Main investigator: A. Lemaître
Permanent staff: L. Largeau and O. Mauguin
PhD student: T. Niazi
Post-doc: J. Curiale
Magnetic anisotropy in GaMnAs
The major contribution to the magnetic anisotropy in GaMnAs is the biaxial expitaxial strain induced by the lattice mismatch between the magnetic layer and the substrate. For a review see T. Dietl et al. Phys. Rev. B 63, 195205 (2001). On GaAs substrate, GaMnAs layers are under a slight compressive strain resulting in, in most cases, a strong in-plane easy axis (for high TC layers). The strain state of the magnetic layer, and therefore the easy axis orientation, can be modified by changing the lattice parameter of either the substrate or the magnetic layer itself.
The first approach, developed at the very beginning of the GaMnAs story by growing the GaMnAs layer on a metamorphic GaInAs substrate (Ohno's group and others, including us). However, even though we achieved very high film quality, the metamorphic growth is inherently associated to the formation of threading dislocations. These defects act as blocking centers for DW propagation.
Epitaxial growth of GaMn(As,P)
To circumvent this problem we have developed a new alloy, GaMnAsP on GaAs substrate, to tune the lattice mismatch at will, hence the magnetic anisotropy. Growth was performed under the same conditions as for GaMnAs, namely a low growth temperature and a quasi III-V stiochiometry.Fig 1 shows as the incorporation of P in substitution to As ions modifies the strain state of the magnetic layer.
The magnetic properties were investigated by SQUID magnetometry, ferromagnetic resonance and magneto-transport (in collaboration with INSP and UMPhy). For low P concentrations (up to 10 %) GaMnAsP layers show very good properties, comparable to P-free GaMnAs, with low resistivity, TC above 100 K and high magnetization. Fig. 2 shows the strong modification of the magnetization curves vs magnetic field as phosphorous is inserted into the matrix. The magnetic field was applied perpendicularly to the sample plane. Without phosphorous no hysterisis cycle is visible, the out-of-plane axis is a hard axis. A large magnetic field is needed to rotate the magnetization out of the plane. With phorphorous (9%), a clear hysterisis loop is visible, the out-of-plane axis is now an easy axis. Hence P incorporation induces a strong modification of the magnetic anisotropy.
Magnetic domain self-organization
Thanks to the reduced number of defects in GaMnAsP compared to GaMnAs grown on a relaxed InGaAs substrate, magnetic domain self-organization has been evidenced by INSP. Fig. 3 shows two Kerr images obtained at LPS and INSP for both systems after an AC demagnetization. In the case of GaMnAsP, the domain organization is governed by the competition between the domain wall energy and the magnetic energy. In the metamorphic approach, the self-organization is limited by numerous defects, mostly threading dislocation impeding the domain wall motion.