ISIS Experimental Report
RB Number:
Rutherford Appleton Laboratory
Date of Report:
11/04/2008
R. Dalgliesh
RB720240
Title of Experiment:
A study of the reduced interfacial magnetisation in a
Heusler/insulator tunnel junction
Local
Contact:
Principal Proposer:
Dr. Martin Jourdan, Institute of Physics,
Johannes Gutenberg University, Mainz, Germany
T. Charlton, S. Langridge, E. Arbelo Jorge, M. Jourdan
Instrument:
CRISP
Date of
Experiment:
26/10/2007
-28/10/2007
Introduction
Some Heusler compounds are predicted to be half
metals, i.e. materials with 100% spin polarisation at
the Fermi energy. Thus magnetic tunneling junctions
with Heusler electrodes are expected to show huge
magnetoresistances. However, at the interface with
the insulating tunneling barrier the spin polarisation
can be strongly reduced compared to the bulk value.
The local magnetisation at the interface compared to
the bulk is an important indicator for possibly
modified interface electronic states.
We investigated epitaxial thin films of the Heusler
compound Co2Cr0.6Fe0.4Al(100), capped with AlOx, or
Al, or epitaxial Cr(100). The AlOx capping
corresponds to an optimized tunneling barrier, the
Al capping corresponds to an underoxidised
tunneling barrier and the epitaxial Cr capping
investigates the effect of a near to perfect metal
interface.
Method
Polarised neutron reflectometry (PNR) was used to
obtain magnetisation profiles of
MgO/Co2Cr0.6Fe0.4Al/capping layer stacks at room
temperature and at 4K. The measured reflectivity
curves for neutrons polarised parallel and
antiparallel to the sample magnetisation were fitted
with an optical reflectivity model including
magnetic moment dependent indices of refraction
(xPolly).
Results
The measurements on AlOx and Al capped
Co2Cr0.6Fe0.4Al (CCFA) samples showed similar
results: The best fits were obtained with only a very
thin (1nm) CCFA interface layer with a reduced
magnetisation. From this two important conclusions
can be drawn:
The AlOx barrier is not overoxidised, i.e. with the
given junction preparation parameters the barrier
oxidation process does not oxidise the CCFA
interface region.
The magnetisation in the CCFA interface region is
not reduced significantly. Thus this necessary
condition for a fully developed spin polarization at
the junction interface is fulfilled. This suggests that
the spin polarisation of 60%, which we determined
previously from tunnelling junctions, is
representative for the bulk as well.
A surprising result was obtained investigating the
fully epitaxial CCFA/Cr sample: The best fit was
obtained if a homogeneous induced ferromagnetic
moment of 0.3B/atom was assumed for the 7nm Cr
layer (fig. 1). This surprising result stimulated
additional X-ray magnetic circular dichroism
investigations (total electron yield) of the same
CCFA/Cr sample. However, no magnetic moment
in the upper 3-4nm of the Cr layer, which are
accessible by this method, was detected. Thus a
contradiction between the PNR and the XMCD-TEY
results remains for the highly application relevant
(spin valves) epitaxial CCFA/Cr system.
Additionally, the total moment of this sample as
determined by SQUID investigations is clearly
smaller then the total moment determined by PNR.
7
6
5
4
3
2
1
0
0
10
Up
Down
-1
10
-2
10
SLD [x10-6 Å-2]
Experimental Team:
Reflectivity
Affiliation:
rho
P
0
-3
10
200
400
600
z [Å]
-4
10
-5
10
-6
10
0.01
0.1
-1
Q [Å ]
Fig. 1: PNR data of a CCFA)/Cr bilayer with the best
fit obtained. For the Cr layer a magnetic moment of
0.3B/atom is obtained. The inset shows the
magnetic (P) and structural (rho) profiles. The
shaded region corresponds to the surface Cr layer.