# Strontium Ferromolybdate-Based Magnetic Tunnel Junctions
## 3 Results and Discussion  
### Selection of Tunneling Barrier Material Based on Electronegativity Differences  
The first challenge is the appropriate selection of barrier material. Tunneling is affected by the properties of the electrode/barrier interface. For instance, the interfacial spin polarization of La\({}_{2/3}\)Sr\({}_{1/3}\)MnO\({}_{3}\) deposited onto various barrier materials (SrTiO\({}_{3}\), TiO\({}_{2}\), LaAlO\({}_{3}\)) significantly depends on the barrier material and is strongly different from the spin polarization of a free surface [64]. On the other hand, suppression of both the Curie temperature \(T_{C}\) and magnetization was observed in a La\({}_{0.7}\)Ca\({}_{0.3}\)MnO\({}_{3}\)/STO superlattice as the thickness of the La\({}_{0.7}\)Ca\({}_{0.3}\)MnO\({}_{3}\) layer decreased, which was attributed to interfacial magnetic disorder [65]. In half-metallic SFMO, only the spin-down (minority) band is present at the Fermi level providing 100% spin polarization [1]. This leads to a negative spin polarization that is characterized by a higher tunneling probability for minority spin electrons [2]. Around the Fermi level, the spin-down band is occupied by both the Mo 4\(d\)\(t_{\rm 2g}\) and the Fe 3\(d\)electrons, which are strongly hybridized with the oxygen \(2p\) states [1]. Here, the attraction of a larger part of the electron density to the SFMO side of the interface increases the occupation of the spin-down states by electrons near the interface (Figure 1). The effective electronegativities of FM electrode materials, as well as of potential barrier materials, calculated using Equation (1), are compiled in Table 1.  
The attraction of more electrons to the SFMO side of the interface stems from the higher \(X\) value of SFMO compared to that of the barrier. This can compensate in part for the weakening of magnetism due to interfacial magnetic disorder obtained in [65]. The opposite occurs for a barrier with a higher \(X\) value than that of SFMO. Consequently, barrier materials with a smaller electronegativity are beneficial for a high TMR. This is in agreement with the values of the TMR of La\({}_{2/3}\)Sr\({}_{1/3}\)MnO\({}_{3}\) on SrTiO\({}_{3}\), LaAlO\({}_{3}\), and TiO \begin{table}
\begin{tabular}{c c c} \hline \hline
**Compound** & **Function** & **\textless{}X\textgreater{}** \\ \hline Fe & Electrode & 1.83 \\ NiFe & Electrode & 1.87 \\ Co & Electrode & 1.88 \\ MgO & Barrier & 2.12 \\ La\({}_{2}\)O\({}_{3}\) & Barrier & 2.18 \\ BaTiO\({}_{3}\) & Barrier & 2.24 \\ SrTiO\({}_{3}\) & Barrier & 2.26 \\ Ce\({}_{0.69}\)La\({}_{0.31}\)O\({}_{1.845}\) & Barrier & 2.31 \\ La\({}_{2/3}\)Sr\({}_{1/3}\)MnO\({}_{3}\) & Electrode & 2.31 \\ La\({}_{0.7}\)Sr\({}_{0.3}\)MnO\({}_{3}\) & Electrode & 2.31 \\ LaAlO\({}_{3}\) & Barrier & 2.35 \\ ZnO & Barrier & 2.38 \\ Sr\({}_{2}\)FeMoO\({}_{6}\) & Electrode & 2.38 \\ Mg\({}_{3}\)B\({}_{2}\)O\({}_{6}\) & Barrier & 2.40 \\ SrMoO\({}_{3}\) & Barrier & 2.42 \\ HfO\({}_{2}\) & Barrier & 2.49 \\ Mn\({}_{2}\)O\({}_{3}\) & Barrier & 2.50 \\ Al\({}_{2}\)O\({}_{3}\) & Barrier & 2.54 \\ SrMoO\({}_{4}\) & Barrier & 2.57 \\ Fe\({}_{5}\)O\({}_{4}\) & Electrode & 2.62 \\ TiO\({}_{2}\) & Barrier & 2.63 \\ MnO\({}_{2}\) & Barrier & 2.64 \\ Ta\({}_{2}\)O\({}_{5}\) & Barrier & 2.71 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Effective electronegativities of ferro and FM electrode materials, as well as of potential barrier materials. Figure 1: Tunneling FM/barrier/FM for X\({}_{\textless{}}\)FMO \(>\) X\({}_{\textless{}}\)barrier (**a**), X\({}_{\textless{}}\)FMO \(=\) X\({}_{\textless{}}\)barrier (**b**), and X\({}_{\textless{}}\)FMO \(<\) X\({}_{\textless{}}\)barrier (**c**). Energy is plotted along the vertical axis, and the density of states of the FM layers is plotted along the horizontal axis. Filled states are shown in blue. A negative bias voltage, \(V\), has been applied to the sample, which effectively raises its Fermi level by \(e\cdot V\) with respect to the Fermi level of the grounded FM electrode.