Neutron and x-ray scattering study of magnetic manganites

Thesis

This thesis presents three investigations of the magnetic and electronic proper- ties of manganese oxide materials. The investigations are performed using a variety of neutron scattering and x-ray scattering techniques.

The electronic ground-state of Pr(Sr_0.1 Ca_0.9)₂ Mn₂O₇ an antiferromagnet with CE-type ordering, is determined using neutron spectroscopy, as opposed to the more usual approach of using diffraction. The Zener polaron model of the elec- tronic ground state of the CE-type magnetic phase is shown to be unsuitable for this material. The ground-state is shown to agree well with the electronic ground state proposed by Goodenough in the 1950’s, but without significant Mn^3+/Mn^4+ disproportionation.

The distribution of the magnetisation density within the unit cell of the CE-type antiferromagnet La_0.5Sr_1.5MnO₄ is determined from a polarised neutron diffraction experiment by analysing the results with the maximum entropy method. The majority of the magnetisation density is found to be located at the Mn site. The investigation shows tentative evidence of a small magnetic moment on the in-plane O site. However, a larger moment is observed at both the La/Sr site and the out-of-plane O site.

The magnetic structure of the magnetoelectric multiferroic DyMn₂O₅ is inves- tigated using resonant magnetic x-ray scattering. The magnetic structure is shown to be similar to other members of the RMn₂O₅ series of multiferroics, but with the key difference that the magnetic moments are closely aligned parallel with the crystallographic b-axis, in contrast to the usual observation of the moments being close to parallel with the a-axis. This study also shows evidence that the electrical polarisation has a significant contribution from the valence electrons of the O ions, agreeing with previous work.

Authors: Johnstone, GE

• Publication date: 2012
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: mulitferroic; neutron scattering; manganese oxide; magnetoelectric; manganite; resonant X-ray
• Subjects: Condensed Matter Physics