EFFECT OF MAGNETIC-FIELD ON THE SUPERPARAMAGNETIC RELAXATION IN GRANULAR CO-AG SAMPLES

In order to study the effect of applied magnetic field on the superpar amagnetic relaxation behavior of small Co particles, magnetization mea surements were carried out on as-prepared and annealed granular sample s of Co20Ag80 and Co25Ag75. Values of the superparamagnetic blocking t emperature T-B(-) were obtained from the characteristic peak in the ze ro-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease of T-B(-) with applied magnetic field is quadratic. An estimate of the magnetic anisotropy ''energy density '' K-u yielded a value which is two orders of magnitude greater than t he value for bulk cobalt. The results reported here underscore the imp ortance of considering the effect of superparamagnetic relaxation on t he performance of nanostructured magnetic materials. (C) 1998 American Institute of Physics.