Controlling Material Surfaces by Electrical Stimuli

Using low-power electric fields to control magnetism (magnetoelectric effect -ME) instead of dissipative flowing currents is considered as a next generation technology to replace conventional methods of data storage and processing, sensing, and actuation. A promising strategy for realization of ME relies on the utilization of composites to couple electric and magnetic polarization at the interface of two functional materials.

The voltage-driven stimulus can be provided by various means, e.g. dielectric, ferroelectric, and electrolyte gating which induces an electric charge at the interface of the functional material leading to charge carrier doping (electric field effect) and thus to a reversibly tuned property change. However going beyond that concept, reversible ion intercalation (reversible electrochemistry), which includes ion transfer across the interface can also effectively alter the materials property. In between these two mechanism, reversible surface chemistry (redox pseudocapacitance), constitutes an effective tools to obtain sizeable surface charges. The different tuning approaches employed in the Tunables group are illustrated schematically in the figure below.

Working Principles of Magnetoelectric Effect (ME) Effect in Artificial Composites

Since in the composite structures the ME effect is of interfacial origin, an understanding of the charging processes that take place at interfaces—or propagate through them—is of crucial importance. The mechanisms acting as mediator of the ME coupling in artificial composites can be divided into three main groups:

• The first one makes use of the stress induced by a piezoelectric actuator to modify the strain state of a magnetic material, and in turn also its magnetic response via magnetostriction.
• The second, which follows the working principle of the field-effect transistor, is based on the accumulation/depletion of charge carriers in a magnet by the polarization of a gate material.
• The third one relies on the insertion/removal of ions into/from the lattice of a host magnet by using an ionic conductor, analogous to the charging/dis-charging processes occurring in electrochemical batteries.