Microstructure, magneto-transport and magnetic properties of Gd-doped magnetron-sputtered amorphous carbon

TL;DR

This study investigates how Gd doping affects the microstructure, magnetic, and magneto-transport properties of amorphous carbon films, revealing significant changes in bonding, morphology, and magnetic behavior with increasing Gd content.

Contribution

It provides new insights into the effects of Gd doping on amorphous carbon's structure and magnetic properties, including the observation of large negative magnetoresistance.

Findings

Gd doping induces evolution from carbon chains to graphitic rings.

Magnetic properties scale with Gd concentration, showing spin-glass behavior.

Transport properties are highly sensitive to Gd content and sp2 ordering.

Abstract

The magnetic rare earth element gadolinium (Gd) was doped into thin films of amorphous carbon (hydrogenated \textit{a}-C:H, or hydrogen-free \textit{a}-C) using magnetron co-sputtering. 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 \textit{a-}Gd$_x$C$_{1-x}$(:H$_y$) 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 $sp^{2}$-bonded carbon atoms evolved from carbon chains to 6-member graphitic rings. Incorporation of H opened up the graphitic rings and stabilized a $sp^{2}$-rich carbon-chain random network. The transport properties not only depended on Gd doping, but were also very sensitive to the $sp^{2}$ ordering. Magnetic properties, such as the spin-glass freezing temperature and susceptibility, scaled with the Gd concentration.