The giant magnetoresistance (GMR) effect in Fe/Cr multilayers [1] has stimulated a great deal of investigation on the magnetotransport properties in various multilayered and sandwiched materials and subsequently in granular materials [2]. Tunnel-MR (TMR) in metal±nonmetal granular systems consisting of nanometer-sized magnetic metal granules embedded in an insulating matrix is one of the large MR effects and is believed to result from spin-dependent tunnelling between magnetic granules through the matrix. Because of the unique microstructure of ultrafine particles, many interesting and potentially useful properties have been found in granular systems.
Up to now, though TMR has been investigated for many kinds of metal±nonmetal granular systems [3], the transport phenomena in granular systems have not been totally understood. So in this work a new Co±Cr±O granular system was further employed to explore the TMR effect and the experimental results are reported here briefly.
Heterogeneous granular Co±Cr±O thin films were co-deposited on glass substrates by rf sputtering in an Ar atmosphere using a composite target of Co and Cr 2 O 3 . The electrical resistivity measurements have been carried out using a dc four-terminal configuration in a magnetic field up to 1.2 T at room temperature (RT) and the temperature dependence of resistivity in range from 5 to 290 K was accomplished by means of a helium close-cycle refrigerator. The film structure was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The composition and chemical bonding were analysed by the Energy Dispersive X-ray analysis (EDX) technique and X-ray photoelectron spectroscopy (PHI 5702 XPS/AES) using a MgK a X-ray source. The magnetic properties were examined at RT with a vibrating sample magnetometer.
It is found by XRD and TEM that the Co 0.35 (Cr 2 O 3 ) 0.65 film (described in volume fraction) consists of nanoscaled Co granules with an average size of 33 nm embedded in a crystalline insulating Cr 2 O 3 matrix, and it is also confirmed by XPS and EDX that the target composition of Cr 2 O 3 is preserved in all the films. As shown in Fig. 1, the TMR ratio (R(0) ± ± R(H))/R(0) of the Co 0.35 (Cr 2 O 3 ) 0.65 film reaches 5.1% under a 1.2 T field, and the magnetization of this film exhibits ferromagnetic properties at RT as shown in Fig. 2.
The temperature dependence of the resistivity r shows a clearly linear relation (shown in Fig. 3) between ln r and T ±1/2 in the temperature range from 191 to 290 K, which well satisfies with the tunneling-type conductance formula rT r 0 exp 2 Cak B T p Y 1 Rapid Research Notes R3 Fig. 1. Field dependence of TMR ratio for Co 0.35 (Cr 2 O 3 ) 0.65 film