Local spin dynamics in geometrically frustrated Mo pyrochlore antiferromagnet Lu$_2$Mo$_2$O$_{5-y}$N$_2$

TL;DR

This study investigates the magnetic ground state of the oxynitride pyrochlore Lu$_2$Mo$_2$O$_{5-y}$N$_2$, revealing the absence of spin freezing and the presence of distinct spin dynamics, supporting its candidacy as a quantum spin liquid.

Contribution

The paper provides the first muon spin rotation/relaxation analysis of Lu$_2$Mo$_2$O$_{5-y}$N$_2$, demonstrating unique spin dynamics and absence of magnetic order, advancing understanding of quantum spin liquids in pyrochlore oxynitrides.

Findings

No spin freezing observed down to 0.3 K.

Detection of two magnetic domains with different spin dynamics.

Evidence of a broad distribution of excitation gaps.

Abstract

The magnetic ground state of oxynitride pyrochlore Lu$_2$Mo$_2$O$_{5-y}$N$_2$, a candidate compound for the quantum spin liquid ($S=1/2$, Mo$^{5+}$), was studied by muon spin rotation/relaxation experiment. In contrast to Lu$_2$Mo$_2$O$_7$ ($S=1$, Mo$^{4+}$) which exhibits the spin-glass behavior with a freezing temperature $T_g\simeq16$ K, no such spin freezing or long range magnetic order was observed down to 0.3 K. Moreover, two separate magnetic domains were detected below $\sim$13 K, which were characterized by differences in spin dynamics. The first is the "sporadic" spin fluctuation seen in frustrated antiferromagnets, where the amplitude of the hyperfine fields suggests that the excitation comprises a local cluster of unpaired spins. The other is rapid paramagnetic fluctuation, which is only weakly suppressed at low temperatures. In place of the paramagnetic fluctuation, the volume fraction for the sporadic fluctuation steadily increases with decreasing temperature, indicating the formation of an excitation gap with a broad distribution of the gap energy involving null gap.