Temperature dependence of the coercivity and the anisotropy of Nd-Fe-B based magnets

The temperature dependence of the coercivity
x
H
c
and the anisotropy field H
A
from Nd-Fe-B based sintered permanent magnets was measured from 100K up to the Curie temperature. A Pr
18
Fe
77
B
8
magnet and a series of the compositions Nd
14.8
Dy
1.8
(Fe
77.8-x
Co
x
)B
7
(x=0,10,20) were studied in order to achieve improvements of the temperature characteristic above room temperature. The Co substitution increases naturally the Curie temperature but it chances H
A
at room temperature only slightly. The coercivity however is reduced so drastically, that the temperature behaviour of the sample with x=20 is comparabel to that of a pure Nd-Fe-B magnet. The best results in this respect were achieved with a heavy rare earth substituted material (x=0) and with the Pr
18
Fe
77
B
8
sample. Additionally it was tried to correlate
x
H
c
(T) and H
A
(T) using two different models in order to get information about the coercivity mechanism. The results thus obtained are in general agreement with the usual picture of the coercivity mechanism valid in Nd-Fe-B based magnets.