Investigations of phase transitions in magnetic materials by magnetic-field- and temperature-dependent x-ray diffraction

The subject of this doctoral work is the assembly of a laboratory-based x-ray diffractometer with magnet and non-ambient sample temperature environment for investigations of magnetic materials. A special focus is on the versatility and accessibility of the instrument. The x-ray diffractometer serves as important analysis tool for the determination of the crystallographic unit cell and derived parameters like strain or expansion as function of the temperature and magnetic field. Some materials undergo structural or magnetic phase transformations under specific conditions, which can be induced within the x-ray diffractometer. These phase transformations, and their effect on crystallographic parameters of the investigated materials can be followed in situ in the instrument. Several case studies of investigations of magnetoelastic and magnetostructural phase transitions at temperatures between 25 to 600 K under various magnetic fields highlight the performance and capabilities of the instrument. The final x-ray diffractometer setup is equipped with a 5.5 T magnet and cryofurnace for non-ambient measurements in the range between 11–700 K. Measurements in transmission geometry result in reliable reflection intensities that allow for the refinement of structural parameters, and even structure solution from the diffraction data. The fast position-sensitive Si strip detector has a large angular coverage, and allows for quick data collections. High resolution data can be collected with the use of a monochromator crystal at the expense of photon flux instead of the high flux x-ray mirror optics. Integration of all diffractometer components into the control software enables the use of macros for automated data collection for a series of different measurement conditions. The first set of case studies is related to materials with a magnetoelastic phase transition, MnB, FeB and LaFe11.4Si1.6, which exhibit no structural change over the course of the magnetic ordering, but a large elastic response. Synchrotron and neutron scattering studies complement the characterization with the lab-based diffractometer with magnet, and reveal the first-order character of the phase transition. Spin fluctuations are the connecting driving force behind the magnetovolume effects and anomalies over the course of the phase transition in MnB, FeB and LaFe11.4Si1.6. The term spin fluctuations, however, describes different phenomena in the investigated materials and is clarified accordingly. The second set of case studies is related to the (La,Ce)Fe12B6 class of materials. They undergo a magnetic-field-induced phase transition at low temperatures that is coupled with a huge magnetostriction. The origin behind this anisotropic expansion over the course of the phase transition is determined as magnetic-field-induced magnetostructural phase transition into a new ferromagnetic structure. The structure is determined from the x-ray diffraction data collected in magnetic fields. A model for the transformation mechanism in the form of a martensitic-like phase transition is proposed. The model also explains the burst-like growth of ferromagnetic phase in the matrix of surrounding grain boundaries. A kinetic arrest of the ferromagnetic structure occurs, and results in a delay and large hysteresis of the magnetostructural phase transition