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

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.

9 pages, 6 figures

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• Softening of breathing elastic mode and trigonal elastic mode in disordered pyrochlore magnet NaCaCo${}_2$F${}_7$
• Glassy disordered ground states in the frustrated pyrochlore and fluorite antiferromagnets NaCd$M_2$F${}_7$ ($M$ = Ni${}^{2+}$, Mn${}^{2+}$)
• Low-temperature high-frequency dynamic magnetic susceptibility of classical spin-ice Dy${}_2$Ti${}_2$O${}_7$