Pressure-enhanced ferromagnetism and metallicity in La_1.24Sr_1.76Mn₂O₇ bilayered manganite system

We report the low temperature magnetization, electrical resistance, and magnetoresistance of La_1.24Sr_1.76Mn₂O₇ bilayer manganite system under hydrostatic pressure. At ambient pressure, the compound shows a sharp ferromagnetic transition (T _C) accompanied by a metal–insulator transition (T _MI) at 130 K. We observe that the T _C and T _MI increase with hydrostatic pressure at a rate of dT _C/dP = 2.08 K/kbar and dT _MI/dP = 2 K/kbar, respectively. Also, we observe an appreciable increase of magnetic moments at low temperatures with increasing pressure. The high temperature regime of temperature dependence of resistivity curves was fitted with the Emin–Holstein’s polaron hopping model and the calculated activation energy values suggest that the applied pressure weakens the formation of Jahn–Teller polarons. The magnetoresistance ratio (MRR) was measured at T _C and at 4.2 K upon an external magnetic field of 5 T. The observed MRR at T _C is about 210 % and the applied pressure increases the MRR significantly. These results can be interpreted by the pressure-enhanced overlap between the orbitals of Mn–O–Mn, which facilitates the charge transfer and hence enhances the ferromagnetism and metallicity.