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

Measurement of switching field reduction of single domain particles in a two-dimensional array

The mechanism of switching of uniaxial, single domain, single crystalline epitaxial garnet particles on a two-dimensional square array was investigated, and the reason for the wide distribution of switching fields was studied. In spite that the particles were found very uniform, the existence of soft magnetic defects, not connected to visible crystalline or manufacturing defects of the material, was found to be responsible for the broad distribution of the switching field, H-c = 280 +/- 85 Oe, as measured on a large number of individual particles. Very good quantitative correlation was found between the strength of the these defects and the switching field

Magnetization process in a classical system of Preisach-type particles

The magnetization process of a regular two-dimensional array of small individual, strongly uniaxial magnetic garnet particles, groups of particles and major loop properties of a ''macroscopic'' sample has been investigated experimentally in an optical magnetometer. These particles correspond to the assumptions of a simple Preisach model. The switching mode is coherent rotation. Each particle has a square hysteresis loop, with no reversible or apparent reversible component. Requirements of wiping-out and congruency properties are satisfied. From measurements of the up- and down switching fields on individual particles, the major loop can be reconstructed, and it is shown to be in nearly exact correspondence with the measured one. The transition from individual to collective behavior is smooth and the properties of a system consisting of 100 particles correspond to the major loop behavior. The interaction field for an assembly of 25 particles was calculated numerically. The switching sequence and the magnetization curve for the 25 particles assembly was derived from the calculated interaction fields and found to be in a very good agreement with the measured values

On the role of statistics in the applicability of the Preisach transformation

Model systems of magnetic elements are investigated theoretically and experimentally. The elements are single domain, they can be switched "up" and "down", they all have the same magnetic moments but different coercive fields and they interact magnetostatically with each other. The analysis shows that such systems generally cannot be exactly represented either by the classical Preisach model or by the Moving Preisach Model as long as the number of the elements is finite. The representation can be considered unambiguous only if the number of elements approaches infinity