Phase problem in the B-site ordering of La2CoMnO6: impact on structure and magnetism

Abstract

Epitaxial double perovskite La₂CoMnO₆ (LCMO) films were grown by metalorganic aerosol deposition on SrTiO₃(111) substrates. A high Curie temperature, T_C = 226 K, and large magnetization close to saturation, M_S(5 K) = 5.8μ_B/f.u., indicate a 97% degree of B-site (Co,Mn) ordering within the film. The Co/Mn ordering was directly imaged at the atomic scale by scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDX). Local electron-energy-loss spectroscopy (EELS) measurements reveal that the B-sites are predominantly occupied by Co²⁺ and Mn⁴⁺ ions in quantitative agreement with magnetic data. Relatively small values of the (1/2 1/2 1/2) superstructure peak intensity, obtained by X-ray diffraction (XRD), point out the existence of ordered domains with an arbitrary phase relationship across the domain boundary. The size of these domains is estimated to be in the range 35–170 nm according to TEM observations and modelling the magnetization data. These observations provide important information towards the complexity of the cation ordering phenomenon and its implications on magnetism in double perovskites, and similar materials.