Inhomogeneous structure and magnetic properties of granular Co10Cu90 alloys - art. no. 014408

Granular Co10Cu90 alloys displaying giant magnetoresistance have been obtai ned by melt spinning followed by an appropriate heat treatment in the range 0-700 degreesC. Their structural and magnetic properties have been studied on a microscopic scale using Co-59 NMR technique and thermoremanent magnet ization measurements. The study reveals that in the as-quenched samples Co is involved in two main structural components: small, irregular, strained C o particles (60% of the entire Co population) and a composition modulated C oCu alloy. A high modulation amplitude of the concentration profile in the alloy subdivides the latter in two parts with distinctly different properti es. One part consists of ferromagnetic alloy (average Cu concentration of a bout 20%) with a blocking temperature of about 35 K (involving 6% of the en tire Co population in a sample). The other part, containing the remaining 3 4% of the entire Co population, is a paramagnetic alloy with a blocking tem perature below 4.2 K. The ferromagnetic alloy is magnetically soft-its tran sverse susceptibility is lower by a factor of 7 than the transverse suscept ibility of the quenched-in Co particles. The latter population has a blocki ng temperature of about 150-200 K. During the heat treatment, each of the t wo main structural components undergoes respective decomposition processes: both of them display two temperature regimes. One process consists in diss olving the quenched-in Co particles after annealing at around 400 degreesC, followed at higher temperatures by a nucleation and growth of the more reg ular in shape Co particles. The other process resembles a spinodal decompos ition of the quenched-in CoCu alloy, resulting in sharpening of the concent ration profile and eventually leading to Co cluster formation in samples an nealed above 450 degreesC. Both processes end at about T-ap = 700 degreesC, in formation of large, pure Co clusters that are ferromagnetic at least up to 400 K.