High-Magnetic-Field Annealing and Coercivity in Sintered Nd-Dy-Fe-B-Type Magnets

Abstract

The effect of a high magnetic field on coercivity was investigated in the annealing process for sintered Nd-Fe-B magnets containing Dy 10 wt.%. In a sample containing Cu 0.13 wt.%, a coercivity value H c of 36 kOe was obtained for an annealing temperature T a of 475˚C under an annealing field H a of 140 kOe. This value of H c is 20% larger than that for the control sample (T a = 475˚C and H a = 0). In samples without Cu, H c values were considerably smaller (< 27 kOe) and no such enhancement of the coercivity by the magnetic fields was observed. The DSC curves exhibited a marked endothermic reaction just below 500˚C only for samples containing Cu. This temperature is attributed to a eutectic point of Nd 70 Cu 30. A possibility of field-induced alignment of Nd-rich grains in a matrix of Nd-Cu liquid is discussed; this would be closely related to the coercivity enhancement.

Effect of high magnetic field on the coercivity in sintered Nd-Fe-B magnets

“…By applying a similar consideration given above, the Al-Cu liquid phase would promote the magnetic orientation of Nd-rich phase for the T a ¼ 550 C sample. This result is consistent with our recent work [2] on a series of 10 wt% Dy samples, in which a large FCR behavior was observed at T a ¼ 550 C only when the sample contains both Al and Cu. …”

“…It is well-known that a heat treatment around 600 C is indispensable to obtain a high coercivity in the sintered Nd-Fe-B magnets. We have been investigating an effect of high magnetic field in this annealing process on the magnetic properties [1,2]. The characteristic value of this experiment is the Fieldenhanced Coercivity Ratio (FCR), which is defined to be a difference of H c with and without annealing field, divided by an original H c value.…”

“…Carnasciali et al [3] reported that there exists an eutectic point at about 500 C in a Nd-Cu binary phase diagram with a composition of Nd 70 Cu 30 . We have so far estimated a magnetic energy [1,2] which drives an alignment of Nd-rich grains against thermal agitation k B T. If there is no disturbance, a Dy-free Nd-rich grain with a diameter larger than about 10 nm can be oriented along the 140 kOe magnetic field [2]. Thus, we may attribute the large FCR observed for the T a ¼ 500 C sample to an orientation of Nd-rich grains in Nd-Cu liquid phase which would make a better lattice matching in the interface with the main Nd 2 Fe 14 B phase.…”

Enhancement of coercivity in Al and Cu added Nd-Fe-B sintered magnets by high feld annealing

“…We first reported an enhancement of Hc of 37% in Dy 3.1% substituted magnets by 140 kOe field annealing [9]. Such a magnetic field effect was found to be sensitive to the presence of small additives such as Al and Cu and to be restricted to the very narrow range of the annealing temperature [10,11]. In this paper, we report on the detail of annealing temperature dependence of the magnetic field effects in Dy-free Nd-Fe-B magnets and their high temperature magnetic properties of the H c enhanced samples.…”