Mott insulator superlattices, ${\text{LaMnO}}_3{\text{-SrMnO}}_3$, grown on lattice-matched ${\text{La}}_{0.3}{\text{Sr}}_{0.7}{\text{Al}}_{0.65}{\text{Ta}}_{0.35}{\text{O}}_3$ substrates were investigated as to the influence of the thicknesses of ${\text{LaMnO}}_3$ ($m$ unit cells [uc], $2\le m\le 10$) and ${\text{SrMnO}}_3$ ($n$ uc, $2\le n\le 6$) layers on the electronic and magnetic properties. The superlattices exhibited dramatic phase evolution and critical behavior when the structural imperfections were significantly diminished. Ground states of the superlattices were mostly ferromagnetic insulator (nonmetal), whereas typical ferromagnetic metal (FM) could be realized for $m>n$ and $n=2$. For $m=2$, the antiferromagnetic insulator (AFI) was stabilized for $n\ge 3$ and an insulating state persisted even down to $n=2$. Around the metal-insulator boundary, the superlattices exhibited magnetorelaxorlike large magnetoresistance and in the case of $m=n=2$, a magnetic field induced an insulator-metal transition, which is unpredictable from ${\text{La}}_{1-x}{\text{Sr}}_x{\text{MnO}}_3$ bulk and alloy films. Detailed analyses of the magnetic field dependences of magnetization and resistivity for the superlattices indicated that the phase separation of FM and AFI may occur at the interface and the AFI state may change to the FM state by applying a magnetic field.

• Received 11 August 2009

DOI:https://doi.org/10.1103/PhysRevB.81.014410