Observation of multiferroic properties in pyroxene NaFeGe ₂O ₆

We report the observation of multiferroicity in a clinopyroxene NaFeGe ₂O ₆ polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T _N1=13K, a second magnetic transition appears at T _N2=11.8K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T _N2 rather than T _N1 and its magnitude increases up to 13μCm ² at 5K, supporting the idea that the ferroelectric state in NaFeGe ₂O ₆ stems from a helical spin order stabilized below T _N2. When a magnetic field of 90kOe is applied, the electric polarization decreases to 9μCm ² and T _N2 slightly increases by 0.5K. At intermediate magnetic fields, around 28 and 78kOe, anomalies in the magnetoelectric current, magnetoelectric susceptibility, and field derivative of magnetization curves are found, indicating field-induced spin-state transitions. Based on these electrical and magnetic properties, we provide a detailed low temperature phase diagram up to 90kOe, and discuss the nature of each phase of NaFeGe ₂O ₆.