Modification of Spin Ice Physics in Ho$_2$Ti$_2$O$_7$ Thin Films

TL;DR

This study investigates how substrate orientation, strain, and defects influence spin ice behavior in Ho$_2$Ti$_2$O$_7$ thin films, revealing that these factors modify the magnetic states and the manifestation of spin ice phenomena.

Contribution

It provides new insights into how growth conditions and structural imperfections affect spin ice physics in thin films, highlighting the importance of orientation and defect control.

Findings

(110) films show the spin ice plateau in magnetization.

Disorder varies with growth orientation, affecting magnetic properties.

Spin ice correlations are suppressed at lower temperatures in thin films.

Abstract

We present an extensive study on the effect of substrate orientation, strain, stoichiometry and defects on spin ice physics in Ho$_2$Ti$_2$O$_7$ thin films grown onto yttria-stabilized-zirconia substrates. We find that growth in different orientations produces different strain states in the films. All films exhibit similar c-axis lattice parameters for their relaxed portions, which are consistently larger than the bulk value of 10.10 \AA. Transmission electron microscopy reveals anti-site disorder and growth defects to be present in the films, but stuffing is not observed. The amount of disorder depends on the growth orientation, with the (110) film showing the least. 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 single crystal. Neutron scattering maps on the more disordered (111) oriented films show the Q=0 phase previously observed in bulk materials, but the Q=X phase giving the plateau state remains elusive. We conclude that the spin ice physics in thin films is modified by defects and strain, leading to a reduction in the temperature at which correlations drive the system into the spin ice state.