Hybrid Spin Valve devices on InAs heterostructures
Marcus Steiner, Alexander van Staa, Guido Meier, Toru Matsuyama
Full-metal spin valve devices have been investigated for about three decades
and are used in modern harddisk read-heads and magnetic random access memories
(MRAM). A tunnel barrier is placed between two ferromagnetic electrodes, which
have defined magnetic orientations. If a current flows through this sandwich
device perpendicular to the material boundaries the electrical resistance is
low when the magnetizations of the electrodes are aligned parallel and is high
when they are aligned antiparallel.
Replacing the tunnel barrier by a two-dimensional electron system (2DES) offers
new possibilities for applications. To give an example, the electron spin in
the semiconductor (InAs) can be controlled via the Rashba effect by applying
an electric field.
Single domain electrodes
For injecting spinpolarized electrons into InAs heterostructures it is important
to know the magnetization of the ferromagnetic electrodes. We work with sputtered
Permalloy electrodes which are 4µm and 8µm long. The shape anisotropy
leads to different switching fields and the orientation of the magnetization
can be controlled by an external field applied parallel to the long axes of
the electrodes. The magnetization can be investigated by magnetic force microscopy
The MFM images in Fig. 1 show that a parallel as well as an antiparallel orientation
of the magnetization is possible. The black and white areas indicate north and
south poles of the electrodes.
Fig. 1: Magnetic configurations of the Permalloy electrodes
The described electrodes are used for experiments with high-mobility InAs heterostructures.
In Fig. 2 a hybrid device is shown, were a 1 µm² InAs mesa is placed
between the electrodes. They are contacted in a further preparation step using
gold contacts to apply a current and to measure the voltage drop across them.
The carrier concentration in the semiconducting InAs mesa can be tuned by application
of a gate voltage.
Fig. 2: Hybrid device of Permalloy and InAs without connecting
First transport measurements show resistance changes of the 2DES channel when
the magnetizations in the electrodes are switched. Currently we analyse in detail,
if these changes are due to the injection of defined spin polarizations.
 M. Julliére, Phys. Lett. 54A, 225 (1975)
 S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)
 C.-M. Hu, Phys. Rev. B 63, 125333 (2001)