Short and long range magnetic ordering in β-MnO2 -: A temperature study

The magnetic long range ordering (LRO) and short range ordering (SRO) in beta-MnO2 has been studied by neutron powder diffraction. Below T-N=92.5(1)k beta-MnO2 has an incommensurate modulated magnetic LRO with temperature dependent propagation vector length k, The values of k, show a maximum, corresponding to k(z) = (2/7) + 0.0120(l), near 90 K. The c lattice constant has a minimum near T-N. The temperature dependence of the ordered magnetic moment shows a critical behavior M(T) alpha (TN - T)(beta) with beta = 0. 18(2). beta-MnO2 has also a magnetic SRO from 50 up to 300 K as conjectured by Sato et al. [Phys. Rev. B 61 (2000) 3563]. The SRO magnetic correlation length of about 9.5(1.0) Angstrom do not change with temperature. The Q-position of the SRO contribution decrease by 20% on warming from 50 to 300 K.