High resolution transmission electron microscopy correlated to in-field Mössbauer spectrometry to investigate exchange coupling behavior and surface frustrated moments
Abstract
In this work, we present a correlation between High Resolution Transmission Electron Microscopy (HRTEM) and in-field Mössbauer spectrometry to point out exchange coupling behavior, surface effects and chemical disorder in CoxFe3-xO4 cobalt ferrite nanoparticles. The samples were synthesized by a solvothermal chemical route. The hysteresis loops, recorded by using Vibrating Sample Magnetometer (VSM), suggest a jump-like behavior nearby zero applied magnetic fields. The HRTEM micrographs illustrate the presence of faceted nanoparticles for x < 1. Moreover, the images show the presence of some crystal defects such as twins. 3D images have been simulated to determine the crystallographic orientation for the different facets. By performing analyses using a TEM corrected spherical aberration (Cs), we have evidenced the presence of twin boundaries viewed along the [110] orientation at atomic scale. These crystal defects, facets and twin boundaries promote important interactions between nanoparticles. The spin frustration has been investigated by in-field Mössbauer spectrometry performed for three samples chosen by respect to zero field analyses. The observed frustrated Fe moments have been referred to three different origins: interactions in the contact surface between faceted nanoparticles, chemical disorder and surface effects for x = 1.6 and 1.8. A canted layer of about 0.35 nm for grain size of about 6 nm has been found for x = 1.6.