Almost a decade ago, an anomaly was observed in neutron scattering experiment on the spin ice material Ho2Ti2O7, and the origin of this anomaly has so far eluded theorists. In a paper published in Phys. Rev. B 93, 180410(R), Prof. Ying-Jer Kao (CS,TG4) and his collaborators simulated the dipolar spin ice model by closely mimicking the experimental setup and found that the anomalous critical scattering is related to the selection of an ordered state, previously known as q=X state. Spin ice is a class of materials with a crystal structure called pyrochlore, which is composed of corner-sharing tetrahedra. The magnetic moments, or spins, are confined to pointing into or out of the center of the tetrahedron. Applying magnetic field along the three-fold crystallographic axis, the field produces a state of finite entropy, known as kagome ice. Titling the field off the high symmetry direction, strong critical scattering was observed at an anti-ferromagnetic wave-vector. Using a simpler model of spin ice, theorists failed to reproduce this result. The transition to the ordered state is driven by the long-range dipolar interaction between spins. This points to the importance of the dipolar interaction in determining ordered states in the spin ice materials.
Figure Caption: Left: The pyrochlore lattice with [111] field orientation (green). Right: The magnetic field vector (gray) is tilted slightly away from the [111] axis.
Reference and link: Field-induced ordering in dipolar spin ice, Wen-Han Kao (高文瀚), Peter C. W. Holdsworth, and Ying-Jer Kao (高英哲), Phys. Rev. B 93, 180410(R) 2016.
