Observation of multiferroic properties in pyroxene NaFeGe2O6

We report the observation of multiferroicity in a clinopyroxene NaFeGe2O6 polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at TN1 = 13 K, a second magnetic transition appears at TN2 = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at TN2 rather than TN1 and its magnitude increases up to ∼13 μC m-2 at 5 K, supporting the idea that the ferroelectric state in NaFeGe2O6 stems from a helical spin order stabilized below TN2. When a magnetic field of 90 kOe is applied, the electric polarization decreases to 9 μC m-2 and TN2 slightly increases by 0.5 K. At intermediate magnetic fields, around 28 and 78 kOe, 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 90 kOe, and discuss the nature of each phase of NaFeGe2O6. (paper)