Magnetic fingerprints of sub-100nmFe dots

Abstract

Sub-$100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ nanomagnets not only are technologically important, but also exhibit complex magnetization reversal behaviors as their dimensions are comparable to typical magnetic domain wall widths. Here we capture magnetic ``fingerprints'' of ${10}^{9}$ Fe nanodots as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method. As the nanodot size increases from $52\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}67\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the FORC diagrams reveal striking differences, despite only subtle changes in their major hysteresis loops. The $52\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ nanodots exhibit single domain behavior and the coercivity distribution extracted from the FORC distribution agrees well with a calculation based on the measured nanodot size distribution. The 58 and $67\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ nanodots exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterflylike features in the FORC distribution and confirmed by micromagnetic simulations. Furthermore, the FORC method gives quantitative measures of the magnetic phase fractions, and vortex nucleation and annihilation fields.