Non-equilibrium cation distribution and enhanced spin disorder in hollow CoFe₂O₄ nanoparticles

We present magnetic properties of hollow and solid CoFe₂O ₄ nanoparticles that were obtained by annealing of Co ₃₃Fe₆₇/CoFe₂O₄ (core/shell) nanoparticles. Hollow nanoparticles were polycrystalline whereas the solid nanoparticles were mostly single crystal. Electronic structure studies were performed by photoemission which revealed that particles with hollow morphology have a higher degree of inversion compared to solid nanoparticles and the bulk counterpart. Electronic structure and the magnetic measurements show that particles have uncompensated spins. Quantitative comparison of saturation magnetization (M_S), assuming bulk Néel type spin structure with cationic distribution, calculated from quantitative XPS analysis, is presented. The thickness of uncompensated spins is calculated to be significantly large for particles with hollow morphology compared to solid nanoparticles. Both morphologies show a lack of saturation up to 7T. Moreover magnetic irreversibility exists up to 7T of cooling fields for the entire temperature range (10-300K). These effects are due to the large bulk anisotropy constant of CoFe₂O₄ which is the highest among the cubic spinel ferrites. The effect of the uncompensated spins for hollow nanoparticles was investigated by cooling the sample in large fields of up to 9T. The magnitude of horizontal shift resulting from the unidirectional anisotropy was more than three times larger than that of solid nanoparticles. As an indication signature of uncompensated spin structure, 11% vertical shift for hollow nanoparticles is observed, whereas solid nanoparticles do not show a similar shift. Deconvolution of the hysteresis response recorded at 300K reveals the presence of a significant paramagnetic component for particles with hollow morphology which further confirms enhanced spin disorder.