Session X41: Chiral Magnetism and Structures II

Stabilizing spin spirals and isolated skyrmions at low magnetic field exploiting vanishing magnetic anisotropy

The stabilization of non-collinear chiral magnetic structure is usually attributed to a large Dzyaloshinskii-Moriya interaction (DMI). DMI is enhanced at surfaces and interfaces via the hybridization of the magnetic atoms with 5d elements. Here, we show that a strong DMI is not a necessary condition to obtain a chiral magnetic order in ultra-thin films. Our spin-polarized scanning tunneling microscopy study show that Co/Ru(0001) possesses a spin spiral ground state, although the DMI is weak. Under moderated magnetic field (≈150 mT) isolated skyrmion can be stabilized. We attribute the stability of this spin texture to the vanishing of the magnetic anisotropy energy.   

Correlating Induced Magnetization and the Dyzaloshinskii-Moriya Interaction in Skyrmion-host Metallic Multilayers

In metallic multilayers, skyrmions can form at interfaces between a 3d elemental ferromagnet (e.g., Co, Fe) and a 5d nonmagnetic element (e.g., Ir, Pd), where broken inversion symmetry and large spin-orbit coupling enable Dzyaloshinskii-Moriya interactions (DMI) to generate chiral magnetic structures. Skyrmions in these systems can be moved through the material by small spin currents and thus present a promising opportunity to realize new, low-energy magnetic storage technologies. Here, we examine prototypical Pt/Co/Pt and Pt/Co/Ir heterostructures to address the question of how the strength of the net DMI of a multilayer structure corresponds to the strength of the proximity-induced magnetism (PIM) that exists in the Stoner-susceptible 5d metals. Specifically, we compare DMI strength, as measured by the spin-orbit torque induced effective field under an in-plane bias magnetic field, to the magnitude of the PIM in Pt and Ir, as determined by X-ray magnetic circular dichroism. We also use resonant reflectometry at the Pt and Ir L₃ edges to extract magnetic depth profiles to understand how PIM changes as a function of Pt and Ir layer thickness.   

Point-Group Dependence of Dzyaloshinski-Moriya Micromagnetics

The micromagnetic realization of Dzyaloshinski-Moriya interactions (DMI) is investigated, with particular reference to spin spirals and skyrmions in crystals with cubic symmetry. The analysis starts with a group-theoretical analysis and explains the micromagnetism by a magnetic gyration tensor that is loosely related to the vector D of the DMI. In contrast to conventional wisdom, the DM free-energy term D M.(▽×M) is not caused by broken inversion symmetry but reflects the presence of chiral building blocks (motifs) in the crystal structure. The scalar D includes not only DMI but also magnetocrystalline and magnetostatic contributions. In micromagnetic calculations, this D should never be used for noncubic structures, such as uniaxial tetragonal, rhombohedral, and hexagonal crystals. Furthermore, D = 0 for the cubic point groups T_h, O_h, and T_d, which include half- and inverse Heusler alloys (T_d), in spite of their broken inversion symmetry and nonzero atomic-scale DMI. The theoretical findings are used to discuss skyrmions in a range of related alloys: MnNiGa and MnNiSn, which crystallize in the centrosymmetric hexagonal Ni₂In structure (point group D_6h), and Heusler-type cubic compounds MnNiIn and MnNiAl.   

Magnetic domain texture and Dzyaloshinskii-Moriya interaction in systems with perpendicular exchange bias

We sputter-deposited polycrystalline multilayers of Pt/Co/IrMn and Pt/Co/FeMn exhibiting perpendicular exchange bias (PEB). These multilayers are of interest because of the coincidence of the Dzyaloshinskii-Moriya interaction (DMI) with a vertical exchange field that could remove the need for an externally applied field to stabilise skyrmions [1]. We measured the exchange bias as a function of the layer thickness of Co, IrMn, and FeMn.

Utilising Kerr microscopy and XMCD-PEEM imaging, the domain morphology of the ferromagnetic (FM) layer (Co) was studied when crossing over the paramagnet to antiferromagnet (AFM) phase transition of the coupled AFM layers (IrMn and FeMn). The domain texture is influenced by the FM-AFM exchange coupling. A domain created by nucleation has rough edges, while a domain cooled through the transition maintains its smooth appearance.

The DMI brings about chiral magnetic order, such as Néel domain walls and skyrmions. The DMI was investigated in these systems, particularly how it is affected by the AFM spin order. The DMI was measured at different phases of the AFM layers. We quantified the DMI by Brillouin light spectroscopy [2].

[1] Fert et al. Nat. Nano. 8, 152 (2013).
[2] J-H Moon et al. PRB 88, 184404 (2013).   

Universality of Defect-Skyrmion Interaction Profiles

Magnetic skyrmions are prime candidates for future spintronic devices. However, incorporating them as information carriers hinges on their interaction with defects present in any device. We map from full ab initio[1], the energy profile of single magnetic skyrmions interacting with single-atom impurities, establishing a generic shape as function of the defect's electron filling. Depending on their chemical nature, foreign 3d and 4d transition metal adatoms or surface-implanted defects can either repel or pin skyrmions in Pd/Fe/Ir(111), which we relate to the degree of filling of bonding and anti-bonding electronic states inherent to the proximity of the non-collinear magnetic structure. Similarities with key concepts of bond theories in catalysis and surface sciences imbue the universality of the shape of the interaction profile and the potential of predicting its interaction. The resulting fundamental understanding may give guidance for the design of devices to generate and control skyrmions.
[1] Lima Fernandes I. et al., Nat. Commun. 9, 4395 (2018)   

Determining the lifetime of metastable skyrmions

The question of the lifetime of metastable skyrmions in a ferromagnetic background is of both fundamental importance and of practical significance for memory applications. We address this question for a chiral magnetic thin film with ferromagnetic exchange J, Dzyaloshinskii-Moriya interaction D, and anisotropy K in an external field B. We present results for skyrmion lifetimes based on novel numerical and variational approaches for analyzing the saddle-point in configuration-space paths that lead to skyrmion decay. Our results highlight the importance of the ratio of the film thickness L to the helical pitch length L_D=2πJ/D in understanding skyrmion decay. We show that Bloch point singularities play a crucial role in skyrmion decay even for small L/L_D. The role of anisotropy and external field in determining skyrmion size and stability will also be discussed and comparison with previous results presented.   

Quantum Collapse of a Skyrmion

We analytically and numerically investigate the quantum collapse of a small skyrmion in a thin magnetic film with Dzyaloshinskii-Moriya interaction (DMI). The stability threshold determined by the DMI, the external magnetic field, and the underlying atomic lattice has been computed. The Lagrangian describing the coupled dynamics of the skyrmion size and the chirality angle has been derived. An instanton solution of the equations of motion that corresponds to the skyrmion underbarrier contraction via quantum tunneling has been obtained. The tunneling rate has been computed and the conditions needed to observe quantum collapse of a skyrmion in a magnetic film have been elucidated [1].

[1] A. Derras-Chouk, E. M. Chudnovsky, and D. A. Garanin, Phys. Rev. B, 98, 024423 (2018)   

Interfacial Dzyaloshinskii-Moriya interaction at an epitaxially grown ferromagnetic and heavy metal interface

We study Dzyaloshinskii-Moriya interaction(DMI) in a series of epitaxially grown CoFe/Pt films using momentum-resolved Brillouin light scattering. The symmetry breaking present in the bilayer allows interfacial DMI, which is expected to depend on atomic alignment at the interface. Different CoFe alloys are studied. Co(25)Fe(75) is of particular interest because it exhibits exceptionally low magnetic damping [ref]. By controlling the chemical composition, thickness and growth conditions, one can tune the DMI parameter along with other magnetic properties of the film. We can measure these different properties using a single technique, Brillouin Light Scattering, allowing for a full characterization of the tunable parameters which could allow for engineering materials with the desired combination of properties.   

Berry Phase and Exchange Contributions in Antiskyrmion Hosting Heusler Compound Mn1.5XY

Recently, the Heusler compounds Mn_1.4PtSn and Mn_1.4Pt_0.9Pd_0.1Sn were shown to stabilize an antiskyrmion lattice above room temperature. This Heusler compound forms in a superstructure with the D_2d symmetry, which allows for an anisotropic Dzyaloshinskii-Moriya interaction (DMI) perpendicular to the tetragonal axis. We use density functional theory calculations of Mn_1.5XY to extract the relevant exchange interactions that determine the rich phase diagrams in these materials. The exchange interactions are between the large moments on the Mn atoms ~4 μ_B, which show magnetic states that are non-collinear ferrimagnetic up to the spin reorientation. The major role of the DMI and anisotropy are due to the X ion, either Rh, Pd, Ir, or Pt which also influence the exchange interactions. The Fermi level can be tuned by the Y ion, either In, Sn or Sb. We last calculate the anomalous Hall effect and topological Hall effects in these regimes, to capture the influence of the electronic structure on the Berry curvature.   

Dzyaloshinski-Moriya Interactions and Berry Curvature in Magnetic Nanoparticles

Noncentrosymmetric crystals, such as B20-ordered MnSi, have long been known to exhibit spin spirals in the bulk and skyrmions with quantized topological charge in thin films. We have theoretically and experimentally investigated the skyrmions of magnetic nanoparticles produced by cluster deposition. Our research includes CoSi, which is nonmagnetic in the bulk but magnetically ordered in nanoparticle form. The size confinement imposed by the nanostructuring suppresses the formation of spin spirals and skyrmions but leads top the formation of curling-type spin structures with nonzero Berry curvature and nonzero contribution to the topological Hall effect (THE). Concerning the nanoparticle material, there is an important distinction between centrosymmetric materials and various types of materials without inversion symmetry, especially polar and chiral crystal structures. With respect to the spin states, the former corresponds to broken chirality, whereas the latter imply a violation of chiral symmetry. It is interesting to note that moderately strong Berry curvatures and unquantized THE contributions may also be caused in nanoparticles with inversion symmetry and due to magnetostatic interactions.   

Signatures of novel spin phases in thermal conductivity of the chiral skyrmion material Cu2OSeO3

We report measurements of thermal conductivity (κ) as a function of magnetic field in single crystals of the chiral skyrmion material Cu₂OSeO₃ in the temperature range 0.5 K ≤ T ≤ 15 K. Traversing phase boundaries associated with the low-temperature skyrmion and tilted conical spin phases, recently identified in small-angle neutron scattering studies,^a,b gives rise to sharp signatures in κ(H) for H ||〈100〉that are attributed to changes in the magnon thermal conductivity. Different crystallographic orientations for the applied field and heat flow will be discussed.

^a A. Chacon et al., Nature Phys. 14, 936 (2018).

^b F. Qian et al., Sci. Adv. 3, 7323 (2018).
  

High pressure and magnetic field induced new phase in skyrmion Cu2OSeO3

In recent years, there has been an enormous experimental research effort on skyrmion materials because of their peculiar topological properties. From a technological perspective, the magnetic skyrmions offer great potentials as information carriers in future robust, high-density, and energy-efficient spintronic devices. Ever since the discovery of skyrmion states in Cu₂OSeO₃ by Y. Tokura’s group in 2012, Cu₂OSeO₃ has attracted considerable interest due to its complex magnetism, which is very sensitive to external perturbations, such as temperature, magnetic field, and pressure. Earlier high pressure studies on Cu₂OSeO₃ reveal that the ferrimagnetic transition temperature increases and the skyrmion state in magnetic H-T phase diagram could be enlarged under pressure up to 1.38 GPa. Recently, our group was able to establish the magnetic phase diagram in Cu₂OSeO₃ under pressure up to 45 GPa. Signatures of high pressure and magnetic field induced new phase in Cu₂OSeO₃ were detected. Structural analysis under high pressure in Cu₂OSeO₃ is ongoing to gain greater insight into the complex magnetism. Details of the experimental results will be shared and discussed in the presentation.