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

Fully epitaxial magnetic tunnel junctions (MTJs) with Co-based Heusler alloy Co2MnSi electrodes and a MgO tunnel barrier were fabricated with various values of Mn composition a for Co2Mn alpha Si in Co2Mn alpha Si/MgO/Co2Mn alpha Si MTJs. The tunnel magnetoresistance (TMR) ratios at both 4.2 K and room temperature (RT) increased systematically with increasing a in Co2Mn alpha Si electrodes from Mn-deficient compositions (alpha < 1) up to a certain Mn-rich composition (alpha > 1), demonstrating high TMR ratios of 1135% at 4.2 K and 236% at RT for MTJs with Mn-rich Co2Mn alpha Si electrodes with alpha = 1.29. Identically fabricated Co2Mn beta Ge delta/MgO/Co2Mn beta Ge delta (delta = 0.38) MTJs showed similar dependence of the TMR ratio on Mn composition beta, demonstrating relatively high TMR ratios of 650% at 4.2 K and 220% at RT for beta = 1.40. The Mn composition dependence of the TMR ratio at both 4.2 K and RT observed commonly for both Co2MnSi/MgO/Co2MnSi and Co2MnGe/MgO/Co2MnGe MTJs can be attributed to suppressed minority-spin in-gap states around the Fermi level for Mn-rich Co2MnSi and Co2MnGe electrodes.