Magnetic Properties and Stability of Cr and Co Added Fe₃B/Nd₂Fe₁₄B Based Exchange Coupled Magnets

Abstract

The effect of co-addition of Cr and Co to Fe₃B/Nd₂Fe₁₄B exchange-spring nanocomposite permanent magnets on the magnetic properties and their stability was investigated. It was found that H_cJ of Fe₃B/Nd₂Fe₁₄B exchange-spring magnets can be controlled in the range from 300 kA/m to 600 kA/m by adjusting the Cr and Nd concentrations. The squareness of the demagnetizion curves is improved by simultaneous addition of Cr and Co. Thermal stability is also improved by Cr addition: irreversible flux losses rate of the 5 at% Cr-doped Fe₃B/Nd₂Fe₁₄B exchange-spring magnets were about 1 % after exposure to a temperature of 100°C. This type of magnets are less susceptible to oxidation because the rare-earth content is lower than that of conventional Nd₂Fe₁₄B isotropic powders. The higher packing density can be obtained because of the round shape of the pulverized Fe₃B/Nd₂Fe₁₄B exchange-spring magnet. Consequently, Fe₃B/Nd₂Fe₁₄B exchange-spring magnet powders are suitable for the injection-molding process in which the magnetic powders are exposed to high temperatures.