Chiral Helimagnet Research Articles

Observation and Characterization of Multiple Resonance Modes in a Chiral Helimagnet CrNb3S6.

The chiral helimagnet CrNb3S6 hosts various temperature- and magnetic-field-stabilized chiral soliton lattices (CSLs) and corresponding exotic collective spin resonance modes, which make it an ideal candidate for future magnetic storage/memory and magnon-based information processing. While most studies have focused on characterizing various static spin textures in this chiral helimagnet, its corresponding collective dynamics have rarely been explored. This study systematically investigates the temperature- and magnetic-field-dependent magnetic dynamics of a single crystal of CrNb3S6 using broadband microwave spectroscopy. We observe an optical mode with a temperature-independent mode number in addition to Kittel-like ferromagnetic resonance (FMR) modes in the CSL phase, consistent with the temperature-independent normalized CSL period L(H)/L(0) based on the 1D chiral sine-Gordon model. Furthermore, combining theoretical model fitting and micromagnetic simulation, we provide a detailed phase diagram and temporal-spatial resolution of dynamic modes, which may help to develop high-frequency exchange-coupling-based spintronic devices.

Spin Transfer Torque and Nonlinear Quantum Electron Transport in Chiral Helimagnets

Spin Transfer Torque and Nonlinear Quantum Electron Transport in Chiral Helimagnets

Continuum of metastable conical states of monoaxial chiral helimagnets

Continuum of metastable conical states of monoaxial chiral helimagnets

Transition Temperature of a Superconductor under a Helical Magnetic field

We consider a bilayer system with a conventional s-wave superconductor and a chiral helimagnet. If we assume that structure of an external magnetic field from the helimagnet is same as the magnetic moment structure in the helimagnet, a helical magnetic field is applied to the superconductor. Our previous simulation showed that vortex structures depend on strongly on structure of the helical magnetic field as well as the amplitude of the field. We study how pitch of the helical magnetic field affects the transition temperature. For this purpose, we formulate the linearized Ginzburg-Landau equation for the helical magnetic field.

Crystal field excitation in the chiral helimagnet YbNi3Al9

A crystal field level scheme of a uniaxial chiral helimagnet $\mathrm{Yb}{\mathrm{Ni}}_{3}{\mathrm{Al}}_{9}$, exhibiting a chiral magnetic soliton lattice state by Cu substitution for Ni, has been determined by inelastic neutron scattering. The ground and the first excited doublets are separated by 44 K and are simply expressed as $\ensuremath{\alpha}|\ifmmode\pm\else\textpm\fi{}7/2\ensuremath{\rangle}+\ensuremath{\beta}|\ensuremath{\mp}5/2\ensuremath{\rangle}$, with $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ nearly equal to $\ifmmode\pm\else\textpm\fi{}1/\sqrt{2}$. The easy axis of the crystal field anisotropy is the $c$ axis when the excited levels are populated at high temperatures and high magnetic fields. On the other hand, the magnetism at low temperatures and low magnetic fields, where only the ground doublet is populated, is described by an easy-plane anisotropy which may be treated as an $S=1/2$ system with an anisotropic $g$ factor, ${g}_{xy}=3.02$ and ${g}_{z}=1.14$. An orbital-dependent exchange interaction is also discussed to explain the temperature dependence of the magnetic susceptibility based on this level scheme.

Temperature- and field angular-dependent helical spin period characterized by magnetic dynamics in a chiral helimagnet MnNb3S6

The chiral magnets with topological spin textures provide a rare platform to explore topology and magnetism for potential application implementation. Here, we study the magnetic dynamics of several spin configurations on the monoaxial chiral magnetic crystal MnNb3S6 via broadband ferromagnetic resonance (FMR) technique at cryogenic temperature. In the high-field forced ferromagnetic state (FFM) regime, the obtained frequency f vs. resonance field Hres dispersion curve follows the well-known Kittel formula for a single FFM, while in the low-field chiral magnetic soliton lattice (CSL) regime, the dependence of Hres on magnetic field angle can be well-described by our modified Kittel formula including the mixture of a helical spin segment and the FFM phase. Furthermore, compared with the sophisticated Lorentz micrograph technique, the observed magnetic dynamics corresponding to different spin configurations allow us to obtain temperature- and field-dependent proportion of helical spin texture and helical spin period ratio L(H)/L(0) via our modified Kittel formula. Our results demonstrated that field- and temperature-dependent nontrivial magnetic structures and corresponding distinct spin dynamics in chiral magnets can be an alternative and efficient approach to uncovering and controlling nontrivial topological magnetic dynamics.

New Gd-based magnetic compoundGdPt2Bwith a chiral crystal structure

Herein, we report the discovery of a novel Gd-based magnetic compound GdPt$_2$B with a chiral crystal structure. X-ray diffraction and chemical composition analyses reveal a CePt$_2$B-type crystal structure (space group: $P6_422$) for GdPt$_2$B. Moreover, we successfully grew single crystals of GdPt$_2$B using the Czochralski method. Magnetization measurements and the Curie$-$Weiss analysis demonstrate that the ferromagnetic interaction is dominant in GdPt$_2$B. A clear transition is observed in the temperature dependence of electrical resistivity, magnetic susceptibility, and specific heat at $T_{\rm O}$ = 87 K. The magnetic phase diagram of GdPt$_2$B, which consists of a field-polarized ferromagnetic region and a magnetically ordered region, resembles those of known chiral helimagnets. Furthermore, magnetic susceptibility measurements reveal a possible spin reorientation within the magnetically ordered phase in magnetic fields perpendicular to the screw axis. The results demonstrate that GdPt$_2$B is a suitable platform for investigating the competing effects of ferromagnetic and antisymmetric exchange interactions in rare-earth-based chiral compounds.

Persistent exchange splitting in the chiral helimagnet Cr1/3NbS2

Using high-resolution angle-resolved photoemission spectroscopy (ARPES) and ab-initio calculation, we systematically investigate the electronic structure of the chiral helimagnet Cr1/3NbS2 and its temperature evolution. The comparison with NbS2 suggests that the electronic structure of Cr1/3NbS2 is strongly modified by the intercalation of Cr atoms. Our ab-initio calculation, consistent with experimental result, suggests strong hybridization between Nb- and Cr-derived states near the Fermi level. In the chiral helimagnetic state (below the Curie temperature Tc), we observe exchange splitting of the energy bands crossing EF, which follows the temperature evolution of the magnetic moment, suggesting an important role of the conduction electrons in the long-range magnetic ordering. Interestingly, the exchange splitting persists far above Tc with negligible temperature dependence, in drastic contrast to the itinerant ferromagnetism described by the Stoner model, indicating the existence of short-range magnetic order. Our results provide important insights into the microscopic mechanism of the chiral helimagnetic ordering in Cr1/3NbS2.

Elastically induced magnetization at ultrafast time scales in a chiral helimagnet

The extreme frustration in the Kagome antiferromagnet makes it possible to realize a large number of closely competing magnetic phases either at the classical or the quantum level. Motivated by recent neutron scattering study on the Kagome antiferromagnet vesignieite BaCu$_{3}$V$_{2}$O$_{8}$(OH)$_{2}$, we have made systematic investigation of the phase diagram of the classical and quantum $J_{1}-J_{3}$ antiferromagnetic Heisenberg model defined on the Kagome lattice(KAFHM). While it is shown previously that a large antiferromagnetic exchange between the third-neighboring spins can drive an emergent $q=4$ Potts order through the order-by-disorder mechanism, it is elusive what will happen in the so called "grey region" where the Luttinger-Tisza criteria fails to predict the classical ordering pattern. Through extensive Monte Carlo simulation, we find that the "grey region" is characterized by competing emergent $q=3$ and $q=4$ Potts order, whose emergence are beyond the description of the conventional order-by-disorder mechanism. Our Schwinger Boson mean field calculation in the "grey region" confirms the existence of the Potts-3 order in the ground state of quantum $J_{1}-J_{3}$ KAFHM. The predicted Potts-3 state is found to take two different forms distinguished by their projective symmetry group character(PSG or quantum order), although they have exactly the same symmetry. A comparison with the exact diagonalization result on a 36-site cluster shows that the "grey region" may indeed host a nematic spin liquid ground state featuring an anisotropic ring structure around the $\mathbf{q}=0$ point in its spinon dispersion relation. We find that such "grey region" of extremely frustrated magnets should better be taken as the playground to study the rich competition between exotic emergent phases, rather than a burden on their theoretical analysis.

Observation of Collective Resonance Modes in a Chiral Spin Soliton Lattice with Tunable Magnon Dispersion.

A chiral spin soliton lattice (CSL), one of the representative systems of a magnetic superstructure, exhibits reconfigurability in periodicity over a macroscopic length scale. Such coherent and tunable characteristics of the CSL lead to an emergence of elementary excitation of the CSL as phononlike modes due to translational symmetry breaking and bring a controllability of the dispersion relation of the CSL phonon. Using a broadband microwave spectroscopy technique, we directly found that higher-order magnetic resonance modes appear in the CSL phase of a chiral helimagnet CrNb_{3}S_{6}, which is ascribed to the CSL phonon response. The resonance frequency of the CSL phonon can be tuned between 16 and 40GHz in the vicinity of the critical field, where the CSL period alters rapidly. The frequency range of the CSL phonon is expected to extend over 100GHz as extrapolated on the basis of the theoretical model. The present results indicate that chiral helimagnets could work as materials useful for broadband signal processing in the millimeter-wave band.

Angular-dependent magnetoresistance in Cr1/3NbS2 single crystals

We present a comprehensive study of angular-dependent magnetoresistance (MR) in the layered chiral helimagnet Cr1/3NbS2 (CNS), which realizes a magnetic soliton lattice. It was found that, at temperatures below Curie temperature, CNS showed a negative magnetoresistance related to the emergence of a chiral soliton lattice state when a magnetic field was applied parallel to the ab-plane (H ‖ ab-plane). When the magnetic field was applied parallel to the c-axis (H ‖ c-axis), however, the negative MR disappeared and a positive MR appeared around zero field at low temperatures. Detailed analysis and modeling indicate that the positive MR is due to the weak anti-localization effect arising from the strong spin–orbit coupling of CNS.

Discrete Magnetic Breathers and Their Stability in a Finite-Size Monoaxial Chiral Helimagnet

We demonstrate that magnetic discrete breathers (DBs) may appear in the monoaxial chiral helimagnet and describe the numerical algorithm of their search. We find out under what conditions these modes occur and examine their stability with the help of the Floquet analysis.

Dark discrete breather modes in a monoaxial chiral helimagnet with easy-plane anisotropy

Nonlinearity and discreteness are two pivotal factors for an emergence of discrete breather excitations in various media. We argue that these requirements are met in the forced ferromagnetic phase of the monoaxial chiral helimagnet $\mathrm{Cr}{\mathrm{Nb}}_{3}\mathrm{S}{}_{6}$ due to the specific domain structure of the compound. The stationary, time-periodic breather modes appear as the discrete breather lattice solutions whose period mismatches with a system size. Thanks to easy-plane single-ion anisotropy intrinsic to $\mathrm{Cr}{\mathrm{Nb}}_{3}\mathrm{S}{}_{6}$, these modes are of the dark type with frequencies lying within the linear spin-wave band, close to its bottom edge. They represent cnoidal states of magnetization, similar to the well-known soliton lattice ground state, with differing but limited number of embedded $2\ensuremath{\pi}$ kinks. The linear stability of these dark breather modes is verified by means of Floquet analysis. Their energy, which is controlled by two parameters, namely, the breather lattice period and amplitude, falls off linearly with a growth of the kink number. These results may pave a path to design spintronic resonators on the base of chiral helimagnets.

Disentangling electronic, lattice, and spin dynamics in the chiral helimagnet Cr1/3NbS2

We investigate the static and ultrafast magneto-optical response of the hexagonal chiral helimagnet $Cr_{1/3}NbS_{2}$ above and below the helimagnetic ordering temperature. The presence of a magnetic easy plane contained within the crystallographic ab-plane is confirmed, while degenerate optical pump-probe experiments reveal significant differences in the dynamic between the parent, $NbS_{2}$, and Cr-intercalated compounds. Time resolved magneto-optical Kerr effect measurements show a two-step demagnetization process, where an initial, sub-ps relaxation and subsequent buildup ($\tau > 50$ ps) in the demagnetization dynamic scale similarly with increasing pump fluence. Despite theoretical evidence for partial gapping of the minority spin channel, suggestive of possible half metallicity in $Cr_{1/3}NbS_{2}$, such a long demagnetization dynamic likely results from spin lattice-relaxation as opposed to minority state blocking. However, comparison of the two-step demagnetization process in $Cr_{1/3}NbS_{2}$ with other 3d intercalated transition metal dichalcogenides reveals a behavior that is unexpected from conventional spin-lattice relaxation, and may be attributed to the complicated interaction of local moments with itinerant electrons in this material system.

Anisotropic microwave propagation in a reconfigurable chiral spin soliton lattice

We investigated microwave propagation in the chiral spin soliton lattice (CSL) phase of micrometer-sized crystals of the monoaxial chiral helimagnet ${\mathrm{CrNb}}_{3}{\mathrm{S}}_{6}$. An advantage of the CSL is that its periodicity can be reconfigured over a macroscopic length scale by means of an external magnetic field. Using a two-antenna microwave spectroscopy technique, we measured the anisotropic response of the transmitted microwaves via the spin dynamics of the CSL. When propagating along the direction parallel to the helical axis, the microwave amplitude increased up to a factor of twenty with decreasing the number of chiral soliton kinks. When the propagation direction was rotated by ${90}^{\ensuremath{\circ}}$ with regards to the helical axis, the microwave amplitude increased by one order of magnitude upon formation of the chiral helimagnetic order in the vicinity of zero magnetic field, exceeding that of the ferromagnetic phase above the critical field. Our findings open a novel route for controlling the characteristics of microwave propagation using noncollinear spin textures.

Magnetotransport properties of chiral helimagnet Cr1/3NbS2 near phase transition

Many interesting physical phenomena emerge in the vicinity of phase transition temperature (Tc) for magnetic materials. However, it remains a big challenge to elucidate the near-Tc behaviors of chiral helimagnets, owing to their diversified magnetic interactions and complicated phase transitions. Here, we have systematically studied the near-Tc properties of a chiral helimagnet Cr1/3NbS2 by magnetotransport measurements with fine tuning of temperature and magnetic field. An anomalous positive magnetoresistance (MR) appears around the critical field (Hc) in a narrow temperature range (~ 128–132 K), which can be attributed to the competing scattering effects on electrons between the chiral magnetic order and thermal fluctuation near critical point. Moreover, the negative MR continues to increase even above Hc, and this behavior becomes more pronounced near Tc. Based on the MR results, a near-Tc magnetic phase diagram of Cr1/3NbS2 is presented, with a particular emphasis on the determination of internal phase boundary of chiral soliton lattice (CSL). This work gains an insight on the near-Tc behaviors of chiral magnetic materials and sheds light on the related electronic and spintronic applications.

Periodically modulated skyrmion strings in Cu2OSeO3

Magnetic skyrmions are vortex-like spin textures, which are usually treated as two-dimensional objects. In their lattice state, they form well-ordered, hexagonal structures, which have been studied in great detail. To obtain a three-dimensional (3D) skyrmion crystal, these planes can be envisaged to be stacked up forming skyrmion strings in the third dimension. Here, we report the observation of a 3D skyrmion phase in Cu2OSeO3 by carrying out reciprocal space mapping in resonant elastic x-ray scattering. We observe regions in the magnetic field-cooling phase diagram in which the skyrmion phase apparently coexists with the conical phase. However, such a coexistence is forbidden due to symmetry arguments. Instead, the skyrmion strings themselves are periodically modulated along their axes, as confirmed by micromagnetic simulations. The periodic modulation is in fact a necessary consequence of the evolution of the skyrmion phase out of the conical state and should therefore be a universal property of skyrmion strings in chiral helimagnets.

Dispersion relation of spin wave in chiral helimagnet under stationary optical vortex radiation

We investigate a dispersion relation of a spin wave of an effectively one-dimensional chiral helimagnet CrNb3S6, which is under stationary radiation of a resonant optical vortex beam coaxial to that spiral magnetic order axis. We derive a classical magnetic structure by solving Landau–Lifshitz–Gilbert equation and calculate the collective excitation on it. We show that the optical vortex radiation makes the energy gap at k ∼ 0, and the exceptional points appear in the dispersion relation. Because such a dispersion relation implies the dynamical properties with instability, our result implies a possibility to control spin wave transport.

Novel vortex structures in a three-dimensional superconductor under a helical magnetic field from a chiral helimagnet

We have investigated vortex structures in three-dimensional superconductors under a helical magnetic field from a chiral helimagnet numerically. In order to obtain vortex structures, we solve the three-dimensional Ginzburg–Landau equations with the finite element method. A spatial distribution of the helical magnetic field corresponds to a spatial distribution of magnetic moments in the chiral helimagnet. Then, the magnetic field is parallel to a yz-plane and helical rotation along a helical axis (x-axis). An arrangement of the vortex is different from the spatial oscillation in each component of the magnetic field. In this calculation, two-types of vortex structures appear. One is an appearance of vortices perpendicular to a xy-plane and a zx-plane of the superconductor. The other arrangement is that vortices are tilted toward the helical axis, although the magnetic field along the helical axis is zero. The arrangement of the tilted vortex depends on a direction of a rotation in the helical magnetic field.

Twisted light-induced spin–spin interaction in a chiral helimagnet

We theoretically investigate how the orbital angular momentum of light can affect a chiral magnetic order. Here, we consider a metallic chiral helimagnet, which is under stationary radiation of a resonant optical vortex beam. We propose a novel interaction between local spins considering microscopic interactions between an optical vortex and electrons. This vortex-induced interaction modulates the chiral magnetic order in an entirely different way than an external magnetic field does. Our spin modulation technique may pave a route to create a unique topological or chiral structure for future opto-spintronics devices.