Effect of disorder on the quantum spin liquid candidate Na$_4$Ir$_3$O$_8$

TL;DR

This study investigates how magnetic and non-magnetic disorder affect the quantum spin liquid candidate Na$_4$Ir$_3$O$_8$ by substituting Ir with Ru or Ti, revealing changes in magnetic properties, heat capacity, and spin freezing behavior.

Contribution

It provides the first detailed experimental analysis of disorder effects on Na$_4$Ir$_3$O$_8$, highlighting how different substitutions modify its magnetic and thermodynamic properties.

Findings

Ru substitution increases Weiss temperature and suppresses magnetic heat capacity anomaly.

Ti substitution reduces Weiss temperature and eliminates the heat capacity anomaly.

Non-magnetic Ti creates orphan spins detectable in magnetic susceptibility.

Abstract

We report on the effects of introducing magnetic and non-magnetic disorder in the hyperkagome iridate quantum spin liquid (QSL) candidate Na$_4$Ir$_3$O$_8$ by partially replacing Ir$^{4+}$ ($S = 1/2$) with Ru$^{4+}$ ($S = 1$) or Ti$^{4+}$ ($S = 0$). Specifically, we synthesized Na$_4$(Ir$_{1-x}$Ru$_x$)$_3$O$_8 (x = 0.05, 0.10, 0.2, 0.3)$ and Na$_4$Ir$_{2.7}$Ti$_{0.3}$O$_8$ samples and measured electrical transport, AC and DC magnetization, and heat capacity down to $T = 1.8$ K. Na$_4$Ir$_3$O$_8$ is associated with a large Weiss temperature $\theta = -650$ K, a broad anomaly in magnetic heat capacity C$_{mag}$ at T $\approx25$ K, low temperature power-law heat capacity, and spin glass freezing below $T_f \approx 6$ K. We track the change in these characteristic features as Ir is partially substituted by Ru or Ti. We find that for Ru substitution, $\theta$ increases and stays negative, the anomaly in C$_{mag}$ is suppressed in magnitude and pushed to lower temperatures, low temperature $C \sim T^\alpha$ with $\alpha$ between $2$ and $3$ and decreasing towards $2$ with increasing $x$, and $T_f$ increases with increase in Ru concentration $x$. For Ti substitution we find that $\theta$ and T$_f$ become smaller and the anomaly in $C_{mag}$ is completely suppressed. In addition, introducing non-magnetic Ti leads to the creation of orphan spins which show up in the low temperature magnetic susceptibility.