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 CuFeO{sub 2}. On cooling from room temperature, CuFeO{sub 2} undergoes two antiferromagnetic phase transitions with incommensurate and commensurate magnetic order at T{sub N1} = 14 K and T{sub 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 {approx}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.