Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCo$_2$F$_7$

TL;DR

This study investigates the magnetic phases and spin dynamics in the bond-disordered pyrochlore NaCaCo$_2$F$_7$ using ESR and magnetization measurements, revealing coexistence of paramagnetic and spin-glass phases and proposing an empirical model for high-field magnetization.

Contribution

It provides new experimental insights into the coexistence of magnetic phases and introduces an empirical model for describing high-field magnetization in bond-disordered pyrochlores.

Findings

Identification of a gapless paramagnetic phase above 2.4 K.

Observation of a spin-glass phase below 20 K with gapped excitations.

Effective g-factors close to 2 for both magnetic modes.

Abstract

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f \sim 2.4$ K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found as well as a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective $g$-factors close to 2 are obtained for both modes in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic $g$-tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective $g$-factors as determined from ESR and the local $g$-tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores.