NaSrMn₂F₇, NaCaFe₂F₇, and NaSrFe₂F₇: novel single crystal pyrochlore antiferromagnets

Abstract

The crystal structures and magnetic properties of three previously unreported A 2 B 2 F 7 pyrochlore materials, NaSrMn 2 F 7 , NaCaFe 2 F 7 , and NaSrFe 2 F 7 are presented. In these compounds, either S=2 Fe 2+ or S=5/2 Mn 2+ is on the B site, while nonmagnetic Na and Ca (Na and Sr) are disordered on the A site. The materials, which were grown as crystals via the floating zone method, display high effective magnetic moments and large Curie-Weiss thetas. Despite these characteristics, freezing of the magnetic spins, characterized by peaks in the susceptibility or specific heat, is not observed until low temperatures. The empirical frustration index, f=-θcw/T f , for the materials are 36 (NaSrMn 2 F 7 ), 27 (NaSrFe 2 F 7 ), and 19 (NaCaFe 2 F 7 ). AC susceptibility, DC susceptibility, and heat capacity measurements are used to characterize the observed spin glass behavior. The results suggest that the compounds are frustrated pyrochlore antiferromagnets with weak bond disorder. The magnetic phenomena that these fluoride pyrochlores exhibit, in addition to their availability as relatively large single crystals, make them promising candidates for the study of geometric magnetic frustration.

Frustrated Quantum Rare-Earth Pyrochlores

“…A family of materials more far afield are the recently synthesized transition metal fluoride pyrochlores [150][151][152]. These transition metal magnets are expected to be more isotropic than their rare-earth counterparts, though there has been some evidence for XY-like features in their excitation spectra [153,154].…”

Quantum and Classical Phases of the Pyrochlore Heisenberg Model with Competing Interactions

“…Their importance stems from the availability of large high-quality single crystals. Prominent candidate spin-liquid examples include the S = 1 NaCaNi 2 F 7 [204], which may be a first realization of a S = 1 quantum spin liquid in three dimensions [205], and the related higher-spin fluoride compounds featuring a high frustration index (f = Θ CW /T c ), such as NaCaCo 2 F 7 [206][207][208], NaCaFe 2 F 7 , NaSrFe 2 F 7 , and NaSrMn 2 F 7 [209], which, nonetheless, either show signs of long-range magnetic order at low temperatures or undergo spin freezing [210]. With the PFFRG formalism in place, it would be useful in such a material context to extend the mapping of the quantum phase diagram in the presence of longer-ranged Heisenberg couplings which will most likely give rise to additional novel phases compared to the seven phases of the classical J 1 -J 2 Heisenberg model, as, for instance, shown in Ref.…”

Single-ion properties of theSeff=12XY antiferromagnetic pyrochloresNaA′Co2Ross1, Brown2, Cava3 et al. 2017Phys. Rev. B505561View full textAdd to dashboardCite

“…[19][20][21][22] The chemical formula and structure are similar to the rare earth oxide pyrochlores, taking the form A 2 B 2 F 7 , where A is a split site of Na + /A (A = Ca 2+ , Sr 2+ ) and B is six-fold coordinated Co 2+ , Ni 2+ , Fe 2+ or Mn 2+ (Figure 1). Unlike the previously studied fluoride-pyrochlore CsNiCrF 6 , 23 the magnetic B-site is chemically uniform, hosting just one magnetic species.…”