Neutron scattering study of the magnetic structure of

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Abstract
The magnetic structure in the ordered phase of the nearly one-dimensional Heisenberg antiferromagnet has been measured using elastic neutron scattering. crystallizes in the orthorhombic Pnma space group with spin chains running along the crystallographic b-direction. Below the ordering temperature the magnetic structure is incommensurate along the chain direction with a temperature-independent ordering wavevector q = (0, 0.472, 0) (rlu}). The occurrence of an incommensurate structure is shown to be the consequence of frustration on the spins induced by the exchange interaction between chains. Group theory is used to determine the possible magnetic structures compatible with the symmetry of the crystal. The results show that at T = 0.3 K the spin ordering is cycloidal with spins rotating in a plane that contains the propagation direction b. A mean-field calculation of the magnetic ground-state energy including exchange anisotropy effects is used to study the stability of the observed structure. Values for the interchain exchange constants that are consistent with the features of the magnetic structure are proposed.