Search for: All records

The anomalous Hall, Nernst, and thermal Hall coefficients of the itinerant ferromagnet Fe3−xGeTe2 display anomalies upon cooling that are consistent with a topological transition that could induce deviations with respect to the Wiedemann–Franz (WF) law. This law has not yet been validated for the anomalous transport variables, with recent experimental studies yielding material-dependent results. Nevertheless, the anomalous Hall and thermal Hall coefficients of Fe3−xGeTe2 are found, within our experimental accuracy, to satisfy the WF law for magnetic fields μ0H applied along its c axis. Remarkably, large anomalous transport is also observed for μ0H||a axis with the field aligned along the gradient of the chemical potential generated by thermal gradients or electrical currents, a configuration that should not lead to their observation. These anomalous planar quantities are found to not scale with the component of the planar magnetization (M||), showing instead a sharp decrease beyond μ0H||= 4 T or the field required to align the magnetic moments along μ0H||. We argue that chiral spin structures associated with Bloch domain walls lead to a field-dependent spin chirality that produces a novel type of topological transport in the absence of interaction between the magnetic field and electrical or thermal currents. Locally chiral spin structures are captured by our Monte Carlo simulations incorporating small Dzyaloshinskii–Moriya and biquadratic exchange interactions. These observations reveal not only a new way to detect and expose topological excitations, but also a new configuration for heat conversion that expands the current technological horizon for thermoelectric energy applications. 

Abstract Magnetic skyrmions exhibit unique, technologically relevant pseudo‐particle behaviors which arise from their topological protection, including well‐defined, 3D dynamic modes that occur at microwave frequencies. During dynamic excitation, spin waves are ejected into the interstitial regions between skyrmions, creating the magnetic equivalent of a turbulent sea. However, since the spin waves in these systems have a well‐defined length scale, and the skyrmions are on an ordered lattice, ordered structures from spin‐wave interference can precipitate from the chaos. This work uses small‐angle neutron scattering (SANS) to capture the dynamics in hybrid skyrmions and investigate the spin‐wave structure. Performing simultaneous ferromagnetic resonance and SANS, the diffraction pattern shows a large increase in low‐angle scattering intensity, which is present only in the resonance condition. This scattering pattern is best fit using a mass fractal model, which suggests the spin waves form a long‐range fractal network. The fractal structure is constructed of fundamental units with a size that encodes the spin‐wave emissions and are constrained by the skyrmion lattice. These results offer critical insights into the nanoscale dynamics of skyrmions, identify a new dynamic spin‐wave fractal structure, and demonstrate SANS as a unique tool to probe high‐speed dynamics. 

Abstract Magnetic insulators, such as the rare‐earth iron garnets, are promising materials for energy‐efficient spintronic memory and logic devices, and their anisotropy, magnetization, and other properties can be tuned over a wide range through selection of the rare‐earth ion. Films are typically grown as epitaxial single crystals on garnet substrates, but integration of these materials with conventional electronic devices requires growth on Si. The growth, magnetic, and spin transport properties of polycrystalline films of dysprosium iron garnet (DyIG) with perpendicular magnetic anisotropy (PMA) on Si substrates and as single crystal films on garnet substrates are reported. PMA originates from magnetoelastic anisotropy and is obtained by controlling the strain state of the film through lattice mismatch or thermal expansion mismatch with the substrates. DyIG/Si exhibits large grain sizes and bulk‐like magnetization and compensation temperature. Polarized neutron reflectometry demonstrates a small interfacial nonmagnetic region near the substrate. Spin Hall magnetoresistance measurements conducted on a Pt/DyIG/Si heterostructure demonstrate a large interfacial spin mixing conductance between the Pt and DyIG comparable to other garnet/Pt heterostructures.