Inhomogeneous structure and magnetic properties of granular Co/sub 10/Cu/sub 90/ alloys

Abstract

Granular Co₁₀Cu₉₀ alloys displaying giant magnetoresistance have been obtained by melt spinning followed by an appropriate heat treatment in the range 0–700 °C. Their structural and magnetic properties have been studied on a microscopic scale using ⁵⁹Co NMR technique and thermoremanent magnetization measurements. The study reveals that in the as-quenched samples Co is involved in two main structural components: small, irregular, strained Co particles (60% of the entire Co population) and a composition modulated CoCu alloy. A high modulation amplitude of the concentration profile in the alloy subdivides the latter in two parts with distinctly different properties. One part consists of ferromagnetic alloy (average Cu concentration of about 20%) with a blocking temperature of about 35 K (involving 6% of the entire Co population in a sample). The other part, containing the remaining 34% of the entire Co population, is a paramagnetic alloy with a blocking temperature below 4.2 K. The ferromagnetic alloy is magnetically soft—its transverse susceptibility is lower by a factor of 7 than the transverse susceptibility of the quenched-in Co particles. The latter population has a blocking temperature of about 150–200 K. During the heat treatment, each of the two main structural components undergoes respective decomposition processes: both of them display two temperature regimes. One process consists in dissolving the quenched-in Co particles after annealing at around 400 °C, followed at higher temperatures by a nucleation and growth of the more regular in shape Co particles. The other process resembles a spinodal decomposition of the quenched-in CoCu alloy, resulting in sharpening of the concentration profile and eventually leading to Co cluster formation in samples annealed above 450 °C. Both processes end at about Tan=700 °C, in formation of large, pure Co clusters that are ferromagnetic at least up to 400 K.