Spin ordering in TbBaCo 2 O 5.5 and its temperature transformation reproducible for differently synthesized samples are studied. First of all, the polymorphism due to the oxygen ordering with the average content close to 5.5 is investigated. One of ceramic samples (I), in addition to the main phase a p × 2a p × 2a p , Pmmm (Z = 2), contained about 25% of the phase a p × a p × 2a p , Pmmm, (Z = 1) with statistical distribution of oxygen over the apical sites, where a p is parameter of perovskite cell. The other sample (II) contained a single phase a p × 2a p × 2a p , Pmmm (Z = 2) with well defined octahedral and pyramidal sublattices. Treatment of neutron diffraction patterns of the sample I itself gives a sophisticated spin structure. Knowing the structure of sample II, one can chose only proper magnetic lines, which give exactly the same results as for sample II. Above the Néel temperature T N ≈ 290 K, there is a structural transition to the phase 2a p × 2a p × 2a p , Pmma. At T N , the spins order with the wave vector k 19 = 0 (phase 1). At T 1 ≈ 255 K, a magnetic transition takes place to the phase 2 with k 22 = b 3 /2. At T 2 ≈ 170 K, the crystal structure changes to 2a p × 2a p × 4a p , Pcca (Z = 4). 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 mutual orientation of the moments in the ordered non-equivalent pyramidal or octahedral positions. Spontaneous moment M 0 = 0.30(3) µ B /Co at T = 260 K is due to ferrimagnetic ordering of the moments M Py1 = 0.46(9) µ B and M Py2 = −1.65(9) µ B in pyramidal sites (Dzyaloshinskii-Moriya canting is forbidden by symmetry). The moments in the non-equivalent octahedral sites are: M Oc1 = −0.36(9) µ B , M Oc2 = 0.39(9) µ B . At T = 230 K, M Py1 = 0.28(8) µ B , M Py2 = 1.22(8) µ B , M Oc1 = 1.39(8) µ B , M Oc2 = −1.52(8) µ B . At T = 100 K, M Py1 = 1.76(6) µ B , M Py2 = −1.76 µ B , M Oc1 = 3.41(8) µ B , M Oc2 = −1.47(8) µ B . The moment values together with the ligand displacements are used to analyze the picture of spin-state/orbital ordering in each phase.

No abstract

The critical exponents g c 0.84͑7͒ of the chiral susceptibility above the Néel temperature, T N , and b c 0.44͑2͒ of the average chirality below T N have been determined for the triangular-lattice antiferromagnet CsMnBr 3 by means of polarized neutron scattering. These first experimental values of chiral critical exponents are in line with theoretical predictions and fulfill their scaling relation. The temperature at which the average chirality appears coincides with the spin-order transition temperature, T N .

No abstract

A single crystal of GdBaCo 2 O 5.47 (2) has been studied by means of X-ray diffraction. Appearance of superstructure reflections at T = 341.5(7) K gives an evidence of continuous transition to the phase with unit cell doubled along the shortest edge a 1 . Critical exponent for the order parameter is found to be 0.33(1) = β .The superstructure reflections are about 2-4 orders of magnitude weaker than the basic ones. Their systematic extinction indicates the crystal symmetry change from to . The integrated intensities allow to calculate displacements of atoms from the positions in the high-temperature phase. The cobalt-ligand distances in the ordered phase are discussed in terms of the spin-state/orbital ordering of Co Pmmm Pmma 3+ ions. PACS numbers: 61.10.+Nz; 61.50.Ks; 61.66.Fn Since discovery of giant magnetoresistance in the oxygen-deficient layered perovskites RBaCo 2 O 5+x , where R is a rare earth [1], these materials attract high interest. Their orthorhombic structure at x ≈ 0.5 with the unit cell a 1 ≈ a p , a 2 ≈ 2a p , a 3 ≈ 2a p , where a p is parameter of the pseudocubic perovskite cell, is usually described by the Pmmm space group [2-5]. One can imagine the structure as a sequence of stacking plains [CoO 2 ][BaO][CoO 2 ][RO x ] along [0,0,1], which results in two types of the cobalt environment: CoO 5 pyramids and CoO 6 . octahedra. The nominal valence of cobalt at x = 0.5 is 3+. It is known [6] that the Co 3+ ion has a non-magnetic, or low-spin ground state (LS, ) as well as two excited states, the intermediate-spin (IS, ) and the high-spin (HS, ) ones. The energy differences are small enough to gain the excited states by the thermal fluctuations or due to the lattice change, which results in spin state transitions [7]. A metal-insulator (MI) transition has been also found, with the transition temperature for GdBaCo 0 6 2 g g e t 1 5 2 g g e t 2 4 2 g g e t 2 O 5+x 350 K < T MI < 370 K being dependent on the oxygen content of 0.4 < x < 0.47 [1, 2 8-10]. The transition is of the first order, which is indicated by a hysteresis of 8 K in resistivity for TbBaCo 2 O 5.4 [8].In spite of numerous studies of these materials, neither Co 3+ spin state nor nature of the MI transition is finally established. A spin-state transition coupled with the orbital degrees of freedom is suggested to be a driving force for the MI transition. The distribution of the IS e g orbitals (3x 2 -r 2 ) in pyramids and (3y 2 -r 2 ) in octahedral sites on cooling has been suggested as an origin of transition on the basis of structural studies of TbBaCo 2 O 5.5 [11]. On the other hand, it has been concluded that the transition to metallic phase in GdBaCo 2 O 5.5 is due to excitation of the LS-state electrons into e g band of the Co HS-state in octahedra; with Co in pyramids having IS both sides of T MI [9]. This conclusion has been made because of octahedron expansion of about 0.012(4) Å and simultaneous pyramid shrinking. The spin-state as well as the orbital ordering among one type of coordinating polyhedra was considered in a num...