Researchers from Intel Corporation and the University of Minnesota and the University of Wisconsin have shown that strained atom-scale films of pure ruthenium (Ru) metal exhibit ferromagnetism at room temperature, openning up the possibility of using the material to build novel magnetic random access memory (MRAM) devices. As per details recently published in Nature Communications (https://doi.org/10.1038/s41467-018-04512-1), Ru thin films with a thickness of 2.5, 6, or 12 nm, were grown on Al2O3 substrates cut along the (110) direction, that had been covered with a 20 nm Mo seed layer. The thin films were grown using a eight-target UHV sputtering system with base pressure of 8 × 10−8 Torr or lower, resulting in the controlled epitaxial growth of strained body-centered tetragonal phase Ru.
From first principles of materials engineering, there should be ways to use different templating materials for this graphoepitaxial process such that silicon-oxide could be used as the substrate instead of aluminum-oxide. If so, then this process could be run on 300mm silicon wafers in today’s leading commercial IC fabs.
The (001) tetragonal Ru plane does not lie perpendicular to the substrates which leads to a soft coercive field, however, if out-of-plane texturing can be achieved high coercivity Ru may be realized. The thickness dependence was also examined, and it was found that due to Ru relaxing into a non-ferromagnetic phase, the magnetization drops with increasing thickness. The 12 nm thick sample showed magnetization of about one-half that of the two thinner samples.
Original Article: https://www.nature.com/articles/s41467-018-04512-1
—E.K.