Magnetization switching of manganite-based thin film heterostructures studied by Cryo Electron Holography

Abstract

Progress in nanofabrication of nanomaterials is pushing the scientific community to search for and develop new tools to measure physical properties at the nanoscale. In the case of magnetic nanomaterials, the measurement of the magnetic properties of individual nanomagnets is of the utmost importance to understand the magnetization processes underway and to correlate the local magnetic behavior with the macroscopic properties expected to lead to new technological applications. Spintronics is a very active area in magnetism for the development of magnetic nanostructures for devices, such as magnetic tunnel junctions (MTJs). Though great advances have been done on the magnetic and magnetotransport macroscopic behavior of MTJs, a great physical insight would be gained by direct visualization of magnetization switching processes with nanometer resolution. High spatial resolution techniques are required for local characterization of magnetization processes in nanostructures. Transmission Electron Microscopy (TEM) techniques such as Electron Holography (EH) allows the quantitative imaging of the magnetization configurations of ferromagnetic (FM) materials with unprecedented nanometer spatial resolution. Furthermore, EH can be combined with in situ TEM experiments applying external constraints such as magnetic and electric fields, temperature, etc. In this work, we use a TEM cryo-holder to image by EH the magnetization states of FM materials whose Curie temperature (TC) is below room temperature while varying in situ an external magnetic field applied on the thin sample We will present the EH study of the magnetization switching of LaxCa1-xMnO3 (LCMO) and LaxSr1-xMnO3 (LSMO) based thin film MTJs (whose TC = 180 and 300 K, respectively) as a function of the temperature and magnetic field applied thanks to the objective lens. The interlayer coupling/decoupling of magnetic electrodes in MTJs will be analyzed by performing hysteresis loops at 100 K, mapping the magnetic configurations of the two FM electrodes. This allows both the switching fields and the magnetization reversal processes to be studied as a function of the multilayer structure and composition. Different strategies to subtract the contribution of the mean inner potential to the total phase shift will be explored in order to optimize the process of magnetic phase retrieval in such cryo-EH conditions.