Ferromagnetic fragmented state in the pyrochlore Ho$_2$Ru$_2$O$_7$

TL;DR

This study investigates the magnetic ordering in Ho2Ru2O7, revealing a complex interplay between Ho and Ru ions leading to a fragmented ferromagnetic state with spin ice correlations, using neutron scattering and thermodynamic measurements.

Contribution

It uncovers the coexistence of ferromagnetic and spin ice states in Ho2Ru2O7 and details the symmetry-breaking transitions driven by Ru-mediated interactions.

Findings

Ru moments order at 95 K into a $b5$ antiferromagnetic state

Ho moments order at 1.55 K into a $b9$ ferromagnetic state with ground state entropy

The ground state resembles a fragmented ferromagnetic state akin to kagome ice

Abstract

The consecutive magnetic ordering of the Ho and Ru ions in the pyrochlore Ho2Ru2O7 and their interplay are investigated by neutron scattering, magnetic and specific heat measurements. The Ru moments order at 95 K into a $\Gamma_5$ easy-plane antiferromagnetic state. At 1.55 K the Ho moments order into an unusual $\Gamma_9$ ferromagnetic state with extensive ground state entropy and structured spin dynamics. It is shown how the internal fields with $\Gamma_5$ and $\Gamma_9$ geometry allow for two symmetry breaking transitions. The lower temperature ordering is driven by ruthenium mediated interactions between holmium moments as spin ice correlations develop. The unsaturated order is compatible with a fragmented ferromagnetic state equivalent to pyrochlore kagome ice.