Magnetic viscosity of modified neodymium iron boron magnets with high coercivities

• Published in: Journal of magnetism and magnetic materials Vol. 153; no. 3; pp. 302 - 310
• Main Authors: Jahn, L., Schumann, R., Rodewald, W.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.1996
• ISSN: 0304-8853
• DOI: 10.1016/0304-8853(95)00550-1

In order to examine the long-term stability of novel NdDyFeCoMoAlB magnets with enhanced temperature stability, after-effect measurements have been carried out in an open circuit in the temperature range 20–230°C. In this temperature range the ln( t) law for the irreversible polarization losses is confirmed and the after-effect constant S and the fluctuation field S v decrease monotonically with temperature for the sample annealed at an optimum. It can be proved that the open circuit measurements of S′ and the irreversible susceptibility, χ′ irr = χ′ tot − χ′ rev, should be corrected by the same the demagnetization coefficient N depending factor and thereafter that the S v values can be derived directly by S v = S′ v = S′ χ′ irr. The determined room-temperature values of S v between 8 and 11 kA m −1 are related to the coercivity approximately within the limits of a Barbier plot (log( S v) = a · log( H cJ) with a ≈ 0.5−1) and do not diminish the advantages of the low-temperature coefficient of the coercivity of the NdDyFeCoMoAlB magnets. The experimentally determined ratio S v/ T yields an activation volume V c for the magnetization reversal corresponding to a critical diameter d c ≈ 7–14 nm, which is 1.5–2.5 times the Bloch wall thickness δ.