Magnetic and Magnetoresistive Properties of Thin Film Alloys Based on Cobalt and Copper

Abstract

This work presents the results of a study of the influence of dimensional and concentration effects on magnetoresistive and magnetic properties and structural-phase state of annealed at Ta  700 K samples of thin film alloys based on Co and Cu with thickness d  40 nm in a wide range of total concentrations of Co (18 ≤ cCo ≤ 69 at. %). The samples were obtained by the method of co-evaporation in vacuum using two independent electron guns. It was found that samples of thin film alloys with a total concentration of cobalt cCo ≤ 22 at. % are characterized mainly by a superparamagnetic state in which hcp-Co granules with size L  5÷12 nm were placed in a matrix of Cu(Co) solid solution (s.s.) based on fcc-Cu. The increase in the total concentration of Co in thin film alloys to cCo  37 at. % led to an increase in the size of the magnetic granules to L  7÷20 nm and a decrease in the distance between them. In samples of thin film alloys with a cobalt concentration of 56 ≤ cCo ≤ 69 at. %, hcp-Co granules with size L  8÷25 nm were combined into ferromagnetic clusters, and the volume between the clusters was filled with fcc-Cu(Co) s.s.. The maximum giant magnetoresistance (GMR) values, measured in an external magnetic field Hmax  15 kOe at room temperature, were obtained for a sample with a total concentration of cobalt cCo  22 at. %. The amplitude of GMR values was 3.8 % and 4.1 % in the perpendicular and longitudinal measurement geometries, respectively. Thin film alloys with a total cobalt concentration of 18 ≤ cCo ≤ 22 at. % were characterized by isotropic magnetoresistance. The increase in the total concentration of cobalt in thin film alloys to cCo ≥ 28 at. % led to the emergence of magnetic anisotropy and anisotropic magnetoresistance. This may be due to the appearance of ferromagnetic interactions between cobalt granules. Low-temperature measurements of magnetoresistance at Тmeas  5 K of the sample with a total cobalt concentration cCo  18 at. % showed an increase in the amplitude of GMR by 2.1 times (GMR  11.1 %) compared with the amplitude of GMR  5.18 % measured at room temperature in an external magnetic field Hmax  90 kOe. With increasing measurement temperature to T  350 K the amplitude of GMR decreased and amounted to 4.5 %. This is due to the increase in electron scattering by phonons with increasing temperature.

Structure and size effects in the magnetic anisotropy factors of Co-Ag nano granular films

“…Another cause is surface anisotropy it could be suggested that the Co particles embedded in the Ag matrix are single-domain fine particles [39]. This perpendicular anisotropy has been observed in other granular films such as sputtered Ag-Co, Cu-Co, Co-Al-O and FeCoZr-CaF2 [14,18,19,[40][41][42][43]. In granulated materials the magnitude of the coercive force Hc depends on the size of magnetic granules [44,45].…”