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(Rh1−xIrx)2Si2. The lattice parameters a and c for x = 0.3, 0.4, and 0.5 jump at around the valence transition temperature Tv. 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 Tv, 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 Bv is proportional to Tv, Bv = αTv 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.