Weak ferromagnetism in LaMnO<Subscript>3</Subscript>

The magnetic properties of (La<Subscript>1 − x </Subscript>Sr<Subscript> x </Subscript>)MnO<Subscript>3</Subscript> crystals have been studied. The ac susceptibility χ=χ′ + iχ″ has been measured at frequencies ranging from 10 Hz to 1 000 Hz. The x=0.25 crystal shows a typical ferromagnetic behaviour with a dimensionless SI susceptibility of 6 below T <Subscript>c</Subscript>=350 K limited by the demagnetizing field. Both x=0 and x=0.06 crystals show very sharp peaks in χ′ and χ″ at the Néel points, 139 K and 135 K respectively. This is quite unlike the usual behaviour of an antiferromagnet. Magnetization measurements on an untwinned single crystal of LaMnO<Subscript>3</Subscript> show a weak moment of 0.18 μ <Subscript>B</Subscript> at 4.2 K which is directed along the c axis. The origin of this canted antiferromagnetic structure is discussed in terms of anisotropy and of Dzyaloshinskii-Moriya interaction. It is concluded that anistropy is insufficient to explain the canted antiferromagnetism. Copyright Società Italiana di Fisica, Springer-Verlag 1999