Confined magnon dispersion in ferromagnetic and antiferromagnetic thin films in a second quantization approach: The case of Fe and NiO

We present a methodology based on the calculation of the inelastic scattering from magnons via the spin-scattering function in confined geometries such as thin films using a second quantization formalism, for both ferromagnetic and antiferromagnetic materials. The case studies are chosen with an aim to demonstrate the effects of film thickness and crystal orientation on magnon modes, using bcc Fe(100) and NiO with (100) and (111) crystallographic orientations as prototypical systems. Due to the quantization of the quasimomentum, we observe a granularity in the inelastic spectra in the reciprocal space path reflecting the orientation of the thin film. This approach also allows for the capture of softer modes that appear due to the partial interaction of magnetic moments close to the surface in a thin film geometry, in addition to bulk modes. The softer modes are also affected by crystallographic orientation, as illustrated by the different surface-related peaks of the NiO magnon density of states at approximately <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>∼</a:mo><a:mn>65</a:mn><a:mspace width="0.16em"/><a:mi>meV</a:mi></a:mrow></a:math> for (100) and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mo>∼</c:mo><c:mn>42</c:mn><c:mspace width="0.16em"/><c:mi>meV</c:mi></c:mrow></c:math> for a (111)-oriented film. Additionally, we explore the role of anisotropy, revealing that anisotropy increases the overall hardness of the magnon modes. The introduction of a surface anisotropy produces a shift of the surface-related magnon DOS peak to higher energies with increased surface anisotropy, and in some cases leads to a surface-confined mode. Published by the American Physical Society 2024