Thin-film perovskite cobaltites have been found to exhibit coercivity values enhanced by almost 2 orders of magnitude in comparison to bulk. In this paper, we have investigated this unexplained coercivity enhancement in detail, focusing on epitaxial SrTiO${}_3$(001)/Nd${}_{0.5}$Sr${}_{0.5}$CoO${}_3$ [SrTiO${}_3=\mathrm{STO}$] films, which display coercivity values up to 40 kOe at low temperatures. Thickness-dependent (10–800 Å) magnetometry and magnetotransport studies demonstrate that nanoscopic magnetic phase separation occurs in the interface region of Nd${}_{0.5}$Sr${}_{0.5}$CoO${}_3$ [consistent with recent work on SrTiO${}_3$(001)/La${}_{1-x}$Sr${}_x$CoO${}_3$], which is responsible for the degradation in magnetic and electronic properties in the very-thin-film limit. The coercivity is shown to be intimately related to the existence of this (70-Å-thick) interfacial phase-separated layer, leading us to advance an explanation for the coercivity enhancement in terms of the pinning of domain walls by interfacial nanoscopic ferromagnetic clusters and a crossover to single domain clusters at very low thickness. Simple estimates of the magnetocrystalline anisotropy (from the maximum coercivity), cluster dimensions (from the superparamagnetic blocking temperature), multidomain to single domain crossover point, and domain-wall width, provide quantitative support for this picture.

• Received 11 April 2011

DOI:https://doi.org/10.1103/PhysRevB.84.024417