Slow relaxation effects in magnetically nonuniform materials

The nonstationary magnetization reversal was investigated as a function of frequency
f
and amplitude
H_{\max}
of the magnetizing field in samples made of magnetite or iron powders, in cold rolled strips of nickel, as well as in multiple films in which nickel laminations are separated by nonmagnetic materials. The magnetization loops were cycled with triangular waveform fields with very low frequencies from 0.005 to 2 c/s, in the wide interval of temperatures from 297° to 4.2°K. The dynamic coercive force of the magnetization loops of these materials increases in approximately the same manner with the product
fH_{\max}
Similar results are also obtained by studying the hysteresis loops of the magnetoresistance effect of the nickel strips. The lower the temperature, the more the relaxational properties are expressed. The investigated materials were considered as magnetically nonuniform because of the expected large density of different internal irregularities. Their intrinsic magnetic structure was described by a nonuniform magnetization, as in pairs of neighboring domain walls. Such a structure should be established by magnetic coupling of small volumes of subdomain sizes. The most likely cause of the reversal is a slow relaxation process introduced by the superparamagnetic properties of the subdomains and the growth of magnetic nuclei. The dependence of the dynamic coercivity of nickel strips on the roll reduction was interpreted from the viewpoint of discrete ordered regions, as in multiple films. These regions may be seen as fine lamellae separated by a matrix of disordered material. Anomalous magnetoresistance loops observed at liquid helium temperatures in some nickel strips with large roll reduction may be an indication of instabilities in the different intrinsic magnetic structures leading to slow transitions from one structure to another. Following the treatment of Kaczér for the interaction between walls, a phenomenological model is outlined for the magnetization hysteresis in laminated materials in which stabilization of domain double walls of different types takes place.