We have measured magnetization processes of a Shastry-Sutherland lattice (SSL) TmB4. For B∥c, plateaus appear at frac(1, 8) and frac(1, 2) of the full moment 7 μB/Tm at 1.7-1.4 and 2-3 T, respectively. For B∥a, a metamagnetic transition occurs at ∼30 T which is followed by a frac(1, 2) plateau at ∼32 T. These plateaus are originated from geometric frustration effect in SSL. Magnetic field and temperature dependences of magnetoresistance and specific heat were also measured in detail below TN=11.7 K. The magnetic phase diagram possesses complex features which have an antiferromagnetic phase, spin flop saturated one, and a few additional phases by long periodic modulations originated from frustration effect in SSL.
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