Effect of defects in Heusler alloy thin films on spin-dependent tunnelling characteristics ofCo2MnSi/MgO/Co2MnSi andCo2MnGe/MgO/Co2MnGe magnetic tunnel junctions

Abstract

Fully epitaxial magnetic tunnel junctions (MTJs) with Co-based Heusler alloyCo2MnSi electrodes and a MgO tunnel barrier were fabricated with various values of Mn compositionα forCo2MnαSi inCo2MnαSi/MgO/Co2MnαSi MTJs. The tunnel magnetoresistance (TMR) ratios at both 4.2 Kand room temperature (RT) increased systematically with increasingα inCo2MnαSi electrodes from Mn-deficient compositions (α < 1) up to a certainMn-rich composition (α > 1), demonstrating high TMR ratios of 1135% at 4.2 K and 236% at RT for MTJs with Mn-richCo2MnαSi electrodes withα = 1.29. Identicallyfabricated Co2MnβGeδ/MgO/Co2MnβGeδ (δ = 0.38) MTJs showed similar dependence of the TMR ratio on Mn compositionβ, demonstrating relatively high TMR ratios of 650% at 4.2 K and 220% at RT forβ = 1.40.The Mn composition dependence of the TMR ratio at both 4.2 K and RT observed commonly for bothCo2MnSi/MgO/Co2MnSi andCo2MnGe/MgO/Co2MnGe MTJs can be attributed to suppressed minority-spin in-gap states around the Fermi level for Mn-richCo2MnSi andCo2MnGe electrodes.