Quantifying symmetric exchange in ultrathin ferromagnetic films with chirality

TL;DR

This study compares multiple methods for measuring exchange stiffness in ultrathin ferromagnetic films, revealing significant discrepancies and providing insights into their respective strengths and limitations for 2D magnetic systems.

Contribution

It offers a comprehensive comparison of experimental and theoretical techniques to quantify exchange interactions in ultrathin films, highlighting challenges and guiding future research.

Findings

Different methods yield up to 5x variation in exchange stiffness values.

Discrepancies are linked to wave-vector ranges and measurement techniques.

The study outlines strengths and limitations of each method for 2D magnetic characterization.

Abstract

The symmetric (Heisenberg) exchange interaction is fundamental to magnetism and assumes critical importance in designing magnetic materials for novel emergent phenomena and device applications. However, quantifying exchange is extremely challenging for ultrathin ($\sim$ 1 nm) magnetic films, as techniques and approximations reliably used for bulk materials are largely inapplicable in the two-dimensional (2D) limit. Here we present and contrast the measurement of exchange stiffness, $A$, by several methods on a series of five Co/Pt-based ultrathin ($1-2$ nm) films. We compare results from (a) spin-wave spectroscopy by Brillouin light scattering (BLS), (b) three analytical models describing the temperature dependence of magnetization obtained by magnetometry, (c) microscopic domain periodicity measurements and simulations, and (d) ab initio density functional theory (DFT) calculations. While different methods present some qualitatively consistent trends across samples, we note, for any given sample, considerable differences (up to $5\times$) in the absolute values of $A$ across the techniques, consistent with discrepancies of $A$ reported in literature for nominally similar samples. We analyze possible sources of the discrepancies across various methods, notably including their relationship to the spin-wave dispersion, and the wave-vector ranges probed. We compare the strengths and limitations of the techniques, and outline directions for their future use in characterizing exchange interactions in ultrathin films.