Dependence of the magnetocrystalline anisotropy constant K1 on composition and temperature was studied. A difference of an entire order of magnitude in the maximum K1 value for the slightly different compositions 8.5 wt% Si, 5.0 wt% Al, bal. Fe sample and the 7.5 wt% Si, 6.8 wt% Al, bal. Fe was found, and the difference between their TK1=0 (temperature at which K1 is zero) was a high 300Â°C. Other results showed that the effect of thermal processing varies dramatically with only slight change in composition.