Memory device: FePd nanocrystals in a magnetic tunneling junction shows smallest voltage controlled magnetic anisotropy memory structure ever reported.

Our work in magnetic and multiferroic materials has been focused on using nanoscale architecture as a tool to induce interesting magnetic behavior and, in some cases, enhance the performance of existing dense magnetic devices. For example, when magnetic particles become small enough, they become extremely susceptible to thermal energy, even at room temperature, such that their spins can easily ‘hop’ between different orientations in what’s known as superparamagnetism. Unlike ferromagnetic materials, which have some amount of magnetocrystalline anisotropy (meaning that it takes more energy to orient magnetization along one crystallographic direction versus another), superparamagnetic materials have net zero inherent magnetocrystalline anisotropy because their spins are ‘hopping’ between random orientations. We have created several systems involving nanoparticles that are on the ‘superparamagnetic limit’, and found many ways to ‘push’ the nanoparticles from superparamagnetic to ferromagnetic so that they do have magnetic anisotropy, such as through strain and changing the dielectric constant of the surrounding media.

Our group is also interested in voltage control of magnetism. Few materials inherently couple electricity and magnetism (so-called magnetoelectric materials), especially at room temperature. One work-around is to couple electricity and magnetism through strain in strain-mediated composites. These so-called strain-mediated magnetoelectric composites couple a piezoelectric material, which strains in response to a voltage, to a magnetostrictive material, which changes magnetization in response to strain from the piezoelectric. An electric field is causes the piezoelectric to strain, which induces a change in magnetization in the magnetostrictive material as the magnetic domains are stretched. In addition to using strain as an avenue for voltage control of magnetism, our group is interested in using electrochemistry to control magnetism in so-called magneto-ionic systems. While magneto-ionics have large magnetic changes, the primary challenge here is the slow speed of diffusion. We are nanostructuring existing magneto-ionic materials to overcome kinetics, which should result in much faster systems. Another area in magneto ionics we are interested in is using proton intercalation to change the coupling between superparamagnetic nanoparticles, which could be used for on/off control of magnetism.