Strong magnetoelastic coupling and unconventional electric polarization in the triangular-lattice multiferroic Li_(0.99)Cu_(0.01)CrO_2

摘要

This work reports an experimental study on the temperature dependence of the structural parameters of LiCrO_2 (LCO) and Li_(0.99)Cu_(0.01)CrO_2 (LCCO) by using a synchrotron x-ray diffraction technique. A significant magnetoelastic coupling is revealed by the anomalies observed in lattice parameters at the magnetic and electric phase transitions, apparent as steplike features in both Cr-O and Li-O bond lengths, as well as in O-Cr-O bond angles. Magnetic, dielectric, and electric polarization measurements reveal the antiferromagnetic and antiferroelectric (AFE) ordering at 119 and 61 K, respectively, for LCCO. Interestingly, a fairly large uncompensated spontaneous electric polarization appears for LCCO in contrast to nearly compensated polarization value for LCO below the AFE ordering. This is correlated to the structurally driven enhancement (～4 times) of the interlayer Cr-O-Li/Cu-O-Cr superexchange interaction for LCCO. We argue that strong magnetoelastic coupling holds the key for the observed uncompensated spontaneous electric polarization in LCCO.