Effects of chemical substitutions and magnetic fields on multiferroic and magnetoelectric materials

We studied the interaction of the magnetic order or magnetic field with polarization or dielectric properties of solids, the so-called magnetoelectric (ME) effect. In this work, we particularly consider three different classes of compounds.

The first class of compound is multiferroic Mn1−xCoxWO4. The frustrated spin helix in the multiferroic phase breaks spatial inversion symmetry and induces ferroelectric polarization. Combining neutron scattering results, we found that the strong Co anisotropy affects the orientation and shape of the spin helix, resulting in two flops of ferroelectric polarization at xc1 = 0.075 and xc2 = 0.15. At xc1 = 0.075, P rotates from the b-axis into the a − c-plane and, at xc2 = 0.15, it flips back to the b-axis. The applied external fields force the normal vector k of spin helix rotating to be parallel to the direction of the magnetic fields. This reorientation of the spin helix causes additional field-induced polarization flops.

The second class of materials is ME borates. RAl3(BO3)4 (R = rare earth) crystallizes in a noncentrosymmetric but also nonpolar lattice structure, not allowing for polarization. Magnetic data show weak coupling of the f -moments in the compounds. However, RAl3(BO 3)4 shows a large ME effect and HoAl 3(BO3)4 sets the record for the highest ME effect in high fields when it was first measured. Upon strong coupling of f -moments to the lattice, the field-induced ionic displacements in a unit cell resulting in a polar distortion and a change in symmetry on the microscopic scale could be the origin of the ME effect.

The third class of compound is polar LiFeP2O7. Macroscopic electric polarization originates from its polar structure. Interestingly, it also has magnetic Fe-moments to form a canted antiferromagnetic ordering at TN ≃ 27 K with a weak ferromagnetic component along the b-axis. The strong internal ME effect is proved by a sharp peak of pyroelectric current at TN, resulting in a sizable polarization decrease at the onset of the antiferromagnetic phase transition. The ME effect in external fields shows a superposition of a linear and quadratic ME effect below TN. Importantly, it proves the existence of strong coupling between magnetic order and lattice polarization, which is well-described by mean field theory.