The effect of boron purity on electric and magnetic properties of CaB_(6)

摘要

We present the consistent electronic and magnetic properties in high-quality defect-controlled single crystals of CaB_(6), based on temperature-dependent transport and isothermal magnetization. Pure CaB_(6) single crystals, synthesized with 99.9999-pure boron, exhibited fully semiconducting characteristics, such as monotonic resistance for 2-300 K. Boron-related defects formed in CaB_(6) single crystals, synthesized with 99.9-pure boron, induced extra free charge carriers. Remarkably, no ferromagnetic signals were detected from single crystals made with 99.9999-pure boron, regardless of stoichiometry, whereas those made with 99.9-boron exhibited ferromagnetism within a finite range of carrier density. This indicates that the boron-related defects generate both the local magnetic moments associated with the exotic ferromagnetism and extra free charge carriers.