Magnetic Properties of the Pyrochlore Ho2Ru2O7

Abstract

<p> In this thesis, we investigated a recent and interesting issue in magnetism; spin ice, which is a term used for systems where there is a analogy between their magnetic structure and the proton structure of water ice. Until now, only three spin ices, Dy2Ti2O7, Ho2Ti2O7, and Ho2Sn2O7, have been discovered. In 2002, Ho2Ru2O7 was proposed as a candidate spin ice by Bansal et al. Given the similar structure and experimental behaviors of Ho2Ru2O7 and known spin ice systems, it has been an issue whether Ho2Ru2O7 is the fourth spin ice.</p> <p> In order to determine whether the new candidate is spin ice or not, the magnetic characteristics of Ho2Ru2O7 have been investigated. The frustrated system Ho2Ru2O7 has a pyrochlore structure with magnetic spins located on lattice of corner sharing tetrahedra. The crystal field originated <1 1 1> anisotropy of this sites and ferromagnetic interaction of spins give the preference of a two spin in and two spin out to a center of the tetrahedra.</p> <p> High quality polycrystalline samples were prepared and crystal growth attempts were made, then various measurements have been conducted. DC susceptibility data were used to determine the effective magnetic moment and value of Weiss temperature(θ). Zero field cooled (ZFC) and field cooled (FC) susceptibility data show a small irreversibility below 95K, which indicate ruthenium antiferromagnetic ordering. AC susceptibility measurements show a strong frequency dependence of the susceptibility which is a feature characteristic of spin glass or superparamagnetic materials.</p> <p> Specific heat experiments were conducted to also confirm the existence of Ru magnetic ordering at 95 K. The Debye temperature is estimated to be ~441 K.</p> <p> μSR measurements were conducted at TRIUMF. The measurements of Ho2Ru2O7 show signatures of ruthenium ordering near 95K and holmium near 1.4K. In agreement with previous neutron scattering experiment we conclude that the ground state of Ho2Ru2O7 is antiferromagnetic rather than spin ice. Presumably the ruthenium ordering acts to preclude the holmium moments entering the spin ice state.</p>