Effect Of Dy Additive On Thermal Stability And Magnetic Domains Of NdFeB Magnet

Due to the excellent cost performance of the sintered Nd-Fe-B permanent magnets,which is called Magneto, there was a rapid development in the past30years. However,few studies on the relationship between intrinsic coercivity, anisotropy and magneticdomain have been reported.In the present work, the sintered Nd-Fe-B samples with nominal compositionNd_33.8-xDy_xFe_64.7B_1.0Al_0.5have been prepared. The magnetic properties of the sinteredNd-Fe-B magnets with different amounts of Dy were measured using NIM-200Ctemperature tester. The temperature coefficient of the intrinsic coercivityβ has beendetermined. The magnetization curves in easy and hard magnetization axes of thesamples have been measured at some temperatures from20°C to180°C using theNIM-200C magnetometer. Then the anisotropy field and the anisotropy field constantwere obtained by extrapolating the easy and hard magnetization curves. The comparisonand analysis of the intrinsic coercivity and anisotropy field for the sintered Nd-Fe-Bmagnets were carried out. The relationship between the intrinsic coercivity and theanisotropy field for the sintered Nd-Fe-B magnets was found to beH_A0².4=2.702+0.3831x_Hcj（x=1.22¹.57）, when the content of Dy is smaller than2.4%.The of same change tendency of the intrinsic coercivity and the anisotropy field wasobserved at different temperatures in the range from20to180°C. However, theanisotropy field of the sintered Nd-Fe-B magnets increases with an increase of Dy forx>2.8%, fitted by the function H_A2.8³.2=16.498-7.872x_Hcj（x=1.57¹.42）. Therelationship between the intrinsic coercivity serving as a structure-sensitive quantity andthe anisotropy constant acting as structure-unsensitive quantity was reported.We also studied magnetic domain in static and dynamic states （including the mag-netizing and demagnetizing processes） for the sintered Nd-Fe-B magnets. It is foundthat the sizes of the intrinsic coercivity and anisotropy constant are different for the magnets with different contents of Dy. Through the observation of the static anddynamic domains in the sintered Nd-Fe-B samples with different contents of Dy, wefound that the change of the magnetic domain becomes small, uniform, and a largenumber of transgranular domains appear. The reason for that was analyzed by comb-ining the phenomenon and the anisotropy constant analyzed. The principles of themagnetizing and demagnetizing processes for the studied samples were investigated byobserving the dynamic magnetic domains.