SPIRAL ORDER IN BA2CUGE2O7

The quasi-two-dimensional square-lattice antiferromagnet Ba2CuGe2O7 wa s studied by neutron scattering and bulk magnetic techniques. An incom mensurate magnetic spiral structure with the propagation vector (1 + x i,xi,0) (xi = 0.027) was observed below T-N = 3.26 K. The spin dynamic s can be adequately described by conventional spin-wave theory with tw o exchange constants: nearest-neighbor in-plane antiferromagnetic coup ling J(1) approximate to 0.48 meV and interplane ferromagnetic interac tion J(3) approximate to 0.013 meV. This set of exchange parameters ap parently fails to explain the spiral order. The non-centrosymmetric cr ystal structure suggests that the incommensurate phase may be the resu lt of a Dzyaloshin-skii-Moriya instability of the Neel ground state.