On some peculiarities of the curves sigma versus temperature on alni and alnico alloys

The curves σ = are taken at a very high heating rate (1000°C/min) on samples of Alni, Alnico 5, and Alnico 8 alloys, heat treated so that their equilibrium state should correspond to a temperature of about 600°C and the α\´ phase particles should be much bigger than after a usual heat treatment. The Curie point occurs respectively at 760, 905, and 920°C. As we think we have avoided the diffusion process between α\´ and α phases, the Curie points should be those of the α\´ phase in almost optimal permanent magnet conditions. From that it is possible to argue that in the Alni alloy the α\´ phase is not pure Fe, but also includes Al and Ni; in Alnico 5 and Alnico 8 alloys, on the other hand, the ferromagnetic phase is not only Fe-Co, but also includes Al or Ni or both. In other curves taken at lower rates (300°C/min) on samples with normal morphology we observed a hysteresis of about 20-30°C between the Curie points during heating and cooling, which may be eventually ascribed to the gap between the solution and spinodal decomposition temperatures.