SPring-8, the large synchrotron radiation facility

Date : 15:00 – February 6, 2008

Place : Kamitsubo Hall

Speaker : Dr. Klaus H. Ploog

Affiliate : Paul Drude Institute for Solid State Electronics

Title : Ferromagnetic Semiconductors with high Curie temperature and unusual magnetic properties - the case of Gd-doped GaN

Abstract :
The emerging field of spintronics (spin electronics) will create new　device functionalities by using the spin in addition to the charge of electrons and holes in device operation. Realistic devices require the electrical injection of spin polarized electrons or holes into　semiconductors at room temperature. The most efficient spin injectors are ferromagnetic semiconductors which allow the realization of all-semiconductor devices, including spin valves and giant-magnetoresistance (GMR) devices. However, despite more than four decades of research efforts, there are only few ferromagnetic semiconductors with Curie temperatures above room temperature. Most promising among them is a family of wide-gap semiconductors which can be labeled "dilute magnetic dielectrics' as they are semi-insulating when doped with transition or rare earth metals.

We have recently observed room-temperature ferromagnetism in Gd-doped semi-insulating GaN films grown on SiC(0001) by molecular beam epitaxy. In addition to the high Curie temperature of >800 K, our measurements have revealed an unexpectedly high magnetic moment per Gd atom and a magnetic anisotropy with the hard axis along the [0001] growth direction and an easy plane parallel to the GaN surface. The temperature dependence of the magnetic properties showed the existence of two ferromagnetic phases with different order temperature. The coexistence of two ferromagnetic phases can be explained within the framework of our phenomenological model which assumes a large polarization of the GaN matrix by the Gd atoms. We also found that the saturation magnetization shows a dependence on the orientation of the magnetic field, which may result from the anisotropy of the polarization induced in the GaN matrix by internal and external magnetic field. It is finally important to note that ferromagnetic GaN films with high Curie temperature can also be produced by Gd-focussed ion beam implantation into as-grown GaN. The observed magnetic moment per Gd atom became even higher in as-implanted samples, and the measured saturation magnetization of as-implanted GaN could be reduced by thermal annealing at 800 C. This  finding points to the important role which defects may play as "mediators" in the exchange coupling between the Gd atoms.

 In addition to the Gd-doped GaN films described here, room-temperature ferromagnetism has also been observed in Co-doped ZnO and anatase TiO(2). In these rather disordered wide-gap semiconductors the standard indirect exchange mechanisms in the absence of free carriers are too short range to overcome the percolation threshold and establish a long-range magnetic order. Intrinsic or extrinsic defects may thus play the part of "mediators" in the inter-impurity exchange coupling.

Organizer : TAKAHASHI (PHS3866)