Microstructure and magnetic properties of melt-spun Sm 2Fe 15− xCo xCr 2C 2 ( x=0–4) nanocomposite magnets

Abstract

The amorphous formation ability, crystallization behavior, microstructure and magnetic properties of melt-spun and heat treated Sm 2Fe 15Cr 2C 2 magnets have been systematically studied using XRD, DSC, TEM and magnetic measurements. It has been found that the as-spun ribbons are comprised of a mixture of α-Fe, a small amount of 2 : 17 phase and an amorphous phase when the wheel speed is 17 m/s and become pure amorphous when the wheel speed is increased up to 61 m/s. The coercivity is dramatically increased by subsequent annealing. A higher annealing temperature combined with a shorter annealing time leads to a higher coercivity and annealing at 900°C for 1 min was found to give the highest coercivity of 12.1 kOe. XRD and TEM studies reveal that the microstructure of the annealed ribbon is composed of 2 : 17 carbide as the majority phase and a small amount of α-Fe as the minority phase. The average grain size at the optimum annealing was found to be about 35 nm for the 2 : 17 and 60 nm for α-Fe. It is further found that a small amount of Co substitution for Fe not only increases the Curie temperature of the 2 : 17 carbides, but also significantly reduces the grain size of α-Fe to about 20 nm, leading to a much smoother hysteresis loop and a better temperature dependence of coercivity.