Dielectric breakdown in underoxidized magnetic tunnel junctions

摘要

Magnetic tunnel junctions (MTJ) consisting of two ferromagnetic (pinned and free) layers separated by an insulating barrier are strong candidates for magnetoresistive sensors in high-density storage devices. Requirements for actual MTJ-sensors include low resistance-area product (RxA<1Ωμm{sup}2) and reasonable tunnel magnetoresistance (TMR>20%). Such goals are being intensively pursuit, usually by using ultrathin barriers (～5-6A). A different approach recently showed that similar TJ-characteristics can be obtained by partially oxidizing thicker (9A) AlO{sub}x-barriers [1]. Here we focus on dielectric breakdown (DB) studies in these low-resistance, underoxidized MnIr/CoFe/AlO{sub}x/CoFe MTJs. We will show that breakdown occurs at localized spots of the barrier, likely where a large concentration of defects (oxygen vacancies due to the underoxidation of the barrier) exists. Furthermore, a relation between TJ-area and breakdown mechanism is seen: While failure in junctions with large areas is usually of an extrinsic nature [2] (smooth R-decrease at DB), MTJs with small areas (A<3μm{sup}2) fail mainly by an extrinsic mechanism (abrupt R-decrease). In fact, with increasing MTJ-area, defects are more likely to appear in the barrier, leading to the observed change in the breakdown mechanism from intrinsic to extrinsic.