Recently, the determination of the topological Hall effect has caused some controversy in the field of spintronics due to the validity of subtracting a magnetization-scaled anomalous Hall component from an experimentally measured Hall resistance. Often the underlying magnetic texture and topology are determined ex-situ of the Hall measurement which can lead to experimental error. Here we report the simultaneous determination of the magnetic texture and Hall effect in a ferrimagnetic FeGd thin film using resonant soft X-ray scattering with in-situ magneto-electrical transport. We find the largest departure of the Hall component at the 6-fold symmetric scattering of the dipole skyrmion state but also an additional smaller feature of opposite sign at higher fields where we observe diffuse scattering just before saturation. We attribute this additional feature to a skew scattering term arising from isolated skyrmions close to saturation.
Tuning the Dzyaloshinskii-Moriya interaction (DMI) using electric (E)-fields in magnetic devices has opened up new perspectives for controlling the stabilization of chiral spin structures. Recent efforts have used voltage-induced charge redistribution at magnetic/oxides interfaces to modulate the DMI. This approach is attractive for active devices but tends to be volatile, making it energy-demanding. Here we demonstrate nonvolatile E-field manipulation of the DMI by ionic-liquid gating of Pt/Co/HfO2 ultra thin films. The E-field effect on the DMI is linked to the migration of oxygen species from the HfO2 layer into the Co and Pt layers and subsequent anchoring. This effect permanently changes the properties of the material, showing that E-fields can be used not only for local gating in devices but also as a material design tool for post growth tuning of the DMI.
Generating and controlling spin currents at magnetic/nonmagnetic layer interfaces using ultrashort laser pulses has triggered the development of new high- operational frequency spintronic devices. Recent studies showed that laser-driven spin currents and opto-magnetic torques acting on spins are most effective when an interface is created between ferromagnetic Co and nonmagnetic Pt thin films. Our study focuses on the role of the Co-Pt interface on laser-induced optical torques in the strongly spin-orbit coupled Co/Pt model system. We varied the average roughness at the interface, in the range of 0.1-1.0 nm, by tuning the deposition pressure conditions during fabrication. With the aid of time-resolved THz-emission spectroscopy we detected both the laser-induced helicity-independent(HI) and helicity-dependent(HD) THz-emission due to spin-Hall and spin-orbit torque effects, respectively. We reveal a dramatic change in the detected THz-signals when the interface roughness is varied. For example, the HD-THz emission is observed only when the roughness is 0.3 nm or above. To study the role of intermixing a CoPt spacer layer, with varying compositions, is introduced at the interface. However, the detected THz-emission signals rule out the intermixing effects in determining the helicity-dependence. Moreover, static spin-hall conductivity measurements provide with new insights in understanding the role of spin-orbit coupling, at the Co/Pt interfaces, in laser-induced optical torques on net magnetization.
This research is funded by DOE:DE-SC001823
We propose modification to gain spectra of semiconductor quantum heterostructures by incorporation of nanostructured metal, paving the way for tailor made “meta-gain” media. We show that the wavelength dependence of the principal direction of energy propagation in media with hyperbolic dispersion leads to blue-shifting of peak photoluminescence (PL), and thereby optical gain, relative to emission from the bare semiconductor. Additionally we show that emission spectra from metal-semiconductor hyperbolic metasurfaces depends strongly upon the polarization of an external optical pump. The simultaneous co-optimization of pump properties and optical and electronic densities of states provides a platform for not only compensating losses in metallic metamaterials, but also designing emission spectra beyond that provided by the constituent quantum heterostructures.
demonstrate that the DW velocity can be significantly increased in antiferomagnetically coupled nanowires. The DW velocity increase is related to the exchange fields and reduction or elimination of the magnetostatic effects, which lead to reduction or elimination of the Walker breakdown. In addition, the reduction of the magnetostatic effects results in the reduction of the effects due to the pinning sites and disorder present in most nanomagnetic systems. The reduction of the pinning site and disorder effects further leads to a steadier DW motion. The study includes an analytical model for explaining how and why the Walker breakdown is overcome as well as numerical study supporting the analytical model and providing insights into the effects of the material and structural disorder. The numerical study is based on micromagnetic simulations solving the Landau-Lifshitz-Gilbert equation with continuous spin transfer torque components. The parameter space considered in the models and simulations includes the material properties, various types of disorder, and the exchange coupling in coupled systems. In addition, we discuss various aspects associated with modeling the DW motion in thin nanowires with disorder, including simulation speed, numerical stability, and the simulation model creation.
Using established nanofabrication techniques, we realize deeply subwavelength multilayer metal-dielectric nanostructures composed of silver and indium gallium arsenide phosphide (InGaAsP). In contrast to most, if not all, subwavelength multilayer metal-dielectric systems to date, the Bloch vector of the fabricated structure is parallel to the plane of the substrate, making it suitable for waveguide integration. InGaAsP multiple quantum wells (MQWs) are epitaxially grown on InP normal to the Bloch vector of the resulting multilayer. The associated carrier population of the MQWs allows for active control of the behavior of the nanostructure via external optical pumping. Individual layer thicknesses of 30nm are repeatedly achieved via electron-beam lithography, reactive ion etching of InGaAsP, and sputter deposition of silver. Resulting 60nm periods of the one-dimensional periodic structure are 25 times smaller than telecommunication wavelengths in vacuum. The realized multilayer nanostructures hold promise as a platform for active and tunable hyperbolic metamaterials at telecommunication frequencies.
We have performed extensive model calculations in order to understand the effect that roughness
has on the X-ray diffraction from multilayers. We have developed models to calculate the low and
the high angle X-ray diffraction spectra, including kinematic and dynamical simulations. These
model calculations were used to understand a variety of systems including crystalline/crystalline,
crystalline/amorphous and amorphous/amorphous multilayers. These model calculations were
compared with the diffraction spectra of multilayered systems prepared using sputtering and
Molecular Beam Epitaxy (MBE) techniques.
Using the experience acquired from these model calculations we have recently developed a
comprehensive nonlinear optimization program to refine the structure of multilayers from X-ray
diffraction spectra. Kinematic formulation is used to refine the high angle data and dynamical
(Fresnel) formalism was used to fit the low angle spectra. A comparison of the results obtained
from the structural refinements with EXAFS and artificially prepared rough multilayers indicates
that this type of approach give a reliable and speedy determination of the roughness, interdiffusion
and lattice parameter variations in multilayers. This work has been described in a number of papers
in the last few years.
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