Magnetic phase separation-induced coercivity enhancement in epitaxial Nd$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films*

Interfacial magneto-electronic phase separation has recently been observed in epitaxial thin films of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ at doping values where no such phase separation exists in bulk. Such systems also display anomalously large coercivity, which is not understood. To achieve a better understanding of this phenomenon we have extended this study to Nd$_{1-x}$Sr$_{x}$CoO$_{3}$ (x = 0.5), the perovskite cobaltite with the largest coercivity in bulk. Thin films of Nd$_{0.5}$Sr$_{0.5}$CoO$_{3}$ are grown via high pressure reactive sputtering on SrTiO$_{3}$ (001) substrates. We have observed a rapid deterioration in magnetization and onset of large intercluster-type magnetoresistance below a critical thickness of 80 {\AA}, signatures of interfacial magneto-electronic phase separation also seen in our earlier work on La$_{1-x}$Sr$_{x}$CoO$_{3}$. The temperature, angular, and thickness dependence of the coercivity ($H_{c})$ was studied using magnetoresistance. Low temperature $H_{C}$ values become very large (up to 3.6 Tesla) at low thickness, and a strong, superlinear $T$ dependence emerges. We propose that the coercivity enhancement arises due to efficient domain wall pinning by the inhomogeneous magnetically phase separated region near the SrTiO$_{3}$ substrate.