Temperature- and magnetic-field-induced change of electric polarization in a multiferroic Mn_0:93Co_0:07W_0:93O _4-δ single crystal

We have investigated magnetic and electrical properties in a multiferroic Mn_0:93Co_0:07W_0:93O_4-δ single crystal grown using the flux method, in which spiral spin orderings are known to be stabilized and thus magnetically induced electric polarization develops consistent with the spin current model. Upon temperature being lowered, two successive magnetic transitions appeared at T_n1 = 13:0 and TN2 = 12:2 K. While there was no development of ferroelectric polarization P below T _n1, a dominant P along the a-axis (P_a) and a small P along the b-axis (P_b) clearly developed below T_n2, suggesting the stabilization of a spiral order with its basal plane close to the ac-plane. The magnetization measurements also support the existence of the spiral order with ac-basal plane below T_N2 by exhibiting the largest spin susceptibility along the b-axis and the smaller ones along the a- and c-axes. Upon further lowering temperature below T^* - 10:1 K, P _a starts to decrease significantly accompanied by a small increase of P_b while the magnetic susceptibility along the c-axis gradually becomes the largest and concomitantly shows a saturation. We have also found that the magnitude of P_a or P_b can be controlled smoothly at low temperatures as a function of magnetic field less than 9 T. These findings show that the magnitude and direction of the electric polarization vector in Mn_0:93Co_0:07W_0:93O _4-δ can be varied as a function of temperature and magnetic field via the changes in the properties of the associated spiral phases.