Strong Temperature Dependence of Magnetoresistance in Co-C Granular Thin Films

Co-C granular thin films with a Co content of 6.4 at.% were deposited at room temperature by co-sputtering. The magnetotransport properties of the thin films were investigated within 2-300 K under magnetic fields up to 90 kOe. A large negative magnetoresistance (MR) of 27.6% was successfully observed at 2 K. As temperature (T) increased, negative MR decreased drastically. Meanwhile, positive MR with an organic MR (OMAR)-like shape emerged in a low magnetic field region. Ordinary MR (OMR) appeared in a high magnetic field region and can be verified by MR ¿ H² fitting. When T reached 10 K, MR became completely positive. The MR effect showed strong temperature dependence. The interesting phenomena may help us gain a new insight into the spin-dependent transport in ferromagnetic metal (FM)/amorphous-carbon (¿-C) system.

I. Introduction

Granular thin films consisting of ferromagnetic metal (FM) nanoparticles (such as Fe, Co, Ni and their alloys) dispersed in a non-magnetic matrix have stimulated extensive interest during the last decades [1]–[4]. As one superior electronic material, amorphous carbon can be tuned in a broad range from metallic to insulating. Different fabrication methods can give rise to various carbon structures. Particularly, a long spin relaxation time is expected for due to its weak spin-orbit coupling and weak hyperfine interaction. Interesting phenomena might be produced in based granular system. A large magnetoresistance (MR) is indispensable for materials applied for spintronics devices. However, as far as we know, most previous studies were mainly focused on magnetic properties of FM/ thin films, where the magnetotransport behavior was not elucidated [5], [6]. Only a few groups once reported negative MR, but the MR value was very small (typically, at 4.2 K, 50 kOe) [7], [8]. Furthermore, the magnetotransport mechanism was unclear.