Spiral phase and spin waves in the quasi-two-dimensional antiferromagnet Ba2CuGe2O7

The quasi-two-dimensional square-lattice antiferromagnet Ba2CuGe2O7 was studied by neutron scattering and bulk magnetic techniques. An incommensurate magnetic spiral structure with the propagation vector (1+zeta,1+zeta,0) (lambda = 0.027) was observed below T-N = 3.26 K. Magnetic ordering occurs with a two-dimensional-like critical exponent beta approximate to 0.15. The spin dynamics can be adequately described by conventional spin-wave theory with two exchange constants: nearest-neighbor in-plane antiferromagnetic coupling J(1) approximate to 0.48 meV and interplane ferromagnetic interaction J(perpendicular to) approximate to 0.013 meV. This set of exchange parameters apparently fails to explain the spiral order. The noncentrosymmetric crystal structure suggests that the incommensurate phase may be the result of a Dzyaloshinskii-Moriya instability of the Neel ground state.