Field-induced ferroelectric state in frustrated magnet CuFe1-xAlxO2

By magnetization and neutron diffraction measurements in an applied magnetic field up to 15 T, we obtained the field ( H) versus temperature ( T) magnetic phase diagram of a geometrically frustrated triangular lattice antiferromagnet CuFe(1-x)AlxO(2) with x = 0.012, and found that the first-field-induced incommensurate noncollinear magnetic phase of CuFeO2 accompanying spontaneous electric polarization continuously connects, in the H - T phase diagram for x = 0.012, with the impurity- induced low- temperature ( LT) phase, in which the spontaneous polarization has recently been observed even in zero field. By pyroelectric measurements on the x = 0.012 sample in zero magnetic field, we also demonstrated that the thermal evolution of ferroelectric polarization corresponds to that of LT magnetic ordering.