NaCaCo2F7 : A single-crystal high-temperature pyrochlore antiferromagnet

Abstract

We report the magnetic characterization of the frustrated transition metal pyrochlore NaCaCo2F7. This material has high spin Co 2+ in CoF6 octahedra in a pyrochlore lattice, and disordered non-magnetic Na and Ca on the large-atom sites in the structure. Large crystals grown by the floating zone method were studied. The magnetic susceptibility is isotropic, the Co moment is larger than the spin-only value, and in spite of the large Curie Weiss theta (-140 K), freezing of the spin system, as characterized by peaks in the ac and dc susceptibility and specific heat, does not occur until around 2.4 K. This yields a frustration index of f = -θCW/Tf ≈ 56, an indication that the system is highly frustrated. The observed entropy loss at the freezing transition is low, indicating that magnetic entropy remains present in the system at 0.6 K. The compound may be the realization of a frustrated pyrochlore antiferromagnet with weak bond disorder. The high magnetic interaction strength, strong frustration, and the availability of large single crystals makes NaCaCo2F7 an interesting alternative to rare earth oxide pyrochlores for the study of geometric magnetic frustration in pyrochlore lattices.

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.…”