic phase boundaries separating it from the fully disordered and magnetically ordered states scale as 1.87 J_2 S^2 and 0.26 J_2 S^2 in the limit of small J_2. The phase transitions are discontinuous. We analytically examine the local stability of the collinear state and obtain a boundary T ~ J_2^2/J_1 in agreement with Monte Carlo simulations.

cond-mat.str-el↗

arXiv2008

Dynamics of a vortex domain wall in a magnetic nanostrip: an application of the collective coordinate approach

The motion of a vortex domain wall in a ferromagnetic strip of submicron width under the influence of an external magnetic field exhibits three distinct dynamical regimes. In a viscous regime at low fields the wall moves rigidly with a velocity proportional to the field. Above a critical field the viscous motion breaks down giving way to oscillations accompanied by a slow drift of the wall. At still higher fields the drift velocity starts rising with the field again but with a much lower mobility dv/dH than in the viscous regime. To describe the dynamics of the wall we use the method of collective coordinates that focuses on soft modes of the system. By retaining two soft modes, parametrized by the coordinates of the vortex core, we obtain a simple description of the wall dynamics at low and intermediate applied fields that describes both the viscous and oscillatory regimes below and above the breakdown. The calculated dynamics agrees well with micromagnetic simulations at low and intermediate values of the driving field. In higher fields, additional modes become soft and the two-mode approximation is no longer sufficient. We explain some of the significant features of vortex domain wall motion in high fields through the inclusion of additional modes associated with the half-antivortices on the strip edge.

cond-mat.mtrl-sci↗

arXiv2008

Dynamics of domain walls in magnetic nanostrips

We express dynamics of domain walls in ferromagnetic nanowires in terms of collective coordinates generalizing Thiele's steady-state results. For weak external perturbations the dynamics is dominated by a few soft modes. The general approach is illustrated on the example of a vortex wall relevant to recent experiments with flat nanowires. A two-mode approximation gives a quantitatively accurate description of both the steady viscous motion of the wall in weak magnetic fields and its oscillatory behavior in moderately high fields above the Walker breakdown.

cond-mat.mtrl-sci↗

arXiv2007

Bose-Einstein Condensation in Magnetic Insulators

The elementary excitations in antiferromagnets are magnons, quasiparticles with integer spin and Bose statistics. In an experiment their density is controlled efficiently by an applied magnetic field and can be made finite to cause the formation of a Bose-Einstein condensate (BEC). Studies of magnon condensation in a growing number of magnetic materials provide a unique window into an exciting world of quantum phase transitions (QPT) and exotic quantum states.

cond-mat.str-el↗