Temperature Dependence of the Hyperfine Magnetic Field at Fe Sites in Ba-Doped BiFeO₃ Thin Films Studied by Emission Mössbauer Spectroscopy

Emission ⁵⁷Fe Mössbauer spectroscopy (eMS), following the implantation of radioactive ⁵⁷Mn⁺ ions, has been used to study the temperature dependence of the hyperfine magnetic field at Fe sites in Ba-doped BiFeO₃ (BFO) thin films. ⁵⁷Mn β decays (t_1/2 = 90 s) to the 14.4 keV Mössbauer state of ⁵⁷Fe, thus allowing online eMS measurements at a selection of sample temperatures during Mn implantation. The eMS measurements were performed on two thin film BFO samples, 88 nm and 300 nm thick, and doped to 15% with Ba ions. The samples were prepared by pulsed laser deposition on SrTiO₃ substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mössbauer spectra of the two films, measured at absorber temperatures in the range 301 K–700 K, comprised a central pair of paramagnetic doublets and a magnetic sextet feature in the wings. The magnetic component was resolved into (i) a component attributed to hyperfine interactions at Fe³⁺ ions located in octahedral sites (B_hf); and (ii) to Fe³⁺ ions in implantation induced lattice defects, which were characterized by a distribution of the magnetic field B_Distr. The hyperfine magnetic field at the Fe probes in the octahedral site has a room temperature value of B_hf = 44.5(9) T. At higher sample temperatures, the B_hf becomes much weaker, with the Fe³⁺ hyperfine magnetic contribution disappearing above 700 K. Simultaneous analysis of the Ba–BFO eMS spectra shows that the variation of the hyperfine field with temperature follows the Brillouin curve for S = 5/2.