Anisotropy study of multiferroicity in the pyroxene NaFeGe${}_2$O${}_6$

Abstract

We present a study of the anisotropy of the dielectric, magnetic and magnetoelastic properties of the multiferroic clinopyroxene NaFeGe${}_2$O${}_6$. Pyroelectric currents, dielectric constants and magnetic susceptibilities as well as the thermal expansion and the magnetostriction were examined on large synthetic single crystals of NaFeGe${}_2$O${}_6$. The spontaneous electric polarization detected below $T_{\mathrm{C}}\simeq 11.6$ K in an antiferromagnetically ordered state ($T_{\mathrm{N}}\simeq 13$ K) is mainly lying within the $ac$ plane with a small component along $b$, indicating a triclinic symmetry of the multiferroic phase of NaFeGe${}_2$O${}_6$. 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.

14 pages, 8 figures. (minor modifications and corrections of the text)