5.4 Experimental Results
137
a sufficiently large positive current results in a sudden rise of resistance from lowerresistance to higher-resistance state. Noteworthy, lower- and higher-resistance states
correspond to the parallel and antiparallel alignments, respectively, of free layer
magnetic moment with respect to the fixed layer moment. It has already been
discussed that positive current corresponds to the flow of electronic current from
the free to the fixed FM layer. As is evident from the above-made discussion, in
this case, parallel orientation between free and fixed layer magnetic moment gets
destabilize and antiparallel alignment between them gets stabilize. Thus, magnetic
moment associated with the free FM layer, in this high-resistance antiparallel configuration, can be reversed back to the parallel orientation by applying a sufficiently
large negative current (Myers et al. 1999). It should be clearly mentioned that such
switching of free layer magnetic moment between parallel and antiparallel configurations with fixed layer moment can be justified and understood with the mechanism
of spin-transfer torques. More subtle point is that such switching is not a mere
consequence of current-induced magnetic fields. This can be confirmed from the
fact that in nanoscopic devices the requisite current levels are so small that it cannot
produce magnetic fields of the desired magnitude to switch the magnetic moment
of free layers. Noteworthy observations in the differential resistance are small peaks
or shoulders (Fig. 5.6b), appearing at the currents I
+
D and I
−
D , before the resistance
exhibits large jumps in its value (Urazhdin et al. 2003). This experimental feature can
be attributed to the turn-on of a dynamical state, in which the magnetic moment associated with free FM layer executes small-angle precession. This subtle experimental
feature further assures us about the origin of magnetization switching in this present
case. It has already been discussed that at the onset, spin-transfer torque drives the
free layer moment into a precessional mode, which is then followed by reversing of
the magnetic moment to attain the final static state.
5.5 Spin-Transfer Torque in Magnetic Multilayer
Nanopillar
Let us imagine an electron system in which exchange interactions take place between
the conduction s-electrons and the magnetic d-electrons, holding local magnetic
moments, as schematically shown in Fig. 5.7a. It is known that the total spin angular
momentum would be conserved during s–d exchange interaction. Thus, it is straightforward to state that a reduction in the sub-total angular momentum associated with
the conduction electrons would be equal to the enhancement of the sub-total angular
momentum corresponding to the magnetic d-electrons. Suppose during transport
through the FM2 layer of a magnetic pillar, spin angular momentum of a conduction
electron changes owing to the s–d interaction. Now, the same amount of angular
momentum, equal to the change experienced by conduction electrons, should be
transferred to the d-electrons in the FM2 layer. Considering the rule for conserving
the spin angular momentum
137
a sufficiently large positive current results in a sudden rise of resistance from lowerresistance to higher-resistance state. Noteworthy, lower- and higher-resistance states
correspond to the parallel and antiparallel alignments, respectively, of free layer
magnetic moment with respect to the fixed layer moment. It has already been
discussed that positive current corresponds to the flow of electronic current from
the free to the fixed FM layer. As is evident from the above-made discussion, in
this case, parallel orientation between free and fixed layer magnetic moment gets
destabilize and antiparallel alignment between them gets stabilize. Thus, magnetic
moment associated with the free FM layer, in this high-resistance antiparallel configuration, can be reversed back to the parallel orientation by applying a sufficiently
large negative current (Myers et al. 1999). It should be clearly mentioned that such
switching of free layer magnetic moment between parallel and antiparallel configurations with fixed layer moment can be justified and understood with the mechanism
of spin-transfer torques. More subtle point is that such switching is not a mere
consequence of current-induced magnetic fields. This can be confirmed from the
fact that in nanoscopic devices the requisite current levels are so small that it cannot
produce magnetic fields of the desired magnitude to switch the magnetic moment
of free layers. Noteworthy observations in the differential resistance are small peaks
or shoulders (Fig. 5.6b), appearing at the currents I
+
D and I
−
D , before the resistance
exhibits large jumps in its value (Urazhdin et al. 2003). This experimental feature can
be attributed to the turn-on of a dynamical state, in which the magnetic moment associated with free FM layer executes small-angle precession. This subtle experimental
feature further assures us about the origin of magnetization switching in this present
case. It has already been discussed that at the onset, spin-transfer torque drives the
free layer moment into a precessional mode, which is then followed by reversing of
the magnetic moment to attain the final static state.
5.5 Spin-Transfer Torque in Magnetic Multilayer
Nanopillar
Let us imagine an electron system in which exchange interactions take place between
the conduction s-electrons and the magnetic d-electrons, holding local magnetic
moments, as schematically shown in Fig. 5.7a. It is known that the total spin angular
momentum would be conserved during s–d exchange interaction. Thus, it is straightforward to state that a reduction in the sub-total angular momentum associated with
the conduction electrons would be equal to the enhancement of the sub-total angular
momentum corresponding to the magnetic d-electrons. Suppose during transport
through the FM2 layer of a magnetic pillar, spin angular momentum of a conduction
electron changes owing to the s–d interaction. Now, the same amount of angular
momentum, equal to the change experienced by conduction electrons, should be
transferred to the d-electrons in the FM2 layer. Considering the rule for conserving
the spin angular momentum