Influence of microstructure and interfacial strain on the magnetic properties of epitaxial Mn3O4/La0.7Sr0.3MnO3 layered-composite thin films

Abstract

Epitaxial Mn3O4/La0.7Sr0.3MnO3 (Mn3O4/LSMO) bilayer thin films were grown on lattice-matched single crystal substrates of SrTiO3 (STO) (100) and MgO (100), with Mn3O4 as the top layer, using a pulsed laser deposition technique. X-ray diffraction (XRD) patterns revealed the single crystalline nature and epitaxial relationship between the layers. A detailed analysis of strains using XRD asymmetric/symmetric scans indicated an increasing in-plane compressive strain in the LSMO layer with increasing thicknesses of the Mn3O4 layer, resulting in a tetragonal distortion of the LSMO lattice in the Mn3O4/LSMO films in comparison to the tensile strains in LSMO single-layer films grown on both STO and MgO substrates. Cross-sectional high resolution transmission electron microscope (HRTEM) images showed atomically sharp interfaces in all films. However, as opposed to a flat interface between LSMO and STO, the Mn3O4 and LSMO interface was undulating and irregular in the bilayer films. Magnetic measurements revealed that relative to LSMO, the presence of Mn3O4 in Mn3O4/LSMO reduced the saturation magnetization at T > 50 K (the ferrimagnetic ordering temperature of Mn3O4) but enhanced it at T < 50 K. The decrease of the saturation magnetization in Mn3O4/LSMO for T > 50 K was associated with the appearance and increase of the compressive strain with the increase in Mn3O4 thickness. These observations point to the importance of a ferromagnetic-ferrimagnetic interfacial coupling between the LSMO and Mn3O4 layers in enhancing the surface magnetism of LSMO in the Mn3O4/LSMO bilayers. Our study provides useful information regarding the development of manganite composite thin films with improved magnetic properties for a wide range of technological applications, such as in spintronics and sensor devices.