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"Thermal and electrical control of perpendicular magnetization"

Relatore: Massimo Ghidini- Department of Materials Science, Cambridge and DIFEST, UniversitÓ di Parma
Note: Seminario di Dipartimento

Aula Newton
20 Dicembre 2012 ore 16.00

Magnetoelectric coupling permits the interconversion between electric and magnetic signals, and therefore it could ultimately be relevant for magnetic field sensing and electric write-magnetic read memories. Several materials (eg. multiferroic oxides) show magnetoelectric coupling, but ferroelectric-ferromagnets where one may attempt electrical control of magnetization are rare (none at room temperature). We have combined macroscopic techniques with magnetic force microscopy (MFM), and photoemission electron microscopy with x-ray magnetic circular dichroism at Diamond Light Source (UK) to investigate ferroelectric and ferromagnetic materials that communicate via strain and we have reported dramatic magnetoelectric effects, and giant magnetocaloric effects. Here I will describe changes of out-of-plane magnetization in polycrystalline films of nickel, from polycrystalline and single-crystal BaTiO3, respectively. Commercial multilayer capacitors display strain-mediated magnetoelectric coupling between Ni electrodes and polycrystalline BaTiO3-based dielectric layers. Magnetic Force Microscopy (MFM) reveals a perpendicularly magnetized feature that exhibits non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field. This magnetization reversal is achieved in a dynamic process triggered by a temporary reduction of perpendicular anisotropy due to fast and reversible ferroelectric switching. In nickel films on BaTiO3 single-crystals, built-in strain yields an out-of-plane magnetization whose sign alternates every 125 nm. These stripe domains may be switched on and off by varying film strain, first by thermally cycling through structural phase transitions in BaTiO3, then by electrically switching the ferroelectric domains. The electrical switching is non-volatile, and could inspire the design of electrically driven nanomagnets.