Coercivity enhancement mechanism in Dy-substituted Nd-Fe-B nanoparticles synthesized by sol-gel base method followed by a reductiondiffusion process

In current work, Nd15-xDyxFe77.5B7.5 (at%) nanoparticles with different Dy-content ( x= 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) were synthesized by sol-gel method followed by a reduction-diffusion process. The effects of Dy on the magnetic properties and the relations between the microstructure and the coercivity of Dy-substituted Nd-FeB nanoparticles have been studied. The coercivity of Nd-Fe-B nanoparticles with the addition of Dy first increase, reaches a maximum, and then starts to decrease. The coercivity of Dy-substituted Nd-Fe-B nanoparticle synthesized by sol-gel method increased from 938.9 to 1663.9 kA/m while the remanence decreased slightly from 1.16 to 1.06 T. The results show that with an increase in Dy content the variation of maximum energy product (( BH) (max)), lowest-order uniaxial magnetocrystalline anisotropy constant (K-u1), and Curie temperature (T-c) had a trend as same as the coercivity. The Henkel plot showed that the existence of exchange coupling interaction between grains, and the exchange coupling interactions increased with increasing x from 0.0 to 2.0 and then decrease with further increasing x= 2.5. The optimum magnetic properties of Nd-FeB nanoparticles with (BH) (max) =40.38 MGOe, H-c=1663.9 kA/m, B-r=1.08 T were obtained by substituted 2.0 at % Dy. The effects of increasing temperature on magnetic properties of Dy-substituted Nd-Fe-B nanoparticle magnets with 2.0 at% Dy was investigated. The reduced spin-reorientation temperature was obtained for Dysubstituted Nd-Fe-B nanoparticles with 2.0 at% Dy. Below 100 K a spin-reorientation transition was takes place. The temperature coefficient of coercivity (beta) was -0.36, -0.46, -0.41, -0.34, -0.29,-0.24, -0.25%/degrees C at different temperature 50, 100, 150, 200, 250, 300, 350 degrees C, respectively. Mssbauer spectroscopy was applied to study the composition and properties of Dy-substituted Nd-Fe-B magnet. Microstructure analysis showed a homogeneous distribution of Dy in produced samples. The possible reason for observed magnetic behavior is improving the intrinsic material parameter and optimizing the microstructure by a uniform enhancement of magnetocrystalline anisotropy by formation the nanocrystalline compound ( Nd,Dy) 2Fe14B.