Magnetic correlations in HoxTb2−xTi2O7

Abstract

Polycrystalline samples of HoxTb2−xTi2O7 (0 < x < 2) have been prepared and characterized for their structural and magnetic properties. The parent compounds of this solid solution are the spin ice Ho2Ti2O7 and the spin liquid Tb2Ti2O7. Specific-heat measurements on HoTbTi2O7 (x = 1.0) reveal the absence of a long-range order state above 0.5 K. The integrated entropies of all HoxTb2−xTi2O7 specimens up to 30 K scale well with the ratio of spin ice and spin liquid in the composition. The neutron diffraction spectrum of HoTbTi2O7 exhibits a dipolar spin-ice pattern and can be well described by mean-field theory for 111 Ising spins, nearest-neighbor exchange, and dipolar interactions. Inelastic neutron scattering on HoTbTi2O7 reveals two dispersionless excitations, one of ∼2.5 meV out of the ground state, and a 4-meV transition out of an excited state. We argue that these data suggest that the very strong single-ion effects of Ho2Ti2O7 and Tb2Ti2O7 persist in the HoxTb2−xTi2O7(0 < x < 2) solid solution, whereas the Tb-Ho correlations are weak, resulting in small shifts in the energy scales but with no dramatic effect on the bulk properties.