Temperature dependence of spin-dependent tunneling resistances of Co2MnSi-based and Co2(Mn, Fe)Si-based magnetic tunnel junctions showing high tunneling magnetoresistances

Summary form only given. A highly efficient spin-polarized electron source is a key element for spintronic devices . One Co-based Heusler alloy in particular, Co₂MnSi (CMS), has attracted much interest [1-5] because of its theoretically predicted half-metallic nature and high Curie temperature of 985 K . To fully exploit the half-metallic character of CMS, the effect of defects associated with off-stoichiometry on its half-metallicity has to be understood . We have recently shown that harmful defects in CMS thin films can be suppressed by appropriately controlling the film composition and demonstrated a high tunneling magnetoresistance (TMR) ratio of 1995% at 4 .2 K (354% at 290 K) for CMS/MgO/CMS MTJs (CMS MTJs) having Mn-rich CMS electrodes [2] . Furthermore, we recently showed that the suppression of Co antisites at nominal Mn/Fe sites is critical to obtaining half-metallicity in quaternary Co₂(Mn,Fe)Si (CMFS) in a similar way as in ternary CMS and demonstrated a giant TMR ratio of 2610% at 4 .2 K (429% at 290 K) for CMFS/MgO/CMFS MTJs (CMFS MTJs) [6] . The purpose of the present study was to investigate how the temperature (T) dependence of the spin-dependent tunneling resistances in CMS MTJs and CMFS MTJs is correlated with the half-metallicity of CMS and CMFS electrodes . The preparation of fully epitaxial CMS MTJs with various values of α in Co₂Mn_αSi electrodes and CMFS MTJs with various values of α´ and β´ in Co₂Mn_α´Fe_β´Si electrodes has been described elsewhere [1,2,6] . The tunneling resistances for the parallel (P) and antiparallel (AP) magnetization configurations, R_P and R_AP, were measured by a dc four-probe method at temperatures from 4 .2 to 290 K . Because of the considerably stronger T dependence of R_AP compared with that of R_P for these fully epitaxial MTJs, the T dependence of- the TMR ratio was almost determined by that of R_AP. In this study, we focused on the T dependence of R_P . Figure 1 plots the T dependence of the normalized R_P (R_P was normalized by its value at 4 .2 K) of the MgO-buffered CMS MTJs prepared on an MgO(001) substrate with various Mn compositions α ranging from 0 .79 to 1 .29 in Co₂Mn_αSi_{1 .0} electrodes . The TMR ratios at 4 .2 K of these MTJs significantly increased with increasing α from 355% for α = 0 .79 to 1035% for α = 1 .29 [1] . Importantly, the T dependence of R_P of α = 1 .0 (almost stoichiometric) and 1 .29 (Mn-rich) featuring higher TMR ratios showed qualitatively different behavior compared with that of α = 0 .79 featuring a much lower TMR ratio . R_P of α = 0 .79 MTJ didn´t show any increase with increasing T but decreased as T increased for T > T₂ of about 100 K (Fig . 1) . This decrease in R_P with increasing T can be qualitatively explained by a model of Zhang et al . [7], in which magnon-assisted tunneling is taken into account under the assumption of T-independent spin polarization at the Fermi level (E_F) . This model predicts that both R_AP and R_P decrease with increasing T . On the other hand, R_P of α = 1 .0 and 1 .29 increased as T increased for a T range from T₁ of about 40 K to T₂ of about 190 K . Then, RP decreased as T increased for T > T₂. Note that the appearance of the increase in R_P for T₁ <; T <; T₂ was observed for CMS MTJs with almost Mn-stoichiometric or Mn-rich CMS electrodes featuring reduced CoMn antisites . Figure 2 plots the T dependence of R_P for a CMFS MTJ having Co₂Mn_1.24Fe_0.16Si_0.84 electrodes showing a giant TMR