Spin Filters as High-Performance Spin
Polarimeters
N. Rougemaille1, G. Lampel1, J. Peretti1, H.-J. Drouhin1, Y. Lassailly1,
A. Filipe1, T. Wirth1 and A. Schuhl2
1
Laboratoire de Physique de la Matière Condensée, UMR 7643 - CNRS,
Ecole Polytechnique, 91128 Palaiseau cedex, France.
2
Laboratoire de Physique des Matériaux, UMR 7556 - CNRS,
Université Henri Poincaré, 54506 Vandoeuvre-Les-Nancy, France.
Abstract. A spin-dependent transport experiment in which hot electrons pass through a
ferromagnetic metal / semiconductor Schottky diode has been performed. A spin-polarized freeelectron beam, emitted in vacuum from a GaAs photocathode, is injected into the thin metal
layer with an energy between 5 and 1000 eV above to the Fermi level. The transmitted current
collected in the semiconductor substrate increases with injection energy because of secondary electron multiplication. The spin-dependent part of the transmitted current is first constant up to
about 100 eV and then increases by 4 orders of magnitude. As an immediate application, the
solid-state hybrid structure studied here leads to a very efficient and compact device for spin
polarization detection.
INTRODUCTION
Electron spin polarization detection yields additional information on the electronic
structure of solids and spin-dependent interactions inside the matter. As spin
measurements can be applied to electron microscopy and spectroscopy techniques,
many efforts are still made to improve the efficiency and the convenience of the spin
detectors [1]. But, the polarimeters in use up to now (Mott, SPLEED or absorbed /
reflected current detectors) suffer from a low spin-discriminating power, a small
collection efficiency and severe operating conditions (surface preparation, high
voltages, large and complex equipment). The Mott polarimeter, based on the spinorbit interaction of electrons with high atomic weight materials, remains the
conventional spin detector for standard measurements but is not convenient enough
for routine application of spin detection.
The spin filter effect in ferromagnetic thin films (the preferential transmission of a
spin direction depending on the relative orientation of the electron spin and the layer
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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magnetization), which originates from exchange interaction, offers a new way for
measuring the free electron spin polarization [2]. Direct transmission experiments of
spin-polarized electrons through free-standing Au/Co/Au films have been already
performed to characterize the spin-filtering efficiency of magnetic thin layers [3].
Such structures containing asymmetrical ferromagnetic cobalt bilayer have shown
interesting properties to realize self-calibrated spin polarimeters [4]. When operated
at very low injection energy (a few eV above the Fermi level), these spin filters
exhibit a large spin-discriminating power (Sherman function) but are limited by a
poor transmission efficiency. Their figure of merit is therefore at best comparable
with the one of the Mott polarimeter. Moreover, it is usually admitted that the
Sherman function of multilayer spin filters should decrease with increasing energy.
Up to now, all the experiments performed at injection energy up to about 100 eV
have indeed confirmed this decrease of the Sherman function and of the figure of
merit.
Starting from a previous experiment of spin-dependent transmission through a thin
ferromagnetic layer deposited on a semiconductor [5], we demonstrate here that,
under operation at injection energy in the keV-range, a ferromagnetic metal /
semiconductor junction constitutes a very efficient and convenient solid-state spin
detector compatible with all standard techniques involving electrons.
EXPERIMENTAL CONDITIONS
The principle of the experiment presented here (Fig. 1) consists in measuring the
spin-dependent transmission of a spin-polarized electron beam through a thin
magnetic layer deposited on a semiconductor substrate.
Pd Fe
GaAs
I0
Ox
IB
IC
FIGURE 1. Schematic representation of the detection principle. The sample is in-plane magnetized
and the spin polarization of the incident electron beam is modulated between -25% and +25%. The
currents IB and IC can then be measured in four configurations depending on the relative orientation
of the spin polarization and on the sample magnetization.
The sample is made of a 3.5 nm-thick iron layer grown onto a n-doped GaAs
substrate. To avoid iron inter-diffusion inside the GaAs, the substrate is previously
oxidized, leading to a typical 2 nm-thick oxide layer. A 4 nm-thick palladium cap
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layer is finally deposited to prevent iron from oxidation. The iron layer exhibits an
in-plane easy-magnetization axis and a square hysteresis loop with a coercive field
of about 20 Oe and a remanence in zero external magnetic field of 90%. Pulsed
operation of magnetic coils allows to control in-situ the magnetization of the iron
layer.
The electron source is a p+-doped GaAs photocathode under optical pumping
conditions. Before measurements the source is activated to Negative Electron
Affinity by co-deposition of cesium and oxygen. Under excitation with a circularly
polarized laser beam of wavelength 780 nm, the source yields an electron beam with
a longitudinal spin-polarization P = 25%.
This beam is then focused onto the sample using electrostatic optics, after a 90°
deflection which converts the longitudinal spin polarization into a transverse one,
parallel to the Fe layer magnetization. A typical current I0 of 200 nA is injected into
the sample. The injection energy of the polarized electrons entering the sample is
changed by varying the voltage between the source and the Schottky diode.
Experiments at injection energies up to 1 keV (referred to the metal Fermi level)
have been performed.
No bias voltage is applied to the Schottky diode. The electrons which have enough
energy to overcome the Schottky barrier are collected in the GaAs substrate and
yields the "transmitted" current IC. The electrons which have an energy lower than
the barrier height are detected in the front contact of the metallic base as a current IB
which is measured independently of IC. We have checked that, in the whole injection
energy range we used, the current balance verifies the relation I0 = IB + IC (no backscattered electrons). The spin-dependent part of IC, DIC, is measured when the
incident spin polarization is modulated between +P and -P.
RESULTS
The "transmission" T=IC/I0 and the spin-dependent transmission DT=DIC/I0 are
plotted in Fig. 2 as a function of the injection energy. Let us remark that T and DT
vary respectively over 6 and 4 orders of magnitude in the studied energy range.
When entering the palladium layer, the incident electrons suffer inelastic
scattering, mainly by electron / electron interaction [6]. The energy lost in the
collisions promotes secondary electrons. This secondary-electron production
together with the increase of the electron mean-free path is responsible for the
increase of the collected current IC.
Due to this cascade process in the palladium layer, the spin polarization of the
electrons before the spin filter is diluted by the unpolarized secondaries which are
generated. Therefore, at low injection energy (below 80 eV), the spin-dependent
transmission DT does not follow the increase of T and is energy independent as
already observed in previous works [3,5]. But very surprisingly, at higher injection
energies, DT is no longer energy independent and increases by 4 orders of
1003
magnitude. The reason of this DT increase is not yet fully understood and will be
discussed elsewhere [7].
IC / I 0
10
1
10
0
DIC / I 0
10
-1
10
-2
10
-3
10
-1
10
-2
-3
10
-4
10
-4
10
-5
10
10
-5
10
-6
0
200
400
600
800
1000
0
200
Injection energy (eV)
400
600
800
1000
Injection energy (eV)
FIGURE 2. The left and right curves show the variations of the transmission T and the spindependent transmission DT respectively as a function of the injection energy in logarithmic scale for
an injected current I0=200 nA. DIC is either obtained by modulating the incident spin polarization
between +P and -P while keeping constant the sample magnetization or modulation the sample
magnetization for a fixed spin polarization.
At the particular energy E0 = 715 eV, the overall transmission reaches unity,
meaning that the collected current IC is exactly equal to the current I0 injected from
the vacuum and consequently IB = 0. As DIC verifies DIC = -DIB, the large magnetic
signal DT observed at high injection energy can thus be conveniently measured in
the metal contact with no background signal. Such a cancelled-background
configuration has already been used in absorbed-current spin detectors [8].
Depending on the relative orientation of the sample magnetization and on the
magnetic moment of the injected electrons, DIB changes its sign.
0,002
b)
a)
c)
DIB / I0
0,001
0
-0,001
-0,002
0
2
4
0
2
4
Time (ms)
0
2
4
FIGURE 3. Magnetic signal measured at injection energy E0 on the metallic base when modulating
the spin polarization between +P and -P at 250 Hz. The signal is obtained in a single shot without any
electronic treatment.
Fig. 3 shows the variations of DIB measured when the incident spin polarization is
reversed at a frequency 250 Hz between ±25% with a magnetization -M (curve a)
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and +M (curve b). Fig. 3c shows that DIB/I0 remains zero when the light polarization
is modulated between two orthogonal linear polarization directions (unpolarized
electron beam), demonstrating that there is no experimental asymmetry. The
measurements of DT presented here have been acquired with a band width of 100
kHz. This indicates that the P=25% spin-polarisation of the incident beam can be
measured in a very short time with a signal-to-noise ratio of the order of 10.
CONCLUSION AND PERSPECTIVES
We have shown here that a ferromagnetic metal / semiconductor Schottky diode
leads to a new kind of spin detector. A spin polarimeter is characterized by its figure
of merit which yields the signal-to-noise ratio of the detection. The figure of merit is
given by the quantity F = S2e [9], where e is the scattering efficiency and S is the
Sherman function. In our case, e = T and S = DT/2TP. At 1 keV injection energy,
F = 10-3. This value has to be compared with the figure of merit of the Mott
polarimeter which is at best 10-4. Moreover, our spin polarimeter present other
advantages: a very small size (few cubic centimeters), a compatibility with ultra-high
vacuum and high vacuum (no specific surface preparation and cleanness are
required), a quite low voltage operation (about 1 kV to be compared with 100 kV for
a self-calibrated Mott detector). Let us also mention that the figure of merit can
again easily be improved. On the one hand, beyond 1 keV injection energy, F still
increases. On the other hand, the noise is here limited by the Johnson noise of the
Schottky junction which can be lowered by decreasing the dark current of the
junction. Finally, this solid-state device is a very efficient spin polarimeter
compatible with all electron spectroscopy and microscopy techniques.
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