Incipient Ferromagnetism in Tb2Ge2O7: Application of Chemical Pressure to the Enigmatic Spin Liquid, Tb2Ti2O7
Abstract
The origin of the spin liquid state in Tb2Ti2O7 has challenged experimentalists and theorists alike for nearly 20 years. To improve our understanding of the exotic magnetism in Tb2Ti2O7, we have synthesized a chemical pressure analog, Tb2Ge2O7. Germanium substitution results in a lattice contraction and enhanced exchange interactions. We have characterized the magnetic ground state of Tb2Ge2O7 with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to Tb2Ti2O7, there is no long-range order in Tb2Ge2O7 down to 20 mK. The Weiss temperature of -19.2(1) K, which is more negative than that of Tb2Ti2O7, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of Tb2Ge2O7 reveals that at 3.5 K liquid-like correlations dominate in this system. However, below 1 K, the liquid-like correlations give way to intense short-range ferromagnetic correlations with a length scale related to the Tb-Tb nearest neighbor distance. Despite stronger antiferromagnetic exchange, the ground state of Tb2Ge2O7 has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in Tb2Ti2O7.
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