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

We have studied the magnetic properties of Nd₄₅Fe₃₀Co₁₅Al₁₀ alloy ribbons with various thicknesses of about 120 (N₁) and 50 μm (N₂) 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 N₂ are greater than those determined for N₁. These results are related to the differences in the critical exponents of N₁ (β=0.418 and γ=1.173), and N₂ (β=0.512 and γ=1.077), which are characteristic for the ferromagnetic nature existing in the alloy ribbons with different thicknesses.