Spin glass behavior in frustrated quantum spin system CuAl2O4 with a possible orbital liquid state

TL;DR

CuAl2O4 exhibits quantum spin glass behavior with no long-range magnetic order or structural distortion down to very low temperatures, suggesting it may host an orbital liquid state due to strong frustration and unique tetrahedral Cu2+ positioning.

Contribution

This study provides the first evidence of a quantum spin glass state in CuAl2O4 and proposes it as a candidate for an orbital liquid, highlighting its unique tetrahedral Cu2+ sites and high frustration.

Findings

Displays spin glass behavior at ~2 K with frequency-dependent susceptibility.

No long-range magnetic order or structural distortion observed down to 0.4 K.

Has a large frustration parameter, f ≈ 67.

Abstract

CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = Theta_CW/Tm ~67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.