Metastable diamagnetic response of 20 nm La1-x MnO3 particles

The magnetic properties of compacted 20 nm La1-x MnO3 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 TC 220 K, below which the relative volume of the ferromagnetic (FM) phase at 5 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 K, HC 400 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.