Entropy and susceptibility of ``stuffed'' spin ice -- Ho$_{2}$(Ho$_{x}$Ti$_{2-x})$O$_{7-x/2}$

The spin ice material Ho$_{2}$Ti$_{2}$O$_{7}$ has been studied extensively due to its apparent residual ground state entropy, which is similar to that seen in water ice. This material has a pyrochlore structure in which the Ho$^{3+}$ and Ti$^{4+}$ cations form two interpenetrating sets of corner sharing tetrahedra. Here we present thermodynamic measurements on Ho$_{2}$(Ho$_{x}$Ti$_{2-x})$O$_{7-x/2}$, with 0 $\le \quad x \quad \le $ 0.67, where we have replaced some Ti with Ho -- effectively stuffing the lattice with more magnetic ions. We find that the zero field magnetic entropy remains essentially unchanged with stuffing. AC susceptibility measurements show the $T$ = 2 K peak associated with the spin ice freezing decreases in magnitude with increasing $x$, indicating that spin freezing has been suppressed. While the residual entropy in Ho$_{2}$Ti$_{2}$O$_{7}$ is reduced with the application of a magnetic field, our measurements show that the entropy becomes less sensitive to applied field as $x$ is increased. This work is supported by the NSF.