Commensurate to incommensurate antiferromagnetic ordering in YBa_1-xSr_xCuFeO₅ (x = 0, 0.5): low temperature neutron diffraction study

YBaCuFeO₅ belongs to the family of oxygen deficient double perovskites which crystallizes in tetragonal structure. This compound is of immense interest because of its magnetic and dielectric properties at high temperatures. It is a type-I multiferroic in which commensurate (C) antiferromagnetic (AFM) ordering (up-up-down-down) sets at ~ 441 K (T_N1) followed by an incommensurate (ICM) antiferromagnetic (AFM) order at ~ 230 K (T_N2). The spiral ICM-AFM ordering at TN₂ is responsible for the polarization. We have studied the structural, magnetic, and dielectric properties of YBa_1-xSr_xCuFeO₅. Low temperature Neutron Powder Diffraction (NPD) study YBa_1-xSr_xCuFeO₅ (x= 0, 0.5) in the temperature range 2 - 300 K collected at wavelength 1.48 Å has been refined using the Fullprof software. The Sr substitution at Ba site leads to the contraction in unit cell and as a result T_N2 towards high temperature side. Current study points towards the independent origins of magnetic and dielectric phenomenon in all compounds