Ion beam assisted deposition of MgO barriers for magnetic tunnel junctions

Abstract

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,+1200V), using assist beams of 40mA (∼130μA∕cm2) with 0to+600V. 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 50Å∕Ru 200Å∕Ta 50Å∕Mn78Ir22 150Å∕Co90Fe10 30Å∕Ru 8Å∕Co56Fe24B20 40Å∕MgO t∕Co56Fe24B20 30Å∕Ru 30Å∕Ta 50Å, 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 100–400Ωμm2, for 11Å thick MgO barriers are obtained using assisted deposition with a +100V assist beam, which is a major improvement of the ∼30% of TMR, if no beam is used.