Hf Doping Effect on Hard Magnetism of Nanocrystalline Zr18-xHfxCo82 Ribbons

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr18-xHfxCo82 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 (Zr2Co11 (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 (M-s) are 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.