Spatially controlled formation of superparamagnetic (Mn,Ga)As nanocrystals in high temperature annealed (Ga,Mn)As/GaAs superlattices

TL;DR

This study investigates how high-temperature annealing induces the formation of (Mn,Ga)As nanocrystals within (Ga,Mn)As/GaAs superlattices, affecting their magnetic properties from ferromagnetic to superparamagnetic states.

Contribution

It demonstrates controlled formation and spatial confinement of (Mn,Ga)As nanocrystals in superlattices through annealing at various temperatures.

Findings

Nanocrystals form inside the superlattice during annealing.

Nanocrystals remain confined at lower annealing temperatures.

Magnetic properties evolve from ferromagnetic to superparamagnetic.

Abstract

The annealing-induced formation of (Mn,Ga)As nanocrystals in (Ga,Mn)As/GaAs superlattices was studied by X-ray diffraction, transmission electron microscopy and magnetometry. The superlattice structures with 50 A thick (Ga,Mn)As layers separated by 25, 50 and 100 A thick GaAs spacers were grown by molecular beam epitaxy at low temperature (250 C), and then annealed at high temperatures of: 400, 560 and 630 C. The high temperature annealing causes decomposition to GaMnAs ternary alloy and formation of (Mn,Ga)As nanocrystals inside the GaAs matrix. The nanocrystals are confined in the planes that were formerly occupied by (Ga,Mn)As layers for up to the 560 C of annealing and diffuse throughout the GaAs spacer layers at 630 C annealing. The corresponding magnetization measurements show the evolution of the magnetic properties of as-grown and annealed samples from ferromagnetic, through superparamagnetic to the combination of both.