This paper reports on the study of deposited ͑Ni 81 Fe 19 / AlO x / Co͒ magnetic tunnel junctions by magnetron sputtering, with the insulating layer obtained by plasma oxidation of Al. Concentration of the tunnel current in small areas of the junctions and low potential barrier heights were identified by fitting, for each individual sample, the room temperature I-V curves with either Simmons' ͓J. Appl. Phys. 34, 1793 ͑1963͒; 35, 2655 ͑1964͒; 34, 2581 ͑1963͔͒ or Chow's ͓J. Appl. Phys. 36, 559 ͑1965͔͒ model. A fast decrease of the tunnel magnetoresistance as a function of the bias voltage is observed, with an inversion of its signal above a critical value. The results are discussed in terms of the quantum coherence factor for low height insulating barriers.