Dipolar-Octupolar Ising Antiferromagnetism in Sm$_2$Ti$_2$O$_7$: A Moment Fragmentation Candidate

TL;DR

This study investigates Sm$_2$Ti$_2$O$_7$, revealing its dipolar-octupolar Ising antiferromagnetic ground state, all-in, all-out magnetic order, and persistent spin fluctuations, suggesting it as a candidate for moment fragmentation physics.

Contribution

First comprehensive characterization of Sm$_2$Ti$_2$O$_7$'s magnetic ground state using neutron scattering and muon spin relaxation, highlighting its potential for moment fragmentation.

Findings

Identified dipolar-octupolar doublet with Ising anisotropy

Observed all-in, all-out magnetic order below 0.35 K

Detected persistent spin fluctuations down to 0.03 K

Abstract

Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm$_2$Ti$_2$O$_7$. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm$_2$Ti$_2$O$_7$. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm$^{3+}$ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm$_2$Ti$_2$O$_7$ orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) $\mu_B$ below $T_N=0.35$ K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm$^{3+}$ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.