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

Abstract

The anisotropic spin-glass transition, in which spin freezing is observed only along the $c$ axis in pseudobrookite ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$, has long been perplexing because the ${\mathrm{Fe}}^{3+}$ moments $({d}^{5})$ are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature ${T}_{g}\ensuremath{\approx}55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, 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 ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$. Strong quasi-elastic magnetic scattering, arising from intrasurfboard correlations, is observed well above ${T}_{g}$. 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 ${T}_{g}$ 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 $T/{T}_{g}\ensuremath{\approx}0.1$, consistent with an energy landscape that is a hybrid between conventional and geometrically frustrated spin glasses.