Study of transport properties of HTSC based Josephson
junctions network with magneto-active barriers

K.A. Shaihutdinov, M.I. Petrov, D.A.Balaev,  S.I. Popkov.

Kirensky Institute of Physics, 660036, Krasnoyarsk, Russia

The aim of this work is to study the effect of magnetic moments in insulator barriers between High Temperature Superconductors (HTSCs) on tunneling of current carriers. Bulk composites HTSC + magneto-active insulator with volume content of the latter component 7.5% and 15% representing a network of Superconductor-Insulator-Superconductor Josephson junctions have been prepared. Paramagnetic NiTiO3 and ferrimagnetic Y3Fe5O12 have been used as materials forming magneto-active barriers between HTSC crystallites. “Nonmagnetic” MgTiO3 and Y3Al5O12 respectively have been used as materials for the “bench mark” composite samples in order to emphasize the effect of magnetic moments of the barriers on the transport properties of the composites.  The temperature dependences of resistivity R(T) at different applied magnetic fields H, the temperature dependences critical current density JC(T), and the current-voltage characteristics (CVCs) at different temperatures are presented. The R(T) dependences of composites Y3/4Lu1/4Ba2Cu3O7 + MgTiO3 are fitted well by the mechanism of Thermally Activated Phase Slippage (TAPS) in Josephson junction using assumption of potential barrier of varying length. The anomalous behavior of R(T) of composites Y3/4Lu1/4Ba2Cu3O7 + paramagnetic NiTiO3, manifesting as a kink on R(T) curves at some temperature Tm » 76K has been observed. In the temperature range Tm < T < TC the CVC are linears, resistivity weakly depends on H, while below Tm the CVCs are non-linear and strong  dependence of R(H) takes place. Anomalous behavior of composites HTSC + NiTiO3 is explained by the effect of magnetic moments of Ni in insulating barriers between HTSC crystallites on the transport of current carriers. The peculiarity observed is interpreted as arisen owing to Abrikosov vortices flow. For the composites Y3/4Lu1/4Ba2Cu3O7 + Y3Fe5O12 similar picture of R(T,H) has been observed. Temperature Tm equals to ~ 40 K. Composites Y3/4Lu1/4Ba2Cu3O7 + Y3Al5O12 demonstrates R(T,H) behavior similar to that predicted by mechanism of TAPS.  Magnetic properties of the composites also have been measured. The magnetization curves of Y3/4Lu1/4Ba2Cu3O7 + Y3Fe5O12 composites represents the superposition of ferrimagnetic and superconducting hysteresis loops. There is a knee on the JC(T) of composite sample Y3/4Lu1/4Ba2Cu3O7 + NiTiO3 correlating with Neel temperature of antiferromagnetic NiTiO3. The knee on the JC(T) of composite sample Y3/4Lu1/4Ba2Cu3O7 + NiTiO3 at Neel temperature of antiferromagnetic NiTiO3 (TN~22K) is observed. This peculiarity can be explained by sharp decreasing of antiferromagnetic correlation length at Neel temperature, so below TN the additional mechanism of reduction of superconducting properties takes place. Thus, the although transport properties of Josephson networks including paramagnetic and ferrimagnetic barriers are quality similar the magnitude of effects (Tm, TC(R=0), JC(T)) is quite different.