Magnetoelectric response in square cupola antiferromagnets

A newly discovered magnetic insulator A(TiO)Cu₄(PO₄)₄ (A = Ba, Sr, etc.) crystalizes into a tetragonal chiral crystal structure with P42₁2 space group [1]. It consists of a unique magnetic M₄O₁₂ cluster with square cupola geometry, in which four corner-sharing MO₄ plaquettes (M = transition metal) form a noncoplanar buckled structure. Here, we report on the experimental results indicating that this asymmetric square cupola cluster provides a source for unique magnetoelectric (ME) effects.
Magnetic susceptibility and powder neutron diffraction data for the Ba system reveal an onset of complex noncollinear spin structure below T = 9.5 K, which can be described by an antiferroic order of magnetic quadrupole moments on Cu₄O₁₂ square cupolas [2]. Moreover, it is found that the dielectric constant exhibits a magnetic-field-induced peak divergent toward T = 9.5 K. This provides an experimental evidence for the ME-activity due to magnetic quadrupole moments [3] and therefore substantiates our design strategy. In the presentation, we will also present the results of other isostructural materials.
[1] K. Kimura et al., Inorg. Chem. 55, 1002 (2016). [2] K. Kimura et al., Nat. Commun. 7, 13039 (2016). [3] Y. Kato et al., Phys. Rev. Lett. 118, 107601 (2017).