Hf Doping Effect on Hard Magnetism of Nanocrystalline Zr$_{18\hbox{-}{\rm x}}$Hf$_{\rm x}$Co$_{82}$ Ribbons

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr
_18-x
Hf
_x
Co
₈₂
ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr
₂
Co
₁₁
, orthorhombic Zr
₂
Co
₁₁
, hcp Co, and cubic Zr
₆
Co
₂₃
phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr
₂
Co
₁₁
(rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (H
_c
) and the saturation magnetization are (M
_s
) found to linearly increases with x (x ≤ 2), then H
_c
slightly increases and M
_s
slightly decreases with increasing x. The maximum energy product (BH)
_max
at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m
³
and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials.