Very large magnetoresistance and coherent switching in half-metallic manganite tunnel junctions

We have fabricated spin polarized tunneling devices based upon half-metallic manganites $\left({\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3\right)$ incorporating ${\mathrm{NdGaO}}_3$ as a barrier material. These devices show high tunnel magnetoresistance (TMR) values above 77 K and coherent switching with a qualitatively different dependence of resistance on magnetic field to previous devices. The electron polarization deduced from measurements at 77 K is higher than the directly measured value at 4.2 K: we suggest an active tunneling mechanism based on percolative phase separation to account for the general TMR temperature dependence in these materials and the high spin polarization in particular.