Nd-Fe-B Based Nanocomposite Magnets And Their Coercivities

(Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆/FeCo magnets were prepared using the procedure of high energy ballmilling, hot pressing following melt spinning. The dependence of magnetic properties on weightpercent of soft magnetic phase and grain size has been studied. The nucleation and self-pinningduring the process of demagnetization, the phase transition thermodynamics ofα-Fe and Nd₂Fe₁₄Band weakened exchange coupling between soft and hard magnetic phase grains have beendiscussed.High energy ball milling (HEBM) makes the powder particles much refine and homogeneous,increases the energy of system, thus improves nucleation rate of nanograins during the period ofcrystallization. Hot pressing also improves nucleation rate of nanograins and suppresses the longrange diffusion of atoms. Owing to the effects of both HEBM and hot pressing on grain size, thenanograins of (Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆ magnets are much finer and its grain size distribution is muchmore centralized. In the nanocrystalline (Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆ magnets with HEBM and hotpressing, where the exchange coupling between grains is much stronger and would make theircoercivities decrease, but it keep the coercivity value up to 1631kA/m, almost equal to that ofnanocrystalline (Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆ magnets without HEBM and hot pressing, 1616kA/m,because of its grain size distribution more centralized and the amount of grain interface relativelymuch greater than the latter. With increasing the weight percent of FeCo, the saturationmagnetization of nanocomposite magnets (Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆/FeCo increases but the coercivitydecreases. In this paper on the (Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆/FeCo nanocomposite magnets with a limitedamount ofα-Fe phase no more than 20 weight percent, the self-pinning is dominant during theprocess of demagnetization with relatively high coercivity value. In the(Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆/FeCo nanocomposite magnets with 10% and 20% of weight percent ofFeCo, the coercivity value is 1357kA/m and 801kA/m respectively. While in the(Nd_10.5Pr_2.5)Fe₈₀Nb₁B₆/FeCo nanocomposite magnets withα-Fe phase more than 30 weight percent,the role of dipolar reaction will play an important role and the nucleation mechanism is dominantduring the process of demagnetization, thus the coercivity of nanocomposite magnets decreasesabruptly. And the coercivity value of nanocomposite magnets with 40 weight percent of FeCo isonly 9.1% of that of nanocrystalline single phase magnets. In the prepared nanocomposite magnets,the grain size of soft magnetic phaseα-Fe, in the range of 40nm to 70nm, is more than that of hard magnetic phase Nd₂Fe₁₄B. This is owed to the big difference of thermodynamics betweenNd₂Fe₁₄B andα-Fe during the period of phase crystallisation, which forcesα-Fe to crystallizefirstly, thusα-Fe grains have much longer time to grow. This is an important reason contributing tothe poor coercivity of nanocomposite magnets especially with higher percent ofα-Fe phase.The defects near grain boundary could weaken the effect of exchange coupling betweengrains, resulting in a decreased coercivity of nanocomposite magnets. To fundamentally improvethe magnetic properties, new methods should been explored to weaken the effects of difference ofthermodynamics between Nd₂Fe₁₄B andα-Fe on the grain size of nanocomposite magnets, thus tokeep the grain size of soft magnetic phase less than the range of hard magnetic one. And newmethods should been taken to reduce the amount of defects, such as structure free volumes nearinterface between grains.Key Words: nanocomposite magnets, melt spinning, high energy ball milling, hotpressing,thermodynamics, coercivity, exchange coupling...