Title: Pure spin currents in submicron metallic structures

Speaker: Yi Ji, University of Delaware

Time and place: 14:00-15:00, 15th, October, 2013, Room 111, Physics Building

Abstract:

Spintronics relies on the functionalities of spin currents. A pure spin current arises when electrons with opposite spins flow in opposite directions. Pure spin currents are less invasive and less dissipative compared to spin polarized charge currents. A pure spin current can be generated in a nonlocal spin valve (NLSV) and transported along the nonmagnetic channel of a NLSV. The interactions between the pure spin currents and a nanoscale magnetic element (a spin detector) yield spin-dependent voltage signals as well as spin-transfer switching and dynamics. In this talk, I will discuss spin transport phenomena in submicron metallic NLSV structures.

The spin diffusion lengths and interfacial spin polarizations are crucial for a high quality NLSV structures. I will show that the spin relaxation process in the nonmagnetic channel is dominated by surface slip scattering from magnetic impurities. The spin-flip rate can be reduced by oxidizing the surface impurities in air. An aluminum oxide barrier (nominally 2 nm thick) between the ferromagnet and the nonmagnetic channel provides a substantial interfacial polarization. The absorption of a pure spin current into the ferromagnet is asymmetric across the finite-size AlOx junctions. This asymmetry leads to an unconventional approach for detecting nonlocal spin accumulation. Spin-transfer effects have been achieved in NLSV structures in a broad range of temperatures from 4.5 K to 200 K. Both full magnetization reversals and evidence for magnetization dynamics have been observed.

A nanometer-sized break-junction gap can be formed between the spin detector and the nonmagnetic channel in a NLSV by electromigration. Large spin signals with both signs (non-inverted or inverted) have been detected. The spin signals are truly nonlocal and are consistent with the delicate nature of a break junction. Theoretically a spin-charge coupling effect across the resistive break-junction leads to a large chemical potential split, which provides conduction channels for pure spin currents across the interface. Therefore an interface which is resistive to a charge current can be actually conductive to a spin current. The signs and magnitudes of the spin signals can be understood by considering the profiles of the electrochemical potentials across the interface on the scale of the charge screening length.

This work was supported by US DOE grant No. DE-FG02-07ER46374.