  We report on the perpendicular magnetic anisotropy (PMA) behavior of heavy
metal (HM)/ Fe alloy/MgO thin film heterostructures after an ultrathin HfO2
passivation layer is inserted between the Fe alloy and the MgO. This is
accomplished by depositing one to two atomic layers of Hf onto the Fe alloy
before the subsequent rf sputter deposition of the MgO layer. This Hf layer is
fully oxidized during the subsequent deposition of the MgO layer, as confirmed
by X-ray photoelectron spectroscopy measurements. As the result a strong
interfacial perpendicular anisotropy energy density can be achieved without any
post-fabrication annealing treatment, for example 1.7 erg/cm^2 for the
Ta/Fe60Co20B20/HfO2/MgO heterostructure. Depending on the HM, further
enhancements of the PMA can be realized by thermal annealing to at least 400C.
We show that ultra-thin HfO2 layers offer a range of options for enhancing the
magnetic properties of magnetic heterostructures for spintronics applications.
