Specific heat anomaly of the holmium garnet Ho 3Al 5O 12 at low temperature

Abstract

Specific heat of the garnet-type single crystals Ho 3Al 5O 12 exhibits two sharp peaks at 0.839 and 0.220 K. The antiferromagnetic transition occurs at 0.839 K with λ-type specific heat anomaly, while the electric quadrupole order may be found at 0.220 K in the insulating state without any conduction electrons. On the assumption that the influence of dominant cubic crystal field on 5I 8 state for Ho 3+ ion, the experimental results of the specific heat have been analyzed. The lowest level of the 5I 8 would be Γ 3 (2) doublet as the non-magnetic ground state, which can lead to novel phenomena such as the electric quadrupole order. The possibility of the electric quadrupole order will be described, where it is essential to assume the ordering of Γ 3-type ( O 2 0) moment. The magnetic moment itself will be induced at higher temperatures through the occupation to the excited doublet-state. These two low-lying doublets reflect the Schottky anomaly in the specific heat, where the broad peak appears around 2.48 K and the entropy change is ΔS/3R= ln 2=0.693 for one Ho 3+ ion.