Anisotropy study of multiferroicity in the pyroxene NaFeGe2O6

We present a study of the anisotropy of the dielectric, magnetic and magnetoelastic properties of the multiferroic clinopyroxene NaFeGe2O6. Pyroelectric currents, dielectric constants and magnetic susceptibilities as well as the thermal expansion and the magnetostriction were examined on large synthetic single crystals of NaFeGe2O6. The spontaneous electric polarization detected below T C ≃ 11.6 K ?> in an antiferromagnetically ordered state ( T N ≃ 13 K ?> ) is mainly lying within the ac plane with a small component along b ?> , indicating a triclinic symmetry of the multiferroic phase of NaFeGe2O6. The electric polarization can be strongly modified by applying magnetic fields along different directions. We derive detailed magnetic field versus temperature phase diagrams and identify three multiferroic low-temperature phases, which are separated by a non-ferroelectric, antiferromagnetically ordered state from the paramagnetic high-temperature phase.