S-shaped configurational magnetic states in square permalloy dots

We experimentally observe both U- and S-shaped zero-field magnetic configurations in single square permalloy dots, with a dot’s configurational state dependent on field history and temperature. Magnetization of 170 nm x 170 nm x 7 nm dots is probed using 4-terminal anisotropic magnetoresistance (AMR) measurements. Room temperature field sweeps parallel to the dot edge yield AMR curves characteristic of both a U-shaped state [1], and of a previously unreported S-shape, for identical field sweep parameters. The S-shape sweep pattern is not observed at low T, implying the state is thermally activated, a result confirmed by micromagnetic simulations. The S-state also occurs when sweeping the field off-axis by several degrees, consistent with minimizing Zeeman interaction energy. The existence of multiple zero-field configurations precludes use of the Stoner-Wolfarth model typically used at this scale, instead necessitating a more complicated configurational picture as dot size decreases.
[1] Endean, et. al, Appl. Phys. Lett. 103, 042409 (2013)

This work supported in part by NSF Grant No. DMR 1609782. Portions of this work conducted in the Minnesota Nano Center, which is supported by the NSF through the National Nano Coordinated Infrastructure Network, Award Number NNCI -1542202.