S-shaped configurational magnetic states in square permalloy particles

We experimentally observe both U- and S-shaped zero-field magnetic configurations in single square permalloy dots, with a given dot’s configuration dependent on field history and temperature. Magnetization of 170 nm x 170 nm x 7 nm dots is probed using 4-terminal resistance measurements via the anisotropic magnetoresistance (AMR). RT field sweeps parallel to the square’s edge produce AMR curves characteristic of both the known U-shaped state [1], as well as curves characteristic of a previously unreported S-shaped state for identical field sweep parameters. Measurements at 4K do not generate the S-state pattern, suggesting thermal activation, a result confirmed by micromagnetic simulation. The S-state is also realized by sweeping an off-axis field, consistent with minimizing Zeeman interaction energy. Existence of multiple zero-field configurations complicates the modeling of dots on this scale, precluding use of the Stoner-Wolfarth model and necessitating a more complicated configurational picture.
[1] Endean, et. al, Appl Phys Lett 103, 042409 (2013)