Zero Point Entropy in Stuffed Spin Ice

TL;DR

This study investigates how chemical modifications in spin ice materials affect their residual entropy and spin dynamics, revealing that added magnetic moments do not change the zero point entropy but influence spin freezing.

Contribution

The paper demonstrates that increasing connectivity in spin ice via chemical stuffing does not alter zero point entropy, challenging existing theories of frustrated magnetic systems.

Findings

Zero point entropy remains unchanged despite added magnetic moments.

Spin freezing is suppressed to lower temperatures with chemical stuffing.

Chemical modification offers new ways to explore frustrated magnetism.

Abstract

The third law of thermodynamics dictates that the entropy of a system in thermal equilibrium goes to zero as its temperature approaches absolute zero. In ice, however, a "zero point" or residual entropy can be measured - attributable to a high degeneracy in the energetically preferred positions of the hydrogen ions associated with the so-called "ice rules".1,2 Remarkably, the spins in certain magnetic materials with the pyrochlore structure of corner-sharing tetrahedra, called "spin ice", have an equivalent degeneracy of energetically preferred states and also have been shown to display a zero point entropy.3,4,5,6,7 Here we report that we have chemically altered Ho2Ti2O7 spin ice by stuffing extra Ho magnetic moments into normally non-magnetic Ti sites surrounding the Ho tetrahedra. The resulting series, Ho2(Ti2-xHox)O7-x/2, provides a unique opportunity to study the effects of increased connectivity between spins on a frustrated lattice. Surprisingly, the measured zero point entropy per spin appears unchanged by these excess spins, and the dynamic freezing of the spins is suppressed to lower temperatures. The results challenge our understanding of the spin ice state, and suggest a new avenue for using chemistry to study both ice-like frustration and the properties of the broad family of geometrically frustrated magnets based on the pyrochlore structure.