Nonlinear Electrical Spin Conversion in a Biased Ferromagnetic Tunnel Contact

TL;DR

This paper reveals that spin detection in biased ferromagnetic tunnel contacts is strongly nonlinear, significantly affecting spin-detection efficiency and providing a new understanding of spin signals in spintronic devices.

Contribution

It demonstrates the nonlinear nature of spin detection under bias in ferromagnetic tunnel contacts and explains its impact on spin signal measurements.

Findings

Spin detection efficiency can exceed 100% under bias.

Nonlinearity arises from energy-dependent tunnel transmission.

The model explains previously puzzling spin signals.

Abstract

The conversion of spin information into electrical signals is indispensable for spintronic technologies. Spin-to-charge conversion in ferromagnetic tunnel contacts is well-described using linear (spin-)transport equations, provided that there is no applied bias, as in nonlocal spin detection. It is shown here that in a biased ferromagnetic tunnel contact, spin detection is strongly nonlinear. As a result, the spin-detection efficiency is not equal to the tunnel spin polarization. In silicon-based 4-terminal spin-transport devices, even a small bias (tens of mV) across the Fe/MgO detector contact enhances the spin-detection efficiency to values up to 140 \% (spin extraction bias) or, for spin injection bias, reduces it to almost zero, while, parenthetically, the charge current remains highly spin polarized. Calculations reveal that the nonlinearity originates from the energy dispersion of the tunnel transmission and the resulting nonuniform energy distribution of the tunnel current, offering a route to engineer spin conversion. Taking nonlinear spin detection into account is also shown to explain a multitude of peculiar and puzzling spin signals in structures with a biased detector, including two- and three-terminal devices, and provides a unified, consistent and quantitative description of spin signals in devices with a biased and unbiased detector.