Metastable diamagnetic response of20nmLa1−xMnO3particles

Abstract

The magnetic properties of compacted $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{MnO}}_{3}$ particles, prepared by the citrate method, in pristine and metastable states have been investigated. It was found that in its pristine state the investigated sample displays a paramagnetic-to-ferromagnetic transition near ${T}_{C}\ensuremath{\approx}220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, below which the relative volume of the ferromagnetic (FM) phase at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ approaches a value of about 24%. Magnetization and ac-susceptibility measurements exhibit a cluster-glass-like behavior characterized by a noticeable difference between zero-field-cooled and field-cooled magnetization and frequency-dependent ac susceptibility. Different metastable states with highly reduced FM phase and ``negative ferromagnetism'' developed after a series of quick coolings of the sample placed in a container filled with silicon oil. The recorded temperature dependence of the negative FM moment appears to be a normalized replica of the corresponding FM dependence. Hysteresis loops of magnetization at low temperatures in both pristine and diamagnetic (DIA) states exhibit the same value of coercive field at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, ${H}_{C}\ensuremath{\approx}400\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$. The abnormal DIA state can only be erased after a few hours storage of the sample at room temperature. These observations are discussed with reference to a model in which the negative ferromagnetism is attributed to the appearance of nondispersive orbital currents which result in a coupling between the core of the FM particles and the surrounding diamagnetic matrix formed during the quick cooling cycles.