GaMnAs |
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Présentation
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Main investigator: A. Lemaître
Permanent Staff: O. Mauguin, L. Largeau
Elaboration of GaMnAs thin films
Within the first years of this activity, we were able to grow layers
showing magnetic properties close to the best results reported in the
literature. In particular, thin films (50 nm thick) with Curie
temperatures in the range of 170-180 K have been obtained after
post-growth low temperature annealing. These annealing were indeed
shown to be highly beneficial to ferromagnetism due to the
out-diffusion of Mn interstitial
atoms. A remarkable
feature of the GaMnAs ferromagnetism is the dependence of the magnetic
anisotropy on the epitaxial strain. It was proved that compressive
strain (GaMnAs on GaAs) induces an in-plane anisotropy while tensile
strain yields a perpendicular easy axis. However the latter
configuration required the growth on a relaxed InGaAs buffer, which
usually initiates a large density of threading dislocations,
detrimental to domain-wall propagation. Therefore we developed a growth
procedure to minimize the density of these defects, using an Indium
composition gradient. Magnetometry (Fig. 1), magnetotransport, and
polar
magneto-optical Kerr effect measurements revealed the high quality of
this layer, manifested in particular by its high Curie temperature (130
K) and a well-defined magnetic anisotropy. Such an optimization
proved crucial for the successful investigation of domain structures
(see below) or quantum interference effects (see
Nanospintronics and
nanomagnetism action) in GaMnAs thin films.
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| Fig 1.: Hystersis cycle (a) and remanent magnetization
vs temperature (b) of a 50 nm thick GaMnAs layer grown on a metamorphic
InGaAs substrate. The Curie temperature is 130 K. |
Tuning of the magnetic properties of hydrogenated GaMnAs
Ferromagnetism in GaMnAs stems from the exchange interaction between
the holes and the Mn magnetic moments. Hydrogenation is a well-known
technique in p-doped GaAs to passivate electrically the acceptors. We
have thus used hydrogenation as a simple and straightforward way
to control the hole density, hence the ferromagnetic properties at
fixed magnetic moment concentration. Indeed complete passivation of
GaMnAs layers resulted in the suppression of ferromagnetism, leaving a
paramagnetic phase. Moreover, subsequent annealing allowed us to
remove progressively, in a controllable manner, the hydrogen atoms from
the layer and thus to adjust finely the hole density. A series of
samples with different hydrogen concentrations was then investigated.
We observed the ferromagnetic phase recovery with increasing hole
density (Fig. 2) along with strong modifications of the magnetic
anisotropy,
consistent with mean-field theory. This technique provides a simple
tool to investigate in detail the mechanisms governing the magnetic
properties of this compound.
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Fig 2.: Remanent magnetization vs temperature for
samples with decreasing hydrogen content (samples 1 to 3 and reference
without hydrogen).
The Curie temperature decreases (resp. increases) with hydrogen content
(resp. hole density). |
Domain structure and magnetization
reversal in GaMnAs
films
with perpendicular anisotropy
Compared to the well-known magnetic behavior of 3d-metallic films,
there are still open questions on the incidence of the specific nature
of the carrier-induced ferromagnetism in GaMnAs and on the dynamics of
the magnetization reversal and related phenomena, such as nucleation
and domain wall motion. A collaboration with J. Ferré and N.
Vernier (LPS, Orsay) was established in an attempt to answer these
questions. For that purpose we investigated the magnetic properties of
a thin GaMnAs film with perpendicular anisotropy using a relaxed
(In,Ga)As buffer, as described above. We show that magnetization
reversal is initiated from a limited number of nucleation centers and
develops by easy domain-wall propagation. Furthermore, Kerr microscopy
allowed us to characterize in detail the magnetic domain structure
(Fig. 3). In
particular, we showed that the domain shape and wall motion are very
sensitive to some defects (probably threading dislocations), which
prevents the expected periodic arrangement of the domains. In
spite of the profound difference in the origin of magnetism in metallic
and DMS films, micromagnetism and magnetization reversal dynamics
observed under quasi-static conditions show obvious similarities.
Nevertheless, we may expect different behaviors for ultra-fast dynamics.
 |
Fig 3.: Magnetization reversal in a GaMnAs thin film
(50 nm) with perpendicular anisotropy, visualized by Kerr microscopy
(2.1 mm x2.7 mm).
Black and white regions are domains with opposite magnetization (up and
down). The movie is a series of snapshots acquired after 5s magnetic
field pulses of 12 Oe. |
Collaborations
- J. von Bardeleben, C.
Gourdon, L. Thevenard, C. Testelin,
E. Peronne at INSP
- V. Jeudy, A. Thiaville and
J. Ferré at LPS
- J.-M. George and H. Jaffrès
at UMPhy
Selected references
Spectra broadening of point-contact Andreev reflection
measurement on GaMnAs, T. W. Chiang, Y. H. Chiu, S. Y. Huang, S. F.
Lee, J. J. Liang, H. Jaffres, J. M. George, and A. Lemaître, J. Appl.
Phys. 105, 07C507 (2009)
Macrospin behavior and superparamagnetism in (Ga,Mn)As
nanodots, J.-P. Adam, S. Rohart, J. Ferré, A. Mougin, N. Vernier, L.
Thevenard, A. Lemaître, G. Faini, and F. Glas, Phys. Rev. B 80, 155313
(2009)
Anisotropic magnetization relaxation in ferromagnetic
Ga1-xMnxAs thin films, Kh. Khazen, H. J. von Bardeleben, M. Cubukcu, J.
L. Cantin, V. Novak, K. Olejnik, M. Cukr, L. Thevenard, and A.
Lemaître, Phys. Rev. B 78, 195210 (2008)
Ferromagnetic resonance of GaMnAs thin films with constant Mn
and variable free-hole concentrations », Kh. Khazen, H. J. von
Bardeleben, J. L. Cantin, L. Thevenard, L. Largeau, O. Mauguin, and A.
Lemaître, Phys. Rev. B 77, 165204 (2008)
Determination of the micromagnetic parameters in (Ga,Mn)As
using domain theory, C. Gourdon, A. Dourlat, V. Jeudy, K. Khazen, H. J.
von Bardeleben, L. Thevenard, and A. Lemaître, Phys. Rev. B 76, 241301
(2007)
Magnetic patterning of (Ga,Mn)As by hydrogen passivation, L.
Thevenard, A. Miard, L. Vila, G. Faini, A. Lemaître, N. Vernier, J.
Ferré, and S. Fusil, Appl. Phys. Lett. 91, 142511 (2007)
Evolution of the magnetic anisotropy with carrier density in
hydrogenated GaMnAs, L. Thevenard, L. Largeau, O. Mauguin, A. Lemaître,
K Khazen, H. J. Von Bardeleben, Phys. Rev. B 75, 195218 (2007)
Domain structure and magnetic anisotropy fluctuations in
(Ga,Mn)As: Effect of annealing , A Dourlat, V. Jeudy, C. Testelin, F.
Bernardot, K Khazen, C Gourdon, L. Thevenard, L. Largeau, O. Mauguin,
A. Lemaître, J. Appl. Phys. 102, 023913 (2007)
Magnetic properties and domain structure of (Ga,Mn)As films
with perpendicular anisotropy , L. Thevenard, L. Largeau, O. Mauguin,
G. Patriarche, A. Lemaître, N. Vernier, J. Ferré, Phys. Rev. B 73,
195331 (2006)
Tuning the ferromagnetic properties of hydrogenated GaMnAs,
L. Thevenard, L. Largeau, O. Mauguin, A. Lemaître, B. Theys, Appl.
Phys. Lett. 87, 182506 (2005)
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