Study On Coercivity Mechanism And Thermal Demagnetization Mechanism Of Hot Deformed NdFeB Magnets

NdFeB magnet has excellent magnetic properties as the third generation rare earth permanent magnetic material.Its theoretical magnetic energy is up to 64 MGOe and the actual one is as high as 59 MGOe.However,as another important index to measure the magnetic properties of magnets,the actual coercivity of NdFeB magnets is much smaller than the theoretical value,and the intrinsic coercivity field is only 20% of its magnetocrystalline anisotropy field.The grain boundary diffusion process(GBDP)is one of the ways of heat treatment of magnets.By changing the microstructure and composition of the magnets,the magnetic separation between the grains is realized,so that the exchange coupling between the grains can be weakened and the coercivity of the magnets can be effectively improved,but there is still a great gap compared with the theoretical coercivity.The Curie temperature of NdFeB magnets is 320 ?,which is much lower than that of SmCo magnets and the temperature stability is poor,which limits its application at high temperature.Thus,the study on the coercivity mechanism and thermal demagnetization mechanism of NdFeB magnets can provide theoretical guidance for further improving the coercivity of magnets or the temperature stability of coercivity.In this paper,the magnetic properties,microstructure and magnetic domain structure of the hot-deformed NdFeB magnet and Nd-Cu diffusion magnet are analyzed and characterized by Physics Property Measurement System and Transmission Electron Microscope.Besides,the coercivity mechanism and thermal demagnetization mechanism are explored.The magnetization and demagnetization process of the hot-deformed NdFeB magnet and the NdCu diffusion magnet are observed by in-situ experiments,and the influence of the magnetic field perpendicular to the plane of the sample in the in-situ experiment on the domain wall energy and the motion of the magnetic domain is analyzed.The in-situ thermal demagnetization of hot deformed NdFeB magnet,Nd-Cu diffusion magnet and sintered NdFeB magnets were studied by using high temperature specimen holders.The main conclusions are as follows:(1)The coercivity is increased from 1.133 MA/m to 1.699 MA/m and the remanence decreased slightly of the hot deformed magnets after the grain boundary diffusion of the low melting point Nd-Cu alloy.The microstructure,domain structure and element distribution indicate that the contents of non-magnetic phase Nd and Cu at the grain boundary increase obviously.(2)During the initial magnetization,both the hot-deformed magnets and Nd-Cu diffusion magnets exhibit some grain boundary pinning behavior,and the pinning effect of the diffusion magnets is stronger.In the demagnetization process,the domain walls of the deformed magnets are continuous and parallel to the easy axes and the demagnetization is realized by the displacement of domain wall.The demagnetization process of NdCu-diffused sample is realized by the magnetization reversal of single grains or grain aggregations.(3)The magnetic field component perpendicular to the sample plane can significantly reduce the domain wall energy and the energy barrier between the grain boundary and the grain,thus weakening the pinning of the domain wall in the grain boundary,resulting in the displacement of the domain wall in a coercive field far smaller than that of the bulk magnet.(4)In in-situ thermal demagnetization experiments,the contrast of domain walls decreases and the domain width decreases with the increase of temperature;this is due to the decrease of magnetocrystalline anisotropy and spontaneous magnetization with the increase of temperature and a sudden aggravation near the Curie temperature.A molten substance appears on the surface of the sample after heating,which may be due to some phase transition of the sample or the precipitation of the low melting point substance in the sample at high temperature.(5)In a single grain with multiple domains,the structure of the domain has changed greatly when the sample is heated to the Curie point,which is because the domain wall energy is uniformly distributed in the single grain.Therefore,the nucleation of magnetic domains is random after de-tempering.The thermal demagnetization of the lamellar domain can result in a closed domain,which is located at the edge of the sample to seal the two ends of the main domain,thus avoiding the appearance of the magnetic pole,thus reducing the demagnetization of the sample.