Nanoscale Mapping of Heterogeneous Strain and Defects in Individual Magnetic Nanocrystals*

Coherent X-ray Diffractive Imaging (CDI), with refraction and elemental sensitivities, is one of the strongest contenders for investigating internal structures (both atomic density and phases) of nanocrystalline materials. The traditional high-resolution imaging techniques such as Transmission Electron Microscopy (TEM) require slicing the specimens to probing very thin cross-sections of the materials. Any internal stresses could possibly be removed when samples are cross-sectioned to produce thin lamellar sections. On the contrary, with CDI, we can probe the entire samples' three-dimensional (3D) internal structure with the native states of the specimens mostly preserved. CDI utilizes advanced iterative reconstruction algorithms in which solutions are to be iterated between real-space and reciprocal-space by forward Fourier transform and reverse Fourier transform, thus retrieving the converged solutions of real-space complex-valued sample wavefunction that satisfy the imposed constraints in both real and reciprocal-spaces. We performed studies on a single 240nm wide Ni/NiO nanowire of a core-shell morphology using Bragg CDI. We utilize the theory of dislocations to compare an individual dislocation obtained from Bragg CDI with theory, with the overall region of the reconstructed core–shell structure shown. The observed symmetrical (and inverse radial distance from dislocation line) decay of the displacement field is consistent with the theory of elasticity.

In conclusion, Bragg CDI is ideal for investigating single crystalline samples in conjunction with in-situ and operando capabilities in the current synchrotron radiation facilities worldwide. We envisage significant advancements in the fundamental and applied nanomaterials research using Bragg CDI shortly.