A Comprehensive Study of Dilute Magnetic ... - OPUS Würzburg

64 5. A voltage controlled spin-valve
transmits current if both RTDs are set to transmit the same spin type (parallel setting).
In case of spin-flips in the area between the two RTDs two more peaks would arise for the
antiparallel settings. Although the plot of I both (VT
RT D , VB
RT D ) in fig. 5.5 shows what could
be interpreted as the four discussed features, the long tails are not a result of a spin-flip
contribution to the resonant transport. They again arise due to characteristic background
of each RTD and the analysis described above. At small applied bias voltages either one
of the RTDs results in a high resistance and the current traversing both RTDs is almost
zero, since most of the current is either coming from or going to the middle contact. As
discussed above, the first peak arises when both RTDs are set to their spin-up resonance,
Increasing VT
RT D while maintaining VB
RT D at the first resonance will put the top RTD
in the valley between its first peak and the increasing background current (and second
resonance). I both is thus given by I 1 until due to the increase of the background current
and/or the second resonance I 2 is greater than I 1 .
We conclude that the single channel analysis does not yield conclusive evidence for a
spin-valve functionality. In the following we therefore discuss a two channel model, which
treats spin-up and spin-down transport channels separately.
5.2 The two channel model
The reason that the data of fig. 5.3a does not show an explicit off state for our spin
valve is that the middle contact ground actually has an active role in device operation.
The middle contact resistance is however a necessary element to allow for independent
control of the two RTDs. Specifically, when one of the spin channels in either diode is
off-resonance, then its resistance is ideally infinite. The middle grounding contact then
provides an alternative current path. Each RTD can be either set to transmit spin-up or
spin-down electrons. Figure 5.6 shows the four resulting possible settings for the spinvalve.
In fig. 5.6a, the bottom RTD is set to transmit only spin down, while the top RTD
is allowing spin up current to flow. The spin up current traversing the top RTD will not
be able to enter the bottom RTD, but instead divert into ground. This does not however
prevent a current from flowing through the bottom RTD, as the same ground contact can
act as a source of spin down current to supply this diode. A similar situation is shown
in fig. 5.6b, with top diode set to spin-down and the bottom diode set to spin-up. For
completeness, fig. 5.6c+d show the two on states of the spin valve, when both RTDs are
set to transmit the same spin species.
A more detailed analysis is therefore necessary to assess the performance of the spin
valve. The essence of the model is contained in the equivalent Kirchhoff circuit of fig. 5.2b.
The device is described in a two channel model [Fert 68], as a spin-up and spin-down path
in parallel, each comprised of two RTDs in series.
A key element is the description of the middle contact, for which two important aspects
must be considered. First, the resistive value of the middle contact to ground is important.