Magnetic anisotropy of ferromagnetic Ga1-xMnxAs1-y Py films with graded composition

We have investigated magnetic anisotropy properties of ferromagnetic semiconductor Ga1-xMnxAs1-yPy films grown by molecular beam epitaxy on GaAs substrates with constant Mn content x of 0.06 and with P content y graded along the growth direction. Two samples were investigated, one with y increasing in the growth direction from 0.03 to 0.24, the other with y decreasing from 0.25 to 0.04. Such grading of phosphorus concentration leads to a continuous variation of strain, and thus of magnetic anisotropy in the film. Although the phosphorus mole fraction in the films was varied by nearly the same amount in both "forward"- and "reverse"-graded samples, their magnetic anisotropy properties are entirely different. Specifically, while the forward-graded specimen can be described in terms of three distinct magnetic layers in which the magnetic easy axes progress from in plane to out of plane as the phosphorus concentration increases, the reverse-graded sample shows only an out-of-plane anisotropy throughout the entire film. This implies that the initial conditions at the nucleation of sample growth determine the magnetic properties of the entire graded Ga1-xMnxAs1-yPy film.