Neutron scattering study of fluctuating and static spin correlations in the anisotropic spin glass Fe2TiO5

The anisotropic spin-glass transition, in which spin freezing is observed only along the c axis in pseudobrookite Fe2TiO5, has long been perplexing because the Fe3+ moments (d5) are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature Tg approximate to 55 K, and a model was proposed in which the freezing of the surfboard magnetization fluctuations leads to the anisotropic spin-glass state. Given this model, we have carried out high resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intrasurfboard spin dynamics on neutron (picosecond) timescales. Here, we report on the temperature-dependence of the spin fluctuations measured from single-crystal Fe2TiO5. Strong quasi-elastic magnetic scattering, arising from intrasurfboard correlations, is observed well above Tg. The spin fluctuations possess a steep energy-wave vector relation and are indicative of strong exchange interactions, consistent with the large Curie-Weiss temperature. As the temperature approaches Tg from above, a shift in spectral weight from inelastic to elastic scattering is observed. At various temperatures between 4 and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of most of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intrasurfboard spin correlations to at least T/Tg approximate to 0.1, consistent with an energy landscape that is a hybrid between conventional and geometrically frustrated spin glasses.

Automated summary

The anisotropic spin-glass transition in pseudobrookite Fe2TiO5, where spin freezing is observed only along the c axis, has been a long-standing puzzle. Recent neutron diffraction experiments reveal the coalescence of surfboard-shaped antiferromagnetic nanoregions above the glass transition temperature. In this study, we conducted high resolution inelastic neutron scattering measurements to understand the temperature dependence of spin dynamics within the surfboard regions on picosecond timescales. The results show that strong quasi-elastic magnetic scattering is observed well above Tg, indicating the presence of strong exchange interactions. The presence of fluctuating intrasurfboard spin correlations is also observed even at base temperature.