BC-05 Current-Induced Circular Dichroism on Metallic Surfaces: A First-Principles Study
Creators: Farzad Mahfouzi
Mark Stiles
Paul Haney
Publisher: name: IEEE
Year: 2025

Circular dichroism, which is the difference in a system’s response to left-hand and right-hand circularly polarized light can be used to characterize magnetic order. We use ab-initio calculations to investigate the current-induced optical response and orbital accumulation at the surfaces of metallic films. These are related by a sum rule that relates the circular dichroic absorption integrated over frequency to the gauge-invariant self-rotation contribution to the orbital magnetization, denoted by MSR. In typical ferromagnets, MSR is a good approximation to the total orbital magnetization. We compute the current-induced MSR for a Pt thin film and compare it to the current-induced orbital accumulation calculated with the atom-centered approximation (ACA). We find significant differences: the size of MSR is generally larger than the ACA orbital magnetization by an order of magnitude and includes substantial finite-size effects. The differences between the two quantities caution against interpreting optical measurements with models utilizing the ACA. Finally, we compute the total MSR and ACA orbital magnetization accumulation as a function of layer thickness. For both quantities, the length scale at which the total surface accumulation saturates is on the order of the mean free path and longer than the length scale of their spatial profiles. Fig. 1 shows the current induced orbital accumulation for the self-rotating αSRyx=δMSRy/eExbias and the ACA type αACAyx=δMACAy/eExbias summed over half of the Pt film. The various curves represent different onsite random potential strengths with normal distributions. We observe that regardless of the impurity strength, both types of orbital accumulations increase monotonically with film thickness and decrease as impurity strength increases.
Circular dichroism, which is the difference in a system’s response to left-hand and right-hand circularly polarized light can be used to characterize magnetic order. We use ab-initio calculations to investigate the current-induced optical response and orbital accumulation at the surfaces of metallic films. These are related by a sum rule that relates the circular dichroic absorption integrated over frequency to the gauge-invariant self-rotation contribution to the orbital magnetization, denoted by MSR. In typical ferromagnets, MSR is a good approximation to the total orbital magnetization. We compute the current-induced MSR for a Pt thin film and compare it to the current-induced orbital accumulation calculated with the atom-centered approximation (ACA). We find significant differences: the size of MSR is generally larger than the ACA orbital magnetization by an order of magnitude and includes substantial finite-size effects. The differences between the two quantities caution against interpreting optical measurements with models utilizing the ACA. Finally, we compute the total MSR and ACA orbital magnetization accumulation as a function of layer thickness. For both quantities, the length scale at which the total surface accumulation saturates is on the order of the mean free path and longer than the length scale of their spatial profiles. Fig. 1 shows the current induced orbital accumulation for the self-rotating αSRyx=δMSRy/eExbias and the ACA type αACAyx=δMACAy/eExbias summed over half of the Pt film. The various curves represent different onsite random potential strengths with normal distributions. We observe that regardless of the impurity strength, both types of orbital accumulations increase monotonically with film thickness and decrease as impurity strength increases.