The role of antiferromagnetic La1/3Ca2/3MnO3 barriers in superconductor/insulator/ferromagnet tunnel junctions

Current transport through thin antiferromagnetic (AF) barriers of the perovskite manganite La1/3Ca2/3MnO3 (LCMO) was studied with respect to its dependence on temperature and voltage. Planar-type La2/3Ca1/3MnO3(∼80 nm)/La1/3Ca2/3MnO3(∼7 nm)/YBa2Cu3O7−δ(∼100 nm) heterojunctions were used as basic structures. The current-voltage (I-V) measurements were carried out on test junctions with a standard area of 20 × 40 μm2 in a four-terminal configuration. In spite of the carefully controlled growth conditions, barriers with the same nominal thickness showed different electrical behavior varying from elastic tunneling to Mott variable range hopping (VRH) via localized states. Fitting the VRH model to the experimental data, allowed for estimating important physical parameters of the barrier as the density of states at the Fermi level N(EF) and with this the average distance between two localized states ℓ0. The different transport characteristics seem to be related to intrinsic difference in microstructure as the average surface roughness of the constituent layers may already be larger than the thickness of the barrier itself. Independent of the barrier quality, the active presence of the diamagnetic and ferromagnetic phases in the heterostructure was corroborated by transport measurements in magnetic fields and in-plane/out-of-plane magnetization hysteresis loops below the superconducting critical temperature, Tc (∼80 K). The values of the critical magnetic field Hc1 estimated from these experiments were in good accordance with those reported in the literature.