Magnetoelectronic properties of Gd-implanted tetrahedral amorphous carbon

Abstract

The structural, electronic, magnetic, and magnetoelectronic properties of tetrahedral amorphous carbon ($ta$-C) thin films doped with gadolinium via ion implantation ($ta$-C${}_{1\ensuremath{-}x}:$Gd${}_{x}$, $x$ = 0.02 \ensuremath{\sim} 0.20) have been studied, both as prepared and after annealing, with Xe-implanted samples as control samples. Gd implantation causes significant increases in electrical conductivity, showing that Gd adds carriers as in other rare earth--semiconductor systems. Gd also provides a large local moment from its half-filled $f$ shell. Carrier-mediated Gd-Gd interactions are strong but very frustrated, causing a spin-glass state 10 K for higher $x$. An enormous negative magnetoresistance (about \ensuremath{-}10${}^{3}$ at 3 K in a 70-kOe field for $x$ = 0.088) is observed at low $T$ (30 K), an indication of carrier--moment interactions that cause magnetic disorder--induced localization and consequent magnetic field--induced delocalization as Gd moments align with the magnetic field. Gd implantation causes substantial changes in Raman intensity, associated with conversion of C-C bonds into Raman inactive bonds, which induce further graphitization after annealing. The changing nature of the C-C bonding with increasing x or with annealing causes the electrical transport properties to depend on Gd concentration $x$ with a nonmonotonic dependence. Systematic but nonmonotonic trends are seen on comparing the magnetic and magnetotransport properties of Gd-doped $a$-C, $a$-Si, and $a$-Ge matrices, suggesting that electron concentration and band gap play separate important roles.