Two spin-canting textures in the antiferromagnetic phase AF1 of MnWO₄based on the new polar atomistic model inP2

Abstract

The low temperature antiferromagnetic (AF) phase of MnWO (the so-called AF1 phase) exhibits different spin-canting configurations at two Mn sublattices of the (3 + 1)-dimensional magnetic structure. The suggested superspace group [Formula: see text] is a significant consequence of the polar space group [Formula: see text]2 true for the nuclear structure of MnWO. Density functional theory calculations showed that its ground state prefers this two spin-canting system. The structural difference between two independent atomic sites for Mn (Mn , Mn ) is too small to allow microscopically detectable electric polarisation. However, this hidden intrinsic polar character allows AF1 two commensurately modulated spin-canting textures. This is considered as the prerequisite onset of the improper ferroelectricity enhanced by the helical spin order in the multiferroic phase AF2 of MnWO.

Theoretical spin-wave dispersions in the antiferromagnetic phase AF1 of MnWO4 based on the polar atomistic model in P2

“…As aforementioned, the most recent study [24] showed that MnWO 4 crystallises in monoclinic P2 structure, and the low temperature magnetic structure is not a collinear spin configuration but two spin-canting textures, as illustrated in Fig. 1.…”

“…The fundamental crystal structure of MnWO 4 is monoclinic, and the corresponding space group has been believed to be P2/c until our structural studies confirmed the true symmetry P2 [21][22][23][24]. For the observation of h0l reflections with l = arbitrary integer (strong when l = even; extremely weak when l = odd), the structure exhibits no gliding mirror plane c perpendicular to the crystallographic b axis, thus cannot belong to P2/c.…”

“…Furthermore, two different spin-canting configurations at two Mn 2+ sublattices are present as a symmetrical consequence of the direct polar subgroup relation between P2 and P2/c [24]. This non-collinear spin-canting texture in AF1 of MnWO 4 is in contrast to the previous collinear magnetic model (↑↑↓↓) where the magnetic moments at two Mn sites are aligned collinearly along the easy axis with a common angle of 35°-37° against a-axis in the (a-c) plane [17,19,20].…”

“…At zero field, three successive antiferromagnetic phase transitions are observed: the commensurate magnetic structure AF1 with propagation vector k = (± 1 4 , 1 2 , 1 2 ) is present below 8 K, the incommensurate elliptical spiral spin structure AF2 existing in 8 ∼ 12.3 K induces ferroelectric order, and the incommensurate collinear sinusoidal spin structure AF3 is only observed in a narrow temperature range below 13.5 K [17,19,20]. The fundamental crystal structure of MnWO 4 is monoclinic, and the corresponding space group has been believed to be P2/c until our structural studies confirmed the true symmetry P2 [21][22][23][24] and the two different spin-canting configurations at two Mn 2+ sublattices, as a symmetrical consequence of the di-rect polar subgroup relation between P2 and P2/c. This noncollinear spin-canting texture in AF1 of MnWO 4 is in contrast to the previous collinear magnetic model (↑↑↓↓) where the magnetic moments at two Mn sites are aligned collinearly along the easy axis with a common angle of 35 • -37 • against a-axis on the (a-c) plane [17,19,20].…”

“…For the observation of h0l reflections with l = arbitrary integer (strong when l = even; extremely weak when l = odd), the structure exhibits no gliding mirror plane c perpendicular to the crystallographic b axis, thus cannot belong to P2/c. As shown extensive neutron and x-ray diffraction including Renninger scans [21][22][23][24], the true space group symmetry for huebnerite (MnWO 4 )-type structures is the polar space group P2. The most relevant finding in our structure model is the presence of faint intrinsic polarity in b, which may be further remedied due to ferrodistortive domains formed about c * , as claimed in [21].…”