Magnetic structure of β-MnO2:: X-ray magnetic scattering study

An X-ray scattering experiment using synchrotron radiation has been performed on rutile-type beta -MnO2 single crystal. Below T-N, satellites due to magnetic scattering appear, which provides direct evidence of a helical magnetic structure. The wave number of the magnetic helix is approximately 0.297c*, incommensurate to the lattice and shows very weak temperature dependence. The extinction rule of the satellites is consistent with the magnetic structure proposed by Yoshimori [J. Phys. Sec. Jpn. 14 (1959) 807] in which the nearest helix chains are coupled in an antiferromagnetic way. The temperature dependence of the scattering intensity indicates that the critical exponent beta is anomalously small for an ordinary three dimensional antiferromagnet; this strongly suggests a large chiral degeneracy effect of helical magnetism.