Temperature characteristics of coercivity and remanence for Fe-Nd-B magnets having different compositions and microstructures have been evaluated. Improved performance of a magnet at elevated temperatures can be achieved by increasing its coercivity. Higher Nd content and a proper concentration of boron, with combination of additional elements such as Dy, Co or A1 increase room and elevated temperature coercivities. High coercivity is also decisive for good temperature characteristics of the remanence. Due to the intrinsic nature of the reversible losses, the relative drop of coercivity is greater for magnets having higher initial coercivity which is mainly related to the phenomena in the area of grain boundaries.
1. Introduction
Neodymium -iron -boron magne.ts exhibit excellent magnetic properties at room temperature but their performance at elevated temperatures is inferior to most of the other permanent magnets. Deterioration of the magnetic properties with increasing temperature results from reversible and irreversible losses of magnetisation. The reversible losses are related to temperature dependence of the magnetic moment and disappear when the magnet is cooled down to room temperature. The irreversible losses can originate from two sources such as changes in magnet microstructure and magnetisation reversal in the presence of magnet own demagnetising field. While the former cannot be cancelled, in the latter case the properties can be restored by additional magnetisation.
Maximum operating temperature for Fe-Nd-B magnets hardly exceeds 100 °C. Within this range the temperature coefficients for sintered magnets are usually -0.1%/°C and -0.5 %/°C for remanence and coercivity, respectively. This is to low for many applications, especially for the automotive industry. Much efforts have been done to improve the properties at elevated temperatures. Partial substitution of Co for Fe and Dy for Nd increase Curie temperature and coercivity of magnets thus improving their performance at higher temperatures [1-3]. However, this is accompanyed by some reduction of the saturation magnetisation. For (Fe,Co,Mo,A1)78(Nd,Dy)14.1B7.9 alloy, operating temperature as high as 200 °C has been reported [3]. Improved temperature characteristics have also been reported for magnets containing Mo [3,4] and V [5].