The bias and temperature dependences of tunneling magnetoresistance (TMR) woe investigated for Fe/Al₂O₃/CoFe with a relatively small tunneling impedance (resistance times area: R × A∼1.8 × 10^-9Ωm²). Despite a rather thinner and lower tunneling barrier, the change in conductance under a magnetic field obeys an a-bT^3/2 dependence, suggesting the dominance of the contribution of the decrease in effective spin polarization with temperature. In addition to a monotonic decrease in the magnitude of the TMR with increasing bias voltage, there is a slight change in the field dependence in low fields.