Highly Anisotropic Phase Diagram of Ho2Ti2O7

Geometrically frustrated systems have garnered much interest in recent times due to the emergence of novel magnetic states in these systems that contrast traditional cooperative magnetic phenomena. The spin ice pyrochlores Ho2Ti2O7 and Dy2Ti2O7 have been extensively studied and have been shown to posses highly degenerate ground states that lack long-range order and furnish an environment in which fractionalized excitations, that behave like magnetic monopoles, have been experimentally realized. These ground states are dictated by the competition between nearest neighbor superexchange (Jnn) and dipolar interactions (Dnn), as well as a strong single-ion anisotropy due to the local crystal field environment of the rare-earth site. In order to determine the effects of epitaxial strain on the emergent properties of spin ice materials, we have grown strained thin films of Ho2Ti2O7 on yttria-stabilized zirconia substrates using pulsed laser deposition. We have performed an extensive study on the effects of substrate orientation, strain, stoichiometry, and defects on spin ice physics in Ho2Ti2O7. Magnetization measurements at 1.8 K show the expected magnetic anisotropy and saturation magnetization values associated with a spin ice for all orientations; shape anisotropy is apparent when comparing in and out-of-plane directions. Significantly, only the (110) oriented films display the hallmark spin ice plateau state in magnetization, albeit less well-defined compared to the plateau observed in a bulk single crystal. Further anisotropy studies performed on bulk single crystals and thin films of Ho2Ti2O7 with capacitive torque magnetometry reveal, for the first time, the rich angular phase diagram associated with the spin ice state. They also furnish a means by which to probe the energy scale associated with the spin-spin interactions (J'eff), when considering the effects of long-range dipolar interactions. We extract J'eff and find that long ranged interactions indeed play an important role. A comparison is made between the angular spin ice phase diagram of the bulk single crystal and the angular response of the thin films. This allows us to probe directly the changes in anisotropy of the films, revealing that the (110) films are the most bulk-like. These studies show that capacitive torque magnetometry is an excellent tool for distinguishing between the different spin textures of highly frustrated materials and possesses the sensitivity to accurately characterize these textures in thin films samples.