Magnetic and magnetotransport properties of Co thin films on Si

A systematic study of magnetic and magnetotransport properties of thin Co f ilms on Si is reported in this work. The Co films of thicknesses 30, 160 an d 440 Angstrom were prepared by magnetron sputtering from a Co target onto a Si(100) substrate held at room temperature. Resistance, magnetoresistance and Hall effect were measured in the temperature range 5-350 K. Magnetizat ion was measured in the same temperature range using a SQUID magnetometer. Complete surface hysteresis loops were measured from 4 to 300 K by means of the magneto-optical Kerr effect, in order to follow the behavior of the co ercivity and magnetic easy axes of the samples. The transport and magnetotr ansport properties display a peculiar effect as functions of temperature. T he films behave as pure metallic Co below 250 K. However, the resistance dr ops with heating from 250 to 280 K, regaining its typical metallic behavior at temperatures higher than 280 K. The thinner the film, the larger is the resistance drop in the temperature interval 250-280 K, reaching a factor o f 4.5 for the 30 A thick film. The Hall effect contains both ordinary and e xtraordinary contributions. From ordinary Hall effect measurements, one fin ds that the conduction is electronic at low temperatures and turns to hole- like above the transition point. Magnetoresistance. changes from negative a t low T to positive at high temperatures. These facts indicate that the obs erved effect is related to the metallic layer, which undergoes a conducting channel switching when the temperature is increased.