Granular Cu-Co alloys as interacting superparamagnets

The anhysteretic magnetization of the granular metallic alloy Cu90Co10 is experimentally studied over a wide temperature range (2-700 K). The measurements definitely exclude that this alloy is a simple superparamagnet, even in the high-temperature limit, although some features of granular systems [such as the typical Langevin-like form of the anhysteretic magnetization curves M(H)] are often taken as evidence of superparamagnetism. A phenomenological theory is proposed, explicitly considering that particle moments interact through long-ranged dipolar random forces, whose effect is pictured in terms of a temperature T*, adding to the actual temperature T in the denominator of the Langevin function argument. This simple formula explains all features of the experimental M(H) curves. The theory indicates that the actual magnetic moments on interacting Co particles are systematically larger than those obtained fitting the magnetic data to a conventional Langevin function. The Cu90Co10 granular alloy is therefore identified as an "interacting superparamagnet" ISR The ISP regime appears as separating the high-temperature, conventional superparamagnetic phase from the low-temperature, blocked-particle regime. In this way, a magnetic-regime diagram can be drawn for each granular system. The competition between single-particle and collective blocking mechanisms is briefly analyzed. The proposed interpretation is thought to be applicable to other fine particle systems; its main features and intrinsic limits are discussed.