Spontaneous spin-lattice coupling in the geometrically frustrated triangular lattice antiferromagnet CuFe O2

We use high-resolution synchrotron x-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet CuFe O2. On cooling from room temperature, CuFe O2 undergoes two antiferromagnetic phase transitions with incommensurate and commensurate magnetic order at TN1 =14 K and TN2 =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 ∼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.