Long-wavelength correlations in ferromagnetic titanate pyrochlores as revealed by small-angle neutron scattering

We have carried out small angle neutron scattering measurements on single crystals of two members of the family of cubic rare-earth titanate pyrochlores that display ferromagnetic Curie-Weiss susceptibilities, Yb2Ti2O7 and Ho2Ti2O7. Ho2Ti2O7 is established as displaying a prototypical classical dipolar spin ice ground state, while Yb2Ti2O7 has been purported as a candidate for a quantum spin ice ground state. While both materials have been well studied with neutron scattering techniques, neither has been previously explored in single crystal form with small angle neutron scattering (SANS). Our results for Yb2Ti2O7 show distinct SANS features below its ΘCW∼0.50K, with rods of diffuse scattering extending along 〈111〉 directions in reciprocal space, off-rod scattering which peaks in temperature near ΘCW, and quasi-Bragg scattering at very small angles which correlates well with TC∼0.26K. The quasi-Bragg scattering corresponds to finite extent ferromagnetic domains ∼140Å across, at the lowest temperatures. We interpret the 〈111〉 rods of diffuse scattering as arising from domain boundaries between the finite-extent ferromagnetic domains. In contrast the SANS signal in Ho2Ti2O7 is isotropic within the (HHL) plane around Q=0, consistent with the expectations of the dipolar spin ice model. However the strength of this overall SANS signal has a temperature dependence resembling that of the magnetic heat capacity, with a peak near 3 K. Below the break between the field-cooled and the zero-field cooled susceptibility in Ho2Ti2O7 at ∼0.60K, the SANS signal is very low, approaching zero.