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 the in-plane configuration than for the out-of-plane configuration, which is indicative of exchange-coupled spins locked along the polar $c$ axis. The sample with the lowest Gd concentration ($2.3\times {10}^{14}\mathrm{c}{\mathrm{m}}^{-2}$) exhibits a saturation magnetization of $41.1{\mu }_B/\mathrm{G}{\mathrm{d}}^{3+}$ 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 is observed. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies nonlinearly 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.

• Received 11 February 2015
• Revised 29 September 2015

DOI:https://doi.org/10.1103/PhysRevB.92.224416