Research

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Introduction

The classification of spin I^{S} and charge I currents in metal and semiconductor spintronic systems corresponding to spatial propagation of spin-\uparrow and spin-\downarrow electronic wave packets carrying spin-resolved currents I^{\uparrow } and I^{\downarrow }: (a) conventional charge current I=I^{\uparrow }+I^{\downarrow }\neq 0 is spin-unpolarized I^{S}={\frac  {\hbar }{2e}}(I^{\uparrow }-I^{\downarrow })\equiv 0; (b) spin-polarized charge current I\neq 0 is accompanied also by spin current I^{S}\neq 0 ; and (c) pure spin current I^{S}={\frac  {\hbar }{2e}}(I^{\uparrow }-I^{\downarrow })\neq 0 arising when spin-\uparrow electrons move in one direction, while an equal number of spin-\downarrow electrons move in the opposite direction, so that total charge current is I\equiv 0.

Over the past two decades, spintronics has emerged as one of the most vigorously pursued areas of condensed matter physics, materials science, and nanotechnology. The rise of spintronics was ignited by basic research on magnetic heterostructures in late 1980s [2], as recognized by the 2007 Nobel Prize in Physics being awarded to A. Fert and P. Grünberg for the discovery of giant magnetoresistance (GMR). The GMR phenomenon also exemplifies one of the fastest transfers of basic physics research into applications where in less than a decade since its discovery it has revolutionized information storage technologies by enabling 100 times increase in hard disk storage capacity.

The current frontiers of spintronics have been reshaped through several intertwined lines of research:

  • ferromagnetic metal devices where the main theme is manipulation of magnetization via electric currents,
  • ferromagnetic semiconductors which, unlike metal ferromagnets, offer additional possibilities to manipulate their magnetic ordering (such as Curie temperature, coercive fields, and magnetic dopants), but are still below optimal operating temperature,
  • paramagnetic semiconductor spintronics focused on all-electrical manipulation of spins via spin-orbit (SO) couplings in solids,
  • spins in semiconductors as building blocks of futuristic solid-state-based quantum computers.

In contrast to non-coherent spin phenomena (such as GMR) of the first generation spintronics, the major themes of the second-generation spintronics are aimed at exploiting quantum-coherent spin states where spin component persists in the direction transverse to external or effective internal magnetic fields. Recent experiments exploring such phenomena include:

  • spin-transfer torque (STT) where spin current of large enough density injected into a ferromagnetic (F) layer either switches its magnetization or generates a dynamical situation with steady-state precessing magnetization,
  • spin pumping as the "inverse" effect of STT where precessing magnetization of a ferromagnetic layer emits pure spin currents into adjacent normal metal layers in the absence of any bias voltage [7-9] (spin pumping and STT can be related by an Onsager reciprocity relation)
  • transport of coherent spins over large distances [10]
  • the direct and inverse spin-Hall effect (SHE) in bulk and low-dimensional semiconductors and metals,
  • generation and detection of pure spin currents which do not transport any net charge and whose harnessing is expected to offer both new functionality and greatly reduced power dissipation.

The research themes of CSB listed below fall in the domain of the second-generation spintronics. The unique feature of the CSB research is synergy between different experimental groups in the Center, as well as with theoretical efforts, which allows us to explore the interplay of pure spin current transport, high frequency spin dynamics, and current/voltage and magnetization noise in a variety of ferromagnet|normal-metal and ferromagnet|insulator nanostructures.

Pure spin current generation and detection via spin pumping, non-local spin valves, and spin Hall effect

Ji nlsv.jpg

Spin-transfer torque driven by pure spin currents

Non-linear effects in high frequency spin dynamics

Electrical noise probing of pure spin currents and magnetization dynamics

Application of magnetic tunnel junctions for biosensors

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