Study on the field-cooling induced magnetic interactions in Gd-doped NiO nanoparticles

We herein investigate the novel magnetic interactions, such as exchange bias (EB), memory effect (ME) and magnetic relaxation dynamics (MRD) in NiO and 10 wt% Gd doped NiO nanoparticles synthesized by hydrothermal process. X-ray diffraction studies showed crystallization of face-centered-cubic structure for both NiO and Gd-doped NiO nanoparticles. Transmission electron microscopy analysis revealed a spherical morphology for both samples and their mean particle sizes were estimated as 24.5 and 10.3 nm for the pure and Gd-doped NiO nanoparticles, respectively. SQUID magnetic measurements demonstrated the occurrence of large EB coupling: 1.04 and 0.767 kOe; and enhanced coercive fields: 1.75 and 1.27 kOe, for the pure and Gd-doped NiO nanoparticles, respectively. The size-dependent magnetic properties such as, EB, ME and MRD is found to vanish at the average blocking temperature and interestingly, these properties set-in on cooling the samples at 5 K under applied field of 5 kOe. The surface defects such as oxygen vacancies and the Gd2O3 impurity phase played a significant role in determining the EB and ME characteristics of NiO nanoparticles.