Magnetic tunnel junctions (MTJs) are increasingly used in applications such as magnetic field sensors, hard drive read heads and non-volatile memory circuits [1]. While most MTJs are fabricated by growing layers with a controlled insulator barrier thickness, they also can be formed naturally in polycrystalline manganite films, in which the grain boundaries (GBs) act as barriers. At low magnetic field strengths (<100 mT) the resistance of such films can be drastically decreased due to the spin-polarized tunneling of charge carriers through the disordered GBs. This effect is known as low-field magnetoresistance (LFMR) [2] and could be used for magnetoresistive sensor applications. At higher magnetic field strengths, the colossal MR effect occurs due to the alignment of the magnetic moments of the Mn ions. This effect does not achieve saturation even at magnetic field strengths of tens of teslas, which is important for the development of high magnetic field sensors [3]. Apart from the magnetoresistive effect, polycrystalline manganite films also exhibit negative electroresistance (ER) since the electric potential can align the energy levels between the magnetic layers separated by the insulating barrier. Therefore, for device applications, it is essential to understand the charge carrier transport mechanisms occurring in the GBs between crystallites.
In this study, we explore the electrical conductivity behavior of vertically aligned La-Ca(Sr)-Mn-O nanocolumn films with disordered GBs while under the influence of synchronized strong pulsed electric and magnetic fields. Nanosecond electric field pulses were employed to analyze the intrinsic properties of the junctions without introducing adverse thermal effects. The pulsed magnetic fields enabled the investigation of the combined magnetic and electric properties under semi-stationary conditions, with fields reaching up to 20 T. It was demonstrated that with increasing electric field strengths the inelastic tunneling through two or more localized states in the GBs becomes dominant and that tunneling behavior is dependent on both the magnetic field and temperature.