Improved giant magnetoresistance in nanogranularCo∕Ag: The role of interparticle RKKY interactions

Abstract

A record 40% room temperature giant magnetoresistance has been achieved in a nanogranular Co/ Ag alloy by optimizing the concentration, sputtering conditions, and cumulative short thermal treatments. Upon annealing, the giant magnetoresistance effect exhibits a local minimum at 230°C before reaching its maximum at 300°C. Assuming the presence of both dipolar and RKKY-like exchange interactions between the particles, these features can be accounted for by considering that the latter correlations are progressively inhibited as the matrix, a supersaturated Ag-Co solid solution, segregates the solute Co atoms. This idea is supported by the temperature dependence of the magnetization in samples submitted to different annealings, where the zero-field cooled magnetization peaks at the lowest temperature in the sample exhibiting the largest magnetoresistance effect.

Memory effect and spin-glass-like behavior in Co-Ag granular films

“…The other two samples, S2 and S3, had similar XRD patterns. After annealing, the fcc Co ͑111͒ diffraction peak obviously emerges, indicating the formation and precipitation of fcc Co-rich particles from the Ag-rich matrix, which is in agreement with the observation by De Toro et al 30 The corresponding change in the morphology and structure of the S1 sample before and after annealing has also been verified by TEM imaging as shown in Fig. 2.…”

“…These results show unambiguously that in the as-deposited Co-Ag film, a fairly large amount of cobalt atoms dissolve in the silver matrix and that the concentration of solute Co atoms is dramatically reduced after annealing at a temperature higher than 300°C due to remarkable segregation of cobalt from the matrix. 30,32 The above arguments may also be supported by the giant magnetoresistance ͑GMR͒ data. Magnetoresistance ͑MR͒ measurements were carried out under a magnetic field of FIG.…”

“…2͒, indicates that the indirect exchange interactions mediated by solute Co atoms weakened as the Co concentration deceased in Ag matrix. 30 We also measured the field-dependent magnetization, M-H, loops for all samples before and after annealing within a temperature range of 5 to 300 K. As was discussed by Allia et al, dipole-dipole interaction between the clusters can be evidenced by a thin, elongated hysteresis loop measured at a temperature much higher than blocking temperature. 34 And noninteracting clusters must show a reversible magnetization curve at a temperature much higher than their blocking temperatures, i.e., the coercive field is zero.…”

“…15 Therefore, this increase can be attributed to the growth of Co-rich particles after they precipitate from the Ag-rich matrix. On the other hand, T p decreases from 30 K to 22 K, which can be ascribed to the reduction of indirect exchange interaction ͓the Ruderman-Kittel-Kasuya-Yosida ͑RKKY͒ interaction͔ 30 as a consequence of decreasing concentration of the solute Co atoms in Ag matrix. When the sample was annealed at 400°C ͑not shown here͒, the bifurcation temperature and T p were both higher than 300 K, indicating that most of the Co clusters are large enough to be stable ͑in a blocked state͒ at room temperature.…”

“…31 Although the mutual solubility equilibrium of Ag and Co is very low, a much larger concentration of solute Co atoms should be expected in Co-Ag films deposited by the nonequilibrium sputtering technique at room temperature. Since the concentration of solute Co atoms in the matrix plays an important role in determining the interactions within the granular films, 30,32 we extracted the concentration of solute Co atoms in the as-deposited and annealed films. The concentration of solute Co atoms can be easily calculated from the shift in the Bragg diffraction peaks of Ag ͑111͒ or ͑222͒ from the bulk solutions if we assume the alloying of Co and Ag follows Vegard's law.…”