  Insertion of layers displaying field-induced metal-to-insulator (M/I)
transition in ferroelectric tunnel junctions (FTJs) has received attention as a
potentially useful way to enlarge junction tunnel electroresistance (TER).
Half-doped manganites being at the verge of metal-insulator character are thus
good candidates to be slave layers in FTJs. However, the phase diagram of these
oxides is extremely sensitive to strain and thus can be radically different
when integrated in epitaxial FTJs. Here we report a systematic study of
large-area (A = 4 to 100 um2) Pt/La0.5Sr0.5MnO3/BaTiO3/La0.7Sr0.3MnO3
(Pt/HD/BTO/LSMO) FTJs, having different thicknesses of the ferroelectric
(2-3nm) and HD layers (1-2nm), grown on substrates imposing either tensile
(SrTiO3) or compressive (LaAlO3) strains. Room-temperature electric
characterization of the FTJs shows polarization-controlled ON/ OFF states.
Clear evidences of field-induced M/I transition (difference between junction
resistance in OFF and ON state is increased of more than one order of
magnitude) are observed in junctions prepared on SrTiO3 but the HD layer is
generally metallic on LaAlO3. Moreover, the M/I transition is only confined in
an interfacial layer of the slave film thus entailing an overall reduction of
TER. The orderly results reported here give some hints towards selection of HD
materials and substrates for optimal FTJ responsiveness.
