Emission Mössbauer spectroscopy study of undoped and Ba-doped BiFeO$_{3}$ thin films

This dissertation presents the use of emission Mössbauer Spectroscopy (eMS), following the implantation of radioactive $^{57}$Mn* which β-decays to the 14.4 keV Mössbauer state of $^{57}$Fe, to study the magnetic behaviour of undoped and Ba-doped BiFeO$_{3}$ (BFO). The measurements were conducted within the Emission Mössbauer Collaboration at the on-line radioactive ion beam facility, ISOLDE, at CERN where the radioactive ions are produced by the fission of a UC$_{2}$ target induced by bombardment with 1.2 GeV protons. After multi-stage laser ionization and electrostatic mass separation, the extracted $^{57}$Mn ions are accelerated to 45 keV energy and implanted into the samples under study. Undoped and doped BFO samples with a Ba content of 15% were prepared by pulsed laser deposition from a target with 20% Bismuth excess. $^{57}$Fe-eMS data of 94 and 300 nm thick BBFO films and an undoped BFO sample were collected as a function of temperature, using a parallel plate avalanche counter in which one stainless-steel electrode was enriched to 90% in $^{57}$Fe. The counter was mounted outside the implantation chamber, at 90$^{o}$ to the Mn beam direction. The eMS spectra were dominated by a broad central doublet together with sextet structure in the wings. Simultaneous analysis of the spectra with the software code VINDA allowed the sextets structure to be resolved into a sextet due to probe Fe ions located at the Fe sites in the BFO and BBFO lattice (Fe$_{BFO}$) and a broad magnetic distribution due to Fe at defect sites such as oxygen vacancies. The main focus of the study was the temperature-dependent behaviour of the magnetic field at the Fe sites in the undoped and Ba-doped samples (Fe$_{BFO}$). The main component of the magnetic sextet was characterized by a hyperfine field with room temperature values 𝐵$_{ℎ𝑓}$ = 46.2 (1) T in the undoped sample and 𝐵$_{ℎ𝑓}$ = 42.9 (6) T in the two Ba-doped samples. The isomer shift for all three samples, $\delta$ = 0.36(1) mm/s, was characteristic of Fe$^{3+}$ in the high spin 5/2 state. Two features characterised the temperature-dependence of 𝐵$_{ℎ𝑓}$ :i) the magnitude of 𝐵$_{ℎ𝑓}$ showed a strong decrease as the temperature was increased, and ii) the intensity (spectral area) decreased rapidly at higher temperatures. Result (ii) made it a lot more difficult to extract the 𝐵$_{ℎ𝑓}$ values at higher temperatures. However, the temperature dependence of the magnetic component of the Ba-doped BiFeO$_{3}$ follows that of a ferromagnetic material and confirms that the anti-ferromagnetic virgin BFO film underwent a phase transformation on doping with Ba ions.