A change in sign brings a sign of change for spin-current devices

Ferromagnetic semiconductors (FMSs), hybrid materials of semiconductors and ferromagnets, are making headway toward mainstream use in electronics as demand continues to grow for scales that continue to shrink. Like many modern components, they exploit quantum mechanically driven electron behavior to store and process information, unlike their predecessors’ classical electric currents. Esaki diodes, for example, demonstrated the first use of (quantum) tunneling current.

For ferromagnetic spin devices, “current” takes the form of coherent, traveling ensembles of electron spins. Their behavior, like any circuit component, ultimately depends on the material’s energy band structure, which maps out allowed energy states and population distributions of its information carriers (spin current). But the band structures of FMSs are particularly complex due to the contribution of a large amount of randomly distributed magnetic dopants. This complexity manifests itself when considering their conductivity response to external magnetic fields (magnetoconductance) and spin polarization effects.

In Applied Physics Letters, such complexity is explored for magnetic thin films, demonstrating a flip of a spin diode’s magnetotransport properties driven by an external bias voltage. The demonstration also presents a novel way to map the energy bands of these structures and engineer future magnetic film devices.

Authors grew spin Esaki diodes consisting of n-type FMS (In,Fe)As and p-type InAs, ensuring atomic-scale flatness between the n- and p-type layers. They measured the magnetoconductance at 3.5 Kelvin under a range of applied bias voltages, and found that an external bias changed magnetoconductance properties, notably even from positive to negative. A two-fluid model provided quantitative descriptions for the observed behavior, deriving the magnetoconductance from the bias-driven band structure changes.

“Different bands have different symmetry and spin polarization, and thus lead to very different magnetoconductance, both in magnitude and sign,” said Le Duc Anh, one of the report’s authors. “This is a new degree of freedom that has never been achieved in single magnetic thin films.” And while such magnetoconductance is typically determined by near-Fermi level carrier properties, here, Anh pointed out, the bands contributing to current were selectable by way of the bias voltage.

Source: “Electrical tuning of the band alignment and magnetoconductance in an n-type ferromagnetic semiconductor (In,Fe)As-based spin-Esaki diode,” by Le Duc Anh, Pham Nam Hai, and Masaaki Tanaka, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5010020 .