Spin structure and magnetic phase transitions in TbBaCo2O5.5 -: art. no. 214407

Spin ordering in TbBaCo2O5.5 and its temperature transformation reproducible for two differently synthesized samples are studied. One of the ceramic samples, in addition to the main phase a(p) x 2a(p) x 2a(p), Pmmm (Z=2), where a(p) is parameter of perovskite cell, contains about 32% of the phase a(p) x a(p) x 2a(p), Pmmm (Z=1) with statistical distribution of oxygen over the apical sites. The other sample is a single phase a(p) x 2a(p) x 2a(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(p) x 2a(p) x 2a(p), Pmma (Z=4). At T-N approximate to 290 K, the spins order with the wave vector k(19)=0 (phase 1). At T-1 approximate to 255 K, a magnetic transition takes place to the phase 2 with k(22)=b(3)/2. The extinction law of magnetic reflections below T-2 approximate to 170 K evidences that the crystal structure changes to 2a(p) x 2a(p) x 4a(p), Pcca (Z=8). The wave vector of the spin structure becomes again k(19)=0 (phase 3). The basis functions of irreducible representations of the group G(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-0=0.30(3)mu(B)/Co at T=260 K is due to ferrimagnetic ordering of the moments M-Py1=0.46(9)mu(B) and M-Py2=-1.65(9)mu(B) in pyramidal sites (Dzyaloshinskii-Moriya canting is forbidden by symmetry). The moments in the nonequivalent octahedral sites are: M-Oc1=-0.36(9)mu(B), M-Oc2=0.39(9)mu(B). At T=230 K, M-Py1=0.28(8)mu(B), M-Py2=1.22(8)mu(B), M-Oc1=1.39(8)mu(B), M-Oc2=-1.52(8)mu(B). At T=100 K, M-Py1=1.76(6)mu(B), M-Py2=-1.76 mu(B), M-Oc1=3.41(8)mu(B), M-Oc2=-1.47(8)mu(B). The moment values together with the ligand displacements are used to analyze the spin-state/orbital ordering in the low-temperature phase.