Magnetoresistance of polycrystalline layered manganites La2-2xSr1+2xMn2O7

Polycrystalline bulk samples of the layered manganites La2-2xSr1+2xMn2O7 sh owing a colossal magnetoresistance (CMR) effect were prepared for doping le vels of x = 0.4 and 0.5 by a solid-state reaction route. Temperature and fi eld dependences of magnetization and of electrical resistivity have been me asured between 5 K and room temperature. Ferromagnetic behavior with a Curi e temperature of T-C = 135 K and a high value of spontaneous magnetization (64 emu g(-1)) was found for x = 0.4. The x = 0.5 compound orders antiferro magnetically near T-N = 215 K, showing a resistivity enhancement there. The resistivity of the x = 0.4 compound shows a field dependence at low temper atures similar to that found for polycrystalline La0.7Sr0.3MnO3. The resemb lance includes the presence of both a low-field and a high-field contributi on to the magnetoresistance, as well as the magnitude of the low-field MR ( -23% at 20 K and -15% at 77 K for H = 1 kOe). The low-field MR is attribute d to the effect of grain boundaries. In contrast to the grain-boundary MR o f La0.7Sr0.3MnO3, it was also observed at temperatures far above the T-C of the layered manganite. (C) 1999 American Institute of Physics. [S0021-8979 (99)20108-9].