High-pressure high-temperature solution growth, structural, and superconducting properties of Fe-substituted MgB2 single crystals

Clarifying the impact of Fe doping on the structural and superconducting properties of MgB2 is crucial, considering that iron is commonly used as a sheath material for the fabrication of metal-clad MgB2 wires and tapes. To date the effects of Fe doping have only been investigated in polycrystalline samples, but the obtained results are controversial. Here, we report the successful growth of Mg1-xFexB2 single crystals in a quaternary Mg-Fe-B-N system using the cubic anvil high-pressure and high-temperature technique. The reaction took place in a closed boron nitride crucible at a pressure of 3 GPa and a temperature of 1960 {\deg}C. The grown crystals exhibit plate-like shapes with sizes up to 0.9 x 0.7 x 0.1 mm3. The variation of the critical temperature Tc of Mg1-xFexB2 crystals with Fe content was found to be different from that observed in polycrystalline samples. For a small Fe doping, up to x < 0.03, the behaviour of Tc(x) is similar to that for the crystals with Al and C substitutions, which suggests that Fe is in non-magnetic state. In this doping range, measurements of the temperature-dependent magnetization performed in high magnetic fields exclude spin states other than S = 0 for the Fe ions. However, for x > 0.03, certain crystals start to show a dramatic decrease in Tc, suggesting that Fe might be in a magnetic state. The M-H dependence for these crystals shows significant increase of magnetization with increasing field in low magnetic field, pointing to a weak ferromagnetism. Overall, the availability of Fe substituted MgB2 single crystals exhibiting such peculiar behaviour offers a unique opportunity to investigate the effect of disorder alone on one hand and the influence of magnetic substituent on the superconducting characteristics on the other.