Low-temperature crystal and magnetic structures of the magnetoelectric material Fe4Nb2O9

Abstract

Fe 4 Nb 2 O 9 was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at T N ≈ 90 K for an antiferromagnetic transition and T str ≈ 77 K of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below T N and uncovered the origin of the second dielectric anomaly as a structural phase transition across T str . In the antiferromagnetically ordered state below T N , 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 T str , 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)μ B at 95 K to 3.83(4)μ B 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 T N and T str are proved to be spin-current and d-p hybridization mechanisms.

Pressure driven re-entrant magnetoelectric transition in honeycomb $Fe_{4}Nb_{2}O_{9}$

“…Powder FNO samples were synthesized through a conventional solid-state synthesis route as described in literature 10 . We have carried out high-pressure XRD and Raman spectroscopy measurements using a piston-cylinder type diamond anvil cell (DAC) from EasyLab Co. (UK) having 300µm culet.…”

“…Dielectric permittivity measurements on the same material show two anomalies at 94 and 80 K, due to magnetic transition and trigonal P − 3c1 to monoclinic C2/c structural phase transition, respectively 10,11,20,21 .…”

“…Apart from type-II multiferroic system, a new class of A 4 B 2 O 9 (where, A = Fe, Co, Mn, and B = Nb, Ta) compounds crystallizing in centrosymmetric crystal structures, exhibit novel linear-ME (LME)effect. Some of the above type of materials exhibit collinear antiferromagnetic structure at the ground state [10][11][12][13][14] . The crystallographic structure of A 4 B 2 O 9 is derived from magnetoelectric corundum-type Cr 2 O 3 , which belongs to the centrosymmetric layered trigonal structure with P − 3c1 space group 15 .…”

“…Recently Mn 4 Nb 2 O 9 , Co 4 Nb 2 O 9 and F e 4 Nb 2 O 9 are reported to belong to LME class 10,[16][17][18] .…”

“…Temperature dependent magnetic susceptibility χ(T ) and neutron diffraction measurements on honeycomb F e 4 Nb 2 O 9 (FNO) show the antiferromagnetic transition at T N = 94K, with a strong magnetic anisotropy suggesting in-plane antiferromagnetic exchange iterations and out of plane ferromagnetic exchange interactions between Fe atoms along c-axis 10,11,19 .…”