Gd-doped GaN: A very dilute ferromagnetic semiconductor with a Curie temperature above 300 K

Abstract

We present a systematic study of growth, structural, and magnetic characterization of $\mathrm{GaN}:\mathrm{Gd}$ layers grown directly on 6H-SiC(0001) substrates by reactive molecular-beam epitaxy with a Gd concentration ranging from $7\ifmmode\times\else\texttimes\fi{}{10}^{15}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. 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 $4000\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ as compared to its atomic moment of $8\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$. Such a colossal magnetic moment can be explained in terms of a long range spin polarization of the GaN matrix by the Gd atoms which is reflected by the circular polarization of magnetophotoluminescence 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.