Ising versus XY Anisotropy in Frustrated R2Ti2O7 Compounds as Seen'' by Polarized Neutrons

We studied the field induced magnetic order in R2Ti2O7 pyrochlore compounds with either uniaxial (R = Ho, Tb) or planar (R = Er, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor {chi(parallel to), chi(perpendicular to)} provides a universal description of the field induced structures in the paramagnetic phase (2-270 K), whatever the field value (1-7 T) and direction. Comparison of the thermal variations of chi(parallel to) and chi(perpendicular to) with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.