Transport Properties of Rare Earth Nitrides

Abstract

In this thesis we investigate the transport properties of SmN, NdN and GdN, members of the rare earth nitride series of intrinsic ferromagnetic semiconductors. GdN is the central member of the series with seven occupied majority spin 4f states and seven empty minority spin 4f states. Both the filled and unfilled 4f states are some few eV away from the conduction and valence band extrema, resulting in transport properties which are dominated by the extended Gd 5d band. The half filled 4f shell, with zero net orbital angular momentum, furthermore simplifies calculations and as such GdN is the most studied both experimentally and in theory. As one moves to lighter members, the filled 4f states become unfilled states in the conduction band and the 4f shell now has a net orbital angular momentum. Calculations concerning these members are now significantly more complicated, and as such there exists a wide range of predictions concerning the conduction band minima in the lighter rare earth nitrides. To inform the current theoretical and experimental literature we report on three studies concerning the transport properties of SmN, NdN and GdN.

To begin we report on the anomalous Hall effect in SmN, NdN and GdN. Under the symmetry of the rock-salt rare earth nitrides the magnitude of the anomalous Hall effect can imply the wave function of the conduction electron (i.e. d or f band). Measurements of the anomalous Hall effect in moderately doped samples are used to show the conduction channel in SmN and NdN is an f band or hybridised f/d band. Furthermore the sign of the anomalous Hall effect can be used to determine the orientation of the spin magnetic moment of the conduction electrons. Optical measurements of SmN, NdN and GdN films are then reported. Optical measurements provide a probe of the band structure of a material via direct transitions between the valence and conduction bands. Measurements of reflectivity and transmission on undoped SmN and NdN films were used to locate the unfilled majority spin 4f bands which form the conduction band minima in each material. Finally a preliminary study of heavily doped SmN, NdN and GdN is discussed. Structural measurements show a reduced lattice parameter while transport results find a significantly enhanced conductivity in heavily doped films. The Curie temperature is found to be enhanced and optical measurements show an increased absorption and red-shifted optical edge in doped films. The superconducting state of SmN is discussed and it is shown only to be present in moderately doped films, i.e. superconductivity is not present in undoped or degenerately doped SmN, within our measurement limits.