Anisotropic defect-induced ferromagnetism and transport in Gd-doped GaN two-dimensional electron gasses

TL;DR

This study demonstrates that Gd doping in GaN 2DEGs induces room-temperature ferromagnetism with anisotropic magnetization, revealing new magnetic properties in semiconductor heterostructures without coupling to conduction electrons.

Contribution

It reports the first observation of anisotropic ferromagnetism in Gd-doped GaN 2DEGs with high Curie temperature and analyzes the magnetic behavior and transport properties in these heterostructures.

Findings

Ferromagnetism observed above room temperature in Gd-doped GaN 2DEGs.

Magnetization is anisotropic, favoring out-of-plane orientation.

No coupling between ferromagnetism and conduction electrons evidenced by Hall measurements.

Abstract

Here we report on the effect of rare earth Gd-doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular beam epitaxy and consist of AlN/GaN heterostructures where Gd is delta-doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane (c-axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for in-plane configuration than for out-of-plane, which is indicative of exchange coupled spins locked along the polar c-axis. The sample with the lowest Gd concentration (2.3 $\times$ $10^{14}$ cm$^{-2}$) exhibits a saturation magnetization of 41.1 $\mu_B/Gd^{3+}$ at 5 K revealing that the Gd ion spins (7 ${\mu}_B$) alone do not account for the magnetization. Surprisingly, control samples grown without any Gd display inconsistent magnetic properties; in some control samples weak ferromagnetism is observed and in others paramagnetism. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies non-linearly with the magnetic field, but does not track the magnetization indicating the lack of coupling between the ferromagnetic phase and the conduction band electrons within the 2DEG.