Novel spin FETs

Novel spin FETs
J. Carlos Egues
Departamento de Física e Informática,
Instituto de Física de São Carlos,
Universidade de São Paulo
The spin field-effect transistor (spin FET) first proposed by Datta and Das [1] more
than a decade ago, highlights the possibility of electric control over an intrinsic
magnetic degree of freedom: the electron spin. The spin FET is arguably the most
popular proposed spintronic device The basic idea of this device (see figure below) is
to rotate the spin of the injected carriers as they travel down a low-dimensional
channel, connecting two ferromagnetic reservoirs (source and drain). Spin rotation is
effected via the so-called Rashba spin-orbit interaction present at the interface where
the 2DEG is formed. This particular type of s-o interaction only arises in
heterojunctions with structural inversion asymmetry. The Rashba s-o interaction
differs fundamentally from its bulk counterpart, the Dresselhaus s-o interaction, in
that it can be externally tuned via proper gate electrodes. Roughly speaking, the
Rashba s-o coupling α is proportional to “average” electric field normal to the
(Phys. Today ‘95)
Gate control of the s-o coupling constant is a crucial ingredient to the operation of a
spin FET. An external gate essentially changes the slope of the confining potential
thus altering α . Datta and Das have shown that upon crossing the length L of the
channel the electrons undergo a rotation by the Rashba angle θ R = 2mαL / η2 . Ideally,
by adjusting the gate voltage the rotation angle can be varied continuously thus
modulating the electric current between the source and drain: when the electron spins
are rotated so as to be anti-aligned to the ferromagnetic drain lead the current is zero.
Other operating modes are possible with source and drain anti-aligned.
The original proposal of Datta and Das assumes a ballistic channel connecting the
source and drain in a FET geometry. Spin-flip and momentum scattering are
detrimental to the operation of a spin transistor since these randomize the spin of the
carriers thus destroying the current spin-polarization, vital for the operation of the
device. Spin flip due to, say, stray magnetic impurities can in principle be
eliminated/minimized. However, the s-o interaction acts as a k-dependent effective
magnetic field which the electrons precess about and momentum scattering changes
this local direction of precession thus randomizing the electron spin (Dyakonov and
Perel mechanism).
In my talk I will present two novel spin-FET proposals, namely: (i) a quasi-onedimensional ballistic setup with s-o induced interband coupling which provides
enhanced spin control [3] and (ii) a robust two-dimensional non-ballistic spin
transistor [4] which benefits from a unique interplay of two distinct s-o interactions,
Rashba and Dresselhaus. The enhanced capability of our setup (i) relies on the
coherent transfer of electrons between two s-o coupled subbands. Spin-polarized
electrons can be further spin rotated via additional side gates controlling the width of
the channel and hence the s-o interband coupling strength. This extra rotation θ d can,
in principle, be tuned independently of that described by θ R . The non-ballistic spin
FET proposal (ii) conveniently relaxes the very stringent constraint of ballistic
channels in the original proposal of Datta and Das. Due to a ‘partial’ cancellation of
the Rashba and Dresselhaus s-o interactions for a particular value of the gatecontrollable α coupling, we can fix the direction of the effective magnetic field the
electrons precess around, thus eliminating the Dyakonov-Perel and Eliott-Yafet types
of spin dephasing processes. In other words, for tuned Rashba and Dresselhaus
couplings the electron spinor is k-independent thus making momentum scattering
ineffective in relaxing the electron spin.
Finally, I should also briefly discuss transport properties in electron beamsplitters and
novel spin-FET geometries with top and side gates [5]. In particular, I will describe
the shot noise characteristics of these devices as a means of detecting the electron spin
polarization and entanglement of electron pairs.
[1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).
[2] J. Nitta T. Akazaki, H. Takayanagi, and T. Enoki, Phys. Rev. Lett. 78, 1335
(1997); G. Engels, J. Lange, Th. Schäpers, and H. Lüth, Phys. Rev. B 55, R1958
(1997); The highest measured value of α reported to date was by Y. sato, T. Kita, S.
Gozu, and S. Yamada, J. Appl. Phys. 89, 8017 (2001).
[3] J. C. Egues, G. Burkard, and D. Loss, Appl. Phys. Lett. 82, 2658 (2003).
[4] J. Schliemann, J. C. Egues, and D. Loss, Phys. Rev. Lett. 90, 146801 (2003).
[5] S. Yamada, private communication.