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.
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.
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.