Self-assembly of an exchange-spring composite via magnetic phase separation in Pr$_{1-x}$Ca$_{x}$CoO$_{3}$

We report structural and magnetic properties in bulk polycrystalline Pr$_{1-x}$Ca$_{x}$CoO$_{3}$ (0.00$\le \quad x \quad \le $0.30) from neutron diffraction, thermogravimetic analysis, magnetometry, and small-angle neutron scattering (SANS). Upon cooling, the Pr$_{0.70}$Ca$_{0.30}$CoO$_{3}$ composition (deep in the FM phase) first undergoes a transition around 250 K where short-range FM clusters emerge with a size of order 1-2 unit cells. The magnetization and SANS intensity slowly increase on cooling to 70 K, where the system undergoes a transition to a long-range ordered FM state, but with low magnetization, indicative of a small FM volume fraction. Magnetometry and SANS data indicate coexistence of the short-range clusters within a network of long-range FM. The coercivities H$_{C}$ of the short-range and long-range FM regions are very different, and a non-monotonic $T$ dependence of the H$_{C}$ reveals clear evidence of FM exchange coupling between the phase-separated regions. In essence the phase separation leads to natural formation of a hard/soft composite, which displays classic exchange spring behavior. Work at UMN supported by DoE.