Development of vortex state in circular magnetic nanodots: Theory and experiment

Abstract

We compare magnetic reversal of nanostructured circular magnetic dots of different sizes. This comparison is based on superconducting quantum interference device ͑SQUID͒ magnetometry, neutron scattering, Monte Carlo simulation, and analytical calculations and is quantified using a parameter which characterizes the variation in the hysteresis curve width. Below a critical dot diameter, the magnetic reversal occurs by coherent rotation and above that diameter, the reversal occurs by formation of a magnetic vortex. The vortex-core diameter is controlled by competing magnetic energy contributions. For 20-nm-thick Fe dots, the values of the critical diameter ͑58-60 nm͒ and the vortex core ͑16-19 nm͒ are in very good agreement between the different experimental and theoretical methods: neutron scattering, SQUID magnetometry, Monte Carlo simulations, and analytical calculations.

Tailoring the magnetic properties of Fe asymmetric nanodots

“…Looking at these snapshots we can conclude that a vortex nucleates during the reversal for any value of δ. In symmetric dots, square loops are a sign of coherent reversal, and the appearance of a neck indicates that the reversal is driven by a vortex nucleation and propagation 22 . However for asymmetric dots reversal by vortex nucleation may lead to a square loop.…”

“…For our simulations we use the typical Fe parameters: saturation magnetization M s = 1.859 × 10 6 A/m, exchange stiffness constant A = 45.78 × 10 −12 J/m, and a mesh size of 2 nm, where spins are free to rotate in three dimensions. Since we are interested in polycrystalline samples, anisotropy is very small and can be safely neglected 22 . In all the cases the damping parameter was chosen as 0.5.…”

“…A symmetric dot is characterized by δ = 1.0, while a semicircular one is represented by δ = 0.0. To simulate the magnetic properties we used micromagnetic simulations, assuming that the dot-dot distance is large enough to consider every dot as independent 22 . For our simulations we use the typical Fe parameters: saturation magnetization M s = 1.859 × 10 6 A/m, exchange stiffness constant A = 45.78 × 10 −12 J/m, and a mesh size of 2 nm, where spins are free to rotate in three dimensions.…”

Influence of shape, size and internal structure on magnetic properties of core-edge nanodots with perpendicular anisotropy

“…28 For much thicker Fe islands (20 nm thick), a critical diameter for CR is shifted much further-up to 60 nm. 29 However, larger dots with uniform magnetization may exhibit a similar behavior to CR law, although their magnetization reversal is far from coherent. It is plausible when demagnetization field is negligible and nucleation volume is much smaller than the dot volume.…”

“…The crossover from the vortex state to the single domain proceeds in annihilation field. 29,[31][32][33] Configurational anisotropy, resulting from the dot shape, forces complex magnetization reversal. In triangular permalloy dots, due to three-fold symmetry, magnetization hysteresis loops recorded along two major directions were clearly different.…”