Magnetism and spin dynamics of Na$_5$Yb(MoO$_4$)$_4$: A weakly interacting rare-earth stretched diamond lattice

TL;DR

This study investigates Na$_5$Yb(MoO$_4$)$_4$, revealing it as a dipolar quantum paramagnet with negligible exchange interactions, persistent spin dynamics, and a well-defined effective spin-1/2 ground state.

Contribution

It provides the first comprehensive experimental and theoretical characterization of Na$_5$Yb(MoO$_4$)$_4$, highlighting its unique magnetic properties dominated by dipolar interactions.

Findings

No long-range magnetic order down to 60 mK.

Yb$^{3+}$ ions have an effective $J_{eff}=1/2$ ground state.

Exchange interactions are negligibly small, consistent with DFT+$U$ calculations.

Abstract

We report a comprehensive investigation of the structural and magnetic properties of Na$_5$Yb(MoO$_4$)$_4$, a member of the stretched diamond magnetic lattice family. Neutron powder diffraction at 3.3~K confirms that the compound crystallizes in the tetragonal \textit{I4$_1$/a} space group, with a large interatomic separation of 6.33~\AA{} between magnetic Yb ions forming a three-dimensional stretched diamond framework. Magnetic susceptibility and specific heat measurements reveal no evidence of long-range magnetic order down to 60~mK. The low-temperature magnetic behavior is governed by an effective $J_{\mathrm{eff}} = 1/2$ Kramers doublet ground state, well separated from excited crystal-field levels, arising from the distorted dodecahedral oxygen coordination of Yb$^{3+}$. Density functional theory calculations within the DFT+$U$ framework indicate that exchange interactions between Yb ions are negligibly small, consistent with the long O--Mo--O super-superexchange pathways. The temperature dependence of the specific heat exhibits signatures of gapped spin excitations, most likely originating from long-range dipolar correlations and further shaped by weak exchange interactions together with the strong single-ion anisotropy of the Yb moments. Muon spin relaxation measurements reveal persistent low-energy spin dynamics, indicating that dipolar correlations remain dynamic and are insufficient to stabilize static magnetic order down to 50~mK. These results identify Na$_5$Yb(MoO$_4$)$_4$ as a rare example of a dipolar quantum paramagnet in which single-ion physics and long-range dipolar interactions dominate, while exchange interactions are suppressed to the millikelvin energy scale.