Weak ferromagnetism in LaMnO3

The magnetic properties of (La1-xSrx)MnO3 crystals have been studied. The a c susceptibility chi = chi' + i chi " has been measured at frequencies rang ing from 10 Hz to 1000 Hz. The x = 0.25 crystal shows a typical ferromagnet ic behaviour with a dimensionless SI susceptibility of 6 below T-c = 350 K limited by the demagnetizing field. Both x = 0 and x = 0.06 crystals show v ery sharp peaks in chi' and chi " at the Neel points, 139 K and 135 K respe ctively. This is quite unlike the usual behaviour of an antiferromagnet. Ma gnetization measurements on an untwinned single crystal of LaMnO3 show a we ak moment of 0.18 mu(B)/Mn 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.