Competing magnetic states in multiferroic BaYFe O 4 : 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 BaYFeO4 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 BaYFeO4 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 Mossbauer 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-N1 similar to 50K, which transforms into the cycloidal AFM order at T-N2 similar to 35K. A spin-glass-like state emerges below T * similar to 17K, and coexists with the long-range cycloidal AFM one in this temperature range. Magnetocaloric and Mossbauer measurements consistently confirm the robustness of both the long-range SDW nd 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 BaYFeO4.