Magnetic tunnel junctions(MTJ), i.e., structures consisting of two ferromagnetic layers (FM_1 and FM_2), separated by a very thin insulator barrier (I), have recently attracted attention for their large tunneling magnetoresistance (TMR) whichi appears when the magnetization of the ferromagnets of FM1 and FM2 change their relative orientation from parallel to antiparallel in an applied magnetic field. Using an ultrahigh vacuum magnetron sputtering system, a variety of MTJ structures have been explored. Double Hc magnetic tunnel junction, NiFe/Al_2O_3/Co and FeCo/Al_2O_3/Co, were fabricated directly using placement of successive contact mask. The tunnel barrier was prepared by in situ plasma oXidation of thin Al layers sputter deposited. For NiFe/Al_2O_3/Co junctions, the maximum TMR value reaches 6.0% at room temperature, the switch field can be less than 10 Oe and the relative step with is about 30 Oe. The junction resistance chantes from hundreds of ohms to hundreds of kilo-ohms and TMR values decrease monotonously with the increase of applied junction voltage bias (under zero magnetic field). For FeCo/Al_2O_3/Co junctions, TMR values exceeding 7% were obtained ato room temperature. It is surprising that an inverse TMR of 4% was observed in FeCo/Al_2O_3/Co. The physics governing the spin polarization of tunneling electrons remains unclear. Structures, FeMn/NiFe/Al_2O_3/NiFe, in which one of the FM layers is exchange biased with an antiferromagnetic FeMn layer, were also prepared by patterning using optical lithography techniques. Thus, the junctions exhibit two well-defined magnetic states in which the FM layers are either parallel or antiparallel to one another. TMR values of 16% at room temperature were obtained. The switch field and step width are less than 10 Oe and larger than 30 Oe, respectively.