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, Co
₂
MnSi (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 Co
₂
(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 Co
₂
Mn
_α
Si electrodes and CMFS MTJs with various values of α' and β' in Co
₂
Mn
_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 Co
₂
Mn
_α
Si
_{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 > T
₂
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 T
₁
of about 40 K to T
₂
of about 190 K . Then, RP decreased as T increased for T > T
₂
. Note that the appearance of the increase in R
_P
for T
₁
<; T <; T
₂
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 Co
₂
Mn
_1.24
Fe
_0.16
Si
_0.84
electrodes showing a giant TMR ratio of 2500% at 4 .2 K (429% at 290 K) and a CMFS MTJ having Co
₂
Mn
_1.0
Fe
_0.40
Si
_0.84
electrodes showing a relatively lower TMR ratio of 1920% at 4 .2 K (328% at 290 K) . Comparing these CMFS MTJs, the relative increase in R
_P
was larger for the MTJ that showed the higher TMR ratio . This correlation observed for both CMS MTJs and CMFS MTJs suggests that the increase in R
_P
with increasing T occurred for MTJs having electrodes being close to be half-metallic . Then, the minority-spin density of states at E
_F
, D
_m
(E
_F
), at the ground state is significantly reduced . The increase in R
_P
with increasing T is qualitatively explained as follows . We take into account the effect of the decrease in the spin polarization at E
_F
, P, arising from thermal spin fluctuation at finite temperatures [8,9] . This leads to the increase in D
_m
(E
_F
) at finite temperatures, resulting in the increase in R
_P
and the decrease in R
_AP
with increasing T . Thus, there are two competitive factors . One factor is an inelastic tunneling process that decreases R
_P
with increasing T due to the model of Zhang et al . [7] Another factor is decreasing P due to thermal spin fluctuation, which increases R
_P
. Because D
_m
(E
_F
) at the ground state is so small for electrodes being close to be half-metallic, the second factor becomes dominant, resulting in the increase in R
_P
with increasing T . In summary, we observed the clear increase in R
_P
for MTJs having electrodes being close to be half-metallic . Our finding suggests the importance of the T dependence of R
_P
to understand the spin-dependent tunneling mechanism of MTJs with electrodes having high spin polarization.