F. Y. Fradin

Blowing magnetic skyrmion bubbles

Skyrmions emerge in trilayers Skyrmions are tiny whirlpools of magnetic spin with potential to act as carriers of information in future devices. Skyrmions have been observed in multiple materials but usually at impractically low temperatures. Jiang et al. used a constriction in a trilayer system to create skyrmions at room temperature (see the Perspective by von Bergmann). The authors pushed elongated magnetic domains through the constriction using an in-plane current, causing individual skyrmion bubbles to form. Science, this issue p. 283; see also p. 234 In-plane current is used to push stripe-shaped magnetic domains through a constriction, creating skyrmions on the other side. [Also see Perspective by von Bergmann] The formation of soap bubbles from thin films is accompanied by topological transitions. Here we show how a magnetic topological structure, a skyrmion bubble, can be generated in a solid-state system in a similar manner. Using an inhomogeneous in-plane current in a system with broken inversion symmetry, we experimentally “blow” magnetic skyrmion bubbles from a geometrical constriction. The presence of a spatially divergent spin-orbit torque gives rise to instabilities of the magnetic domain structures that are reminiscent of Rayleigh-Plateau instabilities in fluid flows. We determine a phase diagram for skyrmion formation and reveal the efficient manipulation of these dynamically created skyrmions, including depinning and motion. The demonstrated current-driven transformation from stripe domains to magnetic skyrmion bubbles could lead to progress in skyrmion-based spintronics.

Quantifying Spin Hall Angles from Spin Pumping: Experiments and Theory

Spin Hall effects intermix spin and charge currents even in nonmagnetic materials and, therefore, ultimately may allow the use of spin transport without the need for ferromagnets. We show how spin Hall effects can be quantified by integrating Ni{80}Fe{20}|normal metal (N) bilayers into a coplanar waveguide. A dc spin current in N can be generated by spin pumping in a controllable way by ferromagnetic resonance. The transverse dc voltage detected along the Ni{80}Fe{20}|N has contributions from both the anisotropic magnetoresistance and the spin Hall effect, which can be distinguished by their symmetries. We developed a theory that accounts for both. In this way, we determine the spin Hall angle quantitatively for Pt, Au, and Mo. This approach can readily be adapted to any conducting material with even very small spin Hall angles.

Detection and quantification of inverse spin Hall effect from spin pumping in permalloy/normal metal bilayers

Spin pumping is a mechanism that generates spin currents from ferromagnetic resonance over macroscopic interfacial areas, thereby enabling sensitive detection of the inverse spin Hall effect that transforms spin into charge currents in nonmagnetic conductors. Here we study the spin-pumping-induced voltages due to the inverse spin Hall effect in permalloy/normal metal bilayers integrated into coplanar waveguides for different normal metals and as a function of angle of the applied magnetic field direction, as well as microwave frequency and power. We find good agreement between experimental data and a theoretical model that includes contributions from anisotropic magnetoresistance and inverse spin Hall effect. The analysis provides consistent results over a wide range of experimental conditions as long as the precise magnetization trajectory is taken into account. The spin Hall angles for Pt, Pd, Au, and Mo were determined with high precision to be 0.013±0.002, 0.0064±0.001, 0.0035±0.0003, and ?0.0005±0.0001, respectively.

Ferromagnetism of ZrZn2

The magnetic properties of ZrZn2 and related alloys and compounds are discussed. In particular, we present a study of the pseudo‐ZrZn2 alloy system ZrCu2−xAlx, where 0.76≤x≤1.5. The results of specific heat, magnetic susceptibility, and nmr measurements are presented to show that the Fermi level of ZrCu1.1Al0.9 lies at the top of a sharp narrow d‐band peak that is about 0.1 eV in half‐width and has a maximum value of 2.7 states (eV‐Zr atom spin)−1. We also show that ZrZn2 has a sharp narrow peak in the d‐band density of states at the Fermi level. This peak has a half‐width of about 0.05 eV and has a maximum value of 5.4 states (eV‐Zr atom spin)−1. The ferromagnetism of ZrZn2 is understandable in terms of this feature of the band structure. The occurrence of a high narrow peak at the Fermi level is the most favorable condition for the existence of itinerant ferromagnetism.

Suppression of spin-pumping by a MgO tunnel-barrier

Spin-pumping generates pure spin currents in normal metals at the ferromagnet (F)/normal metal (N) interface. The efficiency of spin-pumping is given by the spin mixing conductance, which depends on N and the F/N interface. We directly study the spin-pumping through an MgO tunnel-barrier using the inverse spin Hall effect, which couples spin and charge currents and provides a direct electrical detection of spin currents in the normal metal. We find that spin-pumping is suppressed by the tunnel-barrier, which is contrary to recent studies that suggest that the spin mixing conductance can be enhanced by a tunnel-barrier inserted at the interface.

Abstract: Neutron diffraction study of HoRh4B4

Our measurements show that HoRh4B4 is ferromagnetic TC= (6.8±0.2) K with the moment parallel to the unique c tetragonal axis. (AIP)

Research Update: Spin transfer torques in permalloy on monolayer MoS2

We observe current induced spin transfer torque resonance in permalloy (Py) grown on monolayer MoS2. By passing rf current through the Py/MoS2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Therefore our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.

Magnetic interactions in ternary superconductors

Numerous ternary superconductors containing rare‐earth atoms have recently been studied in order to investigate the magnetic interactions between the conduction electrons and local magnetic moments. These investigations range from the evaluation of the strength of spin‐flip scattering of the conduction electrons off the rare‐earth moments to the problem of the coexistence of superconductivity and magnetic order. We present results of our studies on (Eu,Sn)Mo6S8, ErRh4B4, Er1−xHoxRh4B4 and Er1−xGdxRh4B4 using primarily the Mossbauer effect in 151Eu and 166Er, and nuclear magnetic resonance of 95Mo. The paramagnetic hyperfine spectra of the rare‐earth ions indicate a weak coupling of the 4f magnetic moment to the conduction electrons. The enhanced Hc2 value in (Eu,Sn)Mo6S8 is found to be related to a negative s‐band polarization at the Mo site. The magnetic moment on the Er atom in ErRh4B4 (and in related materials) in the magnetically ordered state is found to be about 8.3 μB. This is 30% larger than that ...

Electron-beam tip/sample heating device for a scanning tunneling microscopy

We present an electron-beam heating device for a scanning tunneling microscopy (STM) that can be used for heating both the STM tip and the sample to 2200K. Mounted on a linear bellows with electrical feedthroughs, the device can be readily installed into the transfer path of a vacuum load lock. We demonstrate the heating capability of the device by flash cleaning Ru(0001) and Fe(001) crystals, and W tips. The flash-cleaned W tips are coated with Fe and further used for spin-polarized imaging of the Mn∕Fe(001) system.

Vortex dynamics in patterned ferromagnetic ellipses

The dynamics of individual magnetic vortices and vortex pairs confined in lithographically defined ferromagnetic ellipses were measured using a microwave reflection technique. Resonance frequencies were detected in the subgigahertz range for Ni80Fe20 (Permalloy) ellipses ranging in size from 3 by 1.5μm to 1 by 0.5μm, 40nm in thickness. Micromagnetic simulations indicate that the single mode observed for one vortex is a translational mode and that the two eigenmodes associated with a vortex pair correspond to coupled translational modes of vortices with parallel or antiparallel core polarizations. The single-vortex resonance scales inversely with the ellipse size, in agreement with the simulations and analytical theory.