Single-domain shape anisotropy in near-macroscopic Ni₈₀Fe₂₀ thin-film rectangles

Shape anisotropy provides a simple mechanism to adjust the local bias field in patterned structures. It is well known that for ellipsoidal particles $<1$ $\mu$m in size, a quasi-single domain state can be realized with uniform anisotropy field. For larger patterned ferromagnetic thin-film elements, domain formation is thought to limit the effectiveness of shape anisotropy. In our work we show that very soft lithographically patterned Ni$_{80}$Fe$_{20}$ films with control of induced magnetic anisotropy can exhibit shape anisotropy fields in agreement with single-domain models, for both hysteresis loop measurements at low field and ferromagnetic resonance measurements at high field. We show the superiority of the fluxmetric form over the magnetometric form of anisotropy estimate for thin films with control dimensions from 10 $\mu$m to 150 $\mu$m and in-plane aspect ratios above 10.