Abstract
We map the three-dimensional strain heterogeneity within a single core-shell Ni nanoparticle using Bragg coherent diffractive imaging. We report the direct observation of both uniform displacements and strain within the crystalline core Ni region. We identify non-uniform displacements and dislocation morphologies across the core–shell interface, and within the outer shell at the nanoscale. By tracking individual dislocation lines in the outer shell region, and comparing the relative orientation between the Burgers vector and dislocation lines, we identify full and partial dislocations. The full dislocations are consistent with elasticity theory in the vicinity of a dislocation while the partial dislocations deviate from this theory. We utilize atomistic computations and Landau–Lifshitz–Gilbert simulation and density functional theory to confirm the equilibrium shape of the particle and the nature of the (111) displacement field obtained from Bragg coherent diffraction imaging (BCDI) experiments. This displacement field distribution within the core-region of the Ni nanoparticle provides a uniform distribution of magnetization in the core region. We observe that the absence of dislocations within the core-regions correlates with a uniform distribution of magnetization projections. Our findings suggest that the imaging of defects using BCDI could be of significant importance for giant magnetoresistance devices, like hard disk-drive read heads, where the presence of dislocations can affect magnetic domain wall pinning and coercivity.
Highlights
Dislocation glide can be hindered by grain boundaries, precipitates and other dislocations. These dislocations are currently modeled with continuum mechanics methods, which do not explicitly account for the discrete nature of the material and are even not applicable in the core region of the dislocation that leads to singularities in the amplitudes or divergences in the phases of a scattered complex x-ray wave field
Slip traditionally occurs when the resolved shear stress becomes equal to the critical shear stress, σ f, a parameter that depends upon the mesostructure and the mechanical properties of the sample
Bragg coherent diffraction imaging (BCDI) is used to characterize the mesostructure of our Ni sample from coherent X-ray diffraction (CXD)
Summary
Due to their diverse range of applications—from data storage, energy harvesting and conversion to nano-mechanical devices—nanostructures are of enormous interest to academia and industry [1,2,3,4,5,6,7,8]. These dislocations are currently modeled with continuum mechanics methods, which do not explicitly account for the discrete nature of the material and are even not applicable in the core region of the dislocation (traditionally regions of highest distortions) that leads to singularities in the amplitudes or divergences in the phases of a scattered complex x-ray wave field This core region has abrupt and large displacement gradients in the nanostructures. Understanding and studying defects and dislocations across buried interfaces or in individual nanostructures is challenging Traditional characterization techniques such as laboratory X-ray diffraction (XRD) [16,17], electron microscopy: (SEM [17], EF-TEM [18], HR-TEM [16,17,19]) of Ni/NiO with the core/shell morphology have retrieved information about structure of the core and shell incorporating twinned nature [18] and the texturing and dislocation activity of Ni nanoparticles (NPs) [20].
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