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

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr_18-xHf_xCo₈₂ 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 Zr₂Co₁₁, orthorhombic Zr₂Co₁₁, hcp Co, and cubic Zr₆Co₂₃ phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr₂ Co₁₁ (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/m³ 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.