We study the effect of the magnon-magnon interaction on the nonlinear magnon transport. The magnonmagnon interaction induces nonreciprocal magnon decay when the time-reversal symmetry is broken and leads to nonlinear thermal responses of magnons. We construct a theoretical framework to study the nonlinear thermal responses due to the nonreciprocal magnon decay by using the imaginary Dyson equation and quantum kinetic theory, which is then applied to models of one-dimensional (1D) antiferromagnets and 2D honeycomb ferromagnets with Dzyaloshinskii-Moriya interactions. An order estimate shows that the nonlinear thermal response from the present mechanism is feasible for experimental measurement.
Small angle neutron inelastic scattering measurement has been performed to study the magnon dispersion relation in the field-induced-ferromagnetic phase of the noncentrosymmetric binary compound MnSi. For the magnons propagating parallel or anti-parallel to the external magnetic field, we experimentally confirmed that the dispersion relation is asymmetrically shifted along the magnetic field direction. This magnon dispersion shift is attributed to the relativistic Dzyaloshinskii-Moriya interaction, which is finite in noncentrosymmetric magnets, such as MnSi. The shift direction is found to be …
We study the magnon contribution to the gravitomagnetoelectric (gravito-ME) effect, in which the magnetization is induced by a temperature gradient, in noncentrosymmetric antiferromagnetic insulators. This phenomenon is totally different from the ME effect, because the temperature gradient is coupled to magnons but an electric field is not. We derive a general formula of the gravito-ME susceptibility in terms of magnon wave functions and find that a difference in $g$ factors of magnetic ions is crucial. We also apply our formula to a specific model. Although the obtained gravito-ME susceptibil…
Magnons, the spin-wave quanta, are disturbances that embody a wave propagating through a background medium formed by ordered magnetic moments. In an isotropic Heisenberg system, these disturbances vary in a continuous manner around an ordered spin structure, thus requiring infinitesimal energy as a wavevector approaches a magnetic zone centre. However, competing anisotropic interactions arising from broken symmetry can favour a distinct static and dynamic spin state causing a shift of the minimum point of the magnon dispersion to a nonreciprocal wavevector. Here we report the first direct evid…
The microscopic conditions for the emergence of nonreciprocal magnon excitations in noncentrosymmetric magnets are theoretically investigated. We show that asymmetric magnon excitations appear when a bond magnetic toroidal dipole becomes active, which is defined as a parallel component between the Dzyaloshinskii-Moriya vector and the averaged spin moments at the ends of the bonds. Depending on magnetic structures accompanying the bond magnetic toroidal dipoles, the higher-rank magnetic toroidal multipoles can also be activated in a magnetic cluster, which describes various angle dependences of…
Inelastic neutron scattering (INS) is employed to study damped spin-wave excitations in the noncentrosymmetric heavy-fermion superconductor CePt3Si along the antiferromagnetic Brillouin-zone boundary in the low-temperature magnetically ordered state. Measurements along the (1/2 1/2 L) and (H H 1/2-H) reciprocal-space directions reveal deviations in the spin-wave dispersion from the previously reported model. Broad asymmetric shape of the peaks in energy signifies strong spin-wave damping by interactions with the particle-hole continuum. Their energy width exhibits no evident anomalies as a fun…
We study electric polarization and nonlinear optical effects in spin systems with broken inversion symmetry. We apply strong coupling expansion to the underlying electronic Hamiltonians, and systematically derive expressions for electric polarization in spin systems that are represented in terms of spin operators. The magnon representation of the obtained electric polarization operator allows us to compute linear and nonlinear optical responses by the standard diagrammatic method. We apply our formalism to Heisenberg model with alternating coupling constants and $J_1$-$J_2$ model with inversio…
We have investigated anomalous Hall effect and magnetoresistance in a noncentrosymmetric itinerant magnet Cr$_{11}$Ge$_{19}$. While the temperature- and magnetic-field-dependent anomalous Hall conductivity is just proportional to the magnetization above 30 K, it is more enhanced in the lower temperature region. The magnitude of negative magnetoresistance begins to increase toward low temperature around 30 K. The anisotropic magnetoresistance emerges at similar temperature. Because there is no anomaly in the temperature dependence of magnetization around 30 K, the origin of these observations i…
The feasibility of a single-domain ferromagnet based on uniaxial magnetic ions was examined. For a noncentrosymmetric uniaxial magnetic ion of magnetic moment at a site of local electric dipole moment p, it is unknown to date whether the magnetic moment prefers to be parallel or antiparallel to the electric dipole moment. The nature of this magnetoelectric interaction was probed in terms of analogical reasoning based on the Rashba effect and density functional calculations. We show that the magnetic and electric dipole moments prefer an antiparallel arrangement, predict that Fe-doped CaZnOS is…
Multiple-$Q$ magnetic states often induce nontrivial topological spin textures, such as a skyrmion and a hedgehog. We theoretically investigate yet another multiple-$Q$ state with topological defects, a meron-antimeron crystal (MAX), represented by a periodic array of the meron and antimeron with a half-integer skyrmion number. Performing simulated annealing for an effective spin model of noncentrosymmetric itinerant magnets on a triangular lattice, we show that rectangular-shaped and triangular-shaped MAXs are stabilized by the interplay between the biquadratic interaction arising from the sp…
We theoretically study the optical generation of dc spin current (i.e., a spin-current solar cell) in ordered antiferromagnetic and ferrimagnetic insulators, motivated by a recent study on the laser-driven spinon spin current in noncentrosymmetric quantum spin chains [H. Ishizuka and M. Sato, Phys. Rev. Lett. 122, 197702 (2019)]. Using a non-linear response theory for magnons, we analyze the dc spin current generated by a linearly-polarized electromagnetic wave (typically, terahertz or gigahertz waves). Considering noncentrosymmetric two-sublattice magnets as an example, we find a finite dc sp…
The magnetic hedgehog lattice (HL) is a noncoplanar magnetic texture with a periodic array of magnetic monopoles and anti-monopoles. Despite phenomenological and numerical studies thus far, there remain open issues on the microscopic origin, especially with respect to the recent experimental findings of two different types of HLs even at zero magnetic field. Here, we study the stability of the HLs for an effective spin model with long-range interactions arising from itinerant nature of electrons. By variational calculations and simulated annealing, we find that the HLs are stabilized in the gr…
The Loewdin orthogonalization procedure being the well-known technique, particularly in quantum chemistry, however, gives rise to novel effects missed in earlier studies. Making use of the technique of irreducible tensorial operators we have developed a regular procedure for account of the orthogonalization effects. For illustration we address the emergence of a specific magnetoelectric coupling for noncentrosymmetric 3d or 4f ions.
Magnetic skyrmion, i.e. a topologically stable swirling spin texture, appears as a particle-like object in the two-dimensional (2D) systems, and has recently attracted attention as a candidate of novel information carrier. In the real three-dimensional (3D) systems, a skyrmion is expected to form a string structure along an extra dimension, while its experimental identification has rarely been achieved. Here, we report the direct visualization of 3D shape of individual skyrmion strings, for the recently discovered room-temperature skyrmion-hosting noncentrosymmetric compound Mn1.4Pt0.9Pd0.1Sn.…
Topological spin textures have attracted much attention both for fundamental physics and spintronics applications. Among them, antiskyrmions possess a unique spin configuration with Bloch-type and Néel-type domain walls due to anisotropic Dzyaloshinskii-Moriya interaction (DMI) in the noncentrosymmetric crystal structure. However, antiskyrmions have thus far only been observed in a few Heusler compounds with $D_{2\mathrm{d}}$ symmetry. Here, we report a new material Fe$_{1.9}$Ni$_{0.9}$Pd$_{0.2}$P in a different symmetry class ($S_4$ symmetry), where antiskyrmions exist over a wide temperature…
We predict the phenomenon of antiferroelectric resonance (AFER), in which an ac electric field causes magnetic ions located at noncentrosymmetric positions in a multi-sublattice magnet to undergo magnetic transitions corresponding to exchange collective excitations of the system. We construct a theory of such resonances, and show that in magnets with collinear magnetic structures AFER is caused by relativistic and exchange-relativistic magnetoelectric interactions, while in the noncollinear magnets a significant contribution can also come from the exchange magnetoelectric interaction. We predi…
We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic structure calculations. In MnGe, we observe a transition to a magnetic state at $T_c=275$ K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature one of which we associate with changes to …
The low-energy magnetic excitations of the noncentrosymmetric heavy-fermion superconductor CePt3Si have been measured with inelastic neutron scattering on a single crystal. Kondo-type spin fluctuations with an anisotropic wave vector dependence are observed in the paramagnetic state. These fluctuations do not survive in the antiferromagnetically ordered state below TN=2.2 K but are replaced by damped spin waves, whose dispersion is much stronger along the c-axis than in other directions. No change is observed in the excitation spectrum or the magnetic order as the system enters the superconduc…
We report the observation of the $A_{\rm g}$-symmetric one-magnon Raman peak in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$. Its Raman shift traces the one-magnon energy at the magnetic $Γ$ point, and its intensity is proportional to the squared transverse magnetization. The appearance of the one-magnon Raman scattering originates from the exchange magnon Raman process and reflects the change of the magnetic-state symmetry. Using the bond-operator representation, we theoretically clarify the Raman selection rules, being consistent with the experimental results.
The effect of differently oriented magnetic field on chiral incommensurate helimagnet UPtGe is studied both experimentally and theoretically. The magnetization measurements up to the field above the saturation have revealed an isotropic magnetic response below 20 T and a remarkable nonmonotonic anisotropy in high fields. Moreover, the two principally different phase transitions from the noncollinear incommensurate to the field-induced ferromagnetic state have been observed. These properties are successfully explained by density-functional theory calculations taking into account the noncollinea…
Switching magnetic texture via in-plane magnetic field in noncentrosymmetric dipolar magnets: From skyrmions to antiskyrmions and nontopological magnetic bubbles
We compare the ground-state features of alternating ferrimagnetic chains $(1/2, S)$ with $S=1,3/2,2,5/2$ in a magnetic field and the corresponding Holstein-Primakoff bosonic models up to order $\sqrt{s/S}$, with $s=1/2$, considering the fully polarized magnetization as the boson vacuum. {The single-particle Hamiltonian is a Rice-Mele model with uniform hopping and modified boundaries, while the interactions have a correlated (density-dependent) hopping term and magnon-magnon repulsion.} The magnon-magnon repulsion increases the many-magnon energy and the density-dependent hopping decreases the…
Noncentrosymmetric Triangular Magnet CaMnTeO$_6$: Strong Quantum Fluctuations and Role of s0 vs. s2 Electronic States in Competing Exchange Interactions
We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity.
In order to study the magnetic excitation behaviors in colossal magnetoresistance manganites, a magnon-phonon interacting system is investigated. Sudden broadening of magnon linewidth is obtained when a magnon branch crosses over an optical phonon branch. Onset of the broadening is approximately determined by the magnon density of states. Anomalous magnon damping at the brillouine zone boundary observed in low Curie temperature manganites is explained.
Analogous to the Hall effect, the Nernst effect is the generation of a transverse voltage due to a temperature gradient in the presence of a perpendicular magnetic field. The Nernst effect has promise for thermoelectric applications and as a probe of electronic structure. In magnetic materials, a so-called anomalous Nernst effect (ANE) is possible in zero magnetic field. Here we report a colossal ANE reaching 23 $μ$V/K in the ferromagnetic metal UCo$_{0.8}$Ru$_{0.2}$Al. Uranium's $5f$ electrons provide strong electronic correlations that lead to narrow bands, which are a known route to produci…
In noncentrosymmetric crystals with broken inversion symmetry $\mathcal{I}$, the $I-V$ ($I$: current, $V$: voltage) characteristic is generally expected to depend on the direction of $I$, which is known as nonreciprocal response and, for example, found in p-n junction. However, it is a highly nontrivial issue in translationally invariant systems since the time-reversal symmetry ($\mathcal{T}$) plays an essential role, where the two states at crystal momenta $k$ and $-k$ are connected in the band structure. Therefore, it has been considered that the external magnetic field ($B$) or the magnetic…
The magnetic properties of a pressure induced noncentrosymmetric heavy-fermion superconductor CeIrGe$_3$ have been investigated by muon spin relaxation ($μ$SR), powder neutron diffraction (ND) and inelastic neutron scattering (INS) techniques at ambient pressure. For completeness we have also measured the ac magnetic susceptibility $χ_{\rm ac}(T)$, dc magnetic susceptibility $χ(T)$, dc isothermal magnetization $M(H)$ and heat capacity $C_{\rm p}(T,H)$ down to 2 K. CeIrGe$_{3}$ is known to exhibit pressure induced superconductivity ($T_{\rm c}\approx 1.5$ K) at a pressure of 20 GPa and antiferr…
Helical-to-Fan Transitions under Magnetic Fields in the Noncentrosymmetric Tetragonal Magnet EuRhGe$_3$
Hiroshima University（广岛大学）；Institute of Materials Structure Science, High Energy Accelerator Research Organization（高能加速器研究机构材料结构科学研究所）；University of the Ryukyus（琉球大学）；RIKEN Center for Emergent Matter Science（理化学研究所新兴物质科学中心）
A large number of half-Heusler compounds have been recently proposed as three-dimensional (3D) topological insulators (TIs) with tunable physical properties.However, no transport measurements associated with the topological surface states have been observed in these half-Heusler candidates due to the dominating contribution from bulk electrical conductance. Here we show that, by reducing the mobility of bulk carriers, a two-dimensional (2D) weak antilocalization (WAL) effect, one of the hallmarks of topological surface states, was experimentally revealed from the tilted magnetic field dependen…
The magnon pumping effect in ferromagnetic insulators under an external time-dependent transverse magnetic field is theoretically studied. Generation of a magnon current is discussed by calculating the magnon source term in the spin continuity equation. This term represents the non-conservation of magnons arising from an applied transverse magnetic field. The magnon source term has a resonance structure as a function of the angular frequency of the transverse field, and this fact is useful to enhance the pumping effect.
We study theoretically the dynamics of magnons in the presence of a single skyrmion in chiral magnets featuring Dzyaloshinskii-Moriya interaction. We show by micromagnetic simulations that the scattering process of magnons by a skyrmion can be clearly defined although both originate in the common spins. We find that (i) the magnons are deflected by a skyrmion, with the angle strongly dependent on the magnon wavenumber due to the effective magnetic field of the topological texture, and (ii) the skyrmion motion is driven by magnon scattering through exchange of the momenta between the magnons an…
