  An effective Ising model for the spin-ice type Kondo lattice model is
investigated by the classical Monte Carlo simulation. We clarify the magnetic
phase diagram with four phases: ice-ferro, ice-(0,0,2\pi), 32-sublattice, and
all-in/all-out ordered states. The result well reproduces the phase diagram of
the Kondo lattice model studied previously [H. Ishizuka et al.: J. Phys. Soc.
Jpn. 81 (2012) 113706], which suggests that the RKKY interactions up to third
neighbors are sufficient to describe the magnetic properties of the itinerant
electron model. We discuss the peculiar nature of phase transitions: the
suppression of the critical temperatures down to zero temperature between two
ice phases and the presence of the tricritical points on the phase boundary of
the 32-sublattice ordered state.
