Manganite tunnel structures Ir(25nm)/LCMO(45nm)/Al2O3(1.5nm)/LCMO(25nm)/Al(5nm) were sputtered onto MgO(100) substrates and patterned in size of 50X50 mm2, where additional metallic layers Ir and Al were used to ensure a homogeneous current distribution over the junction area. Epitaxial growth of relevant layers was confirmed by x-ray structural analysis. Resistive measurements revealed a metal-isolating transition temperature of about 240K for the bottom electrode and about 80K for the upper electrode, respectively. A large low-field magnetoresistance (ΔR/Rmax) was found to be ~70 % at 10K, arising from spin dependent tunneling of two separated ferromagnetic LCMO layers. Near 80K, the magnetotransport behavior is dominant by colossal magnetoresistance effect ~80% (7 tesla). Surprisingly, at low bias voltage (< 0.2V), the tunneling current becomes oscillatory with increased magnetic field, showing resistive maxima at 0.5, 2.5 und 5.5 tesla, respectively. Fitting current-voltage (I-V) curves up to 1.4V provides an effective barrier height Φe~1.2eV and an effective width de~1,7nm which corresponds to the barrier layer of Al2O3. Fitting I-V curves within 0.25V, however, supplies Φe~0.3V and de~4.0nm. This additional barrier with a relatively low height can be attributed to the interfacial LCMO.
Supported by DFG-project SA 337/9-1