Preparation of High-Purity Irons and their Electrical Resistivity at Low Temperature

Abstract

Three sources of iron have been purified by floating-zone refining, the dry H₂ treatment, the wet H₂ treatment and the vacuum treatment, and the effects of purification have been discussed. Using the iron specimens with different purities, the effects of various measuring conditions on the electrical resistivity and the longitudinal magnetoresistance effect up to a field of 1000 Oe have been investigated at 4.2°K. The conditions of electrical resistivity measurements which are suitable for specifying the purity of iron have also been considered. The results obtained are summarized as follows.
(1) High-purity iron with the resistivity ratio RRR{=ρ(298°K)⁄ρ(4.2°K)} of 465 [RRR_min{=ρ(298°K)⁄ρ_min(4.2°K)}=2200, Δρ⁄ρ=−84%] has been obtained from the source of iron with the least metallic impurities (Johnson-Matthey iron) by the wet H₂ treatment (30%H₂O-H₂) after floating-zone refining.
(2) The electrical resistivity of purer iron is more influenced by the measuring current.
(3) The longitudinal magnetoresistance of pure iron reaches the minimum value (ρ_min) at an applied magnetic field of 700∼800 Oe. The value of ρ_min is independent of the measuring current density in the range of 50 to 150 A/cm² and is not influenced by the condition of demagnetization and the earth’s magnetic field. RRR_min obtained with ρ_min shows more clearly the difference in the purity of iron. The modified resistivity ratio RRR_min can be used as an appropriate index of purity for iron.