Glassy disordered ground states in the frustrated pyrochlore and fluorite antiferromagnets NaCd$M_2$F$_7$ ($M$ = Ni$^{2+}$, Mn$^{2+}$)

TL;DR

This study investigates the crystal structures and magnetic properties of two frustrated antiferromagnets, revealing glassy disordered ground states caused by structural and magnetic frustration.

Contribution

It reports the synthesis and characterization of NaCdNi$_2$F$_7$ and NaCdMn$_2$F$_7$, highlighting their structural differences and the origin of their glassy magnetic ground states.

Findings

NaCdNi$_2$F$_7$ forms a pyrochlore structure with strong antiferromagnetic interactions.

NaCdMn$_2$F$_7$ adopts a defect-fluorite structure due to ionic size considerations.

Both compounds exhibit glassy magnetic states with high frustration indices.

Abstract

We report the crystal structures, magnetic and thermodynamic properties of two magnetically frustrated $A$'$A$"$M_2$F$_7$-type antiferromagnets, NaCdNi$_2$F$_7$ and NaCdMn$_2$F$_7$. While NaCdNi$_2$F$_7$ forms a stable pyrochlore structure (SG: $Fd \overline{3} m$, #227) with magnetic $S$ = 1 Ni$^{2+}$ ions on the frustrated pyrochlore 16$c$ site and fully disordered non-magnetic Na$^+$/Cd$^{2+}$ ions on the pyrochlore 16$d$ site, NaCdMn$_2$F$_7$ favors the defect-fluorite structure (SG: $Fm \overline{3} m$, #225) with magnetic $S$ = 5/2 Mn$^{2+}$ and non-magnetic Na$^+$ and Cd$^{2+}$ ions fully disordered on the fluorite 4$a$ site. This is a result of the Mn$^{2+}$ ionic radius being too close to the average Na$^+$/Cd$^{2+}$ ionic radius, hindering the cationic site ordering towards the stable pyrochlore structure. In both cases, dominant antiferromagnetic interactions $\theta_{CW,Ni}$ = -91.2(5) K and $\theta_{CW,Mn}$ = -42.4(4) K are noted, with no magnetic transition until $T_{f,Ni}$ = 3.2 K and $T_{f,Mn}$ = 2.0 K, implying substantial frustration, with frustration indices $f_{Ni}$ = 28.5 and $f_{Mn}$ = 21. AC susceptibility measurements and bifurcation of ZFC/FC low-field magnetization indicate a glassy ground-state, precipitated by the magnetic-bond-disorder stemming from the inherent structural disorder.