5 Spintronics and Synchrotron Radiation
141
5.1.3 Magnetization Manipulation without Magnetic Fields
For practical applications, e.g. MRAMs, the possibility to manipulate the magnetization of FM electrodes in spin valves or MTJs without using magnetic field is
required. This corresponds to the next breakthrough with the prediction and the
observation of spin-transfer effects, first providing a new way to reverse the magnetization in a nanostructure but also to generate in some cases steady precession of the
magnetization. In this section, we briefly present the physics of spin-transfer torque
(STT) and also discuss an alternative approach consisting in the manipulation of the
magnetization through the application of a voltage.
5.1.3.1 Spin-Transfer Torque
In the first generation of MRAM devices developed in 2004, the magnetization reversal between the two possible magnetic states was realized by using local magnetic
fields generated by electrical currents flowing in lines close to each magnetic element. This writing process rapidly suffered from both the large energy consumption
needed to generate large enough magnetic fields and from cross-talk problems due to
the difficulty to write a single bit. A solution came out from a major breakthrough in
spintronics in 1996 when it has been proposed that a spin-polarized current flowing
through magnetic multilayers provides a new way to manipulate the magnetization
of a ferromagnet. This new effect, which was called spin-transfer effect, has become
rapidly a very hot topic. Moreover, it is at the basis of a new generation of magnetic
memories, called spin-transfer torque-MRAMs (STT-MRAMs).
The concept of spin-transfer effect was proposed in 1996, concomitantly by
J. Slonczewski [22] and L. Berger [23]. To describe this effect, one can take a standard spintronic structure composed of a fixed FM electrode F 1 and a free FM layer F 2
separated by a NM spacer. When the electrons flow from layer F 1 to F 2 , the current
becomes spin-polarized after passing through F 1 . This non-zero spin polarization is
aligned along the magnetization direction in F 1 and propagates into the NM metallic
spacer or tunnels in the case of an insulating barrier, so that it arrives at the NM/F 2
interface. If the magnetization
− →
M 1 and
− →
M 2 are non-collinear, it results that a component of the spin current transverse to F 2 exists at the NM/F 2 interface [green arrows in
Fig. 5.7a]. When the electrons penetrate into F 2 , the spin of the conduction electron
becomes aligned over a very short distance (within a few atomic distances) toward
the magnetization direction in F 2 because of a strong exchange interaction between
conducting (s) electrons and localized (d) electrons, the latter being responsible of
the magnetic moments.
During all this process, the total spin angular momentum is conserved. Thus the
transverse component of the spin current − → m lost by the electrons when passing
through F 2 is indeed absorbed and transferred to the local magnetization of F 2 .
This transfer of spin angular momentum results in a torque exerted by the spinpolarized current on the local magnetization. For this current polarity, when the
141
5.1.3 Magnetization Manipulation without Magnetic Fields
For practical applications, e.g. MRAMs, the possibility to manipulate the magnetization of FM electrodes in spin valves or MTJs without using magnetic field is
required. This corresponds to the next breakthrough with the prediction and the
observation of spin-transfer effects, first providing a new way to reverse the magnetization in a nanostructure but also to generate in some cases steady precession of the
magnetization. In this section, we briefly present the physics of spin-transfer torque
(STT) and also discuss an alternative approach consisting in the manipulation of the
magnetization through the application of a voltage.
5.1.3.1 Spin-Transfer Torque
In the first generation of MRAM devices developed in 2004, the magnetization reversal between the two possible magnetic states was realized by using local magnetic
fields generated by electrical currents flowing in lines close to each magnetic element. This writing process rapidly suffered from both the large energy consumption
needed to generate large enough magnetic fields and from cross-talk problems due to
the difficulty to write a single bit. A solution came out from a major breakthrough in
spintronics in 1996 when it has been proposed that a spin-polarized current flowing
through magnetic multilayers provides a new way to manipulate the magnetization
of a ferromagnet. This new effect, which was called spin-transfer effect, has become
rapidly a very hot topic. Moreover, it is at the basis of a new generation of magnetic
memories, called spin-transfer torque-MRAMs (STT-MRAMs).
The concept of spin-transfer effect was proposed in 1996, concomitantly by
J. Slonczewski [22] and L. Berger [23]. To describe this effect, one can take a standard spintronic structure composed of a fixed FM electrode F 1 and a free FM layer F 2
separated by a NM spacer. When the electrons flow from layer F 1 to F 2 , the current
becomes spin-polarized after passing through F 1 . This non-zero spin polarization is
aligned along the magnetization direction in F 1 and propagates into the NM metallic
spacer or tunnels in the case of an insulating barrier, so that it arrives at the NM/F 2
interface. If the magnetization
− →
M 1 and
− →
M 2 are non-collinear, it results that a component of the spin current transverse to F 2 exists at the NM/F 2 interface [green arrows in
Fig. 5.7a]. When the electrons penetrate into F 2 , the spin of the conduction electron
becomes aligned over a very short distance (within a few atomic distances) toward
the magnetization direction in F 2 because of a strong exchange interaction between
conducting (s) electrons and localized (d) electrons, the latter being responsible of
the magnetic moments.
During all this process, the total spin angular momentum is conserved. Thus the
transverse component of the spin current − → m lost by the electrons when passing
through F 2 is indeed absorbed and transferred to the local magnetization of F 2 .
This transfer of spin angular momentum results in a torque exerted by the spinpolarized current on the local magnetization. For this current polarity, when the