Modeling of the magnetorefractive effect in Co-Al₂O₃ nanocomposites in the framework of the Bruggeman approximation

Objectives. To investigate the magnetorefractive effect (MRE) in nanocomposites, which consists in changing the reflection, transmittance and light absorption coefficients of samples with large magnetoresistance (MR) upon their magnetization. Materials offering high magneto-optical activity and significant MR include magnetic nanocomposites. These materials are based on a polymer matrix, which includes inorganic magnetic particles, fibers or layered particles, whose nanometer sizes range from 1 to 100 nm in at least one dimension. The main purpose of creating such nanocomposites is to combine the special properties of several components in one material. The presence in such materials of gigantic, colossal and tunneling MR, as well as the giant anomalous Hall effect, is of practical interest. Uses range from magnetic recording, light modulation, and receivers for thermal radiation, while the MRE itself is a promising method for the non-destructive testing of any nanostructures, e.g., measuring MR.

Methods. The use of effective medium theory to describe the optics and magneto-optics of dispersed media provides a means to determine the complex permittivity of a medium through the permittivity of its constituent components or vice versa. The present work considers the example of a Co-Al₂O₃ nanocomposite with a concentration of ferromagnetic metal Co 0.4 near the percolation threshold. This particular case was considered for study, since all the properties of nanocomposites change dramatically near the percolation threshold.

Results. Using the Bruggeman effective medium approximation (EMA) to describe the optical and magneto-optical properties of nanocomposites on the example of Co-Al₂O₃, the characteristics of MRE are obtained, namely, the change in MRE for reflection and transmission of light at normal incidence and at the angle of incidence near the Brewster angle (below the percolation threshold) or the main angle of incidence for metals (above the percolation threshold), which enhances MRE. The advantage of the EMA is the ability to study magneto-optical spectra in the range of average volume concentrations of the metal component.

Conclusions. The obtained values correspond well to the known experimental data. Moreover, the described approach can be used to study any nanostructures.

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