Structural and Magnetic Properties of A_2−xB_xFeMoO₆: Double Perovskite CMR Systems

The structural and magnetic properties of double perovskite CMR systems such as Sr_2−xMg_xFeMoO₆ (x = 0 to 0.5) and Ba_2−xSr_xFeMoO₆ (x = 0 to 1.0) prepared in polycrystalline form are reported herewith. XRD and SEM were used for structural evaluation of the samples. Magnetization measurements were done using vibrating sample magnetometer. XRD results showed that all the samples are single phase except for small amount of second phase in Mg substituted samples (x > 0.3). The magnetization value of Sr_1.9Mg_0.1FeMoO₆ at room temperature and at 85 K are 23 and 39 emu/g respectively. The corresponding values of Sr₂FeMoO₆ are 20 and 35 emu/g. Similarly the magneto-resistance value of Sr_1.9Mg_0.1FeMoO₆ is higher compared to the pure Sr₂FeMoO₆ sample. Samples with Mg content greater than 0.3 does not show any magneto-resistance behavior. The magnetization value of Ba₂FeMoO₆ at room temperature is 13 emu/g and that at 85 K is 37 emu/g. The corresponding values for Ba_1.0Sr_1.0FeMoO₆ are 22 and 33 emu/g respectively. The Curie temperature varies from 317 to 375 K with Ba stoichiometry variation of Ba = 2 to Ba = 1. The magneto-resistance value of Ba₂FeMoO₆ is ∼25% at 77 K while that for BaSrFeMoO₆ is ∼30%. The observed variation in the magnetic and magneto-transport properties are explained by taking into account of the antisite disorder caused by the substitution of smaller cations, i.e., Mg in place of Sr and Sr in place of Ba. Moreover the effective length of Fe–O–Mo bond compresses and it bends due the substitution of the smaller cations.