  The Cu spin magnetism in La2-x-yEuySrxCuO4 (x<=0.17; y<=0.2) has been studied
by means of magnetization measurements up to 14 T. Our results clearly show
that in the antiferromagnetic phase Dzyaloshinsky-Moriya (DM)superexchange
causes Cu spin canting not only in the LTO phase but also in the LTLO and LTT
phases. In La1.8Eu0.2CuO4 the canted DM-moment is about 50% larger than in pure
La2CuO4 which we attribute to the larger octahedral tilt angle. We also find
clear evidence that the size of the DM-moment does not change significantly at
the structural transition at T_LT from LTO to LTLO and LTT. The most important
change induced by the transition is a significant reduction of the magnetic
coupling between the CuO2 planes. As a consequence, the spin-flip transition of
the canted Cu spins which is observed in the LTO phase for magnetic field
perpendicular to the CuO2 planes disappears in the LTT phase. The shape of the
magnetization curves changes from the well known spin-flip type to a
weak-ferromagnet type. However, no spontaneous weak ferromagnetism is observed
even at very low temperatures, which seems to indicate that the interlayer
decoupling in our samples is not perfect. Nonetheless, a small fraction (<15%)
of the DM-moments can be remanently magnetized throughout the entire
antiferromagnetically ordered LTT/LTLO phase, i.e. for T<T_LT and x<0.02. It
appears that the remanent DM-moment is perpendicular to the CuO2 planes. For
magnetic field parallel to the CuO2 planes we find that the critical field of
the spin-flop transition decreases in the LTLO phase, which might indicate a
competition between different in-plane anisotropies. To study the Cu spin
magnetism in La2-x-yEuySrxCuO4, a careful analysis of the Van Vleck
paramagnetism of the Eu3+ ions was performed.
