Stoner-type theory of Magnetism in Silicon MOSFETs

TL;DR

This paper develops a Stoner-type mean-field theory for magnetism in silicon MOSFETs, revealing a ferromagnetic transition at high densities and a possible ferromagnetic instability before the metal-insulator transition at low densities.

Contribution

It introduces a self-consistent approach accounting for finite layer thickness effects, predicting ferromagnetic transitions in silicon MOSFETs.

Findings

Finite thickness enhances susceptibility and leads to ferromagnetism.

Magnetization increases sublinearly with in-plane magnetic field.

Ferromagnetic instability may precede metal-insulator transition at low densities.

Abstract

We consider quasi-two-dimensional gas of electrons in a typical Si-MOSFET, assuming repulsive contact interaction between electrons. Magnetisation and susceptibility are evaluated within the mean-field approach. Finite thickness of the inversion layer results in an interaction-induced electron wave function change, not found in both purely two-dimensional and three-dimensional (bulk) cases. Taking this self-consistent change into account leads to an increased susceptibility and ultimately to a ferromagnetic transition deep in the high-density metallic regime. We further find that in the paramagnetic state, magnetisation increases sublinearly with increasing in-plane magnetic field. In the opposite limit of low carrier densities, the effects of long-range interaction become important and can be included phenomenologically via bandwidth renormalisation. Our treatment then suggests that with decreasing density, the metal-insulator transition is preceded by a ferromagnetic instability. Results are discussed in the context of the available experimental data, and arguments for the validity of our mean-field scheme are presented.