The influence of severe plastic deformation on magnetic properties of Ni48Fe48Zr4, Fe1.5Co0.5BTa0.3 and Co80Zr16B4 alloys

Summary form only given. Since the development of Nd-Fe-B magnets, rare-earth magnets have been the essential components in many fields of technology because of their ability to provide a strong magnetic flux. Nevertheless, the crises of rare-earths during last few years had opened the discussion of rare-earths free permanent magnets once again. The important properties of the permanent magnets include their coercivity, remanence and energy product. There are essentially two ways how to achieve the large values of these properties necessary for today´s applications . First, the microstructure of the material can be optimized (in our case with the help of high pressure torsion) to prevent rotation of ferromagnetic domains . The second factor is the intrinsic spin-orbit coupling of electrons that forces the spins to align along a particular crystallographic direction, giving rise to the magnetocrystalline anisotropy energy of the material . Because the strength of the spin-orbit coupling increases as the fourth power of an element´s atomic number, maximizing the magnetocrystalline anisotropy energy can be accomplished by utilizing heavier elements. Severe plastic deformation has a great effect on magnetic properties of 4-f elements. For instance, in gadolinium a significant increase of the magnetocrystalline anisotropy (up to 2 orders of magnitude) has been observed . Thus, it is the question - is it possible to improve coercivity in 3-d based alloys with the help of severe plastic deformation or not? In our work we have chosen the objects of the investigation based of the following reasons: 1) The meteoritic tetrataenite phase of FeNi has the outstanding magnetic properties as a rare-earth free permanent magnet, but the synthesis of this phase is extremely difficult . Stabilization of the tetrataenite phase could be possible with addition of some extra elements. After preparation of rapidly quenched precursors of Fe₄₈Ni₄₈X₄ (X=Ta, - r, W, Mo, Re) the sample Fe₄₈Ni₄₈Zr₄ shows the highest coercivity and it was the reason to select it as the first object of the investigation. 2) (Fe,Co)₂B alloys have easy-axis magnetic anisotropy and they are promising materials as the rare-earth free permanent magnets. After preparation of rapidly quenched precursors (Fe,Co)₂B alloys with small addition of Ti, Cr, Zr, Nb and Re the highest coercivity has been observed for the composition Fe_1.5Co_0.5BTa_0.3. That was the second compound for current research. 3) In the literature, there are a few works where the magnetic properties of rapidly quenched Co₈₀Zr₁₆B₄ were investigated and an enhancement of the coercivity (up to several kOe) has been reported after a heat treatment. This compound has been selected as the third one . Based on that, the aim of this work is to investigate the influence of high pressure torsion (HPT) on magnetic properties of Ni₄₈Fe₄₈Zr₄, Fe_1.5Co_0.5BTa_0.3 and Co₈₀Zr₁₆B₄ alloys . HPT (Bridgemen´s anvils) was performed under 5GPa pressure with 5 complete turns . Such a high plastic deformation was found to dramatically affect microstructure of the samples by reduction of the grain size down to the nanometer scale . The field dependencies of magnetization for the concerned samples are shown . There are two different curves for each sample measured after HPT and after the heat treatment which consists of heating the sample up to 1000K with 2K/sec followed by cooling the sample in the furnace down to room temperature . No significant change was observed for Ni₄₈Fe₄₈Zr₄ and Fe_1.5Co_0.5BTa_0.3 alloys before and after the heat treatment, but for the Co₈₀Zr₁₆B₄ sample an increase of the co