Sub-100 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 nm to 67 nm, the FORC diagrams reveal striking differences, despite only subtle changes in their major hysteresis loops. The 52 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 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.

Magnetization reversal in nanomagnets via a vortex state, although often investigated at the remanent state, may not necessarily display a zero remanence or a highly pinched hysteresis loop. In contrast, the irreversible nucleation/annihilation events are clear indications of a vortex state. In this work, temperature induced single domain-vortex state transition has been investigated in 67 nm Fe nanodots using a first-order reversal curve (FORC) technique. The two phase coexistence is manifested as different features in the FORC distribution. At lower temperatures, it becomes harder to nucleate and annihilate vortices and the amount of single domain dots increases.

We have used the unique spatial sensitivity of polarized neutron and soft x-ray beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet and and antiferromagnet. The new uncompensated magnetization near the interface responds to applied field, while the uncompensated spins in the antiferromagnetic bulk are pinned, thus providing a means to establish exchange bias.