Mid-infrared optical properties of non-magnetic-metal/CoFeB/MgO heterostructures

TL;DR

This study characterizes the mid-infrared optical properties of non-magnetic metal/CoFeB/MgO heterostructures, revealing interface-specific dielectric behaviors and spin-orbit coupling effects, which are crucial for designing spintronic devices.

Contribution

First mid-infrared dielectric function of CoFeB was extracted, and interface dielectric tensor properties, including spin-orbit coupling effects, were characterized in heterostructures.

Findings

Dielectric function of CoFeB obtained in mid-infrared range.

Detection of a 2DEG at the interface with anisotropic Drude parameters.

Identification of spin-orbit coupling feature affecting optical response.

Abstract

We report on the optical characterization of non-magnetic metal/ferromagnetic (Co$_{20}$Fe$_{60}$B$_{20}$)/MgO heterostructures and interfaces by using mid infrared spectroscopic ellipsometry at room temperature. We extracted for the mid-infrared range the dielectric function of Co$_{20}$Fe$_{60}$B$_{20}$, that is lacking in literature, from a multisample analysis. From the optical modelling of the heterostructures we detected and determined the dielectric tensor properties of a two-dimensional gas (2DEG) forming at the non-magnetic metal and the CoFeB interface. These properties comprise independent Drude parameters for the in-plane and out-of plane tensor components, with the latter having an epsilon-near-zero frequency within our working spectral range. A feature assigned to spin-orbit coupling (SOC) is identified. Furthermore, it is found that both, the interfacial properties, 2DEG Drude parameters and SOC strength, and the apparent dielectric function of the MgO layer depend on the type of the underlying nonmagnetic metal, namely, Pt, W, or Cu. The results reported here should be useful in tailoring novel phenomena in such types of heterostructures by assessing their optical response noninvasively, complementing existing characterization tools such as angle-resolved photoemission spectroscopy, and those related to electron/spin transport.