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Title: Structure and magnetism in the Kagome antiferromagnet RBaCo{sub 4}O{sub 7}.

Abstract

The mixed-valent compound RBaCo4O7 (R=Rare earth, Y), hereafter abbreviated as R-114, is built up of Kagome 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 {yields} 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 Kagome net and a collinear antiferromagnet in the third dimension.

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
971455
Report Number(s):
ANL/MSD/CP-58373
TRN: US1001203
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Conference
Resource Relation:
Conference: 2006 Materials Research Society Fall Meeting; Nov. 27, 2006 - Dec. 1, 2006; Boston, MA
Country of Publication:
United States
Language:
ENGLISH
Subject:
43 PARTICLE ACCELERATORS; DIMENSIONS; MAGNETIC PROPERTIES; MAGNETISM; MAGNETS; NEUTRONS; RESOLUTION; SYMMETRY; SYNCHROTRONS; VALENCE; X-RAY DIFFRACTION

Citation Formats

Mitchell, J. F., Zheng, H., Huq, A., Chapon, L. C., Radaelli, P. G., Stephens, P. W., Materials Science Division, ORNL, Rutherford Appleton Lab., and State Univ. of New York at Stonybrook. Structure and magnetism in the Kagome antiferromagnet RBaCo{sub 4}O{sub 7}.. United States: N. p., 2007. Web.
Mitchell, J. F., Zheng, H., Huq, A., Chapon, L. C., Radaelli, P. G., Stephens, P. W., Materials Science Division, ORNL, Rutherford Appleton Lab., & State Univ. of New York at Stonybrook. Structure and magnetism in the Kagome antiferromagnet RBaCo{sub 4}O{sub 7}.. United States.
Mitchell, J. F., Zheng, H., Huq, A., Chapon, L. C., Radaelli, P. G., Stephens, P. W., Materials Science Division, ORNL, Rutherford Appleton Lab., and State Univ. of New York at Stonybrook. Mon . "Structure and magnetism in the Kagome antiferromagnet RBaCo{sub 4}O{sub 7}.". United States. doi:.
@article{osti_971455,
title = {Structure and magnetism in the Kagome antiferromagnet RBaCo{sub 4}O{sub 7}.},
author = {Mitchell, J. F. and Zheng, H. and Huq, A. and Chapon, L. C. and Radaelli, P. G. and Stephens, P. W. and Materials Science Division and ORNL and Rutherford Appleton Lab. and State Univ. of New York at Stonybrook},
abstractNote = {The mixed-valent compound RBaCo4O7 (R=Rare earth, Y), hereafter abbreviated as R-114, is built up of Kagome 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 {yields} 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 Kagome net and a collinear antiferromagnet in the third dimension.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

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  • The first-order structural phase transition at T{sub S}{approx}240K in the extended kagome antiferromagnet TmBaCo{sub 4}O{sub 7} has been studied by neutron powder diffraction. In order to comprehend the microscopic origin of the transition, a detailed symmetry analysis is performed, based on crystallographic parameters obtained by Rietveld analysis of the neutron data. The results are consistent with the P31c {yields} Pna2{sub 1} symmetry lowering and support a displacive nature of the phase transition. The complex tilting pattern of CoO{sub 4} tetrahedra in both triangular and kagome sublattices is described based on symmetry-adapted pseudovector distortion modes of the parent P6{sub 3}mc hexagonalmore » structure. Our analysis reveals that the unusual topology of the crystal structure does not allow CoO{sub 4} tetrahedra to rotate as rigid units, resulting in their inevitable distortions, whatever the combination of rotational modes considered. A possible analogy between polyhedral distortions and spin frustration in this system is discussed.« less
  • The mixed-valent compound YbBaCo{sub 4}O{sub 7} is built up of Kagome sheets of CoO{sub 4} tetrahedra, linked in the third dimension by a triangular layer of CoO{sub 4} tetrahedra in an analogous fashion to that found in the known geometrically frustrated magnets such as pyrochlores and SrCr{sub 9} {sub x} Ga{sub 12-9} {sub x} O{sub 19} (SCGO). We have undertaken a study of the structural and magnetic properties of this compound using combined high-resolution powder neutron and synchrotron X-ray diffraction. YbBaCo{sub 4}O{sub 7} undergoes a first-order trigonal{sup {yields}}orthorhombic phase transition at 175 K. We show that this transition occurs asmore » a response to a markedly underbonded Ba{sup 2+} site in the high-temperature phase and does not appear to involve charge ordering of Co{sup 2+}/Co{sup 3+} ions in the tetrahedra. The symmetry lowering relieves the geometric frustration of the structure, and a long-range-ordered 3-D antiferromagnetic state develops below 80 K.« less
  • We have experimentally studied the structural and elastic characteristics of rare-earth cobaltites RBaCo{sub 4–x}M{sub x}O{sub 7} (R = Dy–Er, Yb, Y), in which cobalt ions are partly substituted by diamagnetic Al or Zn ions. It was found that small substitution of Co{sup 3+} ions by Al{sup 3+} ions in the YbRBaCo{sub 4–x}M{sub x}O{sub 7} system (x = 0.1, 0.2, 0.5) leads to a rapid decrease and smearing of ΔE(T)/E{sub 0} anomalies of the Young’s modulus in the region of the structural phase transition, which is accompanied by increasing hysteresis. Pure rare-earth cobaltites RBaCo{sub 4}O{sub 7} (R = Dy–Er, Y) exhibitmore » a correlation between the room-temperature structure distortion and hysteresis on the ΔE(T)/E{sub 0} curve in a temperature interval of 80–280 K. In Zn-substituted cobaltites RBaCoZn{sub 3}O{sub 7}, both the hysteresis and ΔE(T)/E{sub 0} anomalies disappear, as do low-temperature sound absorption maxima. This behavior is evidence of the suppression of structural and magnetic phase transitions and the retention of only short-range correlations of the order parameter in Zn-substituted samples.« less
  • Spin correlations in the geometrically frustrated RBaCo{sub 4}O{sub 7} compounds, usually described as an alternating stacking of Kagome and triangular layers on a hexagonal lattice, have been studied by mean-field approach and by Monte Carlo simulations. The behavior of the system was modeled with an isotropic Heisenberg Hamiltonian as a function of the relevant parameter J{sub out}/J{sub in}, representing the ratio between exchange integrals inside the Kagome layers, J{sub in}, and between Kagome and triangular layers, J{sub out}. This ratio can be varied in real systems by appropriate chemical substitutions. At the mean-field level, long-range magnetic order with the wavemore » vector at the K point of symmetry (k=a*/3+b*/3) has been found for J{sub out}/J{sub in} > 0.7. Below this value, the dominant Fourier modes are completely degenerate in the entire Brillouin zone. The Monte Carlo simulations revealed that the long-range ordered configuration found in the mean-field calculations becomes the ground state of the system for J{sub out}/J{sub in} > 1.5. Below this critical ratio, quasi-one-dimensional magnetic ordering along the c axis, involving spins of the triangular sublattice was observed. The correlations in the (ab) plane were found to have a short-range 120{sup o} character with correlation length dependent on J{sub out}/J{sup in}.« less