Effect of Non-Local Growth Dynamics on Magnetic Properties of Composition-Graded Ga1-xMnxAs1-yPy Ferromagnetic Films

We discuss the magnetic properties of Ga1-xMnxAs1-yPy films with phosphorus content y graded along the growth direction from y = 0.03 to y approximate to 0:25. Such grading was achieved by growing the film epitaxially in the form of sublayers with successively increasing values of y. X-ray measurements reveal that the strain in the film changes coherently with increasing phosphorus content, from in-plane compressive at the bottom to in-plane tensile in the upper sublayers of the graded film. The grading of strain arising from the changing composition results in gradual changes of magnetic anisotropy from in-plane in the lower sublayers to out-of-plane as we progress toward the top. Magnetization measurements reveal that the graded film is composed of three regions with different magnetic anisotropies: one with a strong in-plane easy axis, one with a strong out-of-plane easy axis, and in between a region with both in-plane and out-of-plane easy axes. The contribution of the region with an in-plane easy axis to the total magnetization of the film was found to be strongly dominant. This result is quite unexpected, because approximate to 75% of the graded film consists of sublayers under tensile strain, where out-of-plane magnetic anisotropy is expected to dominate. These surprising results arise from effects occurring in epitaxial growth of such graded structures, where properties of a layer being deposited are determined not only by growth conditions at the moment of deposition but also by the presence of layers grown earlier and by deposition of additional layers grown later. This form of growth can be explained in terms of band structure at different locations of the multilayer as the growth proceeds, and we will therefore refer to it as non-local growth.