Development of Rare Earth Free and Rare Earth Balance Permanent Magnets

Abstract

In the quest for a sustainable permanent magnetic material, two material systems were extensively studied. The first part of the thesis investigates the Fe-Sn based systems as potential RE-free candidates. The Fe-Sn binary and several multinary systems such as (FeX)5Sn and (FeX)3(SnY) were extensively screened for the discovery of a new hard magnetic material using high-throughput Reactive Crucible Melting (RCM) technique as well as other non-equilibrium methods. The follow-up experimental screening strategies were supported by theoretical calculations carried out by other scientists. For high-throughput characterization, these synthesis techniques were combined with energy dispersive X-ray spectroscopy as well as magneto-optical Kerr microscopy which enabled the identification of with uniaxial magnetic anisotropy, for example Fe3Sn2 in the binary system. The reliability of the reactive crucible melting method was evaluated by a comparison of the forming in the reactive crucible with appearing in conventionally melted samples. It has been shown that under some circumstances, the phase relations might not always be correctly reproduced. The Fe5Sn3 phase, existing in the equilibrium phase diagram at 800°C and forming in conventionally melted alloys, does not exist in the diffusion zone of the reactive crucible. The problem of missing phases is discussed. In addition, by observation of domain structure and by employing different analytical models based on domain theory, the anisotropies of with uniaxial anisotropy were evaluated. It has been shown that the consideration of the magnitude of anisotropy is crucial for the selection of a proper model and realistic assessment of anisotropy energy. With the example of materials with high and intermediate uniaxial anisotropy, the applicability of 3 analytical models Kittel, Szymczak and Bodenberger-Hubert were investigated. Another major activity to study the Fe-Sn system was the investigation of structural and intrinsic magnetic properties of its binary ferromagnetic compounds by synthesis and characterization of Fe3Sn, Fe5Sn3, and Fe3Sn2 single crystals. Derived from single crystal X-ray diffraction and Transmission Electron Microscopy (TEM), a new structural model is proposed for the Fe5Sn3 crystals - the threefold twinning of an orthorhombic unit cell with (3+1) dimensional space group Pbcm(α00)0s0. The spontaneous magnetization (Ms) and the anisotropy constants K1 and K2 of Fe3Sn, Fe5Sn3, and Fe3Sn2 single crystals were determined in a wide temperature range using M(H) dependencies and the Sucksmith-Thompson technique. A large however planar anisotropy of K1 = -1.16 MJm-3 appreciable for a rare earth free system and a negligible uniaxial anisotropy of K1 = +0.05 MJm-3 were evaluated for Fe3Sn and Fe3Sn2 compounds, respectively. The second part of the thesis investigates the effect of partial substitution of Ce and Co in the Nd-Fe-B system. By synthesis and characterization of (Nd1-xCex)2(Fe1-yCoy)14B single crystals, the structural and intrinsic magnetic properties were investigated for y = 0 and x = 0, 0.15, 0.36, 0.63 and 1 as well as y = 0.1 and x = 0 and 0.15. The effect of doping with Ce and Co on crystal lattice parameters a and c, Ms, Ha, TC, TSPT, and K1 were evaluated. Additionally, for Nd2Fe14B and by analysis of single crystal magnetization curves measured under the field of up to 50 T along four different orientations, five associating anisotropy constants were extracted using theoretical models and based on minimization of the total energy of the system. All intrinsic magnetic properties were gradually decreased by an increase in Ce proportion when it was solely introduced to the structure. Furthermore, Ce substitution resulted in a decrease in spin reorientation transition temperature where anisotropy switches from uniaxial to an easy cone. The addition of Co has not only increased the Curie temperature but also improved the thermal stability of the intrinsic magnetic properties. The reduction of anisotropy energy with Ce substitution could partially be compensated by co-doping together with Co, especially at high temperatures. The improved intrinsic properties of the co-doped Nd2Fe14B was also reflected in their extrinsic magnetic properties by characterization of mechanochemically synthesized sub-micron particles. For the sample with (Nd0.82Ce0.18)2(Fe0.85Co0.15)14B composition, an improved temperature coefficient of coercivity of β = -0.4 %/K was achieved. Comparing these results with single crystal data, it can be verified that the enhanced high-temperature performance of NdFeB-based magnets co-doped with Ce and Co, which have also been observed in various literature, is due to an improvement in intrinsic magnetic properties.