Effects of pressure on charge transport and magnetic properties of La_1.32Sr_1.68Mn₂O₇ layered manganite

We have studied the effects of hydrostatic pressure on the electrical transport and magnetic properties of La_1.32Sr_1.68Mn₂O₇ layered manganite up to 25 kbars. At ambient pressure, the compound exhibits a ferromagnetic transition accompanied by a metal-insulator transition (T_MI¹) at 118 K. Increasing pressure induces a second metal-insulator (T_MI²) transition at a critical pressure of 6≤P_C≤7 kbars in the temperature dependence of resistivity measurement. With further increase in pressure, both T_MI¹ and T_MI² shift to higher temperatures continuously, however, displaying a suppression in the amplitude of the peaks on the resistivity curves. We could not observe any transition corresponding to T_MI² in the temperature dependence of magnetization measurement under pressures up to 10 kbars. However, pressure reduces the magnetic moments at low temperatures and shifts the T_C to higher temperatures at the same rate observed for T_MI¹. A large negative tunneling magnetoresistance was observed around T_C due to the applied magnetic field up to the maximum available value of 5 T, and the pressure reduces the magnetoresistance ratio significantly. This result is due to the canted ferromagnetic order that was established by incr- easing pressure, which leads to an electron localized ferromagnetic insulating phase.