Field dependence of the switching field for nonellipsoidal single domain particles

Experimental data on a model system of a two-dimensional array of single domain garnet particles, switching by incoherent rotation, are presented to show that the switching field of individual particles, H-sw, and the coercivity of the major hysteresis loop for similar to1000 particles, H-c, depend on the previously applied saturating field. For the system measured the asymptotic, "true" value of H-c in large fields is 321 Oe, in contrast with H-c=225 Oe, measured in an applied field of H-sat=188 Oe, i.e., the smallest field adequate to close the major loop. Statistical data were collected on switching of a single particle, with an asymptotic value of H-sw=150 Oe. After the application of H-sat=160 Oe H-sw decreased to 111 Oe. Due to the nonellipsoidal shape of the particles, a significant canting of the magnetization near corners and edges persists up to very high fields. The torque, due to these canted magnetic moments, facilitates premature switching in lower fields. It is proposed that defects are responsible for the irreversibility of the process

Major loop reconstruction from switching of individual particles

Major hysteresis loops of groups of isolated 60 mm square garnet particles of a regular two-dimensional array, have been measured magnetooptically. Individual loops for each particle were measured, and the statistics of the distribution of coercivities and interaction fields was determined. It is shown that from the measured coercivity distribution and calculated magnetostatic interaction fields the major hysteresis loop can be reconstructed. The switching sequence, and the major loop of an assembly of 535 particles were calculated numerically for two cases: first, when calculating the magnetostatic interaction, the 25 particles were assumed to be isolated; second, the major loop of the same 25 particles, embedded into a 939 square, was reconstructed taking into account the interactions among all 81 particles. The numerically simulated major hysteresis loops agree very well with the measured loops, demonstrating the reliability of numerical modeling

Analysis of Surface Roughness Influence in Non-Destructive Magnetic Measurements Applied to Reactor Pressure Vessel Steels

The influence of surface roughness on magnetic measurements of Reactor Pressure Vessel Steels was investigated by applying two types of magnetic, non-destructive measurement on nuclear reactor pressure vessel steel samples: magnetic adaptive testing (MAT) and magnetic Barkhausen noise measurement (MBN). The surface roughness was modified by primary and secondary machine cutting forces. Different settings of machine cutting produced different surface conditions. It was found that for both measurements a monotonic correlation was found to exist between magnetic parameters and surface roughness. Results of the MAT measurements found that the correlation depends on the speed (i.e., on the applied slew rate) of the magnetizing current. In a similar fashion, results from the MBN method show good agreement with MAT, where the response diminishes with an increase in surface roughness. The results show the importance of accounting for surface condition in the interpretation of results of non-destructive magnetic testing

Dipolar random field Ising model: an application to garnet films

The dipolar-random field Ising model (DRFIM) recently introduced displays a behaviour that can be connected to the magnetization of bidimensional magnetic media. Epitaxial magnetic garnet films seem to be the ideal test material for such a model. In this work the results of the measurements performed on garnet samples are presented, as well as the comparisons with simulation results obtained by the DRFIM. The results prove that a variety of hysteresis loops are well described by the DRFIM. This capability does not derive from the fine tuning of a great number of parameters, but by the interplay of exchange and dipolar interactions.Comment: HTML paper, GIF images. To appear on Phys.Rev.B (Brief Reports). Application of the model published on Phys.Rev.B 59 (1999) 985, cond-mat/990707