Helical Magnetic State Research Articles

Helical magnetic state in the vicinity of the pressure-induced superconducting phase in MnP

Helical magnetic state in the vicinity of the pressure-induced superconducting phase in MnP

Creation of single chiral soliton states in monoaxial helimagnets

In monoaxial helimagnets, the Dzyaloshinskii–Moriya interaction favors inhomogeneous distributions of the magnetization with chiral modulations of solitonic character. In addition to the helical magnetic state at zero field, a chiral soliton lattice can be stabilized when a magnetic field perpendicular to the chiral axis is applied. When the magnetic field is increased, the system undergoes a phase transition to the uniform state at a critical field Bc. Above Bc, a single chiral soliton comprises the lowest level excitation over the stable uniform state, surviving as a metastable configuration. How to retain a single chiral soliton metastable state has not been addressed yet. Using micromagnetic simulations, we analyze this possibility by injecting spin polarized currents and put forward a feasible protocol to obtain a state with a single chiral soliton from the chiral soliton lattice. Our proposal could be relevant in the experimental study of metastable solitons for technological applications.

Helical versus collinear antiferromagnetic order tuned by magnetic anisotropy in polar and chiral (Ni,Mn)3TeO6

Polar and chiral ${\mathrm{Ni}}_{3}{\mathrm{TeO}}_{6}$ was found to exhibit a colossal magnetoelectric (ME) effect associated with collinear antiferromagnetic order. We have investigated the evolution of its magnetic state with the substitution of the ${\mathrm{Ni}}^{2+}$ spins by isotropic ${\mathrm{Mn}}^{2+}$ spins, namely ${(\mathrm{Ni},\mathrm{Mn})}_{3}{\mathrm{TeO}}_{6}$ (NMTO). The ground state of NMTO maintains an Ising nature, but a new phase with XY-type magnetic anisotropy is discovered at an intermediate temperature range. Neutron powder diffraction experiments reveal that (1) Mn ions tend to occupy a specific transition-metal site, consistent with the first-principle calculation, and (2) the intermediate phase is an incommensurate (IC) helical magnetic state propagating along the $c$ axis with ${\stackrel{P\vec}{q}}_{\text{IC}}=(0,\phantom{\rule{0.16em}{0ex}}0,\phantom{\rule{0.16em}{0ex}}1.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta})$. We also found that the balance between these two magnetic states can be readily manipulated with magnetic fields, which results in significant ME effects. This selective site occupancy of NMTO allows for a unique competition between collinear and helical magnetic structures with minimal chemical disorder. Thus, NMTO serves as a model system to study site-specific chemical control of noncollinear magnetism and ME coupling.

Field driven recovery of the collective spin dynamics of the chiral soliton lattice

We investigate the magnetic field dependence of the spin excitation spectra of the chiral soliton lattice (CSL) in the helimagnet CrNb3S6, by means of microwave resonance spectroscopy. The CSL is a prototype of a noncollinear spin system that forms periodically over a macroscopic length scale. Following the field initialization of the CSL, we found three collective resonance modes over an exceptionally wide frequency range. Upon further reducing the magnetic field toward 0 T, the spectral weight of these collective modes was disrupted by the emergence of additional resonances whose Kittel-like field dependence was linked to coexisting field polarized magnetic domains. The collective behavior at a macroscopic level was only recovered upon reaching the helical magnetic state at 0 T. The magnetic history of this noncollinear spin system can be utilized to control microwave absorption, with potential use in magnon-driven devices.

Effect of Chromium Substitution for Iron on the Magnetic and Structural Properties of (TmxPr1– x)2Fe17

The (TmxPr1 – x)2Fe17 and (TmxPr1 – x)2Fe16.5Cr0.5 compounds crystallize with the formation of the rhombohedral Th2Zn17-type structure for the compositions with x = 0–0.4 and the hexagonal Th2Ni17-type structure for the compositions with x = 0.8–1 and x = 0.75–1, respectively. Both structures coexist in the (TmxPr1 – x)2Fe17 and (TmxPr1 – x)2Fe16.5Cr0.5 compounds with x = 0.5–0.75 and x = 0.5–0.6, respectively. The (TmxPr1 – x)2Fe17 compositions with 0 < x < 0.6 are ferrimagnets; at x = 0.6–1, additionally a high-temperature helical magnetic state is realized. The substitution of chromium for iron leads to the transformation of the helimagnet to ferrimagnet; in this case, the difference between the Curie temperatures of rhombohedral and hexagonal phases is unexpectedly high (25–28 К). For the composition with x = 0.8, the Curie and Neel temperatures and the ferrimagnet–helimagnet transformation temperature are minimum, and the microdeformations are maximum. The overlap of two peaks in –ΔSM(T), which correspond to the magnetic entropy change at two magnetic phase transformation temperatures, favors the realization of the higher cooling capacity of the two-phase compositions as compared to that of neighboring single-phase compositions.

In situ electron holography of magnetic skyrmions in nanostructures

Understanding the correlations between magnetic skyrmions and the microstructural characteristics of the crystals that host skyrmions is a key issue for fundamental research and practical applications of novel type of magnetic materials. Magnetic skyrmion has received great attention due to its non-trivial topological properties and stability. Here we focus on two important points:1) dimensional confinement effects on magnetic skyrmions in magnetic nanostructures, specifically, the magnetic evolution, its related topological properties and energetic stability in confined nanostructured geometries; 2) effects of crystallographic defects on magnetic skyrmions, such as the pinning effect of magnetic skyrmion by crystal defects, and the effect of crystallographic-magnetic chirality reversal at crystal grain boundaries. For the study of dimensional effects on skyrmions in confined nanoscale geometries, we use state-of-the-art electron holography to directly image the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in a range of 45-150 nm. Our experimental results reveal that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticity in a nanostripe, which are not existent in thin films nor bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We further perform micromagnetic simulations to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices. For studying the effects of crystallographic defects on magnetic skyrmions, we use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. The measurements of spin configurations at grain boundaries reveal the crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, our results show that skyrmion lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. These findings offer an insight into the elasticity of topologically protected skyrmions and their correlation with underlying material defects. Our electron holography results provide a quantitative determination of the fine skyrmionic spin textures in magnetic nanostructures. The resolved spin textures will be correlated with the material microstructures to provide important information about the relationship between the magnetic functions and the material microstructures. Our experiments also highlight the applicability of state-of-the-art electron holography for the study of complex spin textures in nanostructures.

Control of morphology and formation of highly geometrically confined magnetic skyrmions

The ability to controllably manipulate magnetic skyrmions, small magnetic whirls with particle-like properties, in nanostructured elements is a prerequisite for incorporating them into spintronic devices. Here, we use state-of-the-art electron holographic imaging to directly visualize the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in the range of 45–150 nm. We find that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticities in a nanostripe that are absent in both thin films and bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We perform a theoretical analysis based on a three-dimensional general model of isotropic chiral magnets to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices.

Chiral magnetic excitations in FeGe films

Although chiral magnetic materials have emerged as a potential ingredient in future spintronic memory devices, there are few comprehensive studies of magnetic properties in scalably-grown thin films. We present growth, systematic physical and magnetic characterization, and microwave absorption spectroscopy of B20 FeGe thin films. We also perform micromagnetic simulations and analytical theory to understand the dynamical magnetic behavior of this material. We find magnetic resonance features in both the helical and field-polarized magnetic states that are well explained by micromagnetic simulations and analytical calculations. In particular, we show the resonant enhancement of spin waves along the FeGe film thickness that has a wave vector matching the helical vector. Using our analytic model, we also describe the resonance frequency of a helical magnetic state, which depends solely on its untwisting field. Our results pave the way for understanding and manipulating high frequency spin waves in thin-film chiral-magnet FeGe near room temperature.

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.

The roles of chirality and polarity in novel multiferroics: MnSb2O6and Cu3Nb2O8

At room temperature Cu3Nb2O8 has a centrosymmetric, triclinic crystal structure. If cooled below 24 K, the copper magnetic moments order with a complex, generalized helicoidal magnetic structure that breaks inversion symmetry, giving rise to ferroelectricity. Unusually, the direction of the induced electric polarization vector with respect to the helicoidal spin rotation cannot be reconciled by conventional theories of magneto-electric coupling. Instead, we show that the observed multiferroic properties of Cu3Nb2O8 may be explained through a phenomenological analysis based upon coupling between the magnetic chirality, electric polarity, and a structural axial rotation. Trigonal MnSb2O6 crystallizes with a chiral crystal structure. Typically, magnetic materials with a chiral crystal lattice order with a chiral magnetic structure, where the magnetic exchange interactions and anisotropies follow the symmetry of the lattice. The magnetism of MnSi is a classic example of this scenario, in which exotic skyrmion phases emerge out of a helical magnetic state. To the contrary, we show that the low temperature magnetic structure of MnSb2O6 is cycloidal, described by a magnetic polarity as opposed to a chirality. We demonstrate through ab-initio calculations that this magnetic structure is in fact the ground state of the symmetric-exchange Heisenberg spin Hamiltonian, which has higher symmetry than the underlying crystal lattice. Furthermore, the phenomenology may be understood by considering the coupling between structural chirality, magnetic polarity, and a magnetic axial rotation. As a result, we predict MnSb2O6 to be multiferroic with a weak ferroelectric polarization.

GHz magnetic field influence on magnetization reversal in amorphous microwires

AbstractThis work is devoted to the study of the magnetization reversal in the Co‐rich glass covered microwire with nearly‐zero magnetostriction in the presence of super high frequency (SHF) (about 1 GHz) circular magnetic field that is important for the elucidation of GMI mechanism. Magnetic domain imaging has been performed by means of magneto‐optical Kerr effect (MOKE) polarizing microscopy. Local hysteresis loops were obtained from the magneto‐optic intensity as a result of images processing. The increase of the SHF field amplitude causes the change of the magnetization reversal mechanism – the rotation of the magnetization dominates. Also the hysteresis loop has an asymmetric shape that confirms the competition between stable and meta‐stable helical magnetic states. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cascaded spin motive force driven by the dynamics of the skyrmion lattice

We numerically investigate the spin motive force (SMF) driven by the dynamics of a Skyrmion lattice. The rotating mode of the Skyrmion core excited by the AC magnetic field induces the large spin-dependent electric field near the core. Due to the collective dynamics of Skyrmion lattice, the measurable voltage is enhanced by the cascade effect of the SMF. The amplitude of the AC voltage is estimated to 30 μV in a macroscopic sample, where 100 Skyrmions exist between two probes. We also investigate the SMF due to the dynamics of the helical magnetic state, where the enhancement of the SMF does not occur.

Experimental demonstration of basic mechanisms of magnetization reversal in magnetic microwires

We report on the magneto-optical study of surface magnetization reversal in glass covered Co-rich microwires. Four different mechanisms of magnetization reversal were found to exist, depending on external magnetic field configuration. An analysis of the results obtained was performed using the theoretical model which proposes the existence of four magnetization states with different chirality. This model was developed previously to describe current-induced switching in wires with variable inner core radius. The co-existence of stable and meta-stable helical magnetic states on the microwire surface is the basic reason for the variety of mechanisms observed for the magnetization reversal.

Magnetic ordering in (110) Eu films in an applied magnetic field

Below its ordering temperature (T N = 90 K), bulk bcc Eu has a helical magnetic state with propagation vectors along the three equivalent 〈100〉 directions. In contrast, epitaxial (110)Eu films exhibit a unique magnetic ordering: the domain with a magnetic helix propagating along the in-plane [001] direction vanishes on cooling, at the expense of other domains with helices propagating along [100] and [010]. This paper is devoted to the study of the stability of the magnetic domains in an external magnetic field using neutron scattering experiments and macroscopic magnetization measurements. The helix propagating along the [001] direction can be restored by the application of an external field along this direction. On the contrary, when a magnetic field is applied along an intermediate direction, specifically [ $$ \bar 1 $$ 10], the domain with a helix propagating along [001] is suppressed. Both effects depend on the film thickness. They are explained if one considers that, because of the low magnetic anisotropy of Eu, a helix with a propagation vector parallel to (or close to) the applied magnetic field is energetically more favourable than cycloidal structures with unchanged propagation vectors. Finally, the amplitudes of the propagation vectors and their directions (that are modified in films compared to bulk) do not vary under magnetic field.

Heat capacity and thermal expansion of the itinerant helimagnet MnSi

The heat capacity and thermal expansion of a high quality single crystal of MnSiwere measured at ambient pressure at zero and high magnetic fields. Thecalculated magnetic entropy change in the temperature range 0–30 K is less than0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A lineartemperature term dominates the thermal expansion coefficient in the range 30–150 K, whichcorrelates with an enhancement of the linear electronic term in the heat capacity. Asurprising similarity among the variations of the heat capacity, thermal expansioncoefficient and temperature derivative of the resistivity is observed through the phasetransition in MnSi. Specific forms of the heat capacity, thermal expansion coefficient andtemperature derivative of resistivity at the phase transition to a helical magnetic state near29 K are interpreted as the combination of sharp first-order features and broad peaks orshallow valleys of as yet unknown origin. The appearance of these broad satellites probablyhints at a frustrated magnetic state slightly above the transition temperature in MnSi.

Experimental study of the magnetic phase transition in the MnSi itinerant helimagnet

Magnetic susceptibility, heat capacity, thermal expansion, and resistivity of a high-quality single crystal of MnSi were carefully studied at ambient pressure. The calculated change in magnetic entropy in the temperature range 0–30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the behavior of the thermal expansion coefficient in the range 30–150 K, which correlates to a large enhancement of the linear electronic term in the heat capacity. A surprising similarity between variation of the heat capacity, the thermal expansion coefficient, and the temperature derivative of resistivity through the phase transition in MnSi is observed. Specific forms of the heat capacity, thermal expansion coefficient, and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as a combination of sharp first-order features and broad peaks or shallow valleys of yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state in MnSi slightly above the transition temperature. Present experimental findings bring the current views on the phase diagram of MnSi into question.

Magnetic phase transition in the itinerant helimagnetMnSi: Thermodynamic and transport properties

A careful study of thermodynamic and transport properties of a high-quality single crystal of $\mathrm{MnSi}$ at ambient pressure suggests that its transition to a helical magnetic state near $29\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is weakly first order. The heat capacity, temperature derivative of resistivity, thermal expansion, and magnetic susceptibility exhibit a specific structure around the phase transition point, interpreted as a combination of first- and second-order features. Striking mirror symmetry between the temperature derivative of resistivity and the thermal expansion coefficient is observed. Conclusions drawn from these experiments question prevailing views on the phase diagram of $\mathrm{MnSi}$.

Theoretical investigation on electronic properties of topological materials: Möbius nanographite and conjugated polymers

Recent theoretical developments on topological materials, Möbius nanographite and Möbius conjugated polymers, are reported. (I) In nanographite systems with the Möbius boundary condition, there appears a novel magnetic domain. However, this domain state competes against the helical magnetic states. Total energies of the latter states are always lower than that of the former state. Additionally, the domain wall appears in the charge density wave states. (II) Optical properties of Möbius conjugated polymers are studied. We discuss that oligomers with a few structural units are more effective than polymers, in order to measure effects of discrete wave numbers which are shift by the Möbius boundary from those of the periodic boundary. Certain components of the optical absorption for the electric field perpendicular to the polymer axis mix with the absorption spectra for the electric filed parallel with the polymer axis. The polarization dependences of electric field of light can detect whether conjugated polymers have the Möbius boundary or not.

NMR study of MnAs at atmospheric and high pressures

Zero-field NMR of MnAs has been observed as a function of temperature T and pressure. In the ferromagnetic NiAs phase the frequencies of the 55 Mn and 75 As NMR show a T 5/2 dependence below 160 K, which is compared with the empirical T n power law in usual magnetic materials. Above about 4 kbar a broad NMR line is observed and is assigned to the helical magnetic state of MnP type phase.

De Haas-van Alphen Effect of FeP in Double Helical Magnetic State

The de Haas-van Alphen (dHvA) effect has been investigated on high-quality single crystals of FeP in a double helical magnetic state with a 5 c period in the magnetic fields up to 14 T and at tempe...