HfO2 and SiO2 as barriers in magnetic tunneling junctions

Abstract

${\mathrm{SiO}}_{2}$ and ${\mathrm{HfO}}_{2}$ are both high-k, wide-gap semiconductors, currently used in the microelectronic industry as gate barriers. Here we investigate whether the same materials can be employed to make magnetic tunnel junctions, which in principle can be amenable for integration in conventional Si technology. By using a combination of density functional theory and the nonequilibrium Green's functions method for quantum transport we have studied the transport properties of $\mathrm{Co}[0001]/{\mathrm{SiO}}_{2}[001]/\mathrm{Co}[0001]$ and $\mathrm{Fe}[001]/{\mathrm{HfO}}_{2}[001]/\mathrm{Fe}[001]$ junctions. In both cases we found a quite large magnetoresistance, which is explained through the analysis of the real band structure of the magnets and the complex one of the insulator. We find that there is no symmetry spin filtering for the Co-based junction since the high transmission ${\mathrm{\ensuremath{\Delta}}}_{{2}^{\ensuremath{'}}}$ band crosses the Fermi level, ${E}_{\mathrm{F}}$, for both spin directions. However, the fact that Co is a strong ferromagnet makes the orbital contribution to the two ${\mathrm{\ensuremath{\Delta}}}_{{2}^{\ensuremath{'}}}$ spin subbands different, yielding magnetoresistance. In contrast for the Fe-based junction symmetry filtering is active for an energy window spanning between the Fermi level and 1 eV below ${E}_{\mathrm{F}}$, with ${\mathrm{\ensuremath{\Delta}}}_{1}$ symmetry contributing to the transmission.