آثار مغناطوالاستيكي در آهن رباهاي نانو كامپوزيتي ND4FE5/77B5/81

Magnetoelastic effects in Nd4Fe77,5B18.5 nano-composite magnet

In this research, magnetostriction and thermal expansion of polymer-bonded Nd4Fe77.5B18.5 nano-composites are studied using strain gage method. The effect of sintering on samples is also studied. X-ray diffraction patterns and SEM pictures of the original and annealed samples show that the grain size of a-Fe and Fe3B soft magnetic phases in amorphous matrix increases and mechanical hardness of the samples improves after annealing at 700 °C for 20 min.. Overall behavior of the thermal expansion is similar for samples in both cases;-with about 20% larger coefficient for the annealed one. The magnetostriction measurements show huge magnitudes in order of 104 at the presence of relatively weak external magnetic fields. The magnitude of magnetostriction noticeably decreases after annealing which can originate from two different sources. Magnetostriction of the original sample mainly originates from single particle interactions of Nd-sublattice in Nd2Fe14B phase, but the situation is very different for the annealed sample. The complicate variations of the magnetostrictive isotherms of the annealed sample are discussed based on the following three factors: internal stresses, magnetocrystalline anisotropy and mechanisms of coercivity.