Magnetic Ordering in Holmium Ethylsulfate below 215 mK

We report ac and dc susceptibility measurements of single crystals of the axial dipolar paramagnet Ho(C 2 H 5 SO 4 ) 3 ·9H 2 O between 10 mK and 1 K. This material has hexagonal symmetry, with a non-Kramers doublet ground state possessing a very anisotropic g factor, to which the Ising model might be expected to apply: g || =15.41, g ⊥ =0. An antiferromagnetic transition was observed at ∼215 mK. This is in marked contrast to two related compounds with Ising model interactions, Dy(C 2 H 5 SO 4 ) 3 ·9H 2 O and Tb(C 2 H 5 SO 4 )·9H 2 O, both of which order ferromagnetically. A possible reason for the difference is the hyperfine interaction, which is large in holmium.