Gast ::
| Hauptseite > Publikationsdatenbank > Structure, magnetism and excitations in some Mn-based magnetocaloric effect compounds |
| Hauptseite > Publikationsdatenbank > Structure, magnetism and excitations in some Mn-based magnetocaloric effect compounds |
| Dissertation / PhD Thesis/Book | PreJuSER-22832 |
2012
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-874-7
This record in other databases:
Please use a persistent id in citations: http://hdl.handle.net/2128/7590
Abstract: The magnetocaloric effect causes the cooling or heating of a material due to the influence of an applied magnetic field. This mechanism provides an alternative technique for cooling, when using cheap and environment friendly materials. Heating and cooling takes place without moving any mechanical parts. Therefore, this effect attracts the attention of many scientific studies. The magneto caloric effect is characterized by the entropy change. Polycrystalline samples of the compositions Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=0,1,2,3,4 and a single crystal of the composition Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=4 were prepared in order to analyze the magnetocaloric effect in those materials. All samples were characterized with magnetization measurements. Ferroand antiferromagnetic behaviour could be detected. Diffraction patterns were taken on the timeof- flight powder diffractometer POWGEN at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory to analyze the magnetic structures of the compounds. Inelastic neutron data were collected on the single crystal Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=4 on the thermal neutron triple axis spectrometer 2T1 at Laboratoire Léon Brillouin to investigate the dynamic properties. The refined data taken on samples Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=0,1,2,3,4 on POWGEN at room temperature confirm the hexagonal structure. This has already been reported in literature. Anomalies could be identified in several crystallographic parameters as funtion of the Fe-content of the samples Diffraction patterns taken on sample Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=0 between the phase transitions at 70K and 90K could be refined with an orthorhombic unit cell including antiferromagnetism. Below the phase transition at 62K the best refinements could be performed using a monoclinic unit cell. Also, the structure seems to exhibit weak ferromagnetism, which can be annihilated with an applied magnetic field. This mechanism is proposed to cause the negative magnetocaloric effect in this compound. The analysis of diffraction patterns taken in the ferromagnetic phase of Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=4 indicate different behaviours of the magnetic atoms occupying the two crystallographic positions (the third is occupied by Si). This property is proposed to influence significantly the entropy in this material. Phonon branches in the dispersion relation of Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=4 are anisotropic, which is due to the hexagonal structure. In order to investigate the effect of Fe and Mn on the change of the entropy the composition Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=4 was doped with Co (for Mn and Fe). The analysis of the diffraction patterns which were taken on POWGEN identified at least one impurity phase in every sample. Different behaviours of the magnetic moments of the atoms in these compounds could also be verified, which is similar to the composition Mn$_{5−x}$Fe$_{x}$Si$_{3}$ x=4.
|
The record appears in these collections: |
PDF
Fulltext by OpenAccess repository