Magnetic behavior of La-doped Fe₃O₄ studied by perturbed angular correlation spectroscopy with ¹¹¹Cd and ¹⁴⁰Ce

In this paper, the local magnetic properties of La-doped Fe₃O₄ (5% and 10%) bulk and Nanoparticles (NPs) samples were studied by measuring hyperfine interactions in a wide range of temperature from 10 to 900 K with perturbed γ-γ angular correlation spectroscopy using ¹¹¹In(¹¹¹Cd) and ¹⁴⁰La(¹⁴⁰Ce) as probe nuclei. Results for the temperature dependence of the magnetic hyperfine field (B_hf) for bulk and NP samples, pure and doped with La show that its behavior follows a second order Brillouin-like transition from which the Curie temperature (T_C) was determined (T_C ∼ 855 K). Results also show two different regions in NP samples: the core where a minor fraction of probe nuclei with well defined magnetic dipole frequency was observed and the shell where a major fraction with broad distributed electric quadrupolar frequency (surface effect in NP) was observed. The Verwey transition T_V ∼ 120 K, due the order disorder phase, was also observed in all samples. The results are discussed in terms of the magnetic exchange interaction between Fe²⁺ and Fe³⁺ ions in the two regions of NP