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2 Basic Elements of Spintronics
the magnetizations of those two ferromagnetic electrodes are again parallel and the
corresponding device resistance drops. Therefore, a resistance peak is supposed to
be observed between the coercive fields H c1 and H c2 , where the height of this peak is
R = R AP − R P . Likewise, if the magnetic field is scanned in the reverse direction,
the same peak will reappear between the coercive fields. The background resistance
is R P . It is understandable that if the two ferromagnetic electrodes have opposite
signs of spin polarization, then instead of a spin valve ‘peak’, spin valve ‘trough’
would have been observed.
2.10.3 Description of Spin Valve Device Experiments
In this experiment, resistance of the device is measured with the variation of the
applied magnetic field (H). Figure 2.19 gives the pictorial demonstration of the
spin valve response. Let us assume the effect associated with a regular spin valve
in which a low resistance state is associated with parallel configuration, whereas a
high resistance state is associated with that of antiparallel configuration. Let us also
suppose that the coercive fields of those two ferromagnetic electrodes are given by
|H 1 | and |H 2 | with |H 1 | < |H 2 |. Initially, a strong magnetic field (H), such that H
(= H sat ) |H 2 |, is applied to the device. Consequently, as shown in Fig. 2.19, both
the ferromagnetic electrodes become magnetized along the direction of this applied
magnetic field. Corresponding resistance of the device is measured and denoted by
R P , where the subscript (P) indicates ‘parallel’ magnetization configuration. After
that, the applied magnetic field value is decreased, swept through zero and then
reversed. Interestingly, at the very moment when the value of the applied field, i.e.,
H exceeds |H 1 | in the reverse direction (i.e., −|H 2 | < H < −|H 1 |), the ferromagnetic
electrode having lower coercivity (i.e., |H 1 |) flips its magnetization (Fig. 2.19). As a
result, the magnetizations of those two ferromagnetic electrodes become antiparallel
to each other. Corresponding resistance of the device is again measured at this step
and denoted by R AP , where the subscript (AP) indicates ‘antiparallel’ magnetization
configuration. In case of a regular spin valve, as described above, the jump of the
device resistance in increasing direction at H = −|H 1 | implies R AP > R P .
On further increasing the magnetic field in the same reverse direction, at some
point of time it reaches the value of the coercive field of the second ferromagnetic
electrode, i.e., H = −|H 2 |. Quite expectedly, at this point the second ferromagnetic electrode also flips its magnetization direction. As a result, magnetizations of
those two ferromagnetic electrodes once again become parallel. Thus, the corresponding device resistance decreases again to R P at H = −|H 2 |. Therefore, as shown
in Fig. 2.19, during a single scan of magnetic field from H sat to −H sat (blue line),
a rectangular resistance peak appears in between the coercive fields of those two
ferromagnetic electrodes (i.e., between −|H 1 | and −|H 2 |). Likewise, with the variation of the magnetic field from −H sat to H sat , as shown by red line in Fig. 2.19, an
identical rectangular peak in resistance is observed between |H 1 | and |H 2 |. We estimate the relative variation in device resistance, as the magnetization configurations
2 Basic Elements of Spintronics
the magnetizations of those two ferromagnetic electrodes are again parallel and the
corresponding device resistance drops. Therefore, a resistance peak is supposed to
be observed between the coercive fields H c1 and H c2 , where the height of this peak is
R = R AP − R P . Likewise, if the magnetic field is scanned in the reverse direction,
the same peak will reappear between the coercive fields. The background resistance
is R P . It is understandable that if the two ferromagnetic electrodes have opposite
signs of spin polarization, then instead of a spin valve ‘peak’, spin valve ‘trough’
would have been observed.
2.10.3 Description of Spin Valve Device Experiments
In this experiment, resistance of the device is measured with the variation of the
applied magnetic field (H). Figure 2.19 gives the pictorial demonstration of the
spin valve response. Let us assume the effect associated with a regular spin valve
in which a low resistance state is associated with parallel configuration, whereas a
high resistance state is associated with that of antiparallel configuration. Let us also
suppose that the coercive fields of those two ferromagnetic electrodes are given by
|H 1 | and |H 2 | with |H 1 | < |H 2 |. Initially, a strong magnetic field (H), such that H
(= H sat ) |H 2 |, is applied to the device. Consequently, as shown in Fig. 2.19, both
the ferromagnetic electrodes become magnetized along the direction of this applied
magnetic field. Corresponding resistance of the device is measured and denoted by
R P , where the subscript (P) indicates ‘parallel’ magnetization configuration. After
that, the applied magnetic field value is decreased, swept through zero and then
reversed. Interestingly, at the very moment when the value of the applied field, i.e.,
H exceeds |H 1 | in the reverse direction (i.e., −|H 2 | < H < −|H 1 |), the ferromagnetic
electrode having lower coercivity (i.e., |H 1 |) flips its magnetization (Fig. 2.19). As a
result, the magnetizations of those two ferromagnetic electrodes become antiparallel
to each other. Corresponding resistance of the device is again measured at this step
and denoted by R AP , where the subscript (AP) indicates ‘antiparallel’ magnetization
configuration. In case of a regular spin valve, as described above, the jump of the
device resistance in increasing direction at H = −|H 1 | implies R AP > R P .
On further increasing the magnetic field in the same reverse direction, at some
point of time it reaches the value of the coercive field of the second ferromagnetic
electrode, i.e., H = −|H 2 |. Quite expectedly, at this point the second ferromagnetic electrode also flips its magnetization direction. As a result, magnetizations of
those two ferromagnetic electrodes once again become parallel. Thus, the corresponding device resistance decreases again to R P at H = −|H 2 |. Therefore, as shown
in Fig. 2.19, during a single scan of magnetic field from H sat to −H sat (blue line),
a rectangular resistance peak appears in between the coercive fields of those two
ferromagnetic electrodes (i.e., between −|H 1 | and −|H 2 |). Likewise, with the variation of the magnetic field from −H sat to H sat , as shown by red line in Fig. 2.19, an
identical rectangular peak in resistance is observed between |H 1 | and |H 2 |. We estimate the relative variation in device resistance, as the magnetization configurations