The pyrochlore material Ho2Ti2O7 has been suggested to show "spin ice" behaviour. We present neutron scattering and specific heat results that establish unambiguously that Ho2Ti2O7 exhibits spin ice correlations at low temperature. Diffuse magnetic neutron scattering from Ho2Ti2O7 is found to be quite well described by a nearest neighbour spin ice model and very accurately described by a dipolar spin ice model. The heat capacity is well accounted for by the sum of a dipolar spin ice contribution and an expected nuclear spin contribution, known to exist in other Ho 3+ salts. These results settle the question of the nature of the low temperature spin correlations in Ho2Ti2O7 for which contradictory claims have been made.

The magnetic dynamics of the spin ice material Ho2Ti2O7 in its paramagnetic (‘hot’) phase have been investigated by a combination of neutron spin echo and ac-susceptibility techniques. Relaxation at high temperatures (T > 15 K) is proved to occur by a thermally activated single-ion process that is distinct from the process that dominates at lower temperatures (1 K < T < 15 K). It is argued that the low-temperature process must involve quantum mechanical spin tunnelling, as quasi-classical channels of relaxation are exhausted in this temperature range. Our results resolve a mystery in the physics of spin ice: why has a 15 K ac-susceptibility peak been observed in Dy2Ti2O7 but not in Ho2Ti2O7 or Ho2Sn2O7?

Neutron scattering and ac-susceptibility techniques have been performed on the spin ice material Ho 2 Ti 2 O 7 to study the spin relaxation processes in the 'hot' paramagnetic phase (T > 1 K). Neutron spin echo (NSE) proves that above T 15 K the spin dynamics are governed by a thermally activated single-ion process. At lower temperatures (T < 15 K) this cannot account for the spin dynamics found in ac-susceptibility measurements. It is inferred that a second, slower process, with a different thermal signature dominates. We suggest that this is a quantum-mechanical tunnelling process between different spin states separated by a large energy barrier.

We have measured the field-dependent heat capacity in the tetragonal antiferromagnets CeRhIn 5 and Ce 2 RhIn 8 , both of which have an enhanced value of the electronic specific heat coefficient ␥ ϳ400 mJ/mol Ce K 2 above T N . For TϽT N , the specific heat data at zero applied magnetic field are consistent with the existence of an anisotropic spin-density wave opening a gap in the Fermi surface for CeRhIn 5 , while Ce 2 RhIn 8 shows behavior consistent with a simple antiferromagnetic magnon. From these results, the magnetic structure, in a manner similar to the crystal structure, appears more two dimensional in CeRhIn 5 than in Ce 2 RhIn 8 where only about 12% of the Fermi surface remains ungapped relative to 92% for Ce 2 RhIn 8 . When B͉͉c, both compounds behave in a manner expected for heavy-fermion systems as both T N and the electronic heat capacity decrease as field is applied. When the field is applied in the tetragonal basal plane (B͉͉a), CeRhIn 5 and Ce 2 RhIn 8 have very similar phase diagrams which contain both first-and second-order field-induced magnetic transitions.

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