Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

The magnetic rare earth element gadolinium (Gd) was doped into thin films of amorphous carbon (hydrogenated a-C:H, or hydrogen-free a-C) using magnetron cosputtering. The Gd acted as a magnetic as well as an electrical dopant, resulting in an enormous negative magnetoresistance below a temperature (T′). Hydrogen was introduced to control the amorphous carbon bonding structure. High-resolution electron microscopy, ion-beam analysis, and Raman spectroscopy were used to characterize the influence of Gd doping on the a- Gdx C1-x (: Hy) film morphology, composition, density, and bonding. The films were largely amorphous and homogeneous up to x=22.0 at. %. As the Gd doping increased, the s p2 -bonded carbon atoms evolved from carbon chains to 6-member graphitic rings. Incorporation of H opened up the graphitic rings and stabilized a s p2 -rich carbon-chain random network. The transport properties not only depended on Gd doping, but were also very sensitive to the s p2 ordering. Magnetic properties, such as the spin-glass freezing temperature and susceptibility, scaled with the Gd concentration.