Magnetic Metal-doped Amorphous Carbon Films Xiaozhong Zhang, Xinyu Tan, Lihua Wu, Xin Zhang, Xili Gao, Caihua Wan

National Center for Electron Microscopy (Beijing) Laboratory of Advanced Materials Department of Materials Science and Engineering Tsinghai University, Beijing, 100084, People's Republic of China

Motivation:

In our previous work [1], bulk graphite shows a transition from giant positive magnetoresistance (MR) to negative one with decreasing the granular size of graphite. Furthermore, the field dependence of the positive MR changes from sublinear to superlinear with elevating temperature. In order to research influence of dimensionality of the sample on MR, carbon films doped with magnetic metals were deposited on n-type silicon substrates. The as-deposited films show a giant MR as well but its dependence on field and temperature changes a lot. More interesting, it is uncovered that the giant MR has close relation with the silicon substrate and the distribution of the doped magnetic particles.

Experimental:

Graphite and cobalt (99:1 atom) were mixed and hot-pressed into disk which was used as target for pulse laser deposition. The silicon substrates, before put into chamber, were etched in HF (10%) for 10min. Before the deposition, chamber was pumped under the pressure of 5 × 10 -4 Pa and the substrates were kept at some temperatures. Then the target was bombarded in 6Hz for 15min. After the deposition, the films were annealed at high temperature for 20min.

The structures of the films were characterized in TEM, Raman, HADDF and EDS.

I-V

characteristics were measured in Quantum Design SQUID and PPMS with Keithley 2400 source meter and Keithley 182 or 2182 voltage meter. It is worth noting that current-in-plane and current-perpendicular-plane geometry were both adopted in the transport measurement. 2

Main Result:

1 The temperature dependence of resistance of the film C: Co/Si (Fig.2) shows that the transport properties of the film is controlled by 3D variable range hopping under 270K where an intensive Metal-insulator transition occurs. It is found the MR after M-I transition is closely relative to silicon substrates while the MR under the transition is only dependent on carbon films. However, a more intensive explanation for the MR at low temperatures is still being investigated. More details can be found in Ref [2] 4 The field dependence of magnetoresistance measured in CIP geometry evolves significantly with temperature. The field dependence at high temperature is sublinear while it is superlinear at low temperatures (Fig. 1). 3 The MR measured in CPP geometry is ascribed to field dependent recombination of holes and electrons in neutral regions in p-n heterojunctions formed by p-type carbon films and n-type substrates. The mechanism is similar to the one for organic magnetoresistance (OMAR) [4]. The MR is also dependent on distribution of magnetic particles. More details could be found in Ref [3].

I-V

characteristics measured in CPP geometry (Fig.3) show a good rectifying effect. The ellipses label the voltage region where diffusion current dominates transport properties and where MR is significant. Fig.4 shows MR measured in CPP geometry. It is interesting that the MR saturates soon at relatively small field about 0.3T which is much smaller than the field needed before [2].

Reference:

[1] X. Zhang et al, Phys Lett A,

320

, 471 (2004) [2] D. D. Zhu et al, J. Appl. Phys.,

95

, 1906 (2004) [3] X. Zhang et al, Appl. Phys. Lett.,

95

, 022503 (2009) [4] Y. Sheng et al, Phys. Rev. B.,

74

, 045213 (2006)