Singlet state formation and its impact on magnetic structure in tetramer system SeCuO_3

We present an experimental investigation of the magnetic structure in a tetramer system SeCuO_3 using neutron diffraction and nuclear resonance techniques. We establish a non-collinear, commensurate antiferromagnetic ordering with a propagation vector k = (0,0,1 ). The order parameter follows a critical behavior near T_N = 8 K, with a critical exponent β = 0.32 in agreement with a 3D universality class. Evidence is presented that a singlet state starts to form on tetramers at temperatures as high as 200 K, and its signature is preserved within the ordered state through a strong renormalization of the ordered magnetic moment on two non-equivalent copper sites, m_Cu1≈ 0.4 μ_B and m_Cu2≈ 0.7 μ_B at 1.5 K.