Spin structure and magnetic phase transitions inTbBaCo2O5.5

Abstract

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.

X-ray and neutron diffraction studies of coupled structural phase transitions in DyBaCo2O5.5

“…3(a) are mainly due to magnetic scattering as well as those for the Tb material [6]. Two of them with l -odd indicate a spin antitranslation [11] t ′ = c 3 = c 4 /2, while (1, 1, 2) gives an evidence of a translation t = c 3 = c 4 /2.…”

“…It has been suggested [5,6] that the structural changes are driven by the orbital/spinstate ordering of the Co 3+ ions, which may be expected to influence the transport properties. Possible mechanisms, like spin blockade, are suggested in [8,9,10].…”

“…Two of them with l -odd indicate a spin antitranslation [11] t ′ = c 3 = c 4 /2, while (1, 1, 2) gives an evidence of a translation t = c 3 = c 4 /2. To resolve the issue, we have made an assumption [6] that the low-temperature magnetic transition in the Tb material coincides with a structural one that double c axis: c 4 = 2c 3 . Unlike the Tb material, reflection (1, 1, 1) is absent in the case of the DyBaCo 2 O 5.5 below 200 K. However, the other reflection (0, 0, 1) with l -odd still coexists with (1,1,2), and the magnetic transition at In the range of this wide temperature hysteresis, the unusual magnetic properties of field-cooled and zero-field-cooled crystals in the temperature range 120 < T < 210 K [12,13], probably, may be due to coexistence of two phases with different crystal and magnetic structures.…”

“…1). This difference is very important for the magnetic/spinstate/orbital ordering, and the group P mma has been later verified [4] with a twin-free single crystal of Tb 0.9 Dy 0.1 BaCo 2 O 5.54 (3) , the material that is expected to have the properties similar to the Tb ones studied in [6]. The first objective of our work is to check whether this phase transition is typical for the RBaCo 2 O 5.5 materials.…”

“…To explain the magnetic structure at T = 100 K of TbBaCo 2 O 5.53 (1) we have postulated [6] a structural phase transition at about 170 K to the phase with the lattice parameters a 4 ≈ 2a p , b 4 ≈ 2a p , c 4 ≈ 4a p , as shown in Fig. 1.…”