Abstract

We report the tunable magnetic structures in the helimagnet YBa(Cu1-xFex)2O5 by changing various Cu/Fe ratios. The magnetic properties and structures of the samples were studied by using neutron powder diffraction (NPD) and susceptibility measurements. The parent material YBaCuFeO5 exhibits two antiferromagnetic (AFM) transitions at TN1 ~ 450 K and TN2 ~ 175 K. At TN1, YBaCuFeO5 shows a commensurate spin ordering (CM1) with a q– wavevector kc1 = (½ ½ ½ ), which transfers to a spiral magnetic ordering at TN2 with an incommensurate (ICM) q-wavevector (½ ½ ½ ±δ), δ being the incommensurability. Using neutron powder diffraction for samples with 0.490 ≤ x ≤ 0.505, TN2 was observed to systematically increase from 125 to 236 K as a function of the Cu/Fe ratio. For x = 0.510 and 0.515, additional magnetic reflections with a propagation vector kc2 = (½ ½ 0) appear below TN3 ~ 245 and 269 K, respectively. This new commensurate phase (CM2) shows a collinear magnetic ordering. These transition temperatures are consistent with the AFM transitions of the magnetization data. Experiments were also conducted to synthesize the samples with different thermal treatments, demonstrating the TN2 and TN3 to be sensitive to the annealing conditions. These results can be understood within the framework of the random distributions of the dopants/impurities at the B-site of a double perovskite lattice and demonstrate the magnetic ordering in the helimagnet YBaCuFeO5 can be systematically tuned by changing the Cu/Fe ratio.