Wee Tee Soh

Magnetization dynamics in permalloy films with stripe domains

Through a systematic investigation of the field-dependent dynamic magnetization of a series of NiFe films with and without stripe domains in conjunction with the static magnetization process, we demonstrate that the experimental rotatable anisotropy field is not a fixed value but strongly varied with the external in-plane magnetic field, being qualitatively associated with the emergence of stripe domains. Moreover, the frequency linewidth spectra of the films with stripe domains show an abnormal behavior with three distinct regimes which are strongly correlated with both the static magnetization process and the competition between external magnetic field and dynamic anisotropy field. The results are discussed in terms of the effect of inhomogeneous magnetization associated with the formation of stripe domains and the field-dependent dynamic anisotropy that cause the broadening of frequency linewidth.

An angular analysis to separate spin pumping-induced inverse spin Hall effect from spin rectification in a Py/Pt bilayer

Through the inverse spin Hall effect, spin-pumped currents from a Py(Ni80Fe20) layer are converted to direct current (dc) voltages in an adjacent platinum (Pt) layer, which adds to dc voltages generated by the spin rectification effect originating from anisotropic magnetoresistance, ordinary and anomalous Hall effect in the permalloy (Py) layer. Based on a shorted microstrip fixture, we perform angular measurements of rectified dc voltages in a Py(Ni80Fe20)/Pt bilayer to simultaneously separate spin pumping-induced inverse spin Hall voltages from spin-rectified voltages. A systematic approach to accurately determine the spin-Hall angle of Pt is presented.

Note: electrical detection and quantification of Spin Rectification Effect enabled by shorted microstrip transmission line technique.

We describe a shorted microstrip method for the sensitive quantification of Spin Rectification Effect (SRE). SRE for a Permalloy (Ni80Fe20) thin film strip sputtered onto SiO2 substrate is demonstrated. Our method obviates the need for simultaneous lithographic patterning of the sample and transmission line, therefore greatly simplifying the SRE measurement process. Such a shorted microstrip method can allow different contributions to SRE (anisotropic magnetoresistance, Hall effect, and anomalous Hall effect) to be simultaneously determined. Furthermore, SRE signals from unpatterned 50 nm thick Permalloy films of area dimensions 5 mm × 10 mm can even be detected.

Separating inverse spin Hall voltage and spin rectification voltage by inverting spin injection direction

We develop a method for universally resolving the important issue of separating the inverse spin Hall effect (ISHE) from spin rectification effect (SRE) signal. This method is based on the consideration that the two effects depend on the spin injection direction: The ISHE is an odd function of the spin injection direction while the SRE is independent on it. Thus, inversion of the spin injection direction changes the ISHE voltage signal, while SRE voltage remains. It applies generally to analyzing the different voltage contributions without fitting them to special line shapes. This fast and simple method can be used in a wide frequency range, and has the flexibility of sample preparation.

Dependence of dynamic magnetization and magneto-transport properties of FeAlSi films with oblique sputtering studied via spin rectification effect

FeAlSi (Sendust) is known to possess excellent soft magnetic properties comparable to traditional soft magnetic alloys such as NiFe (Permalloy), while having a relatively higher resistance for lower eddy current losses. However, their dynamic magnetic and magneto-transport properties are not well-studied. Via the spin rectification effect, we electrically characterize a series of obliquely sputtered FeAlSi films at ferromagnetic resonance. The variations of the anisotropy fields and damping with oblique angle are extracted and discussed. In particular, two-magnon scattering is found to dominate the damping behavior at high oblique angles. An analysis of the results shows large anomalous Hall effect and anisotropic magneto-resistance across all samples, which decreases sharply with increasing oblique incidence.

Investigation of magnetic properties and microwave characteristics of obliquely sputtered NiFe/MnIr bilayers

A comprehensive investigation of the magnetic properties and high frequency characteristics of NiFe/MnIr bilayers with regards to oblique deposition angle was conducted in conjunction with an analysis based on the Landau-Lifshitz-Gilbert equation. It was found that exchange bias can be significantly enhanced with the variation of oblique deposition angle, which is interpreted in terms of the formation of inclined columnar structure of the films often observed in samples fabricated by this oblique deposition technique. Moreover, the uniaxial magnetic anisotropy field and the resonance frequency are increased with the increasing of oblique deposition angle. The variations of effective Gilbert damping factor and the frequency linewidth with oblique deposition angle are also presented and discussed in details.

Universal dynamical decoupling: Two-qubit states and beyond

Uhrigs dynamical decoupling pulse sequence has emerged as a universal and highly promising approach to decoherence suppression. So far, both the theoretical and experimental studies have examined single-qubit decoherence only. This work extends Uhrigs universal dynamical decoupling from one-qubit to two-qubit systems and even to general multilevel quantum systems. In particular, we show that by designing appropriate control Hamiltonians for a two-qubit or a multilevel system, Uhrigs pulse sequence can also preserve a generalized quantum coherence measure to the order of 1+O(T{sup N+1}) with only N pulses. Our results lead to a very useful scheme for efficiently locking two-qubit entangled states. Future important applications of Uhrigs pulse sequence in preserving the quantum coherence of multilevel quantum systems can also be anticipated.

Spin galvanic effect at the conducting SrTiO3 surfaces

The (001) surface of SrTiO3 were transformed from insulating to conducting after Ar+ irradiation, producing a quasi two-dimensional electron gas (2DEG). This conducting surface layer can introduce Rashba spin orbital coupling due to the broken inversion symmetry normal to the plane. The spin splitting of such a surface has recently been demonstrated by magneto-resistance and angular resolved photoemission spectra measurements. Here we present experiments evidencing a large spin-charge conversion at the surface. We use spin pumping to inject a spin current from NiFe film into the surface, and measure the resulting charge current. The results indicate that the Rashba effect at the surface can be used for efficient charge-spin conversion, and the large efficiency is due to the multi-d-orbitals and surface corrugation. It holds great promise in oxide spintronics.

Inverse spin Hall effect of antiferromagnetic MnIr in exchange biased NiFe/MnIr films

Antiferromagnetic Mn3Ir, which is widely employed in exchange-biased applications, has attracted much attention recently due to its predicted and subsequently observed large spin Hall effect, therefore increasing its potential for spintronic devices in place of conventional paramagnetic 5d spin Hall metals. (Pt, Ta and W) Via the electrical detection of ferromagnetic resonance, we study a series of exchange biased NiFe/MnIr films for various MnIr thicknesses. In these systems, spin-pumped spin currents from NiFe are converted into dc voltages within MnIr via the inverse spin Hall effect (ISHE), which mixes with spin rectification voltages generated from NiFe. Through angular measurements, we separate these different voltage contributions to qualitatively detect non-zero ISHE in MnIr, which coexists with a non-zero unidirectional anisotropy. We find significant extrinsic damping contributions which prevent the accurate quantification of spin pumping-induced ISHE in MnIr films. The results show that spin currents may propagate and dissipate in MnIr films through ISHE in the presence of exchange bias.

Multiple resonance peaks of FeCo thin films with NiFe underlayer

Under zero external magnetic fields, single-layer FeCo thin films exhibit no ferromagnetic resonance (FMR) peaks, while multiple FMR peaks were obtained by growing FeCo thin films on NiFe underlayers with various thicknesses up to 50 nm. Comprehensive investigations of the dynamic magnetic properties and origin of the peaks were conducted through measurements of microwave permeability via a shorted microstrip perturbation technique. Through fitted values of saturation magnetization Ms, uniaxial anisotropy HKsta, and rotatable anisotropy HKrot extracted from the FMR experiments, it was found that two of the three resonance peaks originate from FeCo, and the third from NiFe. The two magnetic phases of FeCo grains are found to have different values of HKrot and explained by the exchange interaction between FeCo and NiFe grains.