Weak magnetism of Aurivillius-type multiferroic thin films probed by polarized neutron reflectivity

Abstract

Unambiguous magnetic characterization of room-temperature multiferroic materials remains challenging due in part to the difficulty of distinguishing their very weak ferromagnetism from magnetic impurity phases and other contaminants. In this study, we used polarized neutron reflectivity to probe the magnetization of $\mathrm{B}{\mathrm{i}}_{6}\mathrm{FeCoT}{\mathrm{i}}_{3}{\mathrm{O}}_{18}$ and $\mathrm{LaB}{\mathrm{i}}_{5}\mathrm{FeCoT}{\mathrm{i}}_{3}{\mathrm{O}}_{18}$ in their epitaxial thin films while eliminating a variety of impurity contributions. Our results show that $\mathrm{LaB}{\mathrm{i}}_{5}\mathrm{FeCoT}{\mathrm{i}}_{3}{\mathrm{O}}_{18}$ exhibits a magnetization of about $0.016\ifmmode\pm\else\textpm\fi{}0.027\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co}$ pair at room temperature, while the $\mathrm{B}{\mathrm{i}}_{6}\mathrm{FeCoT}{\mathrm{i}}_{3}{\mathrm{O}}_{18}$ thin film only exhibits a weak magnetic moment below room temperature, with a saturation magnetization of $0.049\ifmmode\pm\else\textpm\fi{}0.015\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co}$ pair at 50 K. This polarized-neutron-reflectivity study places an upper magnetization limit on the matrix material of the magnetically doped Aurivillius oxides and helps to clarify the true mechanism behind the room-temperature magnetic performance.