Multiferroicity and magnetic structure relationship in noncentrosymmetric Ca2Fe1.1Al0.9O5

The spin-flop transition in noncentrosymmetric Ca2Fe2-xAlxO5 induced by a magnetic field is accompanied by the appearance of a macroscopic electric polarization whose origin was ascribed to a noncollinear spin arrangement in the domain walls between two collinear magnetic phases and/or metal-ligand hybridization effects depending on the spin direction [Abe et al., Phys. Rev. B 89, 054437 (2014)]. Here, we present an extensive study combining experimental and theoretical efforts to propose an alternative mechanism that explains the observed electrical polarization. Our macroscopic magnetic and magnetoelectric as well as neutron diffraction data on a Ca2Fe1.1Al0.9O5 single-crystal sample are in agreement with an angular spin configuration, continuously rotating from being parallel to the c axis at low temperatures and low magnetic fields to being parallel to the a axis either by increasing the temperature or by applying a magnetic field along the c axis. A phenomenological Landau free-energy model emphasizes that the crystal structure belongs to the pyroelectric family with electric polarization along the c axis, which also depends on the spin orientation. It clarifies the origin of the spontaneous and field-induced phase transitions and especially the magnetoelectric features.