Excitation spectrum and low-temperature magnetism in disordered defect-fluorite Ho2Zr2O7

TL;DR

This study investigates the magnetic properties and crystal-field excitations of disordered Ho2Zr2O7, revealing slow spin dynamics without long-range order and highlighting the role of structural disorder in its magnetic behavior.

Contribution

It introduces a model accounting for structural disorder in the crystal-field splitting of Ho2Zr2O7, explaining broad excitations and the persistence of magnetism despite a non-magnetic ground state.

Findings

Signs of slow spin dynamics without glassy behavior.

No long-range magnetic order down to 150 mK.

Structural disorder enables magnetism by mixing low-lying states.

Abstract

In this work, we report on the thermomagnetic characterization and crystalline-electric field (CEF) energy scheme of the disordered defect-fluorite Ho2Zr2O7. This structural phase is distinguished by the coexistence of magnetic frustration and extensive disorder, with Ho3+ and Zr4+ sharing randomly the same 4a site with even 50% occupancy, and an average 1/8 oxygen vacancy per unit cell. AC magnetic susceptibility measurements performed on powder samples down to 0.5 K revealed signs of slowing spin dynamics without glassy behavior, including a frequency dependent peak at 1 K. Yet, no evidence for long-range magnetic order is found down to 150 mK in specific heat. Inelastic neutron scattering measurements show a weak, low-lying CEF excitation around 2 meV, accompanied by a broad level centered at 60 meV. To fit our observations, we propose an approach to account for structural disorder in the crystal-field splitting of the non-Kramers Ho3+. Our model provides an explanation to the broadening of the high-energy, single-ion excitations and suggests that the zirconate ground-state wave function has zero magnetic moment. However, structural disorder acts as guarantor of the magnetism in Ho2Zr2O7, allowing the mixing of low lying states at finite temperatures. Finally, we show that this scenario is in good agreement with the bulk properties reported in this work.