We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+delta}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $Vsim 1$ V we observe a st
ep-like onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly non-equilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long-range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.
We perform a detailed comparison of magnetotunneling in conventional low-$T_c$ Nb/AlAlOx/Nb junctions with that in slightly overdoped Bi$_{2-y}$Pb$_y$Sr$_2$CaCu$_2$O$_{8+delta}$ [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calcula
tions. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign-change of tunneling magnetoresistance from the negative at the sub-gap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field $H_{c2}$. For Nb an extended region of surface superconductivity at $H_{c2}<H<H_{c3}$ is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers and for different mesa sizes, which precludes the influence of self-heating. It is found that $H_{c2}(0)$ for Bi(Pb)-2212 is $simeq 70$ T and decreases significantly at $Trightarrow T_c$. The amplitude of sub-gap magnetoresistance is suppressed exponentially at $T>T_c/2$, but remains negative, although very small, above $T_c$. This may indicate existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at $T_c$.
