Spin structure and magnetic phase transitions in TbBaCo{sub 2}O{sub 5.5}

Journal Article · Wed Jun 01 00:00:00 EDT 2005 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.71.214407· OSTI ID:20719034

Plakhty, V P; Chernenkov, Yu P; Moskvin, E V; Gavrilov, S V ^[1]; Barilo, S N ^[2]; Podlesnyak, A; Pomjakushina, E ^[3]

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1. Petersburg Nuclear Physics Institute, Gatchina, 188300 St. Petersburg (Russian Federation)
2. Institute of Solid State and Semiconductor Physics, National Academy of Sciences, Minsk 220072 (Belarus)
3. Laboratory for Neutron Scattering, ETH Zuerich and Paul Scherrer Institut, CH-5232 Villigen PSI (Switzerland)

Spin ordering in TbBaCo{sub 2}O{sub 5.5} and its temperature transformation reproducible for two differently synthesized samples are studied. One of the ceramic samples, in addition to the main phase a{sub p}x2a{sub p}x2a{sub p}, Pmmm (Z=2), where a{sub p} is parameter of perovskite cell, contains about 32% of the phase a{sub p}xa{sub p}x2a{sub p}, Pmmm (Z=1) with statistical distribution of oxygen over the apical sites. The other sample is a single phase a{sub p}x2a{sub p}x2a{sub p}, Pmmm (Z=2) with well defined octahedral and pyramidal sublattices. Treatment of neutron diffraction patterns of the double-phase sample itself gives a sophisticated spin structure. Knowing the spin structure of the single-phase sample, one can choose only proper magnetic lines, which give exactly the same results for both samples. The spin structure at T=265 K unambiguously indicates the phase 2a{sub p}x2a{sub p}x2a{sub p}, Pmma (Z=4). At T{sub N}{approx_equal}290 K, the spins order with the wave vector k{sub 19}=0 (phase 1). At T{sub 1}{approx_equal}255 K, a magnetic transition takes place to the phase 2 with k{sub 22}=b{sub 3}/2. The extinction law of magnetic reflections below T{sub 2}{approx_equal}170 K evidences that the crystal structure changes to 2a{sub p}x2a{sub p}x4a{sub p}, Pcca (Z=8). The wave vector of the spin structure becomes again k{sub 19}=0 (phase 3). The basis functions of irreducible representations of the group G{sub k} have been found. Using results of this analysis, the magnetic structure in all phases is determined. The spins are always parallel to the x axis, and the difference is in the values and the mutual orientation of the moments in the ordered nonequivalent pyramidal or octahedral positions. Spontaneous moment M{sub 0}=0.30(3){mu}{sub B}/Co at T=260 K is due to ferrimagnetic ordering of the moments M{sub Py1}=0.46(9){mu}{sub B} and M{sub Py2}=-1.65(9){mu}{sub B} in pyramidal sites (Dzyaloshinskii-Moriya canting is forbidden by symmetry). The moments in the nonequivalent octahedral sites are: M{sub Oc1}=-0.36(9){mu}{sub B}, M{sub Oc2}=0.39(9){mu}{sub B}. At T=230 K, M{sub Py1}=0.28(8){mu}{sub B}, M{sub Py2}=1.22(8){mu}{sub B}, M{sub Oc1}=1.39(8){mu}{sub B}, M{sub Oc2}=-1.52(8){mu}{sub B}. At T=100 K, M{sub Py1}=1.76(6){mu}{sub B}, M{sub Py2}=-1.76{mu}{sub B}, M{sub Oc1}=3.41(8){mu}{sub B}, M{sub Oc2}=-1.47(8){mu}{sub B}. The moment values together with the ligand displacements are used to analyze the spin-state/orbital ordering in the low-temperature phase.

• OSTI ID: 20719034
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 71, Issue 21; Other Information: DOI: 10.1103/PhysRevB.71.214407; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
BARIUM COMPOUNDS
CERAMICS
COBALT OXIDES
DISTRIBUTION
FERRIMAGNETIC MATERIALS
IRREDUCIBLE REPRESENTATIONS
LIGANDS
MAGNETIC MOMENTS
MAGNETORESISTANCE
NEUTRON DIFFRACTION
ORIENTATION
ORTHORHOMBIC LATTICES
OXYGEN
PEROVSKITE
PHASE TRANSFORMATIONS
REFLECTION
SPIN
SYMMETRY
TERBIUM COMPOUNDS