Magnetic structures and magnetoelectric coupling in the polar magnets RCrWO₆ (R=Tb, Dy, Ho, Er, Tm, Yb)

Multiferroics and magnetoelectrics are technologically important materials for future memory technologies such as in MERAMs and MESO logic elements and in various sensors. These applications utilize the coupling between the ferroic orders; magnetic ordering (MO) and electric polarization (EP). However, the coupling mechanisms in which an EP is induced from MO are complex. Depending on the coupling strength these are categorized into type I, with negligible coupling, and type II compounds, with MO induced symmetry breaking and emergent EP at a lower temperature (eg. TbMnO₃). In some compounds, the magnetoelectric multiferroics external magnetic field is required to induce EP below MO. Recently another class of compounds -the polar magnets- were reported to be polar (symmetry-broken) already in the paramagnetic phase but the EP emerges below the MO, combining the characteristics of both type I and II. Examples are Mn₂FeMO₆ (M=Nb, Ta, W and Mo), RFeWO₆ and RCrWO₆. We are interested in the magnetic structures and the magnetoelectric coupling in the latter system to investigate the phase transitions. RCrWO₆ crystallizes in the aeschynite structure and MO temperature is reduced from 24 K to 18.5 K moving from Tb to Yb. This is also one of the rare transition metal- R combined oxides together with RFeWO₆, where a simultaneous MO of R and the transition metal moments take place. For the investigations, we use neutron scattering techniques, detailed bulk property measurements and theoretical modeling. Our results indicate that moving towards Tm from Tb, two magnetic phase transitions appear instead of one for larger R which are governed by both exchange interactions and single ion properties of R. We discuss the magnetoelectric properties in terms of these phase transitions.