Structure and Magnetism in the Kagome Antiferromagnet RBaCo 4 O 7
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Structure and Magnetism in the Kagome Antiferromagnet RBaCo4O7 John F. Mitchell1, Hong Zheng2, Ashfia Huq3, Laurent C. Chapon4, Paolo G. Radaelli4, and Peter W. Stephens5 1 Materials Science Division, Argonne National Laboratory, MSD 223, 9700 S. Cass Avenue, Argonne, IL, 60439 2 Materials Science Division, Argonne National Laboratory, MSD 223, 9700 S. Cass Ave., Argonne, IL, 60439 3 Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN, 37831 4 ISIS Facility, Rutherford Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom 5 Physics and Astronomy, SUNY Stonybrook, Stonybrook, NY, 11794
ABSTRACT The mixed-valent compound RBaCo4O7 (R=Rare earth, Y), hereafter abbreviated as R-114, is built up of Kagomé sheets of CoO4 tetrahedra, linked in the third dimension by a triangular layer of CoO4 tetrahedra in an analogous fashion to that found in the known geometrically frustrated magnets such as pyrochlores and SrCr9xGa12-9xO19 (SCGO). We have undertaken a study of the structural and magnetic properties of the Y-114 and Yb-114 compound using combined high resolution powder neutron and synchrotron X-ray diffraction. Both compounds undergo a first order trigonal orthorhombic phase transition that breaks the trigonal symmetry of the structure. We show from Bond Valence Sum arguments that this transition occurs as a response to a markedly underbonded Ba2+ site in the high-temperature phase. The symmetry-lowering transition relieves the geometric frustration of the structure, and a long-range ordered 3-D antiferromagnetic state develops at low temperature. The magnetic structure of the Y compound has been solved and shows a compromise between the well-known 120o structure of the Kagomé net and a collinear antiferromagnet in the third dimension.
INTRODUCTION Transition metal oxides (TMO) offer a wealth of opportunities to study exotic electronic and magnetic states, as evidenced by colossal magnetoresistance in manganites, spin-state transitions in cobaltites, and orbital liquid states in vanadates and titanates. An area in which TMO have been particularly valuable is the field of geometrically frustrated magnets, where oxide spinels, pyrochlores, ladder-compounds, etc. enable one to explore how the global system – spin, charge, lattice – responds to the constraints of structure-enforced frustration. One structural motif commonly associated with geometric frustration is that of the Kagomé net, found in a variety of compounds such as pyrochlores, jarosites, etc. [1-3] Recently, a ternary cobalt compound with formula RBaCo4O7 (hereafter referred to as R-114) has been reported, with R=Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) [4-9]. The crystal structure shown in Figure 1 is built up of Kagomé sheets of CoO4 tetrahedra, linked by triangular layers of CoO4. The presence of a magnetic ion makes the Co sublattice structure of R-114 analogous to that found in the 3-D geometrically frustrated pyrochlore lattice. The structural similarity of R-114 to pyrochlores makes the system a potential new opportunity to explo
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