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 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. Our results for Yb₂Ti₂O₇ show distinct SANS features below its ΘCW ∼ 0.50 K, with rods of diffuse scattering along 111 directions, off-rod scattering which peaks in temperature near ΘCW , and quasi-Bragg scattering at very small angles which correlates well with TC ∼ 0.26 K. 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. 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 ∼ 0.60 K, the SANS signal is very low, approaching zero.