Colossal Magnetic Moment of Gd in GaN

Abstract

We investigate the magnetic properties of epitaxial GaN:Gd layers as a function of the external magnetic field and temperature. An unprecedented magnetic moment is observed in this diluted magnetic semiconductor. The average value of the moment per Gd atom is found to be as high as 4000 micro(B) as compared to its atomic moment of 8 micro(B). The long-range spin polarization of the GaN matrix by Gd is also reflected in the circular polarization of magnetophotoluminescence measurements. Moreover, the materials system is found to be ferromagnetic above room temperature in the entire concentration range under investigation (7 x 10(15) to 2 x 10(19) cm(-3)). We propose a phenomenological model to understand the macroscopic magnetic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd.

Element specific magnetic properties of Gd-doped GaN: Very small polarization of Ga and paramagnetism of Gd

“…The long range magnetic order is accompanied by a huge effective magnetic moment per Gd atom in the ultradilute limit if it is calculated from superconducting quantum interference device (SQUID) magnetometry data, an effect which is even more pronounced in Gd-ion-implanted GaN [3]. The materials for eventual phase segregation are limited to GdN (T N = 56 K), and Gd (T C = 293 K) which makes them unlikely candidates to explain magnetic order above 300 K. The occurrence of magnetic order in wurtzite Gd:GaN was phenomenologically explained by large "spheres of influence" surrounding the Gd dopant atoms inside which the GaN matrix is magnetically polarized [2]. The polarization inside this "sphere of influence" was assumed to be constant and fitting a coalescence model to the SQUID data yields a small polarization of 1.1 · 10 −3 µ B /atom and a large radius of these spheres of about 28 nm [2].…”

“…The materials for eventual phase segregation are limited to GdN (T N = 56 K), and Gd (T C = 293 K) which makes them unlikely candidates to explain magnetic order above 300 K. The occurrence of magnetic order in wurtzite Gd:GaN was phenomenologically explained by large "spheres of influence" surrounding the Gd dopant atoms inside which the GaN matrix is magnetically polarized [2]. The polarization inside this "sphere of influence" was assumed to be constant and fitting a coalescence model to the SQUID data yields a small polarization of 1.1 · 10 −3 µ B /atom and a large radius of these spheres of about 28 nm [2]. The additional polarization for overlapping spheres was marginal [2].…”

“…However, the origin of the long range magnetic order in DMS materials is still controversial. Recently long range magnetic order above room temperature has been demonstrated for wurtzite Gd:GaN [1] even for a dopant concentration as low as 10 16 atoms/cm 3 grown by molecular beam epitaxy (MBE) [2]. The long range magnetic order is accompanied by a huge effective magnetic moment per Gd atom in the ultradilute limit if it is calculated from superconducting quantum interference device (SQUID) magnetometry data, an effect which is even more pronounced in Gd-ion-implanted GaN [3].…”

“…The polarization inside this "sphere of influence" was assumed to be constant and fitting a coalescence model to the SQUID data yields a small polarization of 1.1 · 10 −3 µ B /atom and a large radius of these spheres of about 28 nm [2]. The additional polarization for overlapping spheres was marginal [2]. For a more thorough understanding of the magnetic properties of Gd:GaN it is useful to verify the predicted magnetic polarization of the GaN host and its origin.…”

Magnetoelectronic properties of Gd-implanted tetrahedral amorphous carbon

“…1 and 13. The strong sp 3 covalent bonds, once formed, are very stable under thermal treatments, since the energy barrier for converting sp 3 to sp 2 is large (oxygen can act as a catalyst to lower this barrier, thus annealing in vacuum or an inert gas is essential). Diamond can be annealed in vacuum at a temperature as high as 1800 K without graphitization (see chapter 13.3 of Ref.…”