5 Spintronics and Synchrotron Radiation
137
5.1.2 Tunnelling Magnetoresistance
The pioneer observation of tunnelling magnetoresistance (TMR) in magnetic tunnel
junctions (MTJs) at room temperature in 1995 is considered to be the second breakthrough in spintronics, leading to a second generation of spintronic applications such
as magnetic random access memory (MRAM). In this section, we describe the first
TMR measurements, then the two standard models describing TMR effects.
5.1.2.1 First Experimental and Theoretical Studies
A magnetic tunnel junction is a device in which two FM electrodes are separated
by an ultra-thin insulating barrier. The first MTJ exhibiting TMR has indeed been
reported in 1975 by M. Jullière [12] who measured a TMR ratio of 14% at 4.2K in
a Fe/Ge/Co junction. Twenty years later TMR effects have been observed at room
temperature by J. S. Moodera [13] and T. Miyazaki [14] using amorphous alumina as
tunnel barrier. In Fig. 5.5, the TMR curve obtained for a CoFe/Al 2 O 3 /Co MTJ [13]
is displayed. The TMR ratio is around 12%, at room temperature. This increase in
the resistance difference between the two states allows an easier detection of them
and let appear the potential of such spintronic devices for memory applications.
Such TMR effect has been first explained by M. Jullière in 1975. Keeping the
free-electron approximation, he proposed an additional assumption: the electron spin
conservation during the tunnelling process, meaning that electrons can tunnel from
one FM electrode toward the second FM electrode only into empty states having
identical spins. This simple model easily allows to understand that the tunnelling
current in the (P) and (AP) alignments of magnetizations will differ.
As illustrated in Fig. 5.5, for the parallel (P) configuration of the magnetizations:
when a small voltage is applied, majority-spin (denoted as ↑) electrons from the
injector will tunnel toward majority-spin (↑) empty states of the collector, allowing
"high" current. At the same time, minority-spin electrons from the injector will tunnel
toward minority-spin (↓) empty states of the collector, allowing "low" current.
1 The
conductance then writes
G P ∝ D
↑
1 (E F )D
↑
2 (E F ) + D
↓
1 (E F )D
↓
2 (E F ) .
(5.2)
In the case of antiparallel (AP) configuration of the magnetizations, majority-spin
(↑) electrons from the injector will tunnel toward minority-spin (↓) empty states of the
collector. Minority-spin (↓) electrons from the injector will tunnel toward majorityspin (↑) empty states of the collector. Both currents are then low. The conductance
then writes
1 Note here that the arrow does not define any direction of the spin, but rather the fact that it is
aligned with the local magnetization. In the (AP) case, a spin with one direction will correspond to
(↑) spin in one electrode and (↓) spin in the other.
137
5.1.2 Tunnelling Magnetoresistance
The pioneer observation of tunnelling magnetoresistance (TMR) in magnetic tunnel
junctions (MTJs) at room temperature in 1995 is considered to be the second breakthrough in spintronics, leading to a second generation of spintronic applications such
as magnetic random access memory (MRAM). In this section, we describe the first
TMR measurements, then the two standard models describing TMR effects.
5.1.2.1 First Experimental and Theoretical Studies
A magnetic tunnel junction is a device in which two FM electrodes are separated
by an ultra-thin insulating barrier. The first MTJ exhibiting TMR has indeed been
reported in 1975 by M. Jullière [12] who measured a TMR ratio of 14% at 4.2K in
a Fe/Ge/Co junction. Twenty years later TMR effects have been observed at room
temperature by J. S. Moodera [13] and T. Miyazaki [14] using amorphous alumina as
tunnel barrier. In Fig. 5.5, the TMR curve obtained for a CoFe/Al 2 O 3 /Co MTJ [13]
is displayed. The TMR ratio is around 12%, at room temperature. This increase in
the resistance difference between the two states allows an easier detection of them
and let appear the potential of such spintronic devices for memory applications.
Such TMR effect has been first explained by M. Jullière in 1975. Keeping the
free-electron approximation, he proposed an additional assumption: the electron spin
conservation during the tunnelling process, meaning that electrons can tunnel from
one FM electrode toward the second FM electrode only into empty states having
identical spins. This simple model easily allows to understand that the tunnelling
current in the (P) and (AP) alignments of magnetizations will differ.
As illustrated in Fig. 5.5, for the parallel (P) configuration of the magnetizations:
when a small voltage is applied, majority-spin (denoted as ↑) electrons from the
injector will tunnel toward majority-spin (↑) empty states of the collector, allowing
"high" current. At the same time, minority-spin electrons from the injector will tunnel
toward minority-spin (↓) empty states of the collector, allowing "low" current.
1 The
conductance then writes
G P ∝ D
↑
1 (E F )D
↑
2 (E F ) + D
↓
1 (E F )D
↓
2 (E F ) .
(5.2)
In the case of antiparallel (AP) configuration of the magnetizations, majority-spin
(↑) electrons from the injector will tunnel toward minority-spin (↓) empty states of the
collector. Minority-spin (↓) electrons from the injector will tunnel toward majorityspin (↑) empty states of the collector. Both currents are then low. The conductance
then writes
1 Note here that the arrow does not define any direction of the spin, but rather the fact that it is
aligned with the local magnetization. In the (AP) case, a spin with one direction will correspond to
(↑) spin in one electrode and (↓) spin in the other.