Competing magnetic states in multiferroic BaYFeO4: A high magnetic field study

Spin-driven ferroelectricity phenomena have drawn great interest in the scientific community due to potential application in spintronics and their complex physical mechanisms. A noticeable example of this is multiferroic $\mathrm{BaYFe}{\mathrm{O}}_{4}$ that exhibits an unconventional magnetoelectric (ME) coupling due to the uncorrelated behavior of the ferroelectric and cycloidal states under an applied magnetic field. To shed more light on this spin-driven ME effect, a high-quality sample of $\mathrm{BaYFe}{\mathrm{O}}_{4}$ was synthesized by a standard solid-state reaction method, and its high-field (up to 9 T) magnetic properties have been systematically investigated by means of magnetometry, magnetocaloric effect, and M\"ossbauer measurements over a wide temperature range (5--400 K). In addition, its crystal and magnetic structures have been studied using x-ray and neutron powder diffraction. Results obtained indicate that Fe spins form a long-range spin density wave (SDW) antiferromagnetic (AFM) order at ${T}_{\mathrm{N}1}\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which transforms into the cycloidal AFM order at ${T}_{\mathrm{N}2}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. A spin-glass-like state emerges below ${T}^{*}\ensuremath{\sim}17\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and coexists with the long-range cycloidal AFM one in this temperature range. Magnetocaloric and M\"ossbauer measurements consistently confirm the robustness of both the long-range SDW and cycloidal AFM orders under applied magnetic fields up to 6 T, whereas the spin-glass state is converted into the ferromagnetic (FM) state when the applied magnetic field exceeds 1 T. These findings pinpoint the fact that the magnetic field evolution of spin correlations from the AFM to FM character in the spin-glass state is responsible for the magnetic field dependence of ferroelectricity in $\mathrm{BaYFe}{\mathrm{O}}_{4}$.