Neutron Scattering Study of Fluctuating and Static Spin Correlations in the Anisotropic Spin Glass Fe$_2$TiO$_5$

TL;DR

This study uses neutron scattering to explore how spin correlations in Fe$_2$TiO$_5$ evolve with temperature, revealing persistent intra-surfboard fluctuations and supporting a hybrid spin glass model.

Contribution

It provides detailed inelastic neutron scattering data on intra-surfboard spin dynamics and their temperature dependence in anisotropic spin glass Fe$_2$TiO$_5$, supporting a new hybrid spin glass model.

Findings

Strong quasi-elastic scattering above T_g indicating intra-surfboard correlations.

Spectral weight shifts from inelastic to elastic near T_g.

Persistent fluctuations at low temperatures suggest hybrid spin glass behavior.

Abstract

The anisotropic spin glass transition, in which spin freezing is observed only along the c-axis in pseudobrookite Fe$_2$TiO$_5$, has long been perplexing because the 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$_g$ $\approx$ 55 K, and a model was proposed in which the freezing of the fluctuations of the surfboards' magnetization leads to the anisotropic spin glass state. Given this new model, we have carried out high resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intra-surfboard spin dynamics on neutron (picosecond) time-scales. Here, we report on the temperature-dependence of the spin fluctuations measured from single crystal Fe$_2$TiO$_5$. Strong quasi-elastic magnetic scattering, arising from intra-surfboard 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 K and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of the majority of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intra-surfboard spin correlations to at least T/T$_g$ $\approx$ 0.1 consistent with a description of Fe$_2$TiO$_5$ as a hybrid between conventional and geometrically frustrated spin glasses.