Magnetic ordering and dielectric relaxation in the double perovskite YBaCuFeO5

Using magnetization, dielectric constant, and neutron diffraction measurements on a high quality single crystal of YBaCuFeO5 (YBCFO), we demonstrate that the crystal shows two antiferromagnetic transitions at TN1∼475 K and TN2∼175 K, and displays a giant dielectric constant with a characteristic of the dielectric relaxation at TN2. It does not show the evidence of the electric polarization for the crystal used for this study. The transition at TN1 corresponds with a paramagnetic to antiferromagnetic transition with a magnetic propagation vector doubling the unit cell along three crystallographic axes. Upon cooling, at TN2, the commensurate spin ordering transforms to a spiral magnetic structure with a propagation vector of (h2 k2 l2±δ), where h, k, and l are odd, and the incommensurability δ is temperature dependent. Around the transition boundary at TN2, both commensurate and incommensurate spin ordering coexist.