Magnetic Bicrystal Junctions - a Study of Half-Metallic Manganite Grain Boundaries

Doktorsavhandling, 2004

This thesis presents studies on magnetic bicrystal junctions. In these, films on either side of the junction are structurally and magnetically misoriented. This type of device enables us to study junction magnetoresistance and spin polarized charge transport between ferromagnets with well defined directions of magnetization. The ferromagnets subject to our studies are epitaxial films, prepared by pulsed laser deposition, of La0.7Sr0.3MnO3, suggested to possess half-metallic characteristics. The electrical transport through single and arrays of junctions have been measured as functions of magnetic field and temperature. We have demonstrated that the magnetization reversal processes responsible for the magnetoresistance observed in poly-crystalline films can be studied in detail by using single bicrystal grain boundary junctions. We have shown that when the field is applied perpendicular to a grain boundary, the magnetization reversal process can be well described by a Stoner-Wohlfarth model for coherent rotation of magnetization. Whereas, when the field is applied along the grain boundary, the magnetoresistance data suggest a stochastic reversal process, including splitting into domains. Coherent rotation of magnetization is an essential ingredient when modelling the magnetoresistance hysteresis. We have observed, by high resolution electron microscopy, that the region of structural disorder at the bicrystal interface can be small, with a width as small as down to a unit-cell. Different models have been discussed for the transport mechanism across the interface. Current-voltage characteristics of single junctions are close to Ohmic at low voltage, and become non-linear at voltages higher than 5 - 20 mV, depending on misorientation angle. However, they cannot be fitted to a simple parabolic function. The results are discussed in terms of a tunnelling model.