Magnetic irreversibility and magnetization processes in NiAl/NiO core/shell nanoparticle system.

This work brings out many interesting facets of magnetism in the NiAl/NiO core/shell nanoparticle system. Theandmagnetic irreversibility lines (TWI(H)andTSI(H)) reproduce the previously reported - phase diagram at fieldsH⩽30 Oe, but strong departures occur for > 30 Oe. Comparison with the theoretically predicted - phase diagram allows us to identifywithTCG+SG, where the paramagnetic (PM)-chiral glass () and PM-spin glass () phase transitions occur, andwith, the temperature at which transition to the replica symmetry breakingstate takes place. TheTSI(H)transition line abruptly ends at the point (H≃30 Oe,T≃90K). Asexceeds 30 Oe, a new transition appears which gets completely suppressed at fieldsH>1 kOewhere the magnetic irreversibility ceases to exist. Nolong-range ferromagnetic ordering exists but fields as low as 3 kOe suffice to induce long-range ferromagnetic order. At fixed temperatures, the magnetocrystalline anisotropy fluctuations essentially govern the 'approach-to-saturation' in magnetization for fields in the range 3 - 70 kOe. The present nanocrystalline system behaves as an isotropic system with random easy axis in which the magnetization reversal occurs through the coherent rotation of the magnetizations of weakly-interacting single-domain NiAlparticles. Saturation magnetization, like() atH⩾2 kOe, exhibits an anomalous upturn at temperatures below ≈ 30 K. This upturn is associated with the anomalous softening of spin-wave modes which results in the thermal excitation of a large number of non-equilibrium (finite lifetime) magnons. At sub-Kelvin temperatures, these magnons undergo Bose-Einstein condensation.