Preparation Of C-axis Oriented Strontium Ferrite Thin Film

Introduction Sr hexaferrite &MI thin films with the magnetodumbite type (M-tvpe) are exDected to be high density recording media the same BaM thin fiims becaiie of their-chemical stability and mechanical hardness as well as distinctive magnetic propertie~l'I-[~1. It is possible to use as both longitudinal and perpendicular recording media with arranging the c-axis direction. It has been reported that BaM films are oriented perpendicularly to the film by the substrate heating over 620"C, which is fatally high temperature for the commercial production. In this paper, SrM thin films were prepared at various substrate temperatures (Ts) and their crystal structure and magnetic properties were studied. Experimental film were prepared with a dc magnetron reactive sputtering apparatus. The tarEet with the stoichiometric composition ofM-type, i.e., Sr:Fe = l:12was used. The total ias pressure (PTO*=,) was 2~10-~Torr and the partial oxygen pressure (Poz) was 5x 10-6Torr. The dc power was 7W. The thermally oxidized silicon wafers were used as the substrates and the thickness of the films were about 1500a. The films were prepared at the Ts in the range from 50°C to 600°C. The crystal structure of the films were determined by X-ray diffraction (XRD). The magnetic properties were measured with a vibrating sample magnetometer (VSM). The surface structure was observed by a scanning electron microscope (SEM). Results and Discussion Fig.1 shows XRD diagrams at various Ts. There were found no peaks in the diagram at the Ts lower than 35OoC, which shows the structure of the films was amorphous. As the Ts was increased from 400 to 550"C, the lines from SrM phase were observed. At the Ts of 600"C, the lies only from c-plane were observed. From this figure it was found that SrM films were cystallized at the Ts higher than 400°C and oriented at the Ts of 600°C. The films prepared at the Ts over 400°C had magnetic properties well in accordance with the generation of SrM phase. Fig.2 shows the dependence of the magnetization at magnetic field of lOkOe (M1oko,) on Ts. The M1okoe increased rapidly from about 75 to 200emu/cc when the Ts was increased from 400 to 450°C and was between 200 and 230emu/cc at the Ts between 450 to 600°C while the intensities of the lines in the XRD digram were vehemently increasing with the Ts. Fig.3 shows the dependence of coercivity (Hc) on Ts. When the Ts was increased from 400 to 600°C the Hc decreased from 3.6k to 2.5kOe in the perpendicular direction while that in the in-plane direction decreased from 3.6k to 0.7kOe. The difference of the Hc in the perpendicular and in-plane directions was increased with the increase in the Ts from 400 to 600°C, which is correlated with the orientation of c-axis in hexagonal structure as shown in Fig.1.