We herein report a comprehensive investigation on the magnetic, structural, and electric properties of CoO nanoparticles with different sizes by local inspection through hyperfine interactions measured in a wide range of temperatures (10–670 K) by using radioactive $$^{111}$$111 In( $$^{111}$$111 Cd) tracers with the perturbed angular correlations technique. Small cobalt oxide nanoparticles with the characteristic size of 6.5 nm have been prepared by the wet chemical route that turned out to be essential to incorporate radioactivity tracers during nucleation and growth of the particles. Nanocrystalline samples with 22.1 nm size were obtained by thermal treatments under low pressure of helium at 670 K. The hyperfine data were correlated with X-ray diffraction, ZFC–FC magnetic measurements, and transmission electron microscopy to describe the structure, magnetic properties, size, and shape of samples. An analysis of the temperature evolution of hyperfine parameters revealed that the structural distortion and the magnetic disorder in the core and on the surface layer play an important role in the magnetic behavior of CoO nanoparticles.

Time differential perturbed $\gamma -\gamma$ angular correlation technique has been used to measure the magnetic hyperfine field at both Mn and Ce atom sites in ${\mathrm{CeMn}}_2{\mathrm{Ge}}_2$ and ${\mathrm{CeMn}}_2{\mathrm{Si}}_2$ intermetallic compounds. The probe nucleus ${}^{111}{\overrightarrow{\mathrm{In}}}^{111}\mathrm{Cd}$ was used to investigate the hyperfine interaction at Mn sites, while the ${}^{140}{\overrightarrow{\mathrm{La}}}^{140}\mathrm{Ce}$ was used to measure the magnetic hyperfine field at Ce. Present measurements cover the temperature range from 10 to 460 K in both compounds. Measurements with ${}^{111}\mathrm{Cd}$ probe show pure electric quadrupole interactions above 420 K in both compounds and a combined magnetic dipole and electric quadrupole interaction below respective magnetic transition temperatures. While the temperature dependence of the hyperfine field at ${}^{111}\mathrm{Cd}$ on Mn site in the ${\mathrm{CeMn}}_2{\mathrm{Si}}_2$ showed a standard behavior for an antiferromagnetic compound below $T_N\approx 408\mathrm{K},$ the temperature dependence of magnetic hyperline field (MHF) in ${\mathrm{CeMn}}_2{\mathrm{Ge}}_2$ showed a transition from ferromagnetic to antiferromagnetic phase at around 320 K and from antiferromagnetic to paramagnetic phase at 420 K. No hyperfine field was observed at ${}^{140}\mathrm{Ce}$ in ${\mathrm{CeMn}}_2{\mathrm{Si}}_2$ in the temperature range from 10 K to 400 K. A unique and well-defined magnetic interaction is observed at Ce sites in ${\mathrm{CeMn}}_2{\mathrm{Ge}}_2$ below 320 K, corresponding to a ferromagnetic ordering of Mn moments. The temperature dependence of MHF, however, shows a sharp deviation from an expected Brillouin-like behavior for temperatures below 120 K. The additional magnetic interaction is believed to result from the polarization of Ce spin moments induced by the magnetic field from Mn moments.

The magnetic behavior of Gd₂O₃ nanoparticles, produced by thermal decomposition method and subsequently annealed at different temperatures, was investigated by magnetization measurements and, at an atomic level, by perturbed γ − γ angular correlation (PAC) spectroscopy measuring hyperfine interactions at ¹¹¹In(¹¹¹Cd) probe nuclei. Nanoparticle structure, size and shape were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). Magnetization measurements were carried out to characterize the paramagnetic behavior of the samples. XRD results show that all samples crystallize in the cubic-C form of the bixbyite structure with space group Ia3. TEM images showed that particles annealed at 873 K present particles with highly homogeneous sizes in the range from 5 nm to 10 nm and those annealed at 1273 K show particles with quite different sizes from 5 nm to 100 nm, with a wide size distribution. PAC and magnetization results show that samples annealed at 873 and 1273 K are paramagnetic. Magnetization measurements show no indication of blocking temperatures for all samples down to 2 K and the presence of antiferromagnetic correlations.