Magnetic Transitions in Chemically Synthesized Nanoparticles of CoCr₂O₄

摘要

Bulk CoCr2O4 undergoes a transition from paramagnetic to long-range ferrimagnetic phase at $T_{c}$ (94 K) to a long-range and/or short-range spiral order at $T_{s}$ ( $sim 24$ K), and finally shows a lock-in-transition below 15 K. The spiral component induces an electric polarization and also a spontaneous magnetization for which it is said to be multiferroic. Reducing the size of a CoCr2O4 multiferroic material to $sim 50$ nm by a coprecipitation method, we obtain a pure cubic phase with space group, Fd3m and lattice parameter (8.334 ± 0.003 °A). A rich sequence of magnetic transitions are examined by measuring temperature and field-dependent magnetization and diffused neutron scattering (DNS) using polarized neutron at different temperatures. While paramagnetic to ferrimagnetic transition is enhanced from 97 K in bulk to 99 K at 0.5 kOe field, followed by a decrease in lock-in-transition ( $T_{L}$ ) from 15 K in bulk to 8 K, spiral ordering temperature does not show a significant change. A strong disagreement between paramagnetic moment obtained from the fitting of $chi ^{-1}=({T}/{C})+({1}/{chi _{o}})-({b}/{T-theta })$ and ferrimagnetic moment obtained from the $M$ versus $H$ loop taken at 2 K, nonsaturated magnetization at 50–100 kOe field, two order of magnitude higher coercivity ( $H_{c}$ ), and splitting of ac susceptibly confirm the core–shell structure of the particles. Furthermore, a magnetic scattering analysis clearly shows that while the paramagnetic to ferrimagnetic transition is continuous, the spiral ordering is sharp, short range, and commensurate in contrast to incommensurate spiral order observed single crystal of CoCr2O4.