Charge, spin and orbital order in the candidate multiferroic material LuFe 2 O 4

This thesis is a detailed study of the magnetic, structural and orbital order parameters of the candidate multiferroic material LuFe2O4. Multiferroic oxides with a strong magnetoelectric coupling are of high interest for potential information technology applications, but they are rare because the traditional mechanism of ferroelectricity is incompatible with magnetism. Consequently, much attention is focused on various unconventional mechanisms of ferroelectricity. Of these, ferroelectricity originating from charge ordering (CO) is particularly intriguing because it potentially combines large electric polarizations with strong magneto-electric coupling. However, examples of oxides where this mechanism occurs are exceedingly rare and none is really well understood. LuFe2O4 is often cited as the prototypical example of CO-based ferroelectricity. In this material, the order of Fe valences has been proposed to render the triangular Fe/O bilayers polar by making one of the two layers rich in Fe and the other rich in Fe, allowing for a possible ferroelectric stacking of the individual bilayers. Because of this new mechanism for ferroelectricity, and also because of the high transition temperatures of charge order (TCO ∼320K) and ferro magnetism (TN ∼240K) LuFe2O4 has recently attracted increasing attention. Although these polar bilayers are generally accepted in the literature for LuFe2O4, direct proof is lacking. An assumption-free experimental determination of whether or not the CO in the Fe/O bilayers is polar would be crucial, given the dependence of the proposed mechanism of ferroelectricity from CO in LuFe2O4 on polar bilayers. This thesis starts with a detailed characterization of the macroscopic magnetic properties, where growing ferrimagnetic contributions observed in magnetization could be ascribed to increasing oxygen off-stoichiometry. The main focus is on samples exhibiting a sharp magnetic transition to long-range spin order at TN∼240K accompanied at TLT ∼ 170K by a low temperature phase transition into a phase with glassy magnetic dynamics. It is proposed that this magnetic behavior best approximates the intrinsic defectfree behavior of LuFe2O4. The spin structures of the long-range ordered phases could be refined properly by neutron diffraction as antiferromagnetic (AFM) and ferrimagnetic (fM) spin alignments. The two solutions exhibit a simple geometrical relation, where all spins in half of the bilayers change their sign. Furthermore, it is demonstrated that at TN and H=0 competing AFM and fM spin structures, which correspond respectively to ferro and antiferro stacking of equivalently ordered bilayers, are nearly degenerate. The observation of diffuse magnetic scattering in neutron diffraction far above TN indicates the random stacking of still individually ferrimagnetic ordered bilayers. The first crystal structural refinement, taking into account the superstructure due to the CO in LuFe2O4, was performed on these stoichiometric samples with the help of a monoclinic unit-cell and the C2/m symmetry. By clearly identifying the positions of Fe and Fe valences in this structure with the Bond Valence Sum (BVS) analysis, a completely new and unexpected CO pattern with charged Fe/O bilayers emerges. This new CO arrangement with charged, and consequently non-polar, bilayers is in strong contrast to all previously suggested CO configurations with polar bilayers. The implications of this result on “ferroelectricity from CO” in LuFe2O4 are discussed, addressing the possibility of polarizing the charged bilayers by an external electric or magnetic field, which could not be verified for our samples. In summary, a possible ferroelectric behavior of LuFe2O4 from CO is very unlikely. This is discussed in the light of the so far published work, where some doubt about the “ferroelectricity from CO” scenario is already present. It is worth emphasizing that although the AFM-fM meta-magnetism has not been reported previously and may not be resolvable in the majority of LuFe2O4 samples, our results have strong implications for the general nature of magnetism in this material. Additionally, x-ray magnetic circular dichroism (XMCD) measurements are presented which could link the novel CO configuration with the previously determined spin order, further corroborating both the new CO pattern established by x-ray diffraction and the spin structures refined by neutron diffraction. Finally, the relevance of the strict spincharge coupling to the CO transition is addressed and the possibility of an orbital ordered state in the low temperature phase is discussed.