Phase evolution and critical behavior in strain-tuned LaMnO3-SrMnO3 superlattices

Mott insulator superlattices, LaMnO3-SrMnO3, grown on lattice-matched La0.3Sr0.7Al0.65Ta0.35O3 substrates were investigated as to the influence of the thicknesses of LaMnO3 (m unit cells [uc], 2 <= m <= 10) and SrMnO3 (n uc, 2 <= n <= 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 >= 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 La1-xSrxMnO3 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.