Magnetic irreversibility and magnetization processes in Ni₅Al₃/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 ( T W I ( H ) and T S I ( 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 T WI with T C G + S G , 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. The T S I ( 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 k O e 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 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 ) 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.