5 Spintronics and Synchrotron Radiation
139
compounds are predicted to be half metallic, i.e. to have a 100% spin polarization at
the Fermi level.
Jullière’s model has enjoyed much success in correctly predicting TMR amplitudes in most MTJs with amorphous barriers, until the emergence of MTJs based on
crystalline insulating barriers, notably magnesium oxide (MgO) barriers in the beginning of the 2000s [15, 16]. The model described previously is actually oversimplified
in that it considers an identical tunnelling process of every electron, independently
on their band belonging, hence forgetting about the interplay of the band structures
throughout the MTJ heterostructure.
5.1.2.2 Coherent Tunnelling in Epitaxial Magnetic Tunnel Junctions
Another major breakthrough in the field of spintronics corresponds to the introduction of crystalline MgO tunnel barriers, that is today’s standard in MTJ implemented
in MRAMs, read heads and field sensors working with TMR effect. These experimental developments have been stimulated by some theoretical calculations made
by W. Butler [17] predicting that huge TMR ratios as large as 1600% are anticipated for epitaxially grown Co or Fe electrodes on crystalline, instead of amorphous
MgO. These calculations, developed with ab initio methods, derive the tunnelling
probability of each kind of electrons, depending on their orbital symmetry.
Contrary to amorphous alumina [Fig. 5.6a left], in crystalline MgO tunnel junctions, the electron wave functions in the FM material are coupled with evanescent
wave functions having the same symmetry in the barrier [Fig. 5.6a right]. Through ab
initio calculations, it was then predicted that the tunnelling probability of an electron
strongly depends on the orbital symmetry of the electron (of the band it belongs
to). Beyond the two-current (up-spin and down-spin) model, the system behaves as
if it exists an independent current channel for each band and each spin, leading to
a possible effective symmetry filtering of the tunnelling current. The tunnel barrier
can, therefore, filter the wave functions and thus select the spins in the electronic
transport.
This mechanism of orbital selection for the tunnel conductance is presented in
Fig. 5.6. In Fig. 5.6b, band dispersion of bcc Fe(001) for the minority and majority
spins is shown. 1 Bloch states are present at the Fermi level only for the majority
spins. In Fe(100)/MgO/Fe(100) systems, band structure calculations have demonstrated that majority-spin electrons [see Fig. 5.6c] are mainly filling 1 symmetry
states (hybridized states with spd characters), whereas minority-spin electrons [see
Fig. 5.6d] are filling 2 symmetry states (d type states) and 5 symmetry states
(hybridized pd states). Moreover, this is crucial for getting a large TMR ratio, the
tunnelling exponential decay is much stronger for 2 and 5 states compared to
1 states. For d MgO = 8 monolayers, which is a typical barrier thickness that can
be achieved experimentally, the probability of transmission of 1 electrons is larger
than for 5 electrons by 10 orders of magnitude. Ultimately, only 1 electrons contribute significantly to the current. It is this filtering effect which can explain the large
values of TMR expected on epitaxial or highly textured structures and that has made
139
compounds are predicted to be half metallic, i.e. to have a 100% spin polarization at
the Fermi level.
Jullière’s model has enjoyed much success in correctly predicting TMR amplitudes in most MTJs with amorphous barriers, until the emergence of MTJs based on
crystalline insulating barriers, notably magnesium oxide (MgO) barriers in the beginning of the 2000s [15, 16]. The model described previously is actually oversimplified
in that it considers an identical tunnelling process of every electron, independently
on their band belonging, hence forgetting about the interplay of the band structures
throughout the MTJ heterostructure.
5.1.2.2 Coherent Tunnelling in Epitaxial Magnetic Tunnel Junctions
Another major breakthrough in the field of spintronics corresponds to the introduction of crystalline MgO tunnel barriers, that is today’s standard in MTJ implemented
in MRAMs, read heads and field sensors working with TMR effect. These experimental developments have been stimulated by some theoretical calculations made
by W. Butler [17] predicting that huge TMR ratios as large as 1600% are anticipated for epitaxially grown Co or Fe electrodes on crystalline, instead of amorphous
MgO. These calculations, developed with ab initio methods, derive the tunnelling
probability of each kind of electrons, depending on their orbital symmetry.
Contrary to amorphous alumina [Fig. 5.6a left], in crystalline MgO tunnel junctions, the electron wave functions in the FM material are coupled with evanescent
wave functions having the same symmetry in the barrier [Fig. 5.6a right]. Through ab
initio calculations, it was then predicted that the tunnelling probability of an electron
strongly depends on the orbital symmetry of the electron (of the band it belongs
to). Beyond the two-current (up-spin and down-spin) model, the system behaves as
if it exists an independent current channel for each band and each spin, leading to
a possible effective symmetry filtering of the tunnelling current. The tunnel barrier
can, therefore, filter the wave functions and thus select the spins in the electronic
transport.
This mechanism of orbital selection for the tunnel conductance is presented in
Fig. 5.6. In Fig. 5.6b, band dispersion of bcc Fe(001) for the minority and majority
spins is shown. 1 Bloch states are present at the Fermi level only for the majority
spins. In Fe(100)/MgO/Fe(100) systems, band structure calculations have demonstrated that majority-spin electrons [see Fig. 5.6c] are mainly filling 1 symmetry
states (hybridized states with spd characters), whereas minority-spin electrons [see
Fig. 5.6d] are filling 2 symmetry states (d type states) and 5 symmetry states
(hybridized pd states). Moreover, this is crucial for getting a large TMR ratio, the
tunnelling exponential decay is much stronger for 2 and 5 states compared to
1 states. For d MgO = 8 monolayers, which is a typical barrier thickness that can
be achieved experimentally, the probability of transmission of 1 electrons is larger
than for 5 electrons by 10 orders of magnitude. Ultimately, only 1 electrons contribute significantly to the current. It is this filtering effect which can explain the large
values of TMR expected on epitaxial or highly textured structures and that has made