The formation of a magnetic dead layer at the interfaces of the perovskite oxide La0.67Sr0.33MnO3 (LSMO) is one of the crucial issues for its spintronic applications. In this letter, we report the reduction of the dead layer by growing LSMO on a LaMnO3 (LMO) layer. Furthermore, we detect tunneling magnetoresistance (TMR) in an LSMO/LMO/LSMO heterostructure. The obtained sign of the TMR was negative, but it changed to positive after annealing. This unusual negative TMR can be attributed to the intrinsic structural difference between the upper and lower interfaces of LMO and can be understood by a weak antiferromagnetic metallic thin layer formed at the upper LSMO/LMO interface. This layer is thought to be formed by diffused Sr atoms and oxygen vacancies in the LMO barrier. Our results indicate that control of intermixing of atoms at the interfaces is a key to controlling the TMR.

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