Strain and phase analysis of magnetic nanocrystals in GaδFeN/Al(x)Ga(1-x)N heterostructures
Phase-separated GaδFeN containing ordered arrays of embedded superparamagnetic γ’-Ga(y)Fe(4−y)N nanocrystals (NCs) is expected to be a suitable material system for achieving spintronic devices, as the properties of the NCs can be tuned through the fabrication conditions. In this work, we investigate the effect of strain, induced by adding Al into the GaN buffer, on the structural and magnetic properties of these Fe-rich nanocrystals. The investigated sample series consists of 60 nm GaδFeN layers on top of 1 µm Al(x)Ga(1-x)N buffers with different Al concentrations in the range of 0% < x < 41%, which were grown by metalorganic vapor phase epitaxy on c-plane  Al2O3. Through the addition of Al into the buffer layer, the formation of two different ferromagnetic Fe(y)N phases, ε-Fe3N and γ’-Ga(y)Fe(4−y)N, is promoted.
Conventional TEM micrographs show that the dislocation density increases with the Al concentration, which directly influences the distribution and shape of the embedded nanocrystals. The superposition of the ε -Fe3N and γ’-Ga(y)Fe(4−y)N lattices with the strained GaδFeN matrix result in complex Moiré patterns in high-resolution micrographs. By evaluating these Moiré patterns, we could distinguish between the different NC phases in an efficient way. Using the TVIPS universal scan generator, nanobeam precession electron diffraction (PED) patterns were recorded on a fast 4k × 4k CMOS camera while scanning the beam with a spot size of 2 nm over the area containing a NC. From these PED mappings, strain maps were determined by extracting the positions of diffraction discs automatically by image processing, and subsequently differentiating between reflexes originating from the GaδFeN matrix and the Fe(y)N nanocrystals. This way, we detected highly localized, in-plane strain fields originating from the embedded NCs.
From magnetometry, a perpendicular magnetic anisotropy was confirmed in all samples with Al-containing buffers: the easy axis orientation changed from in-plane for x = 0% to out-of-plane for x > 5%. The crystallographic orientation and the distribution of the two phases in the GaN matrix point at the formation of hexagonal ε-Fe3N NCs elongated along the growth direction as the origin for the observed magnetic anisotropy. These findings open wide perspectives for the implementation of this material in spintronic devices.
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