Ferromagnetic Order in Rapidly Cooled Nd-Fe-Co-Al Alloy Ribbons

We have studied the magnetic properties of Nd 45 Fe 30 Co 15 Al 10 alloy ribbons with various thicknesses of about 120 (N 1 ) and 50 μm (N 2 ) prepared by melt-spinning. Structural analyses based on an X-ray diffractometer and a high-resolution transmission electron microscope revealed an existence of nanocrystals with sizes of 10 ~ 20 nm surrounded by an amorphous host matrix. With decreasing ribbon thickness and nanocrystalline size, magnetic studies upon a vibrating sample magnetometer indicated the decrease of the Curie temperature (T C ), coercive force (H c ), and magnetic entropy change (ΔS m ). In the ferromagnetic region, however, magnetization values determined for N 2 are greater than those determined for N 1 . These results are related to the differences in the critical exponents of N 1 (β=0.418 and γ=1.173), and N 2 (β=0.512 and γ=1.077), which are characteristic for the ferromagnetic nature existing in the alloy ribbons with different thicknesses.