Atomic level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials 1,2. Their effects range from increased chemical reactivity 3,4 to enhanced mechanical properties 5,6. Dislocations have been widely studied using traditional techniques such as X-ray diffraction and optical imaging. Recent advances have enabled atomic force microscopy to study single dislocations 7 in two-dimensions (2D), while transmission electron microscopy (TEM) can now visualise strain fields in three-dimensions (3D) with near atomic resolution 8–10. However, these techniques cannot offer 3D imaging of the formation or movement of dislocations during dynamic processes. Here, we describe how Bragg Coherent Diffraction Imaging (BCDI) 11,12 can be used to visualize in 3D, the entire network of dislocations present within an individual calcite crystal during repeated growth and dissolution cycles. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The nanoscale magnetic structure of FeRh epilayers has been studied by polarized neutron reflectometry. Epitaxial films with a nominal thickness of 500 Å were grown on MgO ͑001͒ substrates via molecular-beam epitaxy and capped with 20 Å of MgO. The FeRh films show a clear transition from the antiferromagnetic ͑AF͒ state to the ferromagnetic ͑FM͒ state with increasing temperature. Surprisingly the films possess a FM moment even at a temperature 80 K below the AF-FM transition temperature of the film. We have quantified the magnitude and spatial extent of this FM moment, which is confined to within ϳ60-80 Å of the FeRh near the top and bottom interfaces. These interfacial FM layers account for the unusual effects previously observed in films with thickness Ͻ100 Å. Given the delicate energy balance between the AF and FM ground states we suggest a metastable FM state resides near to the interface within an AF matrix. The length scale over which the FM region resides is consistent with the strained regions of the film.

Magnetism-the spontaneous alignment of atomic moments in a material-is driven by quantum mechanical exchange interactions that operate over interatomic distances. Some magnetic interactions cause 1,2 , or are caused by 3,4 , a twisting of arrangements of atoms. This can lead to the magnetoelectric e ect, predicted to play a prominent role in future technology, and to the phenomenon of weak ferromagnetism, governed by the so-called Dzyaloshinskii-Moriya interaction 5-8 . Here we determine the sign of the latter interaction in iron borate (FeBO 3 ) by using synchrotron radiation. We present a novel experimental technique based on the interference between two X-ray scattering processes, where one acts as a reference wave. Our experimental results are validated by state-ofthe-art ab initio calculations. Together, our experimental and theoretical approaches are expected to open up new possibilities for exploring, modelling and exploiting novel magnetic and magnetoelectric materials.There is considerable mystery behind the origins of complicated structures. Although the dominant short-range interactions that allow the building blocks to grow are well understood, the much more subtle forces that lead to a particular twisting at larger lengthscales, such as chiral biological molecules and liquid crystals 9 , and canted magnetic systems 3 , remain subjects of topical debate. In this Letter we seek to address this question for the case of magnetism. Our main findings are twofold: first, we demonstrate a novel and elegant experimental method for exploring magnetic materials with weak relativistic spin-orbit interactions, and second, we present a state-of-the-art quantum-mechanical many-body approach to the detailed description of such interactions in crystals. As a touchstone example we selected crystalline iron borate (FeBO 3 ), which is a strongly correlated electron system with a relatively simple crystal structure, nonetheless allowing a nontrivial canted and locally twisted magnetic ordering pattern. Taken together, these two strands demonstrate that modern condensed matter theory is capable of determining the elusive sign of the Dzyaloshinskii-Moriya interaction, and is thus able to elucidate the mechanism for coupling electric and magnetic degrees of freedom in magnetoelectric multiferroics, and to begin to predict the properties of this important class of materials.The interactions between atomic magnetic moments (or spins) are not direct, but mediated by the intervening matter. Coupling can be diminished through screening 10 , or enhanced, for example, by superexchange via oxygen atoms 11 . Moreover, the coupling is a property of the material and, according to Neumann's principle, must therefore possess all of its symmetries. The most general form of the bilinear coupling energy between two spins contains a scalar (isotropic) exchange term, exchange anisotropy (which we will neglect for the present discussion) and an antisymmetric term that reverses with permutation of the spin indices. The latter is the Dzyaloshi...

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The chemical and magnetic structures of the series of compounds Ca 2−x La x RuO 4 [x = 0, 0.05(1), 0.07(1), 0.12(1)] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low-temperature S-P bca space group of the parent compound is retained in all insulating samples [x 0.07(1)], but with significant changes to the atomic positions within the unit cell. These changes can be characterized in terms of the local RuO 6 octahedral coordination: with increasing doping, the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasitetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic k = (0,0,0), b-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of x ≈ 0.11. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favoring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the d 4 Mott insulator Ca 2 RuO 4 , as well as with regard to theoretical studies on the evolution of its electronic and magnetic structure. In particular, our results rule out the presence of a proposed ferromagnetic phase, and suggest that the structural effects associated with La substitution play an important role in the physics of the system.