Novel synthesis of superparamagnetic Ni–Co–B nanoparticles and their effect on superconductor properties of MgB 2 Mislav Mustapic ´ a,⇑ , Josip Horvat a , Md Shariar Hossain a , Ziqi Sun a ,Z ˇ eljko Skoko b , David R.G. Mitchell a , Shi Xue Dou a a Institute for Superconducting and Electronic Materials, AIIM, University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia b Department of Physics, Faculty of Science, University of Zagreb, Bijenic ˇka c. 32, 10000 Zagreb, Croatia Received 12 December 2013; received in revised form 24 February 2014; accepted 24 February 2014 Available online 31 March 2014 Abstract A new procedure for the preparation of amorphous Ni–Co–B nanoparticles is reported, with a detailed investigation of their morphology by X-ray diffraction and transmission electron microscopy, as well as their magnetic properties. Many factors, such as chemical composition, anisotropy, size and shape of the particles, were controlled through chemical synthesis, resulting in the control of morphological and magnetic properties of the nanoparticles. Controlling pH values with ethylenediamine and using sodium dodecyl sulfate surfactant lowered the size of the nanoparticles to below 10 nm. Such a small structure and chemical disorder in nanocrystalline materials lead to magnetic properties that are different from those in their bulk-sized counterparts. The obtained nanoparticles can be used for different purposes, from pharmaceutical applications to implementations in different materials technology. The focus of this research is the synthesis of Ni–Co–B nanoparticles in a new way and studying the reaction of Ni–Co–B nanoparticles with Mg and B precursors and their effect on MgB 2 properties. New nanostructures are formed in the reaction of Ni–Co–B nanoparticles with Mg: Mg 2 Ni, Co 2 Mg and possibly Mg 2 Co. Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Chemical synthesis; Nanocrystalline nanostructure; Magnesium diboride; Differential thermal analysis; X-ray diffraction 1. Introduction Ni–Co–B nanoparticles have been widely prepared by the method of chemical reduction of metallic salts, as reported by several groups [1–5]. Ni–Co–B nanoparticles have been mostly used as catalysts for hydrogenation reaction and dechlorination [2]. The exact preparation pro- cedure for the nanoparticles determines the nanoparticle size, structure and their physico-chemical properties. Another potential use of Ni–Co–B nanoparticles is the enhancement of the superconducting properties of a MgB 2 superconductor. Incorporating Ni–Co–B nanoparticles of 10 nm size into the MgB 2 superconductor is expected to improve its vortex pinning. This is because the coherence length of MgB 2 is of the order of 10 nm. The magnetic moment of these nanoparticles is also deemed useful for further improvement of superconducting properties of MgB 2 . However, the interaction between the magnetic nanoparticles can result in their agglomeration, preventing their homogenous incorporation into MgB 2 matrix. Making Ni–Co–B nanoparticles of small enough size and without agglomeration has been a challenge. Electron d-band occupancy in Ni–Co–B and strong electronegativity may lead to applications of Ni–Co–B nanoparticles in organic chemistry synthesis as well as in pharmacy. The great advantage of Ni–Co–B is its low cost of starting materials, as compared to palladium [6–8], http://dx.doi.org/10.1016/j.actamat.2014.02.040 1359-6454/Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: mislav.mustapic@gmail.com (M. Mustapic ´). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com ScienceDirect Acta Materialia 70 (2014) 298–306