NaCaNi2F7: A frustrated high-temperature pyrochlore antiferromagnet withS=1N

Abstract

Abstract:NaCaNi 2 F 7 is an insulating, frustrated A 2 B 2 F 7 pyrochlore with magnetic S = 1 Ni 2+ on the pyrochlore B site. Non-magnetic Na and Ca are disordered on the A-site. Magnetic susceptibility measurements made on an oriented single crystal, grown in a floating zone furnace, show isotropic behavior at temperatures between 5 and 300 K, with an effective moment of 3.7 µ B /Ni. Despite displaying a large Curie-Weiss theta (-129 K), spin-ordering-related features are not seen in the susceptibility or specific heat until a spin glass transition at 3.6 K. This yields an empirical frustration index of f = -θ CW /T f ≈ 36. The spin glass behavior is substantiated by a shift of the freezing temperature with frequency in the AC susceptibility, bifurcation in the DC susceptibility, and by a broad maximum in the magnetic specific heat. The observations as made on large single crystals suggest that NaCaNi 2 F 7 is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder.

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

“…Several of these fluorine-based pyrochlores have been successfully grown as single crystals and characterized, and yet there exists only a limited amount of information on the properties of materials in this family. 7,8,9 Here we report the crystal structures and magnetic properties of three previously unreported fluoride pyrochlore materials: NaSrMn 2 F 7 , 10 However, the chemical cause of the bond disorder in the fluoride systems investigated here-Na/Ca or Na/Sr mixing on the B site-is much more apparent.…”

“…As observed in many other pyrochlores, the B-site octahedron (FeF 6 or MnF 6 ) is slightly compressed along the <111> direction and has six equal bond lengths ( Figure 2). 7,8,9 This compression results in two F-Mn-F or F-Fe-F bond angles that deviate from the ideal 90° (Table 5). Of the three pyrochlores under investigation here, the angles for NaSrFe 2 F 7 deviate the least, which suggests that the local crystal field around the transition metal is the most regular.…”

“…7,8,9,12,13 The spin glass behavior can be further characterized by fitting the shift in the AC susceptibility to the Vogel-Fulcher law, ! = !…”

“…This peak shape and position are similar to that observed for NaCaCo 2 F 7 , NaCaNi 2 F 7 , and NaSrCo 2 F 7 , which is indicative of spin glass behavior. 7,8,9,11 Both compounds show a saturation of the magnetic heat capacity by roughly 30 K.…”

“…7,8,9 The successful production of these compounds suggests that transition metal fluorides have the potential to develop into a large new family of pyrochlore materials to advance the study of geometric frustration.…”