Experimental probing of the interplay between ferromagnetism and localisation in (Ga,Mn)As

TL;DR

This study investigates how reducing carrier density via gating affects ferromagnetism in (Ga,Mn)As, revealing a transition characterized by phase separation and critical fluctuations, challenging impurity-band models.

Contribution

It provides direct experimental evidence of how carrier depletion influences magnetic order and phase separation in (Ga,Mn)As, highlighting the role of Anderson-Mott localization.

Findings

Curie temperature decreases monotonically with carrier depletion

Transition involves emergence of superparamagnetic-like regions

Evidence of phase separation due to localization effects

Abstract

The question whether the Anderson-Mott localisation enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga,Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed. Here, by direct magnetisation measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin-spin coupling is diminished by the gate electric field in metal/insulator/semiconductor structures of (Ga,Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models. We find that the transition from the ferromagnetic to the paramagnetic state proceeds via the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localisation leads to a phase separation into ferromagnetic and nonmagnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson-Mott quantum transition.