Large magnetoresistance and volume expansion associated with valence transition in Eu(Rh_1-xIr_x)₂Si₂

We report lattice parameters as a function of temperature, magnetization curves, and magnetoresistance of polycrystalline samples of Eu(Rh_1-xIr_x)₂Si₂. The lattice parameters a and c for x = 0.3, 0.4, and 0.5 jump at around the valence transition temperature T_v. For these samples, the high-temperature (larger a, c) and low-temperature (smaller a, c) phases, the volume fraction of which changes with temperature, coexist at around T_v, which means that the valence transition is of the first order. The magnetization curves of x = 0.3, 0.4, and 0.5 at T = 4.2 K exhibit a metamagnetic transition with a large hysteresis, implying a first-order field-induced valence transition. The transition field B_v is proportional to T_v, B_v = αT_v with α = 0.36 T=K. For x = 0.3, a large negative magnetoresistance associated with the field-induced valence transition is observed. The origin of the negative magnetoresistance is discussed.