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

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.

15 pages, 13 figures