Spontaneous spin-lattice coupling in the geometrically frustrated triangular lattice antiferromagnet CuFeO2

We use high-resolution synchrotron x-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet CuFeO2. On cooling from room temperature, CuFeO2 undergoes two antiferromagnetic phase transitions with incommensurate and commensurate magnetic order at T-N1=14 K and T-N2=11 K, respectively. The occurrence of these two magnetic transitions is accompanied by second- and first-order structural phase transitions from hexagonal to monoclinic symmetry. Application of a 6.9 T magnetic field lowers both transition temperatures by similar to 1 K, and induces an additional incommensurate structural modulation in the temperature region where the field-driven ferroelectricity occurs. These results suggest that a strong magneto-elastic coupling is intimately related to the multiferroic effect.