Spin driven emergent antiferromagnetism and metal insulator transition in nanoscale p-Si

TL;DR

This paper demonstrates that spin injection and spin-Hall effects in nanoscale p-Si can induce emergent antiferromagnetism and a metal-insulator transition, revealing new spin-driven phenomena in simple semiconductors.

Contribution

It provides experimental evidence that non-equilibrium spin accumulation can cause phase transitions in non-ferromagnetic semiconductors without relying on large spin-orbit coupling.

Findings

Observation of spin-mediated antiferromagnetism in p-Si

Detection of metal-insulator transition via third harmonic voltage

Identification of spin-Hall effect as the driving mechanism

Abstract

The entanglement of the charge, spin and orbital degrees of freedom can give rise to emergent behavior especially in thin films, surfaces and interfaces. Often, materials that exhibit those properties require large spin orbit coupling. We hypothesize that the emergent behavior can also occur due to spin, electron and phonon interactions in widely studied simple materials such as Si. That is, large intrinsic spin-orbit coupling is not an essential requirement for emergent behavior. The central hypothesis is that when one of the specimen dimensions is of the same order (or smaller) as the spin diffusion length, then non-equilibrium spin accumulation due to spin injection or spin-Hall effect (SHE) will lead to emergent phase transformations in the non-ferromagnetic semiconductors. In this experimental work, we report spin mediated emergent antiferromagnetism and metal insulator transition in a Pd (1 nm)/Ni81Fe19 (25 nm)/MgO (1 nm)/p-Si (~400 nm) thin film specimen. The spin-Hall effect in p-Si, observed through Rashba spin-orbit coupling mediated spin-Hall magnetoresistance behavior, is proposed to cause the spin accumulation and resulting emergent behavior. The phase transition is discovered from the diverging behavior in longitudinal third harmonic voltage, which is related to the thermal conductivity and heat capacity.