Single-ion properties of the Seff = 12 XY antiferromagnetic pyrochlores NaA′Co2 F7 (A′=Ca2+, Sr2+)

K. A. Ross, J. M. Brown, R. J. Cava, J. W. Krizan, S. E. Nagler, J. A. Rodriguez-Rivera, M. B. Stone

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Abstract

The antiferromagnetic pyrochlore material NaCaCo2F7 is a thermal spin liquid over a broad temperature range (≈140 K down to TF=2.4 K), in which magnetic correlations between Co2+ dipole moments explore a continuous manifold of antiferromagnetic XY states [K. A. Ross, Phys. Rev. B 93, 014433 (2016)1098-012110.1103/PhysRevB.93.014433]. The thermal spin liquid is interrupted by spin freezing at a temperature that is ∼2% of the mean-field interaction strength, leading to short-range static XY clusters with distinctive relaxation dynamics. Here we report the low-energy inelastic neutron scattering response from the related compound NaSrCo2F7, confirming that it hosts the same static and dynamic spin correlations as NaCaCo2F7. We then present the single-ion levels of Co2+ in these materials as measured by inelastic neutron scattering. An intermediate spin-orbit coupling model applied to an ensemble of trigonally distorted octahedral crystal fields accounts for the observed transitions. The single-ion ground state of Co2+ is a Kramers doublet with a strongly XY-like g tensor (gxy/gz∼3). The local disorder inherent from the mixed pyrochlore A sites (Na+/Ca2+ and Na+/Sr2+) is evident in these measurements as exaggerated broadening of some of the levels. A simple model that reproduces the salient features of the single-ion spectrum produces approximately 8.4% and 4.1% variation in the z and xy components of the g tensor, respectively. This study confirms that an Seff=12 model with XY antiferromagnetic exchange and weak exchange disorder serves as a basic starting point in understanding the low-temperature magnetic behavior of these strongly frustrated magnets.

Original languageEnglish (US)
Article number144414
JournalPhysical Review B
Volume95
Issue number14
DOIs
StatePublished - Apr 13 2017

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics

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    Ross, K. A., Brown, J. M., Cava, R. J., Krizan, J. W., Nagler, S. E., Rodriguez-Rivera, J. A., & Stone, M. B. (2017). Single-ion properties of the Seff = 12 XY antiferromagnetic pyrochlores NaA′Co2 F7 (A′=Ca2+, Sr2+). Physical Review B, 95(14), [144414]. https://doi.org/10.1103/PhysRevB.95.144414