We have investigated the effects of dipolar coupling on the magnetization configurations of ferromagnetic rectangular particles with rounded-corners. Two arrays of particles with lateral size of 1025×450 nm2 have been patterned by deep UV lithography, followed by the lift-off of a permalloy film of thickness 40 nm. The first array consisted of long chains of interacting nanomagnets put head-to-tail, with 85 nm interdot spacing. In the second array the interdot distance was increased to 700 nm, in order to avoid any effect of dipolar interaction. In this sample the particles could be considered as magnetically isolated and their behavior as a reference for comparison with the interacting ones. Magneto-optical Kerr effect (MOKE) and in-field magnetic force microscopy (MFM) experiments, together with micromagnetic simulations, were performed. MFM measurements clearly showed that closure states characterized by one, two or three vortices occurred in isolated particles at remanence, together with the vortex/antivortex/vortex state. MFM performed on the same particles with a field applied along their hard direction (fig. 1) showed the detail of the nucleation and propagation process of vortices in isolated dots, in particular for the single vortex and the vortex/antivortex/vortex states. Decreasing the field intensity from positive saturation the nucleation process starts at one end of the particle below 200 Oe. The vortices cores then propagate perpendicularly to the field along the dot’s major axis toward the opposite end of the particle where they annihilate. When the field was applied along the major axis of interacting nanomagnets put head-to-tail the cooperation of intrinsic shape anisotropy and configurational anisotropy led to the formation of a regular sequence of single domain states (fig. 2), promoted by dipolar coupling, and the vortex formation was delayed during the reversal process. It turned out that the field range in which vortices exist is much narrower in the case of interacting particles if compared with isolated ones. An interesting result was that dipolar interaction can suppress some magnetic configurations that are present in isolated nanomagnets and favour other states. In particular, the double vortex was not observed in interacting particles, whereas the vortex/antivortex/vortex structure was found to be largely the most probable spin arrangement. A simple explanation of these findings, based on energy balance consideration, will be proposed. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°228673 (MAGNONICS).
Effect of dipolar coupling on vortex states formed in dense chains of ferromagnetic rectangular particles
BISERO, Diego;
2011
Abstract
We have investigated the effects of dipolar coupling on the magnetization configurations of ferromagnetic rectangular particles with rounded-corners. Two arrays of particles with lateral size of 1025×450 nm2 have been patterned by deep UV lithography, followed by the lift-off of a permalloy film of thickness 40 nm. The first array consisted of long chains of interacting nanomagnets put head-to-tail, with 85 nm interdot spacing. In the second array the interdot distance was increased to 700 nm, in order to avoid any effect of dipolar interaction. In this sample the particles could be considered as magnetically isolated and their behavior as a reference for comparison with the interacting ones. Magneto-optical Kerr effect (MOKE) and in-field magnetic force microscopy (MFM) experiments, together with micromagnetic simulations, were performed. MFM measurements clearly showed that closure states characterized by one, two or three vortices occurred in isolated particles at remanence, together with the vortex/antivortex/vortex state. MFM performed on the same particles with a field applied along their hard direction (fig. 1) showed the detail of the nucleation and propagation process of vortices in isolated dots, in particular for the single vortex and the vortex/antivortex/vortex states. Decreasing the field intensity from positive saturation the nucleation process starts at one end of the particle below 200 Oe. The vortices cores then propagate perpendicularly to the field along the dot’s major axis toward the opposite end of the particle where they annihilate. When the field was applied along the major axis of interacting nanomagnets put head-to-tail the cooperation of intrinsic shape anisotropy and configurational anisotropy led to the formation of a regular sequence of single domain states (fig. 2), promoted by dipolar coupling, and the vortex formation was delayed during the reversal process. It turned out that the field range in which vortices exist is much narrower in the case of interacting particles if compared with isolated ones. An interesting result was that dipolar interaction can suppress some magnetic configurations that are present in isolated nanomagnets and favour other states. In particular, the double vortex was not observed in interacting particles, whereas the vortex/antivortex/vortex structure was found to be largely the most probable spin arrangement. A simple explanation of these findings, based on energy balance consideration, will be proposed. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°228673 (MAGNONICS).I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
