Excitation of Uniaxial-Anisotropy Relaxation Processes in Magnetic Films by a Rotating Magnetic Anneal

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Abstract
In hard-axis annealing studies of Permalloy films, which have revealed a number of discrete uniaxial-anisotropy relaxation processes, all processes are annealed simultaneously, making interpretation difficult and ambiguous. In this paper a new method is reported, in which individual processes are excited by a rotating magnetic anneal. Simple physical considerations show that the easy axes of only those processes with relaxation rates comparable to the annealing frequency will lag the annealing field when that field coincides with the easy axis. A theory based on discrete atomic processes predicts Debye peaks for the easy-axis lag angle vs annealing frequency; relaxation rates are determined from the location of these peaks. By varying temperature, activation energies and pre-exponential factors are obtained. The method of detection employs second-harmonic magnetoresistance in a configuration which is insensitive to magnetization dispersion. The measurement is made automatically in ∼0.2 sec twice every cycle; angular sensitivity is ∼10−4 rad. By means of this technique, the anisotropy spectrum in an 83% Ni-17% Fe film is being investigated between 40° and 400°C and over a frequency range 50 μHz to 0.1 Hz.