Shuhan Chen

Reversal and excitations of a nanoscale magnetic domain by sustained pure spin currents

Spin-transfer effects induced by pure spin currents are explored in nonlocal spin valves by using sustained injection currents. Compared to pulsed injection currents used in previous experiments, this approach provides persistent spin-transfer torques and preserves the history of the reversal process. A nanoscale domain in a magnetic wire can be switched reversibly by the sustained pure spin currents. In addition, dips in nonlocal spin signal curves are observed at high magnetic fields for only one polarity of the injection currents. This indicates stable-state magnetization precession around the external field driven by the sustained pure spin currents.

Efficient room temperature spin-Hall injection across an oxide barrier

Spin Hall injection is demonstrated at room temperature using Pt metal and AlOx barriers. A substantial spin accumulation, comparable to that of a magnetic spin injection, is transferred into a mesoscopic Cu wire from an adjacent Pt wire across an AlOx barrier. The Pt spin Hall angle is 0.030 ± 0.007 when assuming a Pt spin diffusion length λpt > 6 nm and 0.09 ± 0.02 when assuming λpt = 2 nm. Nearly (66 ± 6)% of the spin accumulation on the Pt surface is transferred into the Cu across the AlOx, enabling an efficient spin Hall injection scheme.

Nonlocal electrical detection of spin accumulation generated by anomalous Hall effect in mesoscopic Ni81Fe19 films

Spin accumulation generated by the anomalous Hall effects (AHE) in mesoscopic ferromagnetic Ni81Fe19 (permalloy or Py) films is detected electrically by a nonlocal method. The reciprocal phenomenon, inverse spin Hall effects (ISHE), can also be generated and detected all-electrically in the same structure. For accurate quantitative analysis, a series of nonlocal AHE/ISHE structures and supplementary structures are fabricated on each sample substrate to account for statistical variations and to accurately determine all essential physical parameters in-situ. By exploring Py thicknesses of 4 nm, 8 nm, and 12 nm, the Py spin diffusion length {\lambda}_Py is found to be much shorter than the film thicknesses. The product of {\lambda}_Py and the Py spin Hall angle {\alpha}_SH is determined to be independent of thickness and resistivity: {\alpha}_SH*{\lambda}_Py= (0.066 +/- 0.009) nm at 5 K and (0.041 +/- 0.010) nm at 295 K. These values are comparable to those obtained from mesoscopic Pt films.

Low probability of bulk spin-flip in mesoscopic Cu wires

Spin-flip probabilities from bulk defects and phonons are determined for the mesoscopic Cu channels of nonlocal spin valves (NLSVs). The NLSV spin signals are analyzed by a new approach to consistently extract the in situ values of the Cu spin diffusion length, effective spin injection (detection) polarization, and Cu resistivity. The size variations of the spin injectors (detectors) between NLSVs are treated by normalizing the spin signals to be commensurate with a standard injector (detector) size. The microstructure variations are statistically accounted for by measuring many NLSVs on the same substrate. From the Cu resistivity and spin diffusion lengths at 10 K and 295 K, the spin-flip probabilities from bulk defects and phonons are determined to be (3.9 ± 0.8) × 10−4 and (2.8 ± 1.2) × 10−4, respectively.

Spin Hall effects in mesoscopic Pt films with high resistivity

The energy efficiency of the spin Hall effects (SHE) can be enhanced if the electrical conductivity is decreased without sacrificing the spin Hall conductivity. The resistivity of Pt films can be increased to 150-300 {\mu}{\Omega}*cm by mesoscopic lateral confinement, thereby decreasing the conductivity. The SHE and inverse spin Hall effects (ISHE) in these mesoscopic Pt films are explored at 10 K by using the nonlocal spin injection/detection method. All relevant physical quantities are determined in-situ on the same substrate, and a quantitative approach is developed to characterize all processes effectively. Extensive measurements with various Pt thickness values reveal an upper limit for the Pt spin diffusion length: {\lambda}_pt<0.8 nm. The average product of {\lambda}_pt and the Pt spin Hall angle {\alpha}_H is substantial: {\alpha}_H*{\lambda}_pt=(0.142 +/- 0.040)nm for 4 nm thick Pt, though a gradual decrease is observed at larger Pt thickness. The results suggest enhanced spin Hall effects in resistive mesoscopic Pt films.

Robust spin currents in mesoscopic metallic lateral structures

Lateral spin valves, also known as nonlocal spin valves (NLSV), are mesoscopic structures where pure spin currents can be generated, transported over a distance, and detected as clear voltage signals. Due to the unique separation of the spin injector and the spin detector in laterally separated materials stacks, NLSV has been proposed as ultra-high density read head sensors to replace the standard spin valve read head sensors fabricated as single materials stacks. [1] In addition to magnetic recording applications, the lateral structures also provide a platform for novel memory and logic devices.

Absence of high surface spin-flip rate in mesoscopic silver channels

Large spin signals up to 38 mΩ are demonstrated in mesoscopic nonlocal spin valves with silver channels. Scanning electron microscope images reveal Volmer–Weber growth of the silver channels, which consist of coalesced silver islands from 30 to 80 nm in size. The temperature dependence of the spin signals is monotonic, in contrast to previously observed non-monotonic features due to high surface spin-flip rate. We hypothesize that magnetic impurities in the silver are primarily segregated at the bulk grain boundaries instead of near the side surfaces of the channel, resulting in an absence of high surface spin-flip rate.

Large spin accumulation near a resistive interface due to spin-charge coupling

We experimentally and theoretically investigate large spin signals in special nonlocal spin valves, where a vacuum break-junction is formed between the ferromagnetic spin detector and the nonmagnetic channel. The spin signals are clearly nonlocal and can be either non-inverted (meaning high nonlocal resistance for parallel states and low resistance for antiparallel states) or inverted. The magnitudes are significantly larger than those of standard metallic nonlocal devices with similar dimensions. The magnitudes and the signs can be understood by a theory of spin-charge coupling. The coupling between spin accumulation and charge accumulation across a resistive break junction leads to a large interfacial spin accumulation and thereby large spin signals. By analyzing the profiles of electrochemical potentials near the interface, we show that the sign of the spin signal depends on the values of spin-dependent conductivities, diffusion constants, and densities of states. The magnitude of the spin accumulation in the ferromagnetic spin detector can be higher than that in the nonmagnetic channel, enabling a rare amplification effect for spin accumulation.