Annealing at 750 °C for 5 s ͑without figure͒ yields a low
room temperature coercivity H ͑5 K͒ϭ41 mT→H ͑300 K͒
͓
c
c
ϭ1 mT and remanence M (300 K)ϭ0.07•M (5 K) . By
͔
͓
͔
R
R
increasing the annealing time from 5 to 180 s we were able
to increase the MnAs particle size from 10 to 50 nm and
therefore enhance the coercivity and remanence. Figure 2͑b͒
shows the magnetization curves of a sample annealed at
750 °C for 180 s. An increase of the room temperature coer-
civity by
a
factor of
9
is observed
H ͑5 s͒
͓
c
ϭ1 mT→H ͑180 s͒ϭ9 mT . It is remarkable that the room
͔
c
temperature remanence MR remains 40% of the low tem-
perature value. The conductivity of the LT-GaAs:Mn layer
annealed at 750 °C ͑180 s͒ is 110 ⍀Ϫ1 cmϪ1. The reported
values of the coercivity, remanence, and conductivity make
the MnAs nanosize crystallites imbedded in LT-GaAs pos-
sible candidates for hybrid ferromagnetic semiconductor de-
vices if the diffusion of Mn is adequately controlled or used.
In conclusion we have reported the formation of nano-
size ferromagnetic MnAs crystallites in LT-GaAs using Mnϩ
ion implantation and subsequent heat treatment. We have
investigated the structural and the magnetic properties by
TEM, EDX, and SQUID measurements. The best formation
conditions of the MnAs crystallites in LT-GaAs:Mn are
rapid thermal annealing at 750 °C. The MnAs crystallites
exhibit room temperature ferromagnetism. These nanomag-
net characteristics could open the way to novel hybrid mag-
netic semiconductor structures.
We would like to thank D. D. Awschalom for fruitful
discussions. This work has been supported by QUEST, an
NSF Science and Technology center ͑Grant No.
DMR91.20007͒. P.J.W. is a postdoctoral fellow of the Deut-
sche Forschungsgemeinschaft ͑DFG͒ J.M.G. is a postdoc-
toral fellow of the Spanish Ministry of Education and Sci-
ence.
FIG. 3. Plot of the Curie temperature Tc of LT-GaAs:Mn
(͓Mn͔ϭ1•1016 cmϪ2) vs the annealing temperature ͑Tϭ600–900 °C, 5 s͒.
GaAs layer and into the GaAs buffer layer. We already men-
tioned the presence of some precipitates in the buffer layer
when discussing the TEM image of the sample annealed at
750 °C ͑5 s͒ ͓Fig. 1͑a͔͒. In addition we should point out that
there is evidence for a change of the chemical composition of
the crystallites at higher annealing temperatures which may
cause a decrease of the saturation magnetization too.
In Fig. 3 we have plotted the evolution of the Curie
temperature versus the annealing temperature. The Curie
temperature increases rapidly from Tc͑600 °C͒ϭ80 K to
Tc͑750 °C͒ϭ330 K. We attribute this to the growth of the
precipitates and to the formation of the MnAs phase. Starting
at an annealing temperature of 750 °C the Curie temperature
increases further, but now slowly from Tc͑750 °C͒ϭ330 K to
Tc͑900 °C͒ϭ360 K. This behavior is attributed to a change
of the chemical composition of the precipitates. The EDX
analysis already showed that the sample annealed at 750 °C
contains some precipitates with a small amount of Ga ͑pre-
cipitate B in Table I͒. It is known from Shi et al.1 that the
Curie temperature of MnGa precipitates in GaAs ͓annealed
at 920 °C ͑60 s͔͒ have a value above 400 K. From this we
conclude that in our experiment MnAsnGam alloy precipi-
tates may be formed at annealing temperatures higher than
750 °C. The amount of Ga incorporated is supposed to in-
crease with annealing temperature. The Curie temperatures
of the MnAsnGam particles are expected to be in between the
values of MnAs (Tcϭ318 K) and MnGa (TcϾ400 K).
For device applications of the MnAs nanosize ferromag-
nets imbedded in LT-GaAs, it is crucial to study the room
temperature ͑RT͒ characteristics of the hysteresis loop.
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2534 Appl. Phys. Lett., Vol. 71, No. 17, 27 October 1997 Wellmann et al.
93.180.53.211 On: Wed, 05 Feb 2014 20:36:38