Tunneling magnetoresistance in mixed-valence manganite tunnel junctions

An effective tunneling Hamiltonian including spin-flip effect is proposed to account for the tunneling magnetoresistance (TMR) in mixed-valence manganite tunnel junctions and the current-perpendicular-to-plane TMR in layered manganite crystals. It is found that the electron spin-flip tunneling plays an important role in diminishing the amplitude of the TMR ratio. The theoretical results are in agreement with the recent experimental observations in the magnetic trilayer junction structure ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3/{\mathrm{SrTiO}}_3/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ and the bulk crystals ${\mathrm{La}}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_2{\mathrm{O}}_7$. It is suggested that reducing electron spin-flip tunneling is a potential way to gain larger TMR in the manganite tunnel junctions.