  We have succeeded in growing single crystals of SmPt2Si2 and have performed
electrical resistivity, magnetization and specific heat measurements. The
magnetic susceptibility shows that SmPt2Si2 is an Ising magnet with the
crystalline-electric-field ground state of Jz=+-3/2. We have found the
existence of two magnetically ordered states, i.e., an antiferromagnetically
(AFM) ordered phase (I) setting in at TN=5.1 K and a field-induced
magnetization plateau phase (II). In the phase I, unlike the usual AFM states,
a pronounced Curie-Weiss contribution remains in magnetic susceptibility,
indicating the existence of "partially disordered" Sm ions. Largely enhanced
Sommerfeld-coefficient (350 mJ/K2mol) in the phase I can be attributed to
formation of Kondo sublattice with heavy quasiparticles in the partially
disordered Sm ions. These findings may reflect substantial magnetic
frustrations inherent in SmPt2Si2.
