Session L40: Bulk Chiral Magnets

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Visualizing magnetic skyrmions is an essential step in the process of correlating parameters like skyrmion density with other material properties. Lorentz Transmission Electron Microscopy (LTEM) is a well-established technique for imaging magnetic textures like skyrmions, but this approach is challenging to apply to samples in which contrast comes from features besides the magnetic texture, such as grain boundaries in polycrystalline materials, especially if the grains and skyrmions are similar in size. Here, we use Magnetic Force Microscopy (MFM) and LTEM to examine a polycrystalline FeGe thin film grown epitaxially on a Si(111) substrate via molecular beam epitaxy (MBE). Combining MFM and LTEM allows for imaging skyrmions and determining their density as a function of temperature and applied field. Magnetic texture contrast in LTEM is enhanced through use of an image subtraction method to reduce the grain contrast. We also characterize the microstructure and quality of the film in plan view and cross section specimens using Transmission Kikuchi Diffraction (TKD) in a scanning and electron microscope (SEM) and high angle annular dark field (HAADF) scanning (S)TEM imaging. We conclude by showing how the observed skyrmion density correlates with topological Hall measurements.   

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Cubic B20 magnets such as MnSi, (Fe, Co)Si host magnetic skyrmion lattice over a small range of temperature and magnetic fields. Although the qualitative features of the magnetic phase diagram look similar for MnSi and Fe_1-yCo_ySi, there are some differences in the nature of magnetic transition due to the effects of disorder and the difference in the dynamical behavior of spins. We compare MnSi and Fe_1-yCo_ySi based on the change in magnetic entropy during conical to magnetic skyrmion lattice transition. Our results indicate that despite qualitative similarities in the magnetic phase diagrams, the entropic signatures related to such topological transition are significantly reduced in Fe_1-yCo_ySi due to the effect of disorder and the differences in spin dynamical behavior.   

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Lacunar spinels AB₄Q₈ (A=Al, Ga, Ge; B=V, Nb, Mo, Ta; Q=S, Se) display a variety of novel phenomena including skyrmion lattices and pressure-induced insulator-metal transitions and superconductivity. The high temperature structure features unique tetrahedral transition-metal clusters with S=1/2 moments on an FCC lattice. Low temperature distortions of these clusters are crucial to the properties of lacunar spinels. In skyrmion hosts GaV₄S₈ and GaV₄Se₈, a low temperature rhombohedral distortion allows for polar and magnetic order. In GaNb₄Se₈ and GaTa₄Se₈, antiferromagnetic interactions lead to geometric frustration and a quenched magnetization associated with a tetragonal distortion. We present results of local structural characterization on lacunar spinels in connection to their magnetic properties.   

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There has been considerable interest in the magnetic skyrmions, their underlying physics, and the materials in which they are stabilised, due to their potential uses in spintronic devices. Magnetic skyrmions are topological magnetic spin structures that were originally identified in materials belonging to the B20 class but have recently been found in other non-centrosymmetric materials [1]. Both GaV₄S₈ and GaV₄Se₈ have been shown to host Néel type skyrmions [2]. These two materials are structurally similar, forming in a low temperature R3m non-centrosymmetric space group, but exhibit very different magnetic phase diagrams at low temperatures. We have undertaken investigations into the GaV₄S_8-ySe_y family of materials in order to identify the correlations between structure and magnetism across this series with a focus on magnetic skyrmions. We present detailed structural and magnetic property investigations using X-ray and neutron diffraction techniques, and both ac and dc magnetic susceptibility measurements across the entire series of materials [3,4].

[1] S. Mühlbauer, et al., Science 323, 915 (2009).
[2] I. Kézsmárki et al., Nat. Mater. 14, 4402 (2015).
[3] A. Štefančič et al., Sci. Rep. 10, 9813 (2020).
[4] S.J.R. Holt et al., arXiv:2010.13450 (2020).   

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We have investigated the magnetic and charge transport properties of single crystals of Nowotney Chimney Ladder compound Cr11Ge19 and mapped out a comprehensive phase diagram reflecting the complicated interplay between the Dzyaloshinskii-Moriya (DM) interaction, the dipolar interaction, and the magnetic anisotropy. We have identified a set of interesting magnetic phases and attributed a finite topological Hall effect to the recently discovered bi-skyrmion phase. These data also suggest the existence of an anti-skyrmion state at finite fields for temperatures just below the magnetic ordering temperature, Tc, as indicated by a distinct change in sign of the topological Hall effect. Above Tc, we discovered a region of enhanced magnetic response corresponding to a disordered phase likely existing near the ferromagnetic critical point under small magnetic fields. Strong spin chirality fluctuations are demonstrated by the large value of the topological Hall resistivity persisting up to 1 T which is most likely due to the existence of the DM interaction.   

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When neutron scattering is used to probe magnetic excitations in solids, information about electrons' density distribution and interactions is encoded in the energy spectrum, as well as in the reciprocal-space structure of the scattering cross-section. In the local-moment limit that best describes magnetic insulators, the latter quantity translates into the atomic magnetic form factor and the dynamic structure factor of spin waves, which may vary significantly across Brillouin zones (BZs). In the itinerant limit that best describes magnetic metals, however, magnetic excitations consist of both spin waves and Stoner excitations; while the latter directly arise from electronic bands, it remains hitherto unknown how to relate their neutron-scattering cross-section, in particular the variations across BZs, to the electronic band structure and the associated wave functions. Here we present a systematic inelastic neutron scattering study of the prototypical itinerant magnet MnSi, along with first-principles and model-calculational analyses. Our result sheds new light on the local-itinerant dichotomy of magnetism in crystalline solids, by highlighting the notion of magnetic molecular orbitals for describing MnSi.   

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(ND₄)₂[FeCl₅(D₂O)] is a molecular magnet exhibiting direct coupling between magnetism and electric polarization. Here we report a comprehensive inelastic neutron scattering study of the magnetic excitations in the zero-field incommensurate cycloidal phase and the high-field quasi-collinear phase. Accompanying with the field-induced magnetic phase transition, the spontaneous electric polarization direction changes from mostly aligned with the crystallographic a-axis to the c-axis, and the underlying multiferroic mechanism is believed to switch from the inverse Dzyalloshinskii-Moriya model to the p-d hybridization model. Our detailed data analysis using linear spin wave theory reveals that the spin dynamics of both multiferroic phases can be well-described by a Heisenberg Hamiltonian with an easy-plane anisotropy. However, the optimal model parameters show small but clear difference between the two phases, suggesting that delicate reorganizations of electronic degrees of freedom occurs in (ND₄)₂[FeCl₅(D₂O)] in response to external magnetic fields.   

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It was recently reported that the double tungstate LiFe(WO₄)₂ exhibits a multiferroic phase for which the driving mechanism is based on inverse Dzyaloshionskii-Moriya interaction [1]. LiFe(WO₄)₂ is thus beside MnWO₄ only the second multiferroic material in the family of tungstates. Moreover, this system possesses in contrast to related MnWO₄ a simple phase diagram without commensurate phases that interfere with the multiferroic domain dynamics. So far nuclear and magnetic structure investigations were done only on polycrystalline samples. Here we report on our single crystal studies on LiFe(WO₄)₂ and the respective nuclear and magnetic structure determination. Both magnetic phases in LiFe(WO₄)₂ were investigated by neutron diffraction experiments, which revealed a spin-density wave as an intermediate phase and a low-temperature chiral magnetic structure inducing the multiferroic behavior. Furthermore, neutron polarization analysis shows that an unbalanced multiferroic domain distribution develops even in the absence of external fields.
[1] Liu et al. Phys. Rev. B 95, 195134 (2017)   

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Magnetic non-collinear spin textures that have chiral structures are of great current interest. The same type of Dzyaloshinskii-Moriya (DMI) vector exchange interactions that stabilize chiral Néel domain walls in magnetic multilayers^1-3 results in the formation of topological spin textures in bulk compounds. There has been much work on skyrmions but magnetic antiskyrmions were only recently identified, first in Mn_1.4Pt_0.9Pd_0.1Sn⁴, and, subsequently in Mn₂PtSn and Mn₂RhSn, all of which are tetragonal inverse Heusler compounds. The crystalline symmetry of these materials forces the DMI interaction to be more complex than, for example, that which stabilizes skyrmions in the cubic B20 compounds. This also ensures that anti-skyrmions are robust to temperature and magnetic field⁵. Furthermore, this also means that long range magneto-dipole interactions play a significant role in these Heuser compounds. In particular, they strongly influence the diameter of anti-skyrmions that can be varied from nanometers to microns as the thickness of lamellae of the same host material in which they are formed is varied⁶. The magnetic dipole-dipole interactions also allow for the formation of metastable “elliptical skyrmions” in this same material⁷ that have Bloch-like boundaries. Finally, we discuss our recent discovery of novel Néel like skyrmions that exist almost to room temperature in a metallic compound PtMnGa, and that are also tunable in size⁸. The size tunability of these chiral spin textures makes them especially interesting for spintronic applications.

1 Yang, S.-H. et al. Nat. Nano. 10, 221-226, (2015).
2 Yang, S.-H. et al.S. Nat. Phys. 15, 543–548, (2019).
3 Bläsing, R. et al. Nat. Commun. 9, 4984, (2018).
4 Nayak, A. K. et al. Nature 548, 561-566, (2017).
5 Saha, R. et al. Nat. Commun. 10, 5305, (2019).
6 Ma, T. et al. Adv. Mater., 2002043, (2020).
7 Jena, J. et al. Nat. Commun. 11, 1115, (2020).
8 Srivastava, A. K. et al. Adv. Mater. 32, 1904327, (2020).   

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We report a comprehensive neutron scattering study of low energy magnetic excitations in the breathing pyrochlore helimagnetic Cu2OSeO3. Fully documenting the four lowest energy magnetic modes that leave the ferrimagnetic configuration of the “strong tetrahedra” intact (|\hbar ω|<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 complex anisotropic intercluster interactions is obtained.

This work has been published (PhysRevB.101.144411).   

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A skyrmion is a topological stable particle-like object comparable to spin vortex at the nanometre scale. It consists of an about 50 nm large spin rotation which order in a 2 dimensional, typically hexagonal superstructure perpendicular to an applied external magnetic field. Its dynamics has links to flux line vortices as in high temperature superconductors. Cu₂OSeO₃ is a unique case of a multiferroic materials where the skyrmion dynamics could be controlled through the application of an external electric field. The direct control of the skyrmion dynamics through a non-dissipative method would offer technological benefits. Important for technological applications would be a stability range of the skyrmion phase up to room temperature. While room temperature skyrmion materials exist, Cu₂OSeO₃ orders magnetically below 60 K. Our combined small angle neutron scattering and SQUID magnetization measurements did provide direct evidence that the stability range of the skyrmion phase can be extended in Te-doped Cu₂OSeO₃. Furthermore, new aspects about the scaling behavior of the skyrmion and helical length scales are discussed.   

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Electric control of magnetism and magnetic control of ferroelectricity can improve
energy efficiency of magnetic memory and data processing devices. The
magnetoelectric switching requires more than just a coupling between spin and
charge degrees of freedom and is hard to achieve. We show that the application
and subsequent removal of magnetic field along a particular “magic” angle reverses
the electric polarization of the multiferroic GdMn₂O₅, induced by an
antiferromagnetic spin ordering. The polarization reversal appears together with an
unusual 4-state hysteresis cycle, in which one half of all spins undergoes a
unidirectional full-circle rotation in increments of ∼90 degrees.
GdMn₂O₅ thus resembles a microscopy magnetic analogue to a crankshaft,
converting the back and forth variations of the magnetic field into a circular spin
motion. The resulting magnetoelectric switching is independent of the sign of the
magnetic field and does not require magnetoelectric cooling. It is found that the 4-
state hysteresis regime emerges as a topologically distinct region (around the
magic angle) between two more regular 2-state hysteresis regions at high or low
field angles.   

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Chiral magnets have attracted considrable attention in recent years, since the Dzyaloshinskii Moriya (DM) interactions allowed by the lattice chirality could induce various chiral magnetic structures. However, chiral magnets do not necessarily exhibit chiral magnetic structures. For example, the chiral magnet Ni₃TeO₆ exhibits an achiral collinear antiferromagnetic structure below 52 K. Exploring the new methods that could turn the collinear magnetic structure into chiral magnetic structure in chiral magnets is crucial for studying the exotic chiral physics in this family of materials. We synthesized high quality single crystals and powder samples of Mn or Co-substituted Ni₃TeO₆. In this talk, we will discuss our recent neutron scattering results of Mn or Co-substituted Ni₃TeO₆. Our resutls show that an incommensurate helical magnetic structure with spins aligned in the ab plane is induced by Mn or Co substitution. The results obtained from our polarized neutron diffraction experiments show clear magnetic chirality in the magnetic structure. Our results suggest that chemical substitution is an efficient method for creating chiral magnetic structures in chiral magnets.