High-Temperature Ferromagnetism in the Super-Dilute Magnetic Semiconductor GaN:Gd

Abstract

We present a systematic study of the growth, structural, magnetic and magneto-optical properties of GaN:Gd layers grown directly on 6H-SiC(0001) substrates by reactive molecular-beam epitaxy with a Gd concentration ranging from 7 × 10¹⁵ to 2 × 10¹⁹ cm⁻³. The structural properties of these layers are found to be identical to those of undoped GaN layers. However, the magnetic characterization reveals an unprecedented effect. The average value of the magnetic moment per Gd atom is found to be as high as 4000 µB as compared to its atomic moment of 8 µB. This colossal magnetic moment can be explained in terms of a long-range spin polarization of the GaN matrix by the Gd atoms which is also reflected by the circular polarization of the excitonic emission in magneto-photoluminescence measurements. Moreover, the material system is found to exhibit ferromagnetism well above room temperature in the entire concentration range under investigation. We propose a phenomenological model to understand the macroscopic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd. This model also provides a natural explanation for the coexistence of two ferromagnetic phases observed in this material. The microscopic origin for the longrange spin polarization remains unclear at the moment, but recent studies in the literature strongly suggest that native point defects play a crucial role for this phenomenon.