Ion beam assisted deposition of MgO barriers for magnetic tunnel junctions

This work reports for the first time results on MgO tunnel junctions prepared by ion beam. The MgO barrier was deposited from a ceramic MgO target using an assisted beam, following the deposition and assisted beam phase diagram which relate the beam profile with the current and energy. The deposition rate for MgO is calculated with and without assisted beam, and compared with the experimental values. The MgO film growth on Ta/CoFeB/MgO simple stacks was optimized aiming at a (002) preferred orientation for the MgO growth, measured by x-ray diffraction. The optimum assist beam energy was tuned for each deposition beam condition (+800,+1000,+1200 V), using assist beams of 40 mA (similar to 130 mu A/cm(2)) with 0 to +600 V. Without assist beam, no texture is observed for the MgO, while the (002) orientation appears for assisted deposition. The optimum range of assist voltages is large, being limited by the onset of etching at high voltages, reducing the deposition rate. Magnetic tunnel junctions were deposited with the structure Ta 50 angstrom/Ru 200 angstrom/Ta 50 angstrom/Mn78Ir22 150 angstrom/Co90Fe10 30 angstrom/Ru 8 angstrom/Co56Fe24B20 40 angstrom/MgO t/Co56Fe24B20 30 angstrom/Ru 30 angstrom/Ta 50 angstrom, with the MgO barrier deposited with the conditions optimized by x rays. The effect of the assist beam energy on the junction properties (magnetoresistance and magnetization) are discussed. Tunnel magnetoresistance values up to 110%, with RA products of 100-400 Omega mu m(2), for 11 A thick MgO barriers are obtained using assisted deposition with a +100 V assist beam, which is a major improvement of the similar to 30% of TMR, if no beam is used.