Pb2VO(PO4)2
Led vanadyl phosphate belongs to the frustrated vanadium-based square lattice magnets family. Well, the materials that are approximating the frustrated square lattice, as the real structure has a lower symmetry and more inequivalent magnetic interactions. Still, in this quantum ferro-antiferromagnet the "side" and "diagonal" exchanges are different in sign and they are competing for making the ground state either a columnar antiferromagnet or a plain ferromagnet. Antiferromagnetism definitely wins at low field, but closer to saturation some extra unexpected phase appears. The intrigue is, that for a similar setting one may expect a non-dipolar "spin nematic" type of magnetic order. If this is really it, or just an alternative "conventional" ordered structure, future work will tell.
F. Landolt, S. Bettler, Z. Yan, S. Gvasaliya, A. Zheludev, S. Mishra, I. Sheikin, S. Krämer, M. Horvatić, A. Gazizulina , O. Prokhnenko, Pre-saturation phase in the frustrated ferro-antiferromagnet Pb2VO(PO4)2, external pagearXiv:2006.04592
S. Bettler, F. Landolt, Ö. M. Aksoy, Z. Yan, S. Gvasaliya, Y. Qiu, E. Ressouche, K. Beauvois, S. Raymond, A. N. Ponomaryov, S. A. Zvyagin, and A. Zheludev, Magnetic structure and spin waves in the frustrated ferro-antiferro magnet Pb2VO(PO4)2, external pagearXiv:1902.11172v1; external pagePhys. Rev. B 99, 184437 (2019)