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.