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 $\mathbf{\text{k}}=(0,0,1)$. The order parameter follows a critical behavior near $T_N=8$ K, with a critical exponent $\beta =0.32$ 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, $m_{Cu1}\approx 0.4{\mu }_B$ and $m_{Cu2}\approx 0.7{\mu }_B$ at 1.5 K.