Voltage-controlled magnetic tunnel junctions with Gd$_{2}$O$_{3}$ barriers

It is of great importance to investigate magnetic tunnel junction (MTJ) with high-k barriers, with the premise that a large voltage-controlled magnetic anisotropy (VCMA) can be achieved due to the increased charge transfer effect. Gd$_{2}$O$_{3}$ has a dielectric constant of 22, which is substantially larger than that of MgO ($\sim$ 9). It is critical to achieve crystalline barrier with cubic phase in order to obtain symmetry-conserved tunneling as in MgO-based MTJs. We have demonstrated that Cubic Gd2O3 can be grown on amorphous CoFeB by reactive sputtering under proper conditions. In exchanged-biased MTJs with in-plane anisotropy, tunneling magnetoresistance (TMR) up to 12{\%} has been obtained. The sharp switching at near zero field and exchange-bias field higher than 800 Oe indicate the magnetic properties of the CoFeB in these junctions are nearly as good as in MgO-based MTJs. MTJs with interfacial perpendicular magnetic anisotropy (PMA) has been created with TMR $\sim$ 10{\%}. A very interesting VCMA effect in these Gd$_{2}$O$_{3}$-based MTJs has been observed and will be discussed. This work was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.