Electron spin polarization is defined as the ratio of the density of state
of upspin and downspin at the Fermi level. Since magnetoresistance is sensitive
to the spin polarization of ferromagnetic electrode, experimental measurement
of spin polarization is very impoartant. Most commonly used method for
spin polarization measurement is from the tunneling magnetoresistance measurement
from metallic tunneling junctions. According to Julliere's formula, the
MR value of metal tunneling junction (MTJ) should follow MR=P1P2/(1-P1P2), where P1 and P2 are the electron spin polarization of electrode materials [1]. According
to this formula, if we make MTJ using half-metallic materials with spin
polarization 1, the MR value should become infinite. This is the reason
why search of half-metallic thin films receives so much research interest.
The most promissing candiate for half-metallic thin films for MTJ is Heusler
alloys, as some of them have Curie temperatures much higher than room temperature.
To operate MR devices at room temperature, the Curie temperature of the
half-metallic electrode must be much higher than room temperature. In spite
of intense research effort, no MTJ using Heusler alloys exhibit large MR
values exceeding 100% at room temperature. This suggests that the interfacial
structure of the MTJ's is not good enough to obtain spin filtering effect
or the spin polarizaton of the Heusler alloy electrodes are not high enough.
To understand the underlying mechanism for the unexpectedly low MR values
obtained from Heusler alloy MTJ's, we believe that it is desirable to fabricate
Heusler alloy thin films with high spin polarizations before we produce
MTJ's. To do so, direct measurement of spin polarization is necessary.
However, the experimental measurement of spin polarization is not so easy.
De Jong and Beenakker proposed the principle to measure spin polarization
using Andreev reflection that occurs at a superconductor and normal metal contact in 1995. Subsequnetly
Soulen et al. reported the first experimental results of spin polarization
measurement using the point contact Andreev reflection method [3]. However,
the measurement method was based on a simple model GNS/GNN=2(1-P) near
zero vias, so the accuracy of the measurement was not good enough. The
spin polarization measurement using Andreev reflection was futher improved
by Upadhyay et al [4] and Strikers et al. [5] introducing a more reliable
fitting method of the conductance measurement results, and now the point
contact Andreev reflection method is established as a reliable mesurement
method of electron spin polarization.
Figure 1 Appearance and schematic illustration of point contact Andreev
reflection measurement system
We have set up a spin polarization mesurement system using the point contact Andreev reflection (PCAR) method. Figure 1 shows the appearance of the instrument. The point contact sytem using micrometer is dipped into He jewer. The electrical conductance is measured using the four point method by applying bias using a function generator. After dipping the contact mechanism shown in Fig. 2 into the He dewer, the system is left for a few hours for stabilization. Then, point contact is made between the Nb tip and the sample surface using a micrometer installed at the entract of the dewer. Many point contacts are made by moving the tip in three axis so that the data can be plotted as function of contact barrier, z.
Figure 2 Appearance of the point contact mechanism.
Figure 3 shows an example of structure and conductance curves obtained from a Heusler alloys. The data is fit using the modifed BTK theory [5] by changing parameters contact barrier z, superconducting energy gap delta, and temperature T, then P is extrapolated to z=0 to determine P for the ideal contact.
Figure 3 TEM dark filed images, conduction curves, and magnetization and
spin polarization of Co2(Cr1-xVx)Al Heusler alloys.
So far, we have measured spin polarization of various Heusler alloys and
their thin films, the results of which are now in preparation for publications.
Our goal is to process Heusler alloy thin films with high spin polarization.
After confirming a film actually has high polarization, we will process
MTJ's using the film with high spin polarization.
A. Rajanikanth, V. Karthk, Y. K. Takahashi, and K. Hono
