Microstructure and Thermal Stability of ( Fe-CoSi )-( AlF ) Nanogranular Thin Films with Tunnel-Type Magnetroresistance

Developments of magnetic materials with tunnel-type magnetroresistance effect (TMR) have led to significant advances in electronic device technology. In particular, insulating nanogranular thin films, which consisted of magnetic nanogranules (Fe, Co, etc.) embedded in insulating matrix (oxide or fluoride), have promising features of high-frequency magnetic permeability, large magnetroresistance (MR) and highly thermal stability for the device applications [1]. Recently, Kobayashi et. al. developed (Fe-Co)-(Al-F) nanogranular thin films with the MR ratio of 12 % (H=10kOe at room temperature) and the specific electrical resistivity of 1×10 μΩ-cm, indicating between Co-(Al-O) and (Fe-Co)-(Mg-F) films [2]. In addition, thermal stability of the (Fe-Co)-(Al-F) films was found to be higher than that of the (Fe-Co)-(Mg-F) films, and further enhanced by adding a small amount of Si (~5at.%) into the films. In this study, effects of Si addition on microstructures and thermal stability (for electric and magnetic properties) of the (Fe-Co)-(Al-F) nanogranular thin films was investigated in order to understand the mechanism of the property improvement. The thin films were fabricated onto glass substrate (Coaning #7059) by a tandem deposition method using a conventional r.f. magnetron sputtering [3]. The deposition was carried out using FeCo metallic alloy and AlF3 insulating disc targets. The concentrations of Si in the films were varied from 0 to 5 at% by controlling the amounts of Si chips placed on the FeCo alloy target. In particular, the films with a composition of FeCo(3.3 at.%Si)-AlF, which indicated excellent properties, were focused. After depositing, the films were annealed isothermally at 300 °C for 1 hour in a vacuum. The specific electrical resistivity and magnetroresistance in the films were measured by a dc-four terminal method. Microstructures were analyzed by XRD using Cu-Kα radiation, conventional TEM and aberration-corrected STEM equipped with EELS. As for the TMR, the MR ratio of the FeCo-AlF nanogranular film (without Si) was reduced abruptly while that of the FeCo(3.3 %Si)-AlF film increased after annealing at 300 °C. From cross-sectional TEM results (Fig. 1), both FeCo-AlF and FeCo(3.3%Si) -AlF films were observed to consist of uniformly dispersed nanogranules with 2 nm in diameter before annealing. After annealing, however, the nanogranules in the FeCo-AlF film were observed to grow and coalesce between neighboring ones as shown by arrows in Fig. 1(b), resulting in deterioration of the magnetic properties, i.e. reduction of the MR ratio. On the other hand, the nanogranules in the FeCo(3.3%Si)-AlF film did not grow and indicated high thermal stability as shown in Fig. 1(c). This result was consistent with high stability of the magnetic and electric properties at elevated temperatures. In addition, a large amount of Si was observed to distribute locally in the nanogranules by HAADF-STEM observation and EELS elemental mapping analysis as shown in Fig. 2. After annealing, Si did not segregate and diffuse into AlF matrix. As a result, the Si atoms in the FeCo-AlF nanogranular films might play an important role in control of diffusion process (elemental distribution) and atomic structure at the interface between FeCo granule and AlF matrix, which are strongly related to the TMR.