Effect of pressure and magnetic field on the phase transitions in lanthanum-deficient manganites

摘要

We report on the pressure, field and temperature dependencies of magnetization and resistance in manganites of (La_(1-x)Ca_x)_(1-y) Mn_(1+y) O_3 type. The M(T) dependencies is established to exhibit the following peculiarities: the first is connected with the paramagnet (PM)-ferromagnet (FM) transition at 267 K; the peak of M(T) dependence at 263 K is presumably due to an existence of AFM clusters; an anomaly in magnetization (as well as in susceptibility) is connected with the FM - canted phase transition at 42 K and the large irreversibility in the field-cooled and zero field-cooled magnetization. As field is increased above 200 Oe, the sign of the M(T) anomaly changes, namely, the M(T) magnetization increases with increasing field at 42 K. The character of M(T) dependencies does not change under pressure. However, both the Curie temperature, T_C, and the resistance peak (T_(maxR)) corresponding to metal-insulator transition shift in the direction of high temperatures with increasing pressure. A pressure stabilizes the ferromagnetic metallic phase. The temperature of the FM - canting phase transition does not change under pressure. The magnetoresistance effect increases by 15% under pressure of 12.6 kbar. The linear increasing of T_C with the derivatives of dT_C/dP ≈ +1.5 K / kbar in (La_(0.7)Ca_(0.3))_(0.,8)Mn_(1.2)O_3 and dT_C/dP ≈ + 1.9 K / kbar in La_(0.6)Mn_(1.4)O_3 ceramics was observed at pressure up to 10 kbar. The T_(maxR)(P) dependencies display a nonmonothonic behaviour with a peculiarity at P = 5 kbar.