Magnetic irreversibility and magnetization processes in Ni 5 Al 3 /NiO core/shell nanoparticle system.

This work brings out many interesting facets of magnetism in the Ni 5 Al 3 /NiO core/shell nanoparticle system. The weak and strong magnetic irreversibility lines (TWI(H)andTSI(H)) reproduce the previously reported H  -  T phase diagram at fieldsH⩽30 Oe, but strong departures occur for H  > 30 Oe. Comparison with the theoretically predicted H  -  T phase diagram allows us to identify T WI withTCG+SG, where the paramagnetic (PM)-chiral glass ( CG ) and PM-spin glass ( SG ) phase transitions occur simultaneously , and T SI with T SG , the temperature at which transition to the replica symmetry breaking SG state takes place. TheTSI(H)transition line abruptly ends at the point (H≃30 Oe,T≃90K). As H exceeds 30 Oe, a new transition appears which gets completely suppressed at fieldsH>1 kOewhere 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 random easy axis in which the magnetization reversal occurs through the coherent rotation of the magnetizations of weakly-interacting single-domain Ni 5 Al 3 particles. Saturation magnetization, like M ( T ) 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.