Deconvoluting reversal modes in exchange-biased nanodots

Ensemble-averaged exchange bias in arrays of Fe/FeF${}_2$ nanodots has been deconvoluted into local, microscopic bias separately experienced by nanodots going through different reversal modes. The relative fraction of dots in each mode can be modified by exchange bias. Single-domain dots exhibit a simple loop shift, while vortex state dots have asymmetric shifts in the vortex nucleation and annihilation fields, manifesting local incomplete domain walls in these nanodots as magnetic vortices with tilted cores.

Figure 1

Major hysteresis loops measured at 40 K, after separate FC and ZFC procedures, in comparison with that measured at RT. Inset shows a scanning electron micrograph of 67-nm unbiased Fe nanodots.

Figure 2

Families of FORCs, whose starting points are represented by black dots (a) at RT and (c) after FC to 40 K. The corresponding FORC distributions are shown as contour plots in (b) and (d), respectively. The circled regions in (b) and (d) indicate the highly irreversible processes associated with the VS nucleation and annihilation, while the dotted black box highlights the SD reversal mode. (e) Schematic hysteresis loop for unbiased vortices with nucleation field $H_N$ and annihilation field $H_A$ labeled. The inset shows an exchange-biased loop. (f) Schematic FORC distribution indicating the relationship between the three primary FORC peaks in the ($H$, $H_R$) coordinate system and the nucleation/annihilation fields shown in (e).

Figure 3

Projection of the FORC distribution along the $H_B$ axis, $dM$/$d{H}_B$, for the SD reversal mode at RT, after FC, and after ZFC. The shift of the peak along the $H_B$ axis indicates the exchange bias of the SD dots.