Lorentz Microscopy of Pinning Magnets at Variable Temperature

Novel adaptations of magnetic structure investigations in a wide temperature range and numerical micromagnetic simulations will be used for a comprehensive investigation of the domain wall pinning behaviour of the technologically important hard magnets which are suitable of high temperature applications. Systems to be studied include precipitation hardened magnets of the type Sm(Co,Cu)5/Sm(Co,Fe)17 and heavily deformed, hot extruded MnAlC magnets. An important approach to understand the effect of various microstructural features on the extrinsic magnetic properties of pinning controlled bulk magnets is through the use of Lorentz microscopy at various temperatures. The TEM operated in one of the Lorentz imaging modes provides a powerful means of studying the magnetic structures on the mesoscopic and nanoscale level and it has been shown to be a convenient and fast technique capable to give answers to many scientific questions concerning the magnetic domain configuration and the processes during magnetisation reversal processes. Previous studies on the multiphase Sm(Co,Cu,Fe,Zr)7-8 magnets have attributed the high coercive field to domain wall pinning by the Sm(Co,Cu)5-7 cell boundaries. The crystalline anisotropy differentiation at cell boundaries caused by the coherent strain and the local chemical fluctuation are thought to be the reason for domain wall pinning. In order to identify the characteristics of the pinning sites, whether to act as attractive and as repulsive pinning sites, the magnets will also be studied for structural and chemical clarification. Besides the characterisation of the size and shape of the cellular precipitation structure controlled by composition and processing of the magnets the determination of attractive or repulsive domain wall pinning behaviour at certain temperatures for various magnets with different composition will be one of the major tasks of the project. Of special interest will be the transition between ferromagnetic to the paramagnetic state of the cell boundary phase with respect to the pinning behaviour with increasing temperature. The quantitative interaction between magnetisation and microstructure will be calculated by means of a micromagnetic finite element technique which was developed in our group. The micromagnetic simulation of the magnetisation reversal process should reveal exact predictions on the pinning field as function of the microstructural parameters of the system. Questions to be addressed include: how does a nanostructured phase, like the Cu-containing platelet phase, or heavily defected regions, like the planar crystal defects in hot extruded magnets, influence the pinning behaviour of hard magnets. The combination of Lorentz microscopy and micromagnetic simulations will facilitate an innovative study of the domain wall pinning behaviour of a magnet class with an improved coercive field at high temperatures. The search for novel soft and hard magnetic materials for high temperature advanced power applications is worldwide an active area of research. The increase of the operating temperature of motors, generators and other electronic devices leads to an improvement of the efficiency.