Ferromagnetic Ising spin chains emerging from the spin ice under magnetic field

A spin-ice compound, dysprosium titanate, Dy2Ti2O7, is studied by specific heat measurements in magnetic fields applied along the [110] direction of the cubic unit cell. Above a magnetic field 0.4 T, a relatively sharp peak at T = 1.1 K in the specific heat appears from a broad peak associated with spin-ice freezing at low fields. This new peak is almost independent of the magnetic field up to 1.5 T, while another broad, field-dependent peak is detected at a higher temperature. Under a magnetic field along [110]. Ising spins on the pyrochlore lattice can be considered to form two orthogonal sets of chains, parallel and perpendicular to the field direction. It is suggested that these perpendicular chains behave as unique ferromagnetic Ising spin chains without long-range order, giving rise to the observed field-independent peak.