Surface spin disorder and spin-glass-like behaviour in manganese-substituted cobalt ferrite nanoparticles

Manganese-substituted cobalt ferrite nanoparticles coated with triethylene glycol (TREG) have been prepared by the glycothermal reaction. The effect of Mn substitution and coating on temperature-dependent magnetic properties of the TREG-coated Mn _x Co_1−x Fe₂O₄ nanoparticles (0.0 ≤ x ≤ 0.8) with size of ~5–7 nm has been investigated in the temperature range of 10–300 K in a magnetic field up to 9 T. After the irreversible processes of the magnetic hysteresis curves were completed, the high-field regions of these curves were fitted by using a ‘law of approach to saturation’ to extract the magnetic properties, such as the effective anisotropy constant (K _eff) and the anisotropy field (H _A) etc. High coercive field of 12.6 kOe is observed in pure cobalt ferrite coated with TREG at 10 K. The low temperature unsaturated magnetization behaviour indicates the core–shell structure of the Mn _x Co_1−x Fe₂O₄ NPs. Zero-field-cooled (ZFC) and field-cooled (FC) measurements revealed superparamagnetic phase of TREG-coated Mn _x Co_1−x Fe₂O₄ nanoparticles at room temperature. The blocking and irreversibility temperatures obtained from ZFC–FC curves decrease at highest Mn concentration (x = 0.8). The existence of spin-glass-like surface layer with freezing temperature of 215 K was established with the applied field dependence of the blocking temperatures following the de Almeida–Thouless line for the Mn_0.6Co_0.4Fe₂O₄ NPs. The shifted hysteresis loops with exchange bias field of 60 Oe and high-field irreversibility up to 60 kOe in FC M–H curve at 10 K show that spin-glass-like surface spins surrounds around ordered core material of the Mn_0.6Co_0.4Fe₂O₄ NPs. FMR measurement show that all the TREG-coated Mn _x Co_1−x Fe₂O₄ nanoparticles absorb microwave in broad field range of about ten thousands Oe. The spectra for all the samples have broad linewidth because of angular distributions of easy axis and internal fields of nanoparticles.