Neutron Diffraction Studies Of Magnetic Structure

Author : Klaus Antonius Hamacher
Publisher :
ISBN 13 :
Total Pages : 382 pages
Book Rating : 4.:/5 (446 download)

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Download or read book Neutron Diffraction Studies of Organic and Magnetic Layered Structures written by Klaus Antonius Hamacher and published by . This book was released on 1998 with total page 382 pages. Available in PDF, EPUB and Kindle. Book excerpt: Two different neutron diffraction experiments on layered structures have been carried out at the University of Missouri Research Reactor Center. In the first project, the structure of lipid model membranes, consisting of phosphatidylcholine (PC) and cholesterol, and the effect of introducing various alkanes, such as hexane, decane, and toluene, was investigated. The experiments were conducted using PC's with 10 to 20 carbon atoms in the polymethylene chains. By replacing hydrogen with deuterium on parts of the molecules (deuterium labeling) and comparing the scattering amplitude density profiles for the labeled and unlabeled bilayers, it was possible to resolve the spatial arrangement within the bilayer. It was found that the alkanes are localized in a narrow band in the hydrophobic region of the bilayer and that the orientation of their long axis was within the plane of the bilayer. Also, the change of position of the cholesterol molecule as a function of the polymethylene chain length was studied and lead to the unexpected result that it neither aligns itself with the PC head group nor with the end of the polymethylene chains. Additionally, the Debye-Waller factor for the labels on the PC and the cholesterol was determined, and for cholesterol it was found that the disorder (thermal and static) of the cholesterol molecules decreases with increasing temperature. The second project involved the study of the magnetic properties of Dy/Y superlattices with the surface normal between the (1-102) and (1-103) direction. Thus, the c-axis, the propagation direction of the helimagnetic spiral is at an angle of about 35$\sp\circ$ to the surface normal. Neutron diffraction scans showed that the 200nm thick Dy film and the (Dy$\sb\̐vert$Y$\sb5\rbrack\sb6$ superlattice sample behaves like bulk dysprosiun, i.e., having a helimagnetic to ferromagnetic phase transition at T$\sb{C}$ = 85K. The other Dy/Y superlattice samples showed a coexistence of helimagnetic and ferromagnetic phase from low temperatures up to even above T$\sb{C}$. This was the first time that ferromagnetism was observed in Dy/Y superlattices. This coexistence can be attributed to the faceted structure of the samples which consists of two distinct domains, i.e., two mesoscopic superlattices. For all the samples investigated, the magnetic moment, the turnangle of the helimagnetic spiral, and the coherence length as a function of temperature were determined. Also, an attempt was made to model the nuclear scattering, i.e., structure factor calculation. Thus, both experiments focused on layered structures and utilized the neutrons unique capabilities as a probe to study the properties of these thin samples.