Anomaly of Ac Resistance in Magnetic Nanoparticle Alloys at Spin-glass-like Transition

A combined study of magnetic susceptibility and AC resistance was performed on melt-spun Cu-Co granular magnetic ribbons. The AC resistance as a function of temperature has a sharp maximum. We associate it with a diverging correlation length at the temperature of collective freezing of magnetic moments via increasing magnetic losses in the induced non-uniform field. Application of this model to the experimental data allows a direct determination of the critical exponent of correlation length ν on both sides of the transition. Giant AC magnetoresistance is observed at the freezing temperature. 1 Magnetic relaxation in oversaturated magnetic alloys such as Cobalt solutions in Copper, where concentration of Co exceeds several percent, reflects the complexity of the structure where interacting magnetic nanoparticles co-exist with the solid solution phase [1]. One relaxation process present in the system is due to random jumps of moments of individual nanometer-scale ferromagnetic particles over the anisotropy barriers. This process leads to a phenomenon referred to as superparamagnetism [2]. At a given measurement time, or frequency, the particle moment is blocked at temperatures lower than the so-called blocking temperature T B. Magnetic susceptibility of an array of particles of distributed sizes will have a maximum at some effective blocking temperature, which can also depend on dipole-dipole interactions. The latter may contribute to the magnetic relaxation of the ensemble via corrections to the individual magnetic particle barrier [3]. It has been shown that dipole-dipole interactions between magnetic particles can also lead to a spin-glass like collective phenomena [4,5], similar to those in traditional spin glasses [6]. This can cause a susceptibility anomaly at the freezing temperature, which can co-exist with a broader transition associated with blocking of particles [7]. Besides, exchange interactions in a frustrated and disordered system of spins of magnetic atoms dissolved in non-magnetic matrix, may also contribute to the spin-glass behavior [1]. Experimentally, the effects of blocking and spin-glass freezing can be investigated using the frequency (or time) dependence of the temperature-dependent susceptibility [8]. Another possibility is to investigate the scaling of nonlinear susceptibility [5] which is more sensitive to the spin-glass transition than the zero-field susceptibility [6]. In this Letter, we use AC resistance measurements as a tool for unambiguous and direct observation of the spin-glass-like transition in a conducting magnetic granular system. The measurements were performed on melt-spun Cu 0.85 Co 0.15 ribbon samples. While the DC resistivity has no appreciable anomaly at the …