Anisotropy study of multiferroicity in the pyroxene NaFeGe₂O₆

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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.