Abstract
Understanding the ramifications of reduced crystalline symmetry on magnetic behavior is a critical step in improving our understanding of nanoscale and interfacial magnetism. However, investigations of such effects are often controversial largely due to the challenges inherent in directly correlating nanoscale stoichiometry and structure to magnetic behavior. Here, we describe how to use Transmission Electron Microscope (TEM) to obtain Electron Magnetic Circular Dichroism (EMCD) signals as a function of scattering angle to locally probe the magnetic behavior of thin oxide layers grown on an Fe (1 1 0) surface. Experiments and simulations both reveal a strong dependence of the magnetic orbital to spin ratio on its scattering vector in reciprocal space. We exploit this variation to extract the magnetic properties of the oxide cladding layer, showing that it locally may exhibit an enhanced orbital to spin moment ratio. This finding is supported here by both spatially and angularly resolved EMCD measurements, opening up the way for compelling investigations into how magnetic properties are affected by nanoscale features.
Highlights
Half-metallic behavior[2] leading to a near 100% spin polarization[3,4], high chemical stability in ambient conditions, and ability to stabilize very thin films against the onset of superparamagnetism[5]
This means that the sample itself can be used as a beam splitter to break the symmetry of inelastically scattered electrons giving rise to a dichroic effect in the diffraction plane originating from the asymmetry in the density of states inherent to ferromagnetic solids
This dependence - known as an “Electron Magnetic Circular Dichroism (EMCD) strength map” - can be experimentally recorded in the Tranmission Electron Microscope (TEM) either by angularly selecting the inelastically scattered electrons or by energy filtering a series of electron diffraction patterns
Summary
Half-metallic behavior[2] leading to a near 100% spin polarization[3,4], high chemical stability in ambient conditions, and ability to stabilize very thin films against the onset of superparamagnetism[5]. We exploit both of these dynamical diffraction properties to extract mL/ms for two magnetic materials having the same magnetic species but different crystal structures that overlap in the direction of the electron beam Such situations readily arise for metallic TEM lamellae prepared in cross section that have been exposed to atmosphere between sample preparation and transfer to the microscope, forming a thin metal oxide layer on the exposed surfaces. Since both EMCD strength maps vary differently in reciprocal space, when sum rules are applied to the entire EMCD signal map, the measured value for mL/ms will vary as a function of detector position, which can be experimentally determined with high precision We exploit this effect to probe a TEM lamella of bcc iron having an exposed (1 1 0) surface, upon which a thin layer of cubic iron oxide has grown in the {1 1 1} orientation. Our experimental data combined with simulations on the system bring us to the conclusion that these thin Fe3–δO4 layers may locally exhibit a large, uncompensated orbital magnetic moment
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