Multifunctional Spin-Dependent Tunneling: From Tunnel Magnetodielectric to Magneto-Optic and Faraday Effects

Magnetic granular nanocomposites, consisting of magnetic nanogranules dispersed within a host matrix, represent a versatile class of functional materials that enable control over electrical, magnetic, and thermal properties at the nanoscale. Over the past decade, by leveraging electrons as carriers of spin, charge, and heat, these features have enabled the discovery of a family of tunnel related phenomena: tunnel magnetoresistance (TMR), tunnel magneto-Seebeck (TMS), tunnel magnetodielectric (TMD), and most recently tunnel magneto-optical (TMO) effects. Their structural features allow for tuning of granular size, distribution, and intergranular spacing, positioning these materials as promising candidates for miniaturized magnetic field sensors, antennas, microwave devices, and spintronic components.

In this Account, we summarize our recent advances in understanding TMD effects in complex granular nanocomposites over the past decade. We begin by illustrating how key structural parameters, including intergranular spacing, granule distribution, and magnetic granule composition, govern dielectric variations. From a theoretical standpoint, we derive a formula that predicts the maximum achievable dielectric change. Experimentally, we show that introducing small amounts of ferromagnetic species to balance the ferromagnetic and superparamagnetic components in a nanogranular composite greatly enhances low-field sensitivity. Moreover, by integrating silicon into the films to improve interfaces, the TMD response (i.e., the maximum dielectric variation) reaches a record 8.5% under a 10 kOe magnetic field. We also investigate the heterostructures, such as gradient and multilayer architectures, which effectively broaden the TMD frequency range. Based on the established mechanism of spin-dependent charge oscillations, we demonstrate that optical transmittance in these nanocomposites can be regulated via an external magnetic field (the TMO effect). Transparent FeCo–AlF₃ films exhibit magneto-tunable transmittance across the visible-NIR range, while fluoride- and nitride-based nanogranular films yield giant Faraday rotations, which is more than 40 times greater than that of Bisubstituted yttrium iron garnet. Additionally, we have introduced our recent discovery in granular nanocomposites, including giant Faraday rotation as well as electrically tunable dielectric properties. We demonstrate electric-field control of dielectric relaxation in Co–MgF₂, enabling MHz-range tunable capacitors driven by a DC bias.

Finally, we outline the key challenges and future directions in TMD research. Further progress will rely on continued exploration of novel material combinations, including the design of compositionally graded multilayers and heterostructures that couple TMD-active layers with magnonic or photonic elements. Integrating nanogranular films into CMOS-compatible platforms and silicon photonic circuits may open pathways toward miniaturized, tunable devices such as on-chip magnetic capacitors, impedance elements, and optical isolators. At the same time, improving low-field sensitivity, extending the operational frequency into the GHz regime, and ensuring long-term environmental stability remain critical for practical deployment. We hope this Account stimulates further research into the potential of magnetic granular nanocomposites and catalyzes their continued development in spintronics and related interdisciplinary technological fields.