Magnetism of MnBi-Based Nanomaterials

Nanostructured MnBi ribbons doped with impurity elements including B, C, Fe, Hf, Sm and Tb were prepared using the arc melting and melt-spinning techniques. The melt-spun ribbons were annealed in vacuum furnace at 350°C to obtain the intended hexagonal structure. The external impurity doping made a significant change in the magnetic properties of the nanostructured MnBi ribbons including a decrease in saturation magnetization (M_s) and anisotropy energy (K) and an increase in coercivity (H_c). However, Hf and C co-doping showed the opposite effect with a small increase in both M_a and K. Interestingly, the anisotropy energy of the boron doped sample increased by about 15% irrespective of the small decrease in magnetization. A significant increase in H_c of MnBi ribbons was found due to Hf, Tb and Sm doping. H_c as high as 13 kOe was achieved in Hf-doped sample after the sample was aligned in a magnetic field. A thermal hysteresis was observed at the structural phase transition of MnBi, which shifts by about 5 K towards higher temperatures due to impurity doping. The observed magnetic properties of the impurity doped MnBi ribbons are explained as the consequences of the disorder and the competing ferromagnetic and antiferromagnetic interactions.