In this study, we in situ employed low-temperature x-ray diffraction to investigate phase separation in GaN nanowires. Our observations showed that a distinct phase separation developed below 260K, the zinc-blende phase, which is related to short range ordering. The correlation lengths of the zinc-blende phase reached their maximum at 140K but correlation length was still revealed at around 23nm. Our results may be understood using the finite size model and support the conclusion that the phase separation was reversible and occurred through the interaction of the characteristic size of the ordered domain of the GaN nanowires.

We report on the formation and spontaneous self-organization of Cu(2)O/CuO core-shell nanowires from individual copper nanoparticles. The growth process is interpreted using the results of time-dependent in situ x-ray diffraction. High-resolution transmission electron microscopy is used to observe the intermediate state of pearl-necklace-like aggregates that form a chain-like configuration of Cu(2)O nanoparticles intertwined into nanowires. The existence of an amorphous CuO shell is confirmed by the XANES technique and explained through an intensity simulation using a proposed core-shell nanowire model.

We review the phenomenology of the exchange bias and its related effects in core–shell nanocrystals. The static and dynamic properties of the magnetization for ferromagnetic Ni-core and antiferromagnetic NiO-shell cluster glassy nanoparticles are examined, along with the pinning–depinning process, through the measurement of the conventional exchange bias, and associated with different cooling fields and particle sizes. Two significant indexes for the dipolar interaction n and multi-anisotropic barrier β derived from the dynamic magnetization are proposed, which provide a unified picture of the exchange bias mechanism and insight into the influence of the cooling field.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-0925-0) contains supplementary material, which is available to authorized users.

A study of “proteresis (inverted hysteresis)” in core-shell, Cu2O/CuO nanoparticles, is presented. Crystal and characteristic sizes are determined using the x-ray absorption near-edge structure method for the weak ferromagnetic core (Cu2O) and antiferromagnetic shell (CuO) nanoparticles. A core-shell anisotropic energy model is established to describe the observed proteretic behavior in Cu2O/CuO core-shell nanocrystals. The proteresis loop triggered by the applied magnetic field can be tentatively attributed to core-shell exchange coupling induced by the surface of the Cu2O core, hinting at a possible way to tune the strength of the exchange inter-coupling energy that can control the dynamic of proteresis and hysteresis in a core-shell system.

To study the origin of negative thermal expansion effects near the superconducting transition temperature TC in MgB2, low-temperature high-energy synchrotron radiation x-ray diffraction was used to probe the charge redistribution near the boron atoms. Our results reveal that the in-plane hole-distribution of B− hops through the direct orbital overlap of Mg2+ along the c-axis at 50 K and is re-distributed out-of-plane. This study shows that the out-of-plane π-hole distribution plays a dominant role in the possible origin of superconductivity and negative thermal effects in MgB2.