J Supercond Nov Magn DOI 10.1007/s10948-016-3780-z ORIGINAL PAPER The Effect of Fe Diffusion on Some Physical and Superconducting Properties of MgB 2 Asaf Tolga Ulgen 1 · Ibrahim Belenli 1 Received: 24 May 2016 / Accepted: 26 August 2016 © Springer Science+Business Media New York 2016 Abstract The iron (Fe) diffusion in superconducting MgB 2 bulk samples has been studied over the temperature range of 650–900 ◦ C for 1 h. Fe coating on bulk polycrstalline super- conducting MgB 2 samples was performed in two ways, i.e., on pressed pellets without sintering (set 2 ) and on pressed and sintered pellets (set 1 ). For both sets, a 50 μm thick Fe layer was coated on MgB 2 by evaporation in vac- uum. Effects of Fe diffusion on the crystal structure and superconducting properties of MgB 2 have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and resistivity measurements. Fe diffusion coeffi- cients were determined from lattice parameter c and room temperature resistivity values. The temperature dependence of the Fe diffusion coefficient in this temperature range is described by the Arrhenius relation. It has been found that the Fe diffusion coefficient increases with increasing sin- tering temperature, as expected. The plausible explanations for the observed improvement in microstructure and super- conducting properties of the samples due to Fe diffusion are discussed. Keywords MgB 2 · Fe diffusion · Diffusion coefficient and activation energy  Asaf Tolga Ulgen ulgen at@ibu.edu.tr 1 Department of Physics, Faculty of Arts and Science, Abant Izzet Baysal University, 14030 Bolu, Turkey 1 Introduction The discovery of superconductivity in MgB 2 [1] have aroused spacious interest in the applied superconductor researchers. The practical applications of MgB 2 have strong potential due to its distinctive properties, such as being low cost, low mass density, simple chemical composition, and high critical current density (J c ). It also has relatively higher critical temperature (T c ) compared to other binary super- conductors. Bulk, wire, and film forms of superconducting MgB 2 samples were intensely investigated in recent years. The powder in tube method is generally used to fabricate MgB 2 wires in various metal tubes including soft iron [2, 3]. When Fe or stainless steel sheathing material comes into contact with superconducting MgB 2 core, iron diffusion becomes an important issue. Since iron diffusion rate into MgB 2 is a temperature-dependent phenomena, it will be an important parameter for sintering of MgB 2 wires. The Fe doping has been reported to decrease critical temperature with the increasing dopant concentration [4, 5]. Although, Jin et al. have studied the effect of Fe doping on MgB 2 wires and found that 5 wt% Fe doping has a slight negative effect on the J c values but no effect on T c [6]. Diffusion of some metals in high-temperature supercon- ductors has been studied and reported in the literature with detailed calculations of diffusion coefficient and activa- tion energy values [7–11]. Also, the literature contains few papers concerning the diffusion in MgB 2 [12–14]. The vac- uum evaporation method is ascendant of the others as a consequence of the simple reproducible operation in diffu- sion studies and the controlled doping at low concentrations [15, 16]. This method of coating also ensures that the grains of MgB 2 at the surface are coated all around.