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Although there are a number of different magnetic semiconductors, in the
short time since its invention (Ga,Mn)As has become the most
popular and widely studied for a number of reasons. Firstly, it is
based on the world's second favorite semiconductor, GaAs, and as such is readily
compatible with existing semiconductor technologies. Secondly,
many dilute magnetic semiconductors
(DMSs), such as the majority of those based on II-VI semiconductors, are only paramagnetic.
(Ga,Mn)As, on the other hand, is ferromagnetic, and hence exhibits hysteretic magnetization behavior. This
memory effect is of importance for the creation of persistent
devices. A third key feature of (Ga,Mn)As is that not only do the
manganese atoms provide a magnetic moment, each also acts as an acceptor, making it a
p-type material. The presence of carriers allows the material to be used
for spin-polarized currents. In contrast,
many other ferromagnetic DMSs are strongly insulating[2
and so do not possess free carriers. When all these factors
are taken together, (Ga,Mn)As appears to be an exceptionally good
candidate as a spintronic material.
Like other DMSs, (Ga,Mn)As is formed by doping a standard semiconductor with
magnetic elements. This is done using the growth technique molecular beam epitaxy (MBE), whereby
crystal structures can be grown with atom layer precision. In
(Ga,Mn)As the manganese substitute into gallium sites in the GaAs crystal and
provide a magnetic moment. Because manganese has a low solubility
incorporating a sufficiently high concentration for ferromagnetism to
be achieved proves challenging. In standard MBE growth, to ensure that a good
structural quality is obtained, the temperature the substrate is
heated to, known as the growth temperature, is normally high,
typically ~600°C. However, if a large flux of manganese is used in
these conditions, instead of being incorporated, segregation occurs
where the manganese accumulate on the surface and form complexes
with elemental arsenic atoms.
This problem was overcome using the technique of low-temperature MBE. It was found, first in (In,Mn)As
and then later used for (Ga,Mn)As,
that by utilising non-equilibrium crystal growth techniques larger
dopant concentrations could be
successfully incorporated. At lower temperatures, around 250°C,
there is insufficient thermal energy for surface segregation to
occur but still sufficient for a good quality single crystal alloy
In addition to the substitutional incorporation of manganese,
low-temperature MBE also
causes the inclusion of other impurities. The two other common
impurities are interstitial manganese 
and arsenic antisites.[9
] The former is where the manganese atom sits
between the other atoms in the zinc-blende lattice structure and
the latter is where an arsenic atom occupies a gallium site. Both
impurities act as double donors, removing the holes provided by
the substitutional manganese, and as such they are known as
compensating defects. The interstitial manganese also bond antiferromagnetically to
substitutional manganese, removing the magnetic moment. Both these
defects are detrimental to the ferromagnetic properties of the
(Ga,Mn)As, and so are undesired.
The temperature below which the transition from paramagnetism to ferromagnetism
occurs is known as the Curie temperature, TC.
Theoretical predictions based on the Zener model suggest that the
Curie temperature scales with the quantity
of manganese, so TC above 300 K is possible if
manganese doping levels as high as 10% can
After its discovery by Ohno et al., 
the highest reported Curie temperatures in (Ga,Mn)As rose from
60 K to 110 K. 
However, despite the predictions of room-temperature ferromagnetism,
no improvements in TC were made for several
As a result of this lack of progress, predictions started to be
made that 110 K was in fact a fundamental limit for (Ga,Mn)As. The
self-compensating nature of the defects would limit the possible hole
concentrations, preventing further gains in
The major breakthrough came from improvements in post-growth
annealing. By using annealing temperatures comparable to the growth
temperature it was possible to pass the 110 K barrier. [13
These improvements have been attributed to the removal the highly
mobile interstitial manganese.
Currently, the highest reported values of TC
in (Ga,Mn)As are around 173 K,[18
] still well below the much sought
room-temperature. As a result, measurements on this material must
be done at cryogenic temperatures, currently precluding any
application outside of the laboratory. Naturally, considerable
effort is being spent in the search for an alternative DMS that does not share this
] In addition to this, as MBE techniques and equipment are refined
and improved it is hoped that greater control over growth
conditions will allow further incremental advances in the Curie
temperature of (Ga,Mn)As.
Regardless of the fact that room-temperature ferromagnetism
has not yet been achieved, DMS materials such as (Ga,Mn)As,
have shown considerable success. Thanks to the rich interplay of
physics inherent to DMSs a variety of novel
phenomena and device structures have been demonstrated. It is
therefore instructive to make a critical review of these main
A key result in DMS technology is gateable ferromagnetism,
where an electric field is used to control the ferromagnetic
properties. This was achieved by Ohno et al.[24
] using an insulating-gate field-effect transistor with
(In,Mn)As as the magnetic channel. The magnetic properties were
inferred from magnetization dependent Hall measurements of the channel. Using the
gate action to either deplete
or accumulate holes in the channel it was possible to
change the characteristic of the Hall response to be either that of a paramagnet or of a
When the temperature of the sample was close to its
TC it was possible to turn the ferromagnetism on
or off by applying a gate voltage which could change
the TC by ±1 K.
A similar (In,Mn)As transistor device was used to provide
further examples of gateable ferromagnetism.
In this experiment the electric field was used to modify the
coercive field at which magnetization reversal occurs. As a result
of the dependence of the magnetic hysteresis on the gate bias the electric field
could be used to assist magnetization reversal or even demagnetize
the ferromagnetic material. The combining of
magnetic and electronic functionality demonstrated by this
experiment is one of the goals of spintronics and may be expected to have a
great technological impact.
Another important spintronic functionality that has been
demonstrated in DMSs is that of spin injection.
This is where the high spin polarization inherent to these
magnetic materials is used to transfer spin polarized carriers into a
In this example, a fully epitaxial heterostructure was
used where spin polarized holes were injected
from a (Ga,Mn)As layer to an (In,Ga)As quantum well where they combine with
unpolarized electrons from an n-type substrate. A
polarization of 8% was measured in the resulting electroluminescence. This is again
of potential technological interest as it shows the possibility
that the spin
states in non-magnetic semiconductors can
be manipulated without the application of a magnetic field.
(Ga,Mn)As offers an excellent material to study domain wall mechanics
because the domains can have a size of the order of 100 µm.
Several studies have been done in which lithographically defined lateral
] or other pinning points[29
] are used to manipulate domain walls. These experiments are crucial
to understanding domain
wall nucleation and propagation which would be necessary for
the creation of complex logic circuits based on domain wall
Many properties of domain walls are still not fully understood
and one particularly outstanding issue is of the magnitude and size
of the resistance associated with current passing through domain walls. Both
values of domain
wall resistance have been reported, leaving this an open area
for future research.
An example of a simple device that utilizes pinned domain walls is
provided by reference .
This experiment consisted of a lithographically defined narrow island
connected to the leads via a pair of nanoconstrictions. While the
device operated in a diffusive regime the constrictions would pin
resulting in a giant magnetoresistance (GMR)
signal. When the device operates in a tunnelling regime another magnetoresistance (MR) effect is
observed, discussed below.
A further interesting property of domain walls is that of current induced domain wall motion.
This reversal is believed to occur as a result of the spin-transfer torque exerted by a
] It was demonstrated in reference [35
] using a lateral (Ga,Mn)As device containing
three regions which had been patterned to have different coercive
fields, allowing the easy formation of a domain wall. The central region was
designed to have the lowest coercivity so that the application of
current pulses could cause the orientation of the magnetization to
be switched. Interestingly, this experiment showed that the current
required to achieve this reversal in (Ga,Mn)As was two orders of
magnitude lower than that of metal systems. It has also been
demonstrated that current-induced magnetization reversal can occur
across a (Ga,Mn)As/GaAs/(Ga,Mn)As vertical tunnel junction.[36
Another novel spintronic effect, which was first observed
in (Ga,Mn)As based tunnel devices, is tunnelling anisotropic
magnetoresistance (TAMR). This effect arises from the intricate
dependence of the tunnelling density of states on the
magnetization, and can result in MRs of several orders of magnitude.
This was demonstrated first in vertical tunnelling structures
and then later in lateral devices.
This has established TAMR as a generic property of ferromagmetic
tunnel structures. Similarly, the dependence of the single electron
charging energy on the magnetization has resulted in the
obersvation of another dramatic MR effect in a (Ga,Mn)As device, the
so-called Coulomb blockade anisotropic
There are many excellent review articles about the properties
and applications of DMSs and (Ga,Mn)As in
particular. If further information is required on the topic, the
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