Electronic International Standard Serial Number (EISSN)
Fe4Nb2O9 was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at TN ≈90K for an antiferromagnetic transition and Tstr ≈77K of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below TN and uncovered the origin of the second dielectric anomaly as a structural phase transition across Tstr. In the antiferromagnetically ordered state below TN, both Fe1 and Fe2 magnetic moments lying within the weakly and strongly buckled honeycomb layers are arranged in a fashion that the three nearest neighbors are directed oppositely. Upon cooling below Tstr, the symmetry of crystal structure is lowered from trigonal P-3c1 to monoclinic C2/c, in which a weak sliding of the metal octahedral planes introduces a monoclinic distortion of ∼1.7◦. The magnetic structure is preserved in the low-temperature monoclinic phase, and the Fe magnetic moment increases from 2.1(1)muB at 95 K to 3.83(4)muB at 10 K assuming an equal moment conﬁguration at Fe1 and Fe2 sites. The magnetic point group and linear magnetoelectric tensor at each temperature region are determined. From a symmetry-related tensor analysis, the microscopic origins of the magnetoelectric effects between TN and Tstr are proved to be spin-current and d-p hybridization mechanisms.
magnetic order; magneto-dielectric effect; antiferromagnets; multiferroics; crystal structures; neutron diffraction