Competition between stripy and 120-degree spin correlations in the spin glass ground state of the triangular quantum antiferromagnet ErMgGaO₄

ErMgGaO₄ is a quantum antiferromagnet wherein pseudospin 1/2, Er³⁺ degrees of freedom

decorate two-dimensional triangular planes separated by disordered non-magnetic bilayers of Mg²⁺

and Ga³⁺. Its sister compound, YbMgGaO₄, has attracted much interest as a quantum spin liquid

ground state candidate, although the presence of the disordered Mg-Ga bilayers add complexity to

this description. In contrast, ErMgGaO₄ shows a clear spin glass transition near T_g ∼ 2 K, about 1/3

of its Curie-Weiss temperature. We have carried out new inelastic neutron scattering measurements

on powder ErMgGaO₄ samples, which show the frozen elastic component of the scattering to develop

below T_g. It is separated by a ∼ 0.25 meV gap from a continuum of dynamic spectral weight

with a ∼ 0.75 meV bandwidth. The frozen (elastic) and fluctuating (inelastic) spin correlations are

analysed separately, using a combination of reverse Monte Carlo and Warren lineshape analysis.

These are consistent with fluctuating 120-degree spin correlations at all temperatures, and the development

of competing stripy static correlations below T_g.