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The anhysteretic magnetization of the granular metallic alloy Cu 90 Co 10 is experimentally studied over a wide temperature range ͑2-700 K͒. 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 Cu 90 Co 10 granular alloy is therefore identified as an ''interacting superparamagnet'' ISP. 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.

Slow magnetic relaxation and hysteresis effects of dynamic origin have been observed above liquid helium temperature in a chain compound (see picture), comprising CoII centers and organic radicals, without any evidence of phase transition to three‐dimensional magnetic order. These results are the first evidence of the slow dynamics predicted for one‐dimensional magnetic systems with Ising anisotropy, and they open the possibility of storing information in a single magnetic nanowire.

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The magnetic properties of Cu97Co3 and Cu90Co10 granular alloys were measured over a wide temperature range (2 to 300 K). The measurements show an unusual temperature dependence of the coercive field. A generalized model is proposed and explains well the experimental behavior over a wide temperature range. The coexistence of blocked and unblocked particles for a given temperature rises difficulties that are solved here by introducing a temperature dependent blocking temperature. An empirical factor gamma (γ) arise from the model and is directly related to the particle interactions. The proposed generalized model describes well the experimental results and can be applied to other single-domain particle system.

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