Doping effect on magnetic properties of high-temperature multiferroic compound YBaCuFeO₅

We have investigated the impurity effects on magnetic properties for the oxygen-deficiency ordered perovskite YBaCuFeO5, which has multiferroic phenomenon at T-N2 = 319 K. The measurements of the magnetic susceptibility chi have been carried out for the polycrystalline samples of YBaCu1-xMxFeO5 (M = Ni, Mn, and Zn), where the Cu2+ spins (S = 1/2) are partially substituted by the nonmagnetic and magnetic ions. From anomalies of the temperature dependence of chi, the doping concentration dependences of the magnetic transition temperatures such as T-N1 and T-N2, were determined. With increasing the doping concentration for YBaCu1-xMxFeO5 (M = Ni, Mn, and Zn), the value of T-N1 decreases, and that of T-N2 increases, and two magnetic transitions become one at x = x(c). In the doping region of x > x(c), the magnetic behaviors of the magnetic ions doped systems are different from that of the nonmagnetic one. In particular, we found that two magnetic transitions newly appear for the Ni- and Mn-doped systems in the region of x > x(c). Based on the obtained results, we discussed the physics of impurity effects on the magnetic transitions for the high-temperature multiferroic compound YBaCuFeO5.