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 Co(2)MnSi electrodes and a MgO tunnel barrier were fabricated with various values of Mn composition α for Co(2)Mn(α)Si in Co(2)Mn(α)Si/MgO/Co(2)Mn(α)Si MTJs. The tunnel magnetoresistance (TMR) ratios at both 4.2 K and room temperature (RT) increased systematically with increasing α in Co(2)Mn(α)Si electrodes from Mn-deficient compositions (α < 1) up to a certain Mn-rich composition (α > 1), demonstrating high TMR ratios of 1135% at 4.2 K and 236% at RT for MTJs with Mn-rich Co(2)Mn(α)Si electrodes with α = 1.29. Identically fabricated Co(2)Mn(β)Ge(δ)/MgO/Co(2)Mn(β)Ge(δ) (δ = 0.38) MTJs showed similar dependence of the TMR ratio on Mn composition β, demonstrating relatively high TMR ratios of 650% at 4.2 K and 220% at RT for β = 1.40. The Mn composition dependence of the TMR ratio at both 4.2 K and RT observed commonly for both Co(2)MnSi/MgO/Co(2)MnSi and Co(2)MnGe/MgO/Co(2)MnGe MTJs can be attributed to suppressed minority-spin in-gap states around the Fermi level for Mn-rich Co(2)MnSi and Co(2)MnGe electrodes.