Spin dynamics of the ordered dipolar-octupolar pseudospin-$^1/_2$ pyrochlore Nd$_2$Zr$_2$O$_7$ probed by muon spin relaxation

TL;DR

This study uses muon spin relaxation to investigate the persistent slow spin dynamics and fluctuations in the ordered dipolar-octupolar pyrochlore Nd$_2$Zr$_2$O$_7$, revealing complex magnetic behavior below 0.4 K.

Contribution

It provides new insights into the dynamical magnetic properties of Nd$_2$Zr$_2$O$_7$, highlighting persistent fluctuations and the role of multipole interactions in the ordered state.

Findings

Persistent slow spin dynamics below the ordering temperature.

Fluctuation rate decreases with temperature and saturates.

Field-induced spin-flip transition observed.

Abstract

We present a muon spin relaxation study on the Ising pyrochlore Nd$_2$Zr$_2$O$_7$ which develops an "all-in-all-out" magnetic order below 0.4~K. At 20~mK far below the ordering transition temperature, the zero-field muon spin relaxation spectra show no static features and can be well described by a dynamical Gaussian-broadened Gaussian Kubo-Toyabe function indicating strong fluctuations of the ordered state. The spectra of the paramagnetic state (below 4.2~K) reveal anomalously slow paramagnetic spin dynamics and show only small difference with the spectra of the ordered state. We find that the fluctuation rate decreases with decreasing temperature and becomes nearly temperature independent below the transition temperature indicating persistent slow spin dynamics in the ground state. The field distribution width shows a small but sudden increase at the transition temperature and then becomes almost constant. The spectra in applied longitudinal fields are well fitted by the conventional dynamical Gaussian Kubo-Toyabe function, which further supports the dynamical nature of the ground state. The fluctuation rate shows a peak as a function of external field which is associated with a field-induced spin-flip transition. The strong dynamics in the ordered state are attributed to the transverse coupling of the Ising spins introduced by the multipole interactions.