Low temperature magnetic properties of frustrated pyrochlore ferromagnet Ho₂Ir₂O₇

We have performed low temperature magnetic susceptibility, specific heat, electrical resistivity and neutron diffraction measurements to study the magnetic properties of frustrated pyrochlore ferromagnet Ho₂Ir₂O₇. The magnetization and dc resistivity measurement indicate metal-insulator transition near 140 K. The accumulated value of observed entropy calculated from heat capacity data is Rln2, indicating the Ho³⁺ doublet ground state. The Ir sublattice order is antiferromagnetic which is evident from the magnetization measurements. The best fit of the neutron diffraction spectra at 1.8 K shows the magnetic structures with two types of components. One of the magnetic structure is all-in-all-out type, which is due to the strong coupling between Ho³⁺ and Ir⁴⁺ ions and develops below 18 K. It suggests that the magnetic moment of Ho³⁺ ions align in the field of Ir⁴⁺, ordering along local <111> directions. There is also a Ho³⁺ magnetic component perpendicular to <111> below 100 K. This perpendicular component may originate from the defects in the Ir sub-lattice. Defects such as Ir deficiency or an Ir atom pointing at the opposite direction will cause the emergent of the perpendicular component.