  The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 are experimental and theoretical
exemplars of highly frustrated magnetic materials. However, the effects of an
applied uniaxial pressure are not well studied, and here we report
magnetization measurements of Ho2Ti2O7 under uniaxial pressure applied in the
[001], [111] and [110] crystalline directions. The basic features are captured
by an extension of the dipolar spin ice model. We find a good match between our
model and measurements with pressures applied along two of the three
directions, and extend the framework to discuss the influence of crystal
misalignment for the third direction. The parameters determined from the
magnetization measurements reproduce neutron scattering measurements we perform
under uniaxial pressure applied along the [110] crystalline direction. In the
detailed analysis we include the recently verified susceptibility dependence of
the demagnetizing factor. Our work demonstrates the application of a moderate
applied pressure to modify the magnetic interaction parameters. The knowledge
can be used to predict critical pressures needed to induce new phases and
transitions in frustrated materials, and in the case of Ho2Ti2O7 we expect a
transition to a ferromagnetic ground state for uniaxial pressures above 3.3
GPa.
