Magnetic irreversibility and magnetization processes in Ni5Al3/NiO core/shell nanoparticle system.

Abstract

This work brings out many interesting facets of magnetism in the Ni5Al3/NiO core/shell nanoparticle system. The weak and strong magnetic irreversibility lines (TWI(H) and TSI(H)) reproduce the previously reported H - T phase diagram at fields H ≤ 30 Oe, but strong departures occur for H > 30 Oe. Comparison with the theoretically predicted H - T phase diagram allows us to identify TWIwith TCG+SG, where the paramagnetic (PM)-chiral glass (CG) and PM-spin glass (SG) phase transitions occur simultaneously, and TSIwith TSG, the temperature at which transition to the replica symmetry breaking SG state takes place. The TSI(H) transition line abruptly ends at the point (H ≃ 30 Oe, T ≃ 90 K). As H exceeds 30 Oe, a new transition appears, which gets completely suppressed at fields H > 1 kOe, where the magnetic irreversibility ceases to exist. No intrinsic long-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 a random easy axis in which the magnetization reversal occurs through the coherent rotation of the magnetizations of weakly-interacting single-domain Ni5Al3particles. Saturation magnetization, like M(T) at H ≥ 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.&#xD.