Because the magnetization is a vector: observing magnetism at the nanometer scale with low-energy electrons

Abstract

Nowadays there is a strong effort to harness spintronics in order to provide the next generation storage (and maybe even computation) devices. Those efforts typically rely on the control of magnetic domains and magnetic domain walls through novel effects such as spin-transfer or spin-orbit torque instead of pure magnetic fields. However, there are not too many techniques that allow determining the magnetization vector from the surface of a magnetic material with nanometer resolution. In this talk I will discuss how low-energy electrons can provide such information through the use of different techniques, mostly x-ray magnetic circular dichroism in photoemission microscopy (XMCD-PEEM) and spin-polarized low-energy electron microscopy (SPLEEM). The materials on which we will apply such techniques have a long history in magnetic applications. Cubic ferrites, of which magnetite is the oldest known member, span a wide range of properties: ferrimagnetic/antiferromagnetic, conductive/insulating, etc. They share the same spinel structure, with the cation in either octahedral or tetrahedral sites. While magnetite is a soft magnet, cobalt ferrite is (relatively) hard. One problem for their use in spintronic applications is that films of cubic ferrites usually present growth defects that strongly affect the magnetic properties, giving rise to small domains in remanence. In this talk, we will first map the magnetization vector with nanometer resolution on the surface of bulk crystals of magnetite [1]. We will show how the magnetic domains evolve with temperature due to changes in the magnetocrystalline anisotropy, both through the Verwey transition [2,3](see Fig. 1) or in the absence of any phase transition [4]. Then we will turn to domains in magnetite films with antiphase domain boundary defects [5], and finally to micrometric wide nanometer thick islands of magnetite free of such domains [6]. We will end discussing the relative limitations and advantages of these techniques (and related ones [7]) for the observation of surface magnetism.