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.

Abstract. This paper presents results of a recent study of multiferroic CuCrO 2 by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at T N1 = 24.2 K and T N2 = 23.6 K. The low temperature magnetic structure below T N2 is unambiguously determined to be a fully 3-dimensional proper screw. Between T N1 and T N2 antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the (H, K) plane, whereas nearest neighbors along the L-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.

12In this paper detailed neutron scattering measurements of the magnetic excitation spectrum of CuCrO2 in the ordered state below TN1 = 24.2 K are presented. The spectra are analyzed using a model Hamiltonian which includes intralayer-exchange up to the next-next-nearest neighbor and interlayer-exchange. We obtain a definite parameter set and show that exchange interaction terms beyond the next-nearest neighbor are important to describe the inelastic excitation spectrum. The magnetic ground state structure generated with our parameter set is in agreement with the structure proposed for CuCrO2 from the results of single crystal diffraction experiments previously published. We argue that the role of the interlayer exchange is crucial to understand the incommensurability of the magnetic structure as well as the spin-charge coupling mechanism.

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In this paper we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite YbFeO 3 at temperatures below and above the spin-reorientation (SR) transition T SR = 7.6 K, in magnetic fields applied along the a, b and c axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of ∼60 meV, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of ∼1 meV within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at T SR drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinon-like fluctuations. At T < T SR , the Yb spin chains have a well defined field-induced ferromagnetic (FM) ground state, and the spectrum consists of a sharp single-magnon mode, a two-magnon bound state, and a two-magnon continuum, whereas at T > T SR only a gapped broad spinon-like continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem J Yb-Yb , mainly neglected in previous studies, creates unusual quantum spin dynamics on the low energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be "hidden" by a dominating interaction. entropy evolution [9], laser-pulse induced ultrafast spinreorientation [10-12] etc. Magnetic property investigations of the rare-earth orthoferrites RFeO 3 have shown that the Fe 3+ moments (S = 5 2 ) are ordered in a canted AFM structure Γ 4 at high temperature with T N ≈ 600 K (details of the notations are given in [13]), and the spin canting gives a weak net ferromagnetic moment along the c axis [ Fig. 1(c)] [13][14][15]. Furthermore, symmetry analysis and careful neutron diffraction measurements have found a second "hidden" canting along the b-axis, which is symmetric relative to the ac-plane and does not create a net moment [16,17]. With decreasing temperature, a spontaneous spin-reorientation (SR) transition from Γ 4 to the Γ 2 magnetic configuration occurs in many orthoferrites with magnetic R-ions [13,14] in a wide temperature range from T SR ≈ 450 K for SmFeO 3 down to T SR ≈ 7.6 K for YbFeO 3 , and the net magnetic moment rotates from the a to the c axis [see Fig. 1(c-e)]. Most of previous work that was devoted to the investigation of the SR transition in RFeO 3 , associated this phenomenon with the R-Fe exchange interaction, because orthoferrites with nonmagnetic R =La, Y or Lu preserve the Γ 4 magnetic structure down to the lowest temperatures.Taking into account three characteristic t...