Helimagnetic Research Articles

Spin-Hall magnetoresistance in multidomain helical spiral systems

We study the spin-Hall magnetoresistance (SMR) in multidomain helical spiral magnet Cu2OSeOPt heterostructures. We compare the SMR response of Cu2OSeO3 at 5 K, when the magnetic domains are almost frozen, to that at elevated temperatures, when domain walls move easily. At 5 K the SMR amplitude vanishes at low applied magnetic fields, while at 50 K it does not. This phenomenon can be explained by the effect of the magnetic field on the domain structure of Cu2OSeO3. At elevated temperatures the system can reach the thermodynamic equilibrium state, in which a single domain that has a minimal energy for a given field direction occupies the whole sample and gives rise to a nonzero SMR signal. In contrast at 5 K, the three types of domains with mutually orthogonal spiral wave vectors have equal volumes independent of the field direction, which leads to the cancellation of the SMR signal at low fields. In the single-domain conical spiral and collinear ferrimagnetic states, the angular and field dependence of the SMR is found to be same at all temperatures ( K). This behavior can be understood within the framework of the SMR theory developed for collinear magnets.

螺旋型磁石を用いない磁気ねじアクチュエータの提案

This paper presents a novel magnetic lead screw actuator without helical permanent magnets. Magnetic lead screw actuator is high efficiency due to its non-contact transmission. It is expected to be applicable to interactive robots with human because of its compliance to external forces. The proposed actuator has simple structure comparing to the conventional magnetic screws. The result of the force calculation and structure optimization employing 3-D finite element method is reported.

Realizing topological stability of magnetic helices in exchange-coupled multilayers for all-spin-based system.

Topologically stabilized spin configurations like helices in the form of planar domain walls (DWs) or vortex-like structures with magnetic functionalities are more often a theoretical prediction rather than experimental realization. In this paper we report on the exchange coupling and helical phase characteristics within Dy-Fe multilayers. The magnetic hysteresis loops with temperature show an exchange bias field of around 1.0 kOe at 10 K. Polarized neutron reflectivity reveal (i) ferrimagnetic alignment of the layers at low fields forming twisted magnetic helices and a more complicated but stable continuous helical arrangement at higher fields (ii) direct evidence of helices in the form of planar 2π-DWs within both layers of Fe and Dy. The helices within the Fe layers are topologically stabilized by the reasonably strong induced in-plane magnetocrystalline anisotropy of Dy and the exchange coupling at the Fe-Dy interfaces. The helices in Dy are plausibly reminiscent of the helical ordering at higher temperatures induced by the field history and interfacial strain. Stability of the helical order even at large fields have resulted in an effective modulation of the periodicity of the spin-density like waves and subsequent increase in storage energy. This opens broad perspectives for future scientific and technological applications in increasing the energy density for systems in the field of all-spin-based engineering which has the potential for energy-storing elements on nanometer length scales.

Josephson junction through a disordered topological insulator with helical magnetization

We study supercurrent and proximity vortices in a Josephson junction made of disordered surface states of a three-dimensional topological insulator with a proximity induced in-plane helical magnetization. In a regime where the rotation period of helical magnetization is larger than the junction width, we find supercurrent $0\ensuremath{-}\ensuremath{\pi}$ crossovers as a function of junction thickness, magnetization strength, and parameters inherent to the helical modulation and surface states. The supercurrent reversals are associated with proximity induced vortices, nucleated along the junction width, where the number of vortices and their locations can be manipulated by means of the superconducting phase difference and the parameters mentioned above.

Chiral Magnetism in an Itinerant Helical Magnet, MnSi — An Extended 29Si NMR Study —

The microscopic magnetism in the helical, the conical and the ferro-magnetically polarized phases in an itinerant helical magnet, MnSi, has been studied by an extended 29Si NMR at zero field and under external magnetic fields. The temperature dependence of staggered moment, M_Q(T), determined by the 29Si NMR frequency, nu(T), and nuclear relaxation rate, 1/T_1(T) is in general accord with the SCR theory for weak itinerant ferromagnetic metals and its extension. The external field dependence of resonance frequency, nu(H), follows a vector sum of the contributions from atomic hyperfine and macroscopic fields with a field induced moment characteristic to the itinerant magnets. A discontinuous jump of the resonance frequency at the critical field, H_c, between the conical and the polarized phases has also been found that suggests a first order like change of the electronic states at H_c.

Observation of spin textures in La1−xSrxMnO3 (x = 0.175)

We have investigated topological spin textures in the ferromagnetic metallic phase of La0.825Sr0.175MnO3 with the centrosymmetric crystal structure by small-angle electron diffraction (SmAED) and low-temperature Lorentz transmission electron microscopy (TEM) experiments. In-situ Lorentz TEM and SmAED experiments revealed that type-I and type-II magnetic bubbles evolved from magnetic stripe domains with the Bloch-type domain wall by applying vertical magnetic field. Type-I magnetic bubbles with left-handed and right-handed spin helicity were randomly distributed and simultaneously type-II magnetic bubbles are formed locally. The important point about type-I and type-II magnetic bubbles is that their emergence depends strongly on whether perpendicular magnetic field is applied parallel to the magnetic easy axis along the [001] direction. Our experimental results suggested that the stabilization of magnetic bubbles should originate from the long-range dipole-dipole interactions, as opposed to the Dzyaloshinskii-Moriya interaction in helical magnets.

Rashba Torque Driven Domain Wall Motion in Magnetic Helices.

Manipulation of the domain wall propagation in magnetic wires is a key practical task for a number of devices including racetrack memory and magnetic logic. Recently, curvilinear effects emerged as an efficient mean to impact substantially the statics and dynamics of magnetic textures. Here, we demonstrate that the curvilinear form of the exchange interaction of a magnetic helix results in an effective anisotropy term and Dzyaloshinskii–Moriya interaction with a complete set of Lifshitz invariants for a one-dimensional system. In contrast to their planar counterparts, the geometrically induced modifications of the static magnetic texture of the domain walls in magnetic helices offer unconventional means to control the wall dynamics relying on spin-orbit Rashba torque. The chiral symmetry breaking due to the Dzyaloshinskii–Moriya interaction leads to the opposite directions of the domain wall motion in left- or right-handed helices. Furthermore, for the magnetic helices, the emergent effective anisotropy term and Dzyaloshinskii–Moriya interaction can be attributed to the clear geometrical parameters like curvature and torsion offering intuitive understanding of the complex curvilinear effects in magnetism.

Phase diagram of the chiral magnetCr1/3NbS2in a magnetic field

We construct the phase diagram of the chiral magnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$ in a dc magnetic field $({H}_{\mathrm{dc}})$ using ac magnetic susceptibility measurements. At ${H}_{\mathrm{dc}}=0$, the ac response at the transition from the helical magnetic (HM) state to the paramagnetic (PM) state consists of a giant third-order harmonic component $({M}_{3\ensuremath{\omega}})$ and a first-order harmonic component $({M}_{1\ensuremath{\omega}})$. By applying ${H}_{\mathrm{dc}}$ perpendicular to the $c$ axis, the HM state is transformed to the chiral soliton lattice (CSL) state, which is a superlattice tuned by ${H}_{\mathrm{dc}}$. The above giant ${M}_{3\ensuremath{\omega}}$ is markedly suppressed at small ${H}_{\mathrm{dc}}$. The CSL state is found to consist of CSL-1, with dominant helical texture and a poor ferromagnetic array, and CSL-2, with a large ferromagnetic array. The transition between CSL-1 and the PM state causes a linear magnetic response, the dominant component of which is the in-phase ${M}_{1\ensuremath{\omega}}$. With increasing temperature, CSL-2 is transformed into the forced ferromagnetic (FFM) state, and ultimately the PM state is reached. The transition between CSL-2 and the FFM state consists of a large ${M}_{3\ensuremath{\omega}}$ and large out-of-phase ${M}_{1\ensuremath{\omega}}$ as well as in-phase ${M}_{1\ensuremath{\omega}}$. The transition between the FMM and PM states also yields a linear magnetic response, like the CSL-1--PM-state transition. Five typical magnetic dynamics in the transitions among the HM state, CSL-1, CSL-2, FFM state, and PM state were identified according to the equivalent dynamical motion equation of a nonlinear spring model.

Effect of Yttrium doping on structural and magnetic properties of Dysprosium

A comparative structural and magneto transport study has been performed on the Dysprosium (Dy) and Yttrium (Y) doped Dysprosium (Dy–Y) alloys in order to study the effect of Y concentration, temperature and magnetic field on the magnetic states and transitions of Dy–Y alloys. The magnetic state of Dy and Dy–Y alloys having lower Y substitutions change from Paramagnetic (PM) to Helimagnetic (HM) state via second order phase transition and from Helimagnetic state to Ferromagnetic (FM) state via first order phase transition. Small change in lattice parameters, strain and micro-strain is observed with X-ray diffraction on replacement with Y ions. Neel temperature and Curie temperature both show a decreasing trend on diluting Dy with non-magnetic Y in small concentrations. PM to HM and HM to FM transitions in the lower substitutional alloys discussed in this manuscript show a direct transition from PM to FM state at fields above 1.5T.

Direct observation of Σ7 domain boundary core structure in magnetic skyrmion lattice.

Skyrmions are topologically protected nanoscale magnetic spin entities in helical magnets. They behave like particles and tend to form hexagonal close-packed lattices, like atoms, as their stable structure. Domain boundaries in skyrmion lattices are considered to be important as they affect the dynamic properties of magnetic skyrmions. However, little is known about the fine structure of such skyrmion domain boundaries. We use differential phase contrast scanning transmission electron microscopy to directly visualize skyrmion domain boundaries in FeGe1-x Si x induced by the influence of an "edge" of a crystal grain. Similar to hexagonal close-packed atomic lattices, we find the formation of skyrmion "Σ7" domain boundary, whose orientation relationship is predicted by the coincidence site lattice theory to be geometrically stable. On the contrary, the skyrmion domain boundary core structure shows a very different structure relaxation mode. Individual skyrmions can flexibly change their size and shape to accommodate local coordination changes and free volumes formed at the domain boundary cores. Although atomic rearrangement is a common structural relaxation mode in crystalline grain boundaries, skyrmions show very unique and thus different responses to such local lattice disorders.

Stiffness in vortex-like structures due to chirality-domains within a coupled helical rare-earth superlattice.

Vortex domain walls poses chirality or ‘handedness’ which can be exploited to act as memory units by changing their polarity with electric field or driving/manupulating the vortex itself by electric currents in multiferroics. Recently, domain walls formed by one dimensional array of vortex—like structures have been theoretically predicted to exist in disordered rare-earth helical magnets with topological defects. Here, in this report, we have used a combination of two rare-earth metals, e.g. superlattice that leads to long range magnetic order despite their competing anisotropies along the out-of-plane (Er) and in-plane (Tb) directions. Probing the vertically correlated magnetic structures by off-specular polarized neutron scattering we confirm the existence of such magnetic vortex—like domains associated with magnetic helical ordering within the Er layers. The vortex—like structures are predicted to have opposite chirality, side—by—side, and are fairly unaffected by the introduction of magnetic ordering between the interfacial Tb layers and also with the increase in magnetic field which is a direct consequence of screening of the vorticity in the system due to a helical background. Overall, the stability of these vortices over a wide range of temperatures, fields and interfacial coupling, opens up the opportunity for fundamental chiral spintronics in unconventional systems.

らせん状磁石による結合・変形機構を備えたモジュラーロボットの開発

らせん状磁石による結合・変形機構を備えたモジュラーロボットの開発

Magnetoelectric coupling and spin-induced electrical polarization in metal–organic magnetic chains

We report first-principles predictions of magnetoelectric coupling in organic magnetic helices and clarify the microscopic mechanism of spin-induced electric polarization.

Stabilization of magnetic helix in exchange-coupled thin films

Based on micromagnetic simulations, we report on a novel magnetic helix in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers with different anisotropies. We show that such a confined helix stays stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal magnetic field created by the exchange interaction. We show that this internal field stores a magnetic energy density of a few kJ/m3. We also find that it dramatically modifies ferromagnetic resonances, such that the helix can be used as a ferromagnetic resonance filter and a fast acting attenuator.

Symmetry of Magnetostatic Fields Generated by Toroidal Helicoidal Magnets

In this paper, the axial symmetry of the magnetic field generated by a permanent magnet of helicoidal toroidal kind is shown. In the first part of this paper, we illustrate the shape of the magnet and the number of areas where the field is calculated to demonstrate the symmetry. We define quantitatively the size of the toroidal helical magnet and the regions where the magnetostatic field is evaluated. The field is carried out for each angular sector that represents the regions where the magnetic flux density is computed. This calculation is performed with reference to a matrix of points belonging to each sector. Two sets of evaluations are performed. The first one is referred to a less dense matrix of points relative to all the regions. The aim of this computation is to demonstrate the axial symmetry of the field. The second set of calculations concerns the field evaluation using a much higher dense matrix of points. Using these data, we are able to interpolate the same field with a high precision. This second evaluation of the field is carried out with reference to only the flat region facing the first coil of the helical toroidal magnet. The use of an interpolation surface through the final points of the magnetic induction vectors previously computed allows a very fast evaluation of the field virtually in all the infinite points of the angular sector. The symmetry enables us to drastically reduce the time computation of the magnetostatic field in the points of interest.

Manipulation of the spin helix in FeGe thin films and FeGe/Fe multilayers

Magnetic materials without structural inversion symmetry can display the Dzyaloshinskii-Moriya interaction, which manifests itself as chiral magnetic ground states. These chiral states can interact in complex ways with applied fields and boundary conditions provided by finite sample sizes that are of the order of the length scale of the chiral states. Here we study epitaxial thin films of FeGe with a thickness close to the helix pitch of the helimagnetic ground state, which is about 70 nm, by conventional magnetometry and polarized neutron reflectometry. We show that the helix in an FeGe film reverses under the application of a field by deforming into a helicoidal form, with twists in the helicoid being forced out of the film surfaces on the way to saturation. An additional boundary condition was imposed by exchange-coupling a ferromagnetic Fe layer to one of the interfaces of an FeGe layer. This forces the FeGe spins at the interface to point in the same direction as the Fe, preventing node expulsion and giving a handle by which the reversal of the helical magnet may be controlled.

Band Structure of Helimagnons in MnSi Resolved by Inelastic Neutron Scattering.

A magnetic helix realizes a one-dimensional magnetic crystal with a period given by the pitch length λh. Its spin-wave excitations-the helimagnons-experience Bragg scattering off this periodicity, leading to gaps in the spectrum that inhibit their propagation along the pitch direction. Using high-resolution inelastic neutron scattering, the resulting band structure of helimagnons was resolved by preparing a single crystal of MnSi in a single magnetic-helix domain. At least five helimagnon bands could be identified that cover the crossover from flat bands at low energies with helimagnons basically localized along the pitch direction to dispersing bands at higher energies. In the low-energy limit, we find the helimagnon spectrum to be determined by a universal, parameter-free theory. Taking into account corrections to this low-energy theory, quantitative agreement is obtained in the entire energy range studied with the help of a single fitting parameter.

Torsion-induced effects in magnetic nanowires

Magnetic helix wire is one of the most simple magnetic systems which manifest properties of both curvature and torsion. There exist two equilibrium states in the helix wire with easy-tangential anisotropy: a quasi-tangential magnetization distribution in case of relatively small curvatures and torsions, and an onion state in opposite case. In the last case the magnetization is close to tangential one, deviations are caused by the torsion and curvature. Possible equilibrium magnetization states in the helix magnet with different anisotropy directions are studied theoretically. The torsion also essentially influences the spin-wave dynamics, acting as an effective magnetic field. Originated from the curvature induced effective Dzyaloshinskii interaction, this magnetic field leads to the coupling between the helix chirality and the magnetochirality, it breaks mirror symmetry in spin-wave spectrum. All analytical predictions on magnetization statics an dynamics are well confirmed by the direct spin lattice simulations.

Conductance and shot noise in the helimagnet tunnel junction

As a result of sinusoidal spatial modulation, helimagnet induces spin-dependent diffracted transmission. In this work, we propose a general scattering matrix treatment to the transport properties in helimagnets with arbitrary helical structures. Multiferroic properties can be considered by taking electric polarization and magnetic helix together into account. A monolayer magnetic helix can be treated as a δ-barrier by diffraction theory. The conductance and shot noise properties of the normal metal/helimagnet/normal metal toy model are investigated. It is found that the shot noise is suppressed and demonstrates rise-and-fall variations as a result of interference between different diffracted channels. Sharp change of the shot noise occurs when one and both diffracted beams disappear into evanescent modes at the helimagnet spiral wave vector equal to one and two times the electron Fermi wave vector q=kF and 2kF. We also considered the conductance and shot noise properties in a real multiferroic helimagnet TbMnO3 with ferroelectricity and helimagnetism coexistent. It is found that clockwise and counterclockwise spin helices are distinctly separated in the conductance and shot noise spectra. Their variation pattern is a combined result of helimagnetism and electric polarization.

Raman phonon spectrum of the Dzyaloshinskii-Moriya helimagnetBa2CuGe2O7

The Raman spectrum of Ba2CuGe2O7, a tetragonal insulator which develops Dzyaloshinsky- Moriya helical magnetism below TN = 3.2 K, has been detected at temperatures varying from 300 to 80 K in a single crystal, with the radiation polarized either in the ab plane or along the c axis of its tetragonal cell. 29 phonon lines out of the 35 allowed by the Raman selection rules for the present geometry were observed, and their vibrational frequencies were found in overall good agreement with those provided by shell-model calculations. Together with the previous report on the infrared-active phonons [A. Nucara et al., Phys. Rev. B 90, 014304 (2014)] the present study provides an exhaustive description, both experimental and theoretical, of the lattice dynamics in Ba2CuGe2O7.