Vibrating coil magnetometry of the spin ice state in Ho2Ti2O7

The 'spin ice' model highlights the strong analogy between spin ordering in Ising magnets and proton ordering in ice. Recently, it has been proposed that the excitations may be described in terms of magnetic point defects or emergent magnetic monopoles. The cubic pyrochlore systems Dy2Ti2O7 and Ho2Ti2O7 are excellent experimental realizations of spin ice, where the local left angle 111 right angle Ising anisotropy of the rare earth sites in combination with net ferromagnetic interactions give rise to a highly degenerate ground state. The similarities with water ice are the observation of a first order transition terminating in a critical point in the magnetization of Dy2Ti2O7 for applied magnetic fields along the global left angle 111 right angle direction. We report a comprehensive study of the magnetization at mK temperatures of Ho2Ti2O7 by means of a vibrating coil magnetometer. We observe considerable qualitative similarities with Dy2Ti2O7 but also distinct differences. We consider the role of strong hyperfine interactions associated with Ho compounds.