Critical behavior of resistivity in the pressure-induced first to second order transition in Pr_0.6Ca_0.4Mn_0.96B_0.04O₃ (B=Co and Cr) polycrystals

We have investigated the hydrostatic pressure (P) dependence of the resistivity on Pr_0.6Ca_0.4Mn_0.96B_0.04O₃ (B = Co and Cr) polycrystals. At ambient pressure, the temperature dependence of resistivity [ρ(T)] of both the samples show a first order paramagnetic insulator-ferromagnetic metallic transition at T=T_IM. The application of P on both the samples increases the T_IM, reduces the resistivity, and suppresses the hysteresis width, indicating a crossover from first to second order transition. The critical pressure, where the first-second order crossover takes place, are 2.02 and 2.40 GPa for Co and Cr doped samples respectively. The critical property of both systems around second order transition is investigated using Fisher-Langer relation and Suezaki-Mori method. The estimated critical exponents are close to the three-dimensional Heisenberg model for the Co doped sample suggesting short range interaction, and the exponents for the Cr doped sample follow the mean field theory suggesting long range ferromagnetic order. Further, the application of P suppresses the high temperature resistivity by reducing high temperature polarons in the case of the Cr doped sample, but it does not happen for the Co doped sample. The application of P helps to examine the stability of polarons in the high temperature regime