LOCAL ORDER AND GLOBAL DISORDER IN BIDISPERSE FERROFLUIDS

We study the magnetization response of a frozen ferrofluid as large-di ameter (10-nm) magnetite particles are added to a dilute, random distr ibution of small-diameter (5-nm) moments. The small dipole moments are free to relax down to low temperatures, while the large dipoles becom e immobilized when the solvent freezes, acting as a random local field . The glass transition temperature is a sensitive function of the rand om field concentration, permitting the investigation of the crossover from spin-glass to random-field behavior.