Impurity bands and the character of the electronic states in ferromagnetic GaMnAs layers

TL;DR

This paper investigates how disorder and spin polarization influence impurity bands and electronic states in ferromagnetic GaMnAs layers, providing a phase diagram that predicts metallic or insulating behavior and explaining the maximum Curie temperature.

Contribution

It introduces a figure of merit to classify hole states and links impurity band properties to ferromagnetic transition temperatures in GaMnAs.

Findings

Samples with high figures of merit have a specific ratio of extended hole density to Mn concentration.

The study reproduces observed metal-insulator transitions with increasing Mn concentration.

An explanation is provided for the maximal Curie temperature in ferromagnetic GaMnAs.

Abstract

The interplay between disorder and spin polarization in a GaMnAs thin layer results into spin-polarized impurity hole bands. A figure of merit is defined to label the hole state as being extended or localized. The calculation leads to a phase diagram determining the metallic or non-metallic character of the sample. It is shown that samples with the highest figures of merit have a ratio between the extended hole density and the Mn concentration near 0.2, in agreement with the ratio of 0.1-0.25 known to occur among samples produced with the highest Curie temperatures. Both the non-metal-to-metal and the metal-to-non-metal transitions experimentally observed in the ferromagnetic regime are obtained, as the Mn concentration increases. An explanation is given for the occurrence of a maximal Curie temperature in ferromagnetic GaMnAs samples.