Schematic of the electrolytic cell containing the FePt or FePd film within an applied magnetic field H. The potential profile E due
to the applied potential U is indicated by the red line. The potential drop at the Pt electrode side is much lower (as compared
to that of the sample surface) as a result of the Pt electrode’s large
Magnetic memories can store information forever, but writing or modifying this information is very costly, in terms of energy. In collaboration with CNRS-Institut Néel and INPG-LEPMI,we have shown that the magnetic properties of metallic ferromagnetic alloys can be modified by an electrical field.
Magnetic nanosystems could be driven in this way, and thus use less power than by using an electrical current. An essential parameter for recording is the coercivity of the magnetic layer, which is the magnetic field that has to be applied in order to change the direction of the magnetization. Thin films of orderedFePd and FePt with high magnetic coercivity, have been grown by epitaxy on a MgO substrate. The samples are immersed in an electrolyte (see figure). This arrangement permits the surface application of an intense electric field which modifies the electronic state of the layer and thus its magnetic properties.
The electric field does not penetrate inside the metal, so the excess or lack of induced charges is localized at the surface. With a 2 nm-thick layer, and under a bias of 1 V, the coercivity variation is – 4 % for FePt and + 1 % for FePd. The electronic structure calculations for both alloys account well for this effect, both for its sign and its intensity. In the future, we will replace the liquid electrolyte by a solid-state tunnel barrier which will allow for better control of the surface electric field.
M. Weisheit et al., Science, 315 349 (2007).
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