Simon Sawatzki

Coercivity enhancement in hot-pressed Nd-Fe-B permanent magnets with low melting eutectics

Nd-Fe-B melt-spun ribbons with optimized composition for hot workability have been hot-compacted together with low melting DyCu, DyNiAl, NdCu, and NdAl powders to enhance coercivity. Annealing at 600 °C leads to an interdiffusion of Dy and Nd at the interfaces between the Nd-Fe-B flakes and the Dy-rich phase. This modifies the grain boundary phase and thus further enhances coercivity without decreasing remanence. The higher coercivity for DyCu compared to DyNiAl was attributed to the lower melting point obtained by differential scanning calorimetry. For NdCu and NdAl, annealing was found to be ineffective.

On the synthesis and microstructure analysis of high performance MnBi

Highly anisotropic MnBi powder with over 90 wt% low-temperature phase can be prepared using conventional arc-melting and 2 hour-low energy ball milling (BM) followed by magnetic separation. After proper alignment, the purified Mn55Bi45(Mn45Bi55) powder show remarkable magnetic properties: mass remanence of 71(65) Am2/kg and coercivity of 1.23(1.18) T at 300 K. The nominal maximum energy product of 120 kJ/m3 is achieved in the purified 2h-BM Mn55Bi45 powder, close to theoretical value of 140.8 kJ/m3. The Mn55Bi45(Mn45Bi55) bulk magnets show the highest volume remanence of 0.68(0.57) T at 300 K, while they were consolidated at 573(523) K by a pressure of 200 MPa for 5 minutes using hot-compaction method. In addition to the observed grain size, the coercivity of the hot-compacted samples at 300 K was found to be strongly related to the amount of metallic Mn and Bi residue at the grain-boundary. Our study proves that the magnetic properties of the Mn45Bi55 bulk magnets are stable up to 500 K, and the nominal (BH)max values are still above 40 kJ/m3 at 500 K showing the potential ability for high-temperature applications.

The Nucleation of the Spin Spiral in Epitaxial

Recently, exchanged coupled hard/soft magnetic trilayers based on the hard magnetic SmCo5 phase and a high moment soft Fe phase reached largely improved energy densities, (BH)max, of above 300 kJ/m3 by a combination of fully epitaxial growth and an optimized soft layer thickness. In order to evaluate quantitatively the effect of Fe-layer thickness on the nucleation field Hn in these epitaxial trilayers, micromagnetic calculations were performed in a one-dimensional model using OOMMF assuming strong coupling at the interface. Calculating the hysteresis on the basis of the nominal layer architecture the experimental saturation polarization and remanence Jr are underestimated, the nucleation field is overestimated and the onset of the spin spiral is too sharp. On the other hand, considering a texture spread via a tilt angle of 3° reduces Hn slightly and leads to a realistic smoothening of the transition. Including diffusion of Co into the Fe layer with a diffusion zone of 2 nm at each interface increases the soft layer thickness by 4 nm on cost of the SmCo5 layer thickness. Such a layer architecture results in a further reduction of Hn together with an increase of Jr and leads to a very reasonable agreement with the experimental data. The modified model furthermore reproduces the shape of the reversible demagnetizing branch of the hysteresis in the exchange-spring regime and is thus suitable for describing the magnetic behavior of existing exchange-coupled multilayer.

{\rm SmCo}_{5}/{\rm Fe/SmCo}_{5}

A detailed understanding of the magnetization processes in an exchange coupled SmCo5/Fe/SmCo5 trilayer is derived from imaging the domain configuration of the sample in a succession of remanent states by magnetic force microscopy. Domain nucleation and domain wall movement processes are identified and are interpreted in terms of the dominating coercivity mechanism. As in single hard magnetic SmCo5 layers, strong pinning of domain walls governs the magnetization reversal and determines coercivity. The coupling to the Fe layer, however, leads to an increased domain size in the thermally demagnetized state and to a larger nucleation density prior to the irreversible magnetization switching. Due to slightly different switching fields of the two SmCo5 layers, a compensated state can be adjusted, in which the moments of the Fe layer form 180° spin spirals with laterally varying chirality. The magnetic contrast arising from these spin spirals is imaged and the irreversible changes in a small in-plane field perpendicular to the easy axis are studied.