Oscillation of Low-Bias Tunnel Conductance With Applied Magnetic Field in Manganite/Alumina Tunnel Structures

We present magnetotransport measurements on tunnel structures Ir/LCMO/Al₂O₃/LCMO/Al, where the Ir and Al layers were used to guarantee the homogeneity of tunnel current. Epitaxial growth of Ir and LCMO layers on MgO(001) was confirmed by X-ray diffractions. At TLtT_c, the junction exhibits a large tunneling magnetoresistance (TMR) of about 210% (5 K, low-field effect). However, near T_c the tunneling conductance G becomes oscillatory with increased magnetic field H, as the bias voltage V drops down below 0.2 V, accompanied with a huge large-field magnetoresistance (MR) (1220% at 14 mV and 70 kOe) and a large extra low-field MR (430% at 1 mV and 5 kOe). For V>0.2 V, the magnetotransport behavior becomes simply colossal MR (CMR)-like. The novel behaviors observed near T_c could be attributed to the field-induced enhancement in density of, for example, electron state near the Fermi level