130
5 Spin-Transfer Torque
5.2.2 Double Ferromagnetic (FM) Layers
Let us consider a device geometry consisting of a NM/FM/NM/FM/NM multilayer
nanopillar structure, where NM is a non-magnetic and FM is a ferromagnetic metal
as shown in Fig. 5.2. In order to attain spin-transfer torque effect in this multilayer
nanopillar, special structural engineering is done on this structure. The special characteristics of this multilayer structure is such that one of its FM layer is deposited
as thinner than the consecutive FM layer. Thus, in a multilayer nanopillar structure
the successive FM layers are having different thicknesses, i.e., one is thin and other
is thick. The thin FM layer is called as a free layer and the thick layer as a fixed
layer. It is understandable that the magnetic moment associated with the free FM
layer could be more freely and easily reoriented by spin-transfer torques, whereas
magnetic moment of thicker FM layer is more resistant and rigid to be oriented by the
torques. The thick FM layer is constructed by high magnetic moment material and
thereby serves as the polarizer. This means during transmission through and reflection from this thick FM layer, electron spins can be filtered. Thus, this layer can
produce spin-polarized electrons, which eventually reach and act on the magnetization of the free FM layer. Let us consider the thick FM layers are positive polarizers,
i.e., thick FM layers preferentially transmit majority-spin electrons. In this context,
taking into considerations the thermal fluctuations in the system, let us assume that
there is initially a misalignment angle θ, between magnetic moments of those two
FM layers. If the magnetic moments in the two FM layers are aligned exactly parallel,
then spin-transfer torque would be zero.
Now, two cases can be framed:
Fig. 5.2 Schematic drawing
of spin-transfer torque effect
modulating magnetization
direction in samples having
two ferromagnetic layers.
a Electrons flowing from the
thicker (fixed) ferromagnetic
layer to the thinner (free)
layer (negative current),
b electrons flowing from the
thinner (free) ferromagnetic
layer to the thicker (fixed)
layer (positive current)
5 Spin-Transfer Torque
5.2.2 Double Ferromagnetic (FM) Layers
Let us consider a device geometry consisting of a NM/FM/NM/FM/NM multilayer
nanopillar structure, where NM is a non-magnetic and FM is a ferromagnetic metal
as shown in Fig. 5.2. In order to attain spin-transfer torque effect in this multilayer
nanopillar, special structural engineering is done on this structure. The special characteristics of this multilayer structure is such that one of its FM layer is deposited
as thinner than the consecutive FM layer. Thus, in a multilayer nanopillar structure
the successive FM layers are having different thicknesses, i.e., one is thin and other
is thick. The thin FM layer is called as a free layer and the thick layer as a fixed
layer. It is understandable that the magnetic moment associated with the free FM
layer could be more freely and easily reoriented by spin-transfer torques, whereas
magnetic moment of thicker FM layer is more resistant and rigid to be oriented by the
torques. The thick FM layer is constructed by high magnetic moment material and
thereby serves as the polarizer. This means during transmission through and reflection from this thick FM layer, electron spins can be filtered. Thus, this layer can
produce spin-polarized electrons, which eventually reach and act on the magnetization of the free FM layer. Let us consider the thick FM layers are positive polarizers,
i.e., thick FM layers preferentially transmit majority-spin electrons. In this context,
taking into considerations the thermal fluctuations in the system, let us assume that
there is initially a misalignment angle θ, between magnetic moments of those two
FM layers. If the magnetic moments in the two FM layers are aligned exactly parallel,
then spin-transfer torque would be zero.
Now, two cases can be framed:
Fig. 5.2 Schematic drawing
of spin-transfer torque effect
modulating magnetization
direction in samples having
two ferromagnetic layers.
a Electrons flowing from the
thicker (fixed) ferromagnetic
layer to the thinner (free)
layer (negative current),
b electrons flowing from the
thinner (free) ferromagnetic
layer to the thicker (fixed)
layer (positive current)