Ferromagnetism and colossal magnetic moment in Gd-focused ion-beam-implanted GaN

Abstract

The structural and the magnetic properties of Gd-focused ion-beam-implanted GaN layers are studied. Gd3+ ions were uniformly implanted in molecular beam epitaxy grown GaN layers at room temperature with an energy of 300keV at doses ranging from 2.4×1011to1.0×1015cm−2 which corresponds to an average Gd concentration range of 2.4×1016–1.0×1020cm−3. The implanted samples were not subjected to any annealing treatment. No secondary phase related to Gd was detected by x-ray diffraction in these layers. Magnetic characterization with superconducting quantum interference device reveals a colossal magnetic moment of Gd and ferromagnetism with an order temperature above room temperature similar to that found in epitaxially grown Gd-doped GaN layers. The effective magnetic moment per Gd atom in these samples is, however, found to be an order of magnitude larger than that found in epitaxially grown layers for a given Gd concentration which indicates that the defects play an important role in giving rise to this effect.

Effect of annealing on the magnetic properties of Gd focused ion beam implanted GaN

“…Interestingly enough, the magnetic moment per Gd atom in these samples is found to be even larger as compared to that found in epitaxially grown layers for a given Gd concentration. 6 Since the implanted samples are expected to have a larger density of defects, this finding suggests that, defects play an important role in giving rise to the effect. 6 Annealing at high temperatures is expected to reduce the overall concentration of defects in the implanted layers.…”

“…6 Since the implanted samples are expected to have a larger density of defects, this finding suggests that, defects play an important role in giving rise to the effect. 6 Annealing at high temperatures is expected to reduce the overall concentration of defects in the implanted layers. It would be thus interesting to study the change in the magnetic properties of these layers because of annealing.…”

“…It has to be noted that these are the same set of samples, whose magnetic and the structural properties have been investigated in Ref. 6. Our study suggests that Ga and/or N interstitials might be responsible in giving rise to the effects of colossal magnetic moment of Gd and associated ferromagnetism observed in Gd:GaN.…”

“…More details about the implantation could be found in Ref. 6. Implanted samples are rapid thermally annealed in flowing N 2 gas in two steps; at 800°C for 30 s and then at 900°C for 30 s. Before and after each annealing step, all the necessary structural and the magnetic characterizations are performed.…”

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

“…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 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].…”