Association

2.3. Magnetic properties of EuO 17
For spin-selective tunneling due to different barrier heights Φ ↑↓ , spin polarizations up to
100% are expected theoretically. In practice, spin selection by using a magnetic insulator
was first reported by Moodera et al. (1988) for the magnetic tunnel barrier EuS. The europium
chalcogenide EuS – the electronic equivalent to EuO – has shown a spin polarization
of ∼86%. 71 In an initial experiment of polycrystalline EuO tunnel contacts a metal electrode,
a spin polarization of 29% has been observed by Santos and Moodera (2004). 72 Recently,
Miao and Moodera (2012) have realized polycrystalline EuO/MgO/Si magnetic tunnel junctions.
73 We, however, follow the approach to integrate EuO directly on Si with high crystalline
quality, thus keeping the tunnel path minimal and well-defined.
Benefits of EuO for spin filter tunneling
We decide to use the magnetic oxide EuO out of the set of magnetic insulators, because EuO
brings unique benefits for a possible application as spin-functional tunnel contact to silicon.
For EuO, the lower edge of the conduction band is exchange-split by 2ΔE ex ≈ 0.6 eV – almost
double the value of EuS. Moreover, the Curie temperature of EuO is four times higher than
for EuS. This renders the fundamental properties of EuO, among the Eu chalcogenides, best
suitable for future spin filter tunnel barriers.
An integration of the magnetic insulator EuO with silicon combines established semiconductor
technology with spin functionality. EuO provides fundamental prerequisites for this
approach: it is the only binary magnetic oxide which is thermodynamically stable in direct
contact with silicon. 14 This, in principle, already allows the fabrication of functional EuO
tunnel contacts directly on silicon, thus paving the tunneling pathway for spin electronic devices
like a spin-FET. 7 Moreover, EuO can resistively be tuned over a large range by means of
electron doping. This reduces Schottky barriers at the spin injection interface, 74 and allows
for a conductance match between EuO and silicon. This is one requirement for high-efficiency
spin injection into silicon. 11
Finally, comparable band gaps of EuO (1.12 eV) and Si (1.1 eV) allow for a possible band
matching at the spin injection interface. In this way, an epitaxial integration of EuO on Si
(001) with a crystalline interface quality is promising for an advanced tunneling approach in
the framework of symmetry bands: coherent tunneling of spin-polarized electrons directly
into Bloch states of the silicon electrode.
An advanced possibility: coherent tunneling in EuO
An advanced tunnel approach exploits the matching of majority or minority symmetry bands
of the magnetic oxide with the Bloch states of the electrodes. This allows for efficient spin
filter tunneling, referred to as “coherent tunneling”.
A well-known example is the lattice-matched epitaxial Fe/MgO/Fe trilayer. Here, high TMR
values could be observed by spin filtering through Δ symmetry bands by Yuasa et al. (2004)
and Parkin et al. (2004). 12,13 Recently, predictions of spin filtering by symmetry bands for
EuO have been reported. 75,76 In EuO, both the real and complex bands are spin-dependent
(Fig. 2.10). The studies conclude that spin filter tunneling will occur mainly via Δ 1 symmetry
bands in EuO/bcc metal interfaces. In particular, in epitaxial Cu/EuO/Cu tunnel junctions
(Fig. 2.10b), a coherent tunnel efficiency of ∼100% is predicted. 76