Coercivity extrema in melt-spun CuCo ribbons: Effects of the magnetic moment distribution

Measurements of magnetization loops on melt-spun CuCo ribbons revealed a minimum in the temperature dependence of the coercivity. A coherent interpretation was given through Monte Carlo simulations of a dispersed system of noninteracting, uniaxial magnetic granules embedded in a nonmagnetic matrix. The coercivity is implicitly defined by the balance between the negative magnetization of superparamagnetic granules and the remaining magnetization of blocked granules after saturation in the positive field direction. When the temperature rises in a system made of a large amount of small granules and a small amount of big granules, unblocking predominates over thermal fluctuations and the coercivity decreases until a certain temperature at which most of the small granules are superparamagnetic; above this temperature, thermal fluctuations predominate, and the coercivity increases almost linearly with the temperature until the final unblocking of the big granules. (C) 1999 American Institute of Physics. [S0021-8979(99)04618-6].