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 $T_{N1}\sim 475$ K and $T_{N2}\sim 175$ K, and displays a giant dielectric constant with a characteristic of the dielectric relaxation at T _N2. It does not show the evidence of the electric polarization for the crystal used for this study. The transition at T _N1 corresponds with a paramagnetic to antiferromagnetic transition with a magnetic propagation vector doubling the unit cell along three crystallographic axes. Upon cooling, at T _N2, the commensurate spin ordering transforms to a spiral magnetic structure with a propagation vector of ($\frac{h}{2}$ $\frac{k}{2}$ $\frac{l}{2}\pm \delta$), where $h$, $k$, and $l$ are odd, and the incommensurability δ is temperature dependent. Around the transition boundary at T _N2, both commensurate and incommensurate spin ordering coexist. (paper)