
resistance for SDT devices must be at the lower end
of the resistance range. Parkin et al. found that the
resistance-area products of magnetic tunnel junctions
can be varied from 10
9
Omm
2
to as low as 60 Omm
2
by
varying the aluminum thickness and properly oxidiz-
ing it (Parkin et al. 1999). Incomplete oxidation leads
to the presence of metallic aluminum in the barrier,
which results in a rapid suppression of the tunnel
junction magnetoresistance. Based on these facts, the
RC time constant for the junctions should be of the
order of nanoseconds (Wong et al. 1998, Sousa et al.
1999). Nevertheless, such a SDT-MRAM has to be
read with a silicon sense amplifier. This means a
mixed silicon-tunneling structure: the X-Y decoders/
sense amplifiers in an array in silicon with ‘‘holes’’ to
be filled in by tunneling array. Thus, the speed of the
device will probably be limited by silicon technology.
(Today, the speed of experimental silicon devices has
improved to 1 ns and will be expected to dip below
this in future.) There is no significant access-time ad-
vantage for the tunneling design as compared to the
bipolar devices.
As well as the continuing developments which are
always to be expected in semiconductor memory
technology, it is also interesting to note other novel
technologies, such as the spin-dependent tunneling
(SDT) effect. In the immediate future DRAM will
continue as the densest semiconductor memory, but
MRAM using the SDT effect looks set to take the
lead in the medium and longer term and is a very
important development for applications where non-
volatility, higher density, radiation hardness and low-
er power are required. Prototypes are out, technology
is maturing, and the advances mentioned in this ar-
ticle would speed up the pace of the application of
MRAM, since a microstructured junction might serve
as a high-capacity and low-power substitute for con-
ventional semi-conductor memories.
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F. Z. Wang
University of North London, UK
Rare Earth Intermetallics: Thermopower of
Cerium, Samarium, and Europium
Compounds
The thermoelectric power, S, is known as a transport
property that is very sensitive to details of the elec-
tronic structure at the Fermi energy. Cerium com-
pounds exhibit widely different types of anomalous
physical properties owing to the hybridization of ce-
rium 4f electrons and conduction electrons. There is
already an accumulation of experimental data for S
for a large number of cerium compounds; however,
there has been no review article concerning these
data, as far as is known. The aim of this article is to
give an overview and discussion of these data for S
for different types of cerium compounds.
This article is based on previous review papers by
Sakurai (1993), and Sakurai et al. (1996). Anomalous
physical properties of cerium compounds are of in-
terest with respect to highly correlated electron sys-
tems (see Electron Systems: Strong Correlations).
Doniach (1977) considered a competition of two in-
teractions: the Kondo interaction between the cerium
4f electrons and the conduction electrons (character-
istic temperature T
K
) and the RKKY interaction be-
tween the 4f electrons on different lattice sites via the
conduction electrons (characteristic temperature T
N
1078
Rare Earth Intermetallics: Thermopower of Cerium, Samarium, and Europium Compounds