Influence of spherical assembly of copper ferrite nanoparticles on magnetic properties: orientation of magnetic easy axis

Abstract

The magnetic properties of copper ferrite (CuFe₂O₄) nanoparticles prepared via sol–gel auto combustion and facile solvothermal method are studied focusing on the effect of nanoparticle arrangement. Randomly oriented CuFe₂O₄ nanoparticles (NP) are obtained from the sol–gel auto combustion method, while the solvothermal method allows us to prepare iso-oriented uniform spherical ensembles of CuFe₂O₄ nanoparticles (NS). X-ray diffractometry (XRD), atomic absorption spectroscopy (AAS), infra-red (IR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), ⁵⁷Fe Mössbauer spectroscopy and vibrating sample magnetometer (VSM) are used to investigate the composition, microstructure and magnetic properties of as-prepared ferrite nanoparticles. The field-dependent magnetization measurement for the NS sample at low temperature exhibits a step-like rectangular hysteresis loop (M_R/M_S ∼ 1), suggesting cubic anisotropy in the system, whereas for the NP sample, typical features of uniaxial anisotropy (M_R/M_S ∼ 0.5) are observed. The coercive field (H_C) for the NS sample shows anomalous temperature dependence, which is correlated with the variation of effective anisotropy (K_E) of the system. A high-temperature enhancement of H_C and K_E for the NS sample coincides with a strong spin–orbit coupling in the sample as evidenced by significant modification of Cu/Fe–O bond distances. The spherical arrangement of nanocrystals at mesoscopic scale provokes a high degree of alignment of the magnetic easy axis along the applied field leading to a step-like rectangular hysteresis loop. A detailed study on the temperature dependence of magnetic anisotropy of the system is carried out, emphasizing the influence of the formation of spherical iso-oriented assemblies.