Temperature dependence of the hyperfine magnetic field at Fe sites in Ba-Doped BiFeO_(3) thin films studied by emission Mossbauer spectroscopy

Abstract

Emission Fe-57 Mossbauer spectroscopy (eMS), following the implantation of radioactive Mn-57(+) 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 fi decays (t(1/ 2) = 90 s) to the 14.4 keV Mossbauer state of Fe-57, 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 SrTiO_3 substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mossbauer 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 Fe^(3+) ions located in octahedral sites (B_(hf)); and (ii) to Fe^(3+) 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 Bhf 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.