Nonquasiparticle States in ${\mathrm{Co}}_2\mathrm{MnSi}$ Evidenced through Magnetic Tunnel Junction Spectroscopy Measurements

We investigate the effects of electronic correlations in the full-Heusler ${\mathrm{Co}}_2\mathrm{MnSi}$, by combining a theoretical analysis of the spin-resolved density of states with tunneling-conductance spectroscopy measurements using ${\mathrm{Co}}_2\mathrm{MnSi}$ as electrode. Both experimental and theoretical results confirm the existence of so-called nonquasiparticle states and their crucial contribution to the finite-temperature spin polarization in this material.

Figure 1

Spin-resolved density of states of the ${\mathrm{Co}}_2\mathrm{MnSi}$ full-Heusler alloy obtained by LSDA and $\mathrm{LSDA}+\mathrm{DMFT}$. The $\mathrm{LSDA}+\mathrm{DMFT}$ results are obtained for $U=3\mathrm{eV}$ and $J=0.9\mathrm{eV}$ (in both Co and Mn), and different temperatures.

Figure 2

(a) Tunnel conductance for the ${\mathrm{Co}}_2\mathrm{MnSi}/\mathrm{Al}\mathrm{\text{−}}\mathrm{O}/{\mathrm{Co}}_2\mathrm{MnSi}$ antiparallel magnetic tunnel junction and (b) TMR ratio (normalized to $T=2K$, $V=0$), as a function of voltage for different temperatures. The inset of panel (a) shows the conductance normalized to zero bias voltage.

Figure 3

Comparison of the TMR ratio and magnetization ($M$) as a function of temperature for a $\mathrm{FeCo}/\mathrm{Al}\mathrm{\text{−}}\mathrm{O}/{\mathrm{Co}}_2\mathrm{MnSi}$ tunnel junction as obtained from experimental data and from $\mathrm{LSDA}+\mathrm{DMFT}$. Triangle-down/up represents the measured polarization and the fit to the analytic expression $1-P(T)/P(0)\propto T\ln T/T^{*}$ [18].

Figure 4

Schematic representation of an electron tunneling process at $\Delta V\approx 0$ and at finite bias voltage in a ${\mathrm{Co}}_2\mathrm{MnSi}/\mathrm{Al}\mathrm{\text{−}}\mathrm{O}/{\mathrm{Co}}_2\mathrm{MnSi}$ magnetic tunnel junction. At $T\approx 0\mathrm{K}$ (a), NQP states vanish at $E_F$, while for $T>0$ (b) (see text) NQP states extend across $E_F$ and an additional tunneling channel opens.