Correlation of structural imperfections and exchange bias in vacuum-annealed LaMnO3 films

Lanthanum manganite (LaMnO 3 , LMO) exhibits a versatile capacity to accommodate multiple manganese valence states, enabling widely tunable magnetic and electrical properties with various applications. While stoichiometric orthorhombic LMO is an antiferromagnetic insulator, the incorporation of excess oxygen induces a transition to ferromagnetism. In this work, a detailed structural and magnetic investigation was performed on oxygen-rich and oxygen-deficient LMO films prepared on lanthanum strontium aluminum tantalate (LSAT) substrates. The multiple valence of Mn 3+ –Mn 4+ and Mn 3+ –Mn 2+ was confirmed in the as-grown and vacuum-annealed films respectively, using X-ray photoelectron spectroscopy (XPS) and electron energy loss spectrometry (EELS). Atomic-resolution (scanning) transmission electron microscopy revealed structural changes in the crystalline structure of the LMO/LSAT films upon vacuum annealing. In particular, Ruddlesden-Popper faults are formed in the vacuum-annealed film, which transformed LMO to La 2 MnO 4 . Additionally, oxygen vacancies ( V o ) and octahedral rotations were identified. These structural changes, and the presence of multiple valence Mn in the films, provide solid support to explain the occurrence of exchange bias in vacuum-annealed LMO films, the evidence of which was rather scant in previous studies of similar systems. The atomic-resolution insight bridges the existing gap between macroscopic magnetic observations and microscopic structural and chemical changes induced by post-annealing.