On the origin of magnetism in (Ga,Mn)As: from paramagnetic through superparamagnetic to ferromagnetic phase

TL;DR

This study investigates the evolution of magnetic phases in (Ga,Mn)As, revealing that changes in the valence band occur at very low Mn levels and that the transition from paramagnetic to ferromagnetic states involves nanoscale hole density fluctuations rather than band structure modifications.

Contribution

It provides new insights into the magnetic phase transition mechanisms in (Ga,Mn)As, emphasizing the role of nanoscale hole density fluctuations over band structure changes.

Findings

Valence band modification occurs at very low Mn content.

Paramagnetic to ferromagnetic transition involves nanoscale hole density fluctuations.

Fermi level is influenced by carrier concentration and many-body interactions.

Abstract

The high-spectral-resolution spectroscopic studies of the energy gap evolution, supplemented with electronic, magnetic and structural characterization, show that the modification of the GaAs valence band caused by Mn incorporation occurs already for a very low Mn content, much lower than that required to support ferromagnetic spin - spin coupling in (Ga,Mn)As. Only for n-type (Ga,Mn)As with the Mn content below about 0.3% the Mn-related extended states are visible as a feature detached from the valence-band edge and partly occupied with electrons. The combined magnetic and low-temperature photoreflectance studies presented here indicate that the paramagnetic - ferromagnetic transformation in p-type (Ga,Mn)As takes place without imposing changes of the unitary character of the valence band with the Fermi level located therein. The whole process is rooted in the nanoscale fluctuations of the local (hole) density of states and the formation of a superparamagnetic-like state. The Fermi level in (Ga,Mn)As is coarsened by the carrier concentration of the itinerant valence band holes and further fine-tuned by the many-body interactions.