SrRuO_3 is an itinerant ferromagnet (T_c ~ 150 K) characterized by large uniaxial magnetocrystalline anisotropy (K_1 ~ 7.7 × 10~6 erg/cm~3) and relatively low saturation magnetization (M_s = 213 emu/cm~3) - properties which make SrRuO_3 an extremely hard magnetic material. We have patterned arrays of rectangular nanoislands of a high quality epitaxial film of SrRuO_3 with sides ranging between 50 and 500 nm, and studied their magnetization reversal at 4 K using a low temperature magnetic force microscope. We find that the nucleation field for many of the nanoislands is very close to that expected by the Stoner-Wohlfarth model (~ 3.8 T) and from nanoislands which exhibit partial reversals we can determine an upper bound for the nucleation volume on the order of 100 × 100 × 10 nm~3. We also find that domain wall pinning in the nanoislands is extremely high and in some cases the depinning field exceeds ~ 3 T.
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