Spin-Polarized Tunneling Spectroscopy in Tunnel Junctions with Half-Metallic Electrodes

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ including the energy gap $\delta$ between the Fermi level and the bottom of the $t_{2g}$ minority-spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.

Figure 1

Bias dependence of TMR (normalized to the value found at $V_{\mathrm{dc}}=-10\mathrm{mV}$) at $T=4\mathrm{K}$ for a LSMO/STO/LSMO junction from R(H) (open circles) and $\mathrm{P}/\mathrm{AP}$ $I(V_{\mathrm{dc}})$ data (solid circles). Inset: spin asymmetry ${\Delta }_{\mathrm{spin}}=(I_P-I_{\mathrm{AP}})/(I_{\mathrm{P}}+I_{\mathrm{AP}})={\mathrm{P}}^2$.

Figure 2

Temperature evolution of the parallel conductance from 4 K (solid squares) to higher temperatures (open symbols). Inset: bias dependence of the parallel conductance derivative at 4 and 70 K.

Figure 3

(a) Bias dependence of the parallel and antiparallel conductances $G_{\mathrm{P}}=(dI/dV{)}_{\mathrm{P}}$ and $G_{\mathrm{AP}}=(dI/dV{)}_{\mathrm{AP}}$ at $T=4\mathrm{K}$. (b) Bias dependence of the conductance derivatives showing the onset of tunneling into the minority-spin $t_{2g}$ subband for the antiparallel conductance at 350 mV. Dotted lines are guides to the eye. The conductance derivative in the AP configuration is plotted vs $(V_{\mathrm{dc}}-\delta {)}^{3/2}$ in the inset of (a); symbols are experimental data and the line is a linear fit. (c) Schematic representation of tunneling towards a half-metallic collecting electrode for ${\mathrm{eV}}_{\mathrm{dc}}$ smaller (left panel) and larger (right panel) than $\delta$.

Figure 4

Spin-resolved spectra taken at 100 K for a STO/LSMO interface. A smoothing of experimental data at 100 K is plotted for the two spin channels.