Magnetism and charge ordering inFe3O2BO3studied byFe57Mössbauer spectroscopy

Abstract

The homometallic $\mathrm{Fe}$ ludwigite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{2}{\mathrm{BO}}_{3}$, is an oxyborate with a chain structure of oxygen octahedra occupied by ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ with four different crystallographic sites. We report $^{57}\mathrm{Fe}$ M\ossbauer spectroscopic studies on the magnetic spin arrangements and on the charge ordering in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{2}{\mathrm{BO}}_{3}$, between $4$ and $450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. From the temperature dependence of the isomer shifts and quadrupole interactions, together with the information from previous x-ray and transport data, we are able to discuss the structural arrangement of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ and the dynamics of the electronic configurations. Below ${T}_{N1}=112\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the M\ossbauer spectra show the onset of antiferromagnetic order only for a fraction of the $\mathrm{Fe}$ ions, the complementary fraction remaining paramagnetic down to $74\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Below $74\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ all the sample becomes magnetically ordered, and an increase and a canting of the magnetic hyperfine field for each $\mathrm{Fe}$ ion is observed. This canting is related to the presence of weak ferromagnetism, as proposed from magnetization measurements. Below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ antiferromagnetic order is found. From M\ossbauer spectra taken under applied external magnetic fields, there is strong evidence for magnetic short-range order above ${T}_{N1}$ involving ferromagnetic clusters. The observed magnetic transitions appear to be clearly related to charge ordering.