Tunnel magnetodielectric effect: Theory and experiment

Abstract

The recently discovered tunnel magnetodielectric (TMD) effect—the magnetic field-induced increase in the dielectric permittivity (ε′) of nanogranular composites caused by the spin-dependent quantum mechanical charge tunneling—is of interest for both the scientific value that combines the fields of magnetoelectric and spintronics and multifunctional device applications. However, little is known about how large the maximum dielectric change Δε′/ε′ can achieve and why the Δε′/ε′ variations obey the dependence of square of normalized magnetization (m2), which are critically important for searching and designing materials with higher Δε′/ε′. Here, we perform approximate theoretical derivation and reveal that the maximum Δε′/ε′ can be estimated using intrinsic tunneling spin polarization (PT) and extrinsic normalized magnetization (m), that is, Δε′/ε′ = 2PT2m2. This formulation allows predicting over 200% of theoretical limit for m = 1 and accounts for the observed m2 dependence of Δε′/ε′ for a given PT. We experimentally demonstrate that x-dependence of Δε′/ε′ in (CoxFe100−x)–MgF2 films is phenomenologically consistent with this formulation. This work is pivotal to the design of ultra-highly tunable magnetoelectric applications of the TMD effect at room temperature.