Instability of magnetism in Pr0.5Ca0.5Mn1-xCrxO3 (x=0.015,0.03):: Competition between pressure and thermal cycling effects

Magnetic properties of polycrystalline Pr0.5Ca0.5Mn1-xCrxO3 (x=0.015,0.03) samples have been investigated in the temperature range 1.5-300 K, magnetic field up to 16 kOe and under hydrostatic pressures up to 11 kbar. The studies involved sequential zero field cooled (ZFC) magnetization (M) measurements followed by measurements during cooling in the same magnetic field (H). Additional measurements of magnetization vs H were carried out at ambient and applied pressure. The volume of the ferromagnetic phase for x=0.015 sample exhibits a considerable decrease upon thermal cycling even after many cycles. On the other hand, both the Curie temperature (T-C) and the width of thermal hysteresis remain practically unchanged during the thermal cycling. In contrary to the effect of thermal cycling, applied pressure enhances T-C and the volume of the ferromagnetic phase below T-C and reduces considerably the width of thermal hysteresis. A ferromagnetic contribution to magnetization due to short range ferromagnetic coupling was observed in the vicinity of the charge order transition temperature T-CO approximate to 220 K; it appears to enhance under pressure. In the case of Pr0.5Ca0.5Mn0.97Cr0.03O3 an applied pressure also increases the magnetization and T-C with a pressure coefficient of dT(C)/dP approximate to 1.8 K/kbar, at the same time the hysteresis in the vicinity of T-C narrows. The results observed are related to the interfacial elastic energy at the phase boundaries of the phase separated Cr doped manganites. The effect of the suppression of the Jahn-Teller distortions and of the electron-phonon coupling in interfacial regions and in ferromagnetic domains under an applied hydrostatic pressure is also discussed.