Abstract
The magnetic long range ordering (LRO) and short range ordering (SRO) in β-MnO 2 has been studied by neutron powder diffraction. Below T N =92.5(1) K β-MnO 2 has an incommensurate modulated magnetic LRO with temperature dependent propagation vector length k z . The values of k z show a maximum, corresponding to k z = (2/7) + 0.0120(1), 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 )∝( T N - T ) β with β=0.18(2). β-MnO 2 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) A do not change with temperature. The Q -position of the SRO contribution decrease by 20% on warming from 50 to 300 K.
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