Multiferroicity in the frustrated spinel cuprate ${\mathrm{GeCu}}_2{\mathrm{O}}_4$

Different from other magnetically frustrated spinel systems, ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ is a strongly tetragonal distorted spinel cuprate in which edge-sharing ${\mathrm{CuO}}_2$ ribbons are running along alternating directions perpendicular to the $c$ axis. Here, ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ samples of high quality were prepared via high-pressure synthesis (at 4 GPa) and the corresponding magnetic and dielectric properties were investigated. We observed a ferroelectric polarization emerging at $T_N\sim 33$ K. Although the ferroelectric polarization is weak in ${\mathrm{GeCu}}_2{\mathrm{O}}_4\left(P\sim 0.2\mu \mathrm{C}/{\mathrm{m}}^2\right)$, the existence of spin-induced multiferroicity provides a strong constraint on the possible ground-state magnetic structures and/or the corresponding theoretical models of multiferroicity for ${\mathrm{GeCu}}_2{\mathrm{O}}_4$.

Figure 1

(a) A photo of our synthesized ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ single crystals. (b) Representation of the crystal structure of ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ as obtained from our single-crystal x-ray diffraction measurement. The Cu-oxygen plaquettes are indicated by the blue areas; red, green, and blue ellipsoids denote Ge, Cu, and O ions, respectively (99% probability ellipsoids). (c)–(e) X-ray scattering intensities in the $0KL,H0L$, and $HK0$ planes of reciprocal space indicating the single-crystalline nature of our ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ single crystal.

Figure 2

(a) The temperature dependence of the magnetic susceptibility $\chi \left(T\right)$ of ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ (black) and ${\chi }^{-1}$ (red) measured in an external field $H$ of 0.1 T. The dashed line denotes a Curie-Weiss fit. In the inset the derivative $\left(d\chi /dT\right)$ is shown. (b) $\chi$ measured under different applied fields $H$ ranging from 0.1 to 7 T). For clarity, all the curves are shifted vertically except for $H=0.1\mathrm{T}$.

Figure 3

Temperature dependence of the dielectric constant $\epsilon \left(T\right)$ of polycrystalline ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ under different magnetic fields $H=0-9$ T. (b) The corresponding tangential loss $\tan \delta$. All these curves have been measured at the frequency of 10 kHz. (c) The temperature dependence of the electric polarization (obtained by integration of the pyroelectric current) which was measured after different poling processes (+2, 0, and $-2$ MV/m).