Interplane Tunneling Magnetoresistance in a Layered Manganite Crystal

The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La_2-2xSr_1+2xMn_2O_7 (x = 0.3), which is composed of the ferromagnetic-metallic MnO_2 bilayers separated by nonmagnetic insulating block layers. The CPP-MR is extremely large (10^4 percent at 50 kilo-oersted) at temperatures near above the three-dimensional ordering temperature (T_c ≈ 90 kelvin) because of the field-induced coherent motion between planes of the spin-polarized electrons. Below T_c, the interplane magnetic domain boundary on the insulating block layer serves as the charge-transport barrier, but it can be removed by a low saturation field, which gives rise to the low-field tunneling MR as large as 240 percent.