Hole-doping effect on the Verwey-type transition and magnetoresistivity of Ba(Sm,Ca)Fe₂O_5+δ

Two series of samples of the oxygen-deficient double-perovskite phase, Ba(Sm,Ca)Fe₂O_5+δ were synthesized employing a sample encapsulation technique that utilizes Fe metal as an oxygen getter. The hole-doping level on the FeO₂ plane was controlled by varying the amount of excess oxygen or the Ca concentration at the Sm site. Earlier it had been found that at room temperature five-coordinated Fe²⁺ and Fe³⁺ species form pairs by sharing a d electron, leading to the formation of a Fe^2.5++ fluctuating valence state [Phys. Rev. B 60 (1999) 15,251]. At T_V, a Verwey-type transition occurred, signifying the charge separation of the Fe^2.5+ fluctuating valence state into high-spin Fe²⁺ and Fe³⁺. Moreover, related to the Verwey-type transition a negative magnetoresistance peak with a magnitude of a few percent was observed [Appl. Phys. Lett. 77 (2000) 1683]. The position of the peak was found to correspond to the jumps seen in the susceptibility and resistivity vs. temperature curves. For the present samples, it is found that the value of T_V is severely decreased upon increasing the Ca concentration, as seen by susceptibility and magnetoresistivity measurements. Introduction of excess oxygen leads to a less severe decrease of the transition temperature. The observed behavior of T(V) was reproduced by a simple model based on combinatorial-entropy calculations.