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Complex cobalt oxides with perovskite structure attract significant current interest owing to the possi bility of different spin states in the Co 3+ ion and their closely related magnetic and transport characteristics [1]. For example, in layered LnBaCo 2 O 5.5 (Ln = lan thanide) cobaltites, metal-insulator and antiferro magnet-"ferromagnet" transitions were observed. The latter transition is accompanied by anomalies in transport properties and by the giant magnetoresistive effect [2,3]. The magnetoresistive effect manifests itself in single crystals along only one crystallographic axis [4] and has little in common with the isotropic colossal magnetoresistance in magnetic semiconduc tors and manganites. In this class of compounds, the ferromagnetic component is not due to the existence of ferromagnetic clusters in the antiferromagnetic host material since it produces a coherent magnetic contri bution to neutron scattering [5,6]. In addition, the antiferromagnet-ferromagnet transition has clearly pronounced cooperative features [4], which are incompatible with the behavior of the system of clus ters. However, in contrast to magnetic semiconductors and manganites [7], cobaltites exhibit neither signifi cant anomalies in the electrical conductivity nor a pronounced magnetoresistive effect near the Curie point [8,9]. It is important to note the absence of a complete set of solid solutions between the LaCoO 3 and LaBaCo 2 O 5.5 phases [10]. Therefore, it is reason able to state that the compound with the nominal composition Gd 0.9 Ba 0.1 CoO 3 described in [11] is actu ally a mixture of the GdCoO 3 and GdBaCo 2 O 5.5 phases. Another class of anion deficient layered cobaltites was produced quite recently. It has the chemical composition Sr 3 LnCo 4 O 10.5 + δ (its reduced chemical formula is Sr 0.75 Ln 0.25 CoO 3 -γ ), where the rare earth ions can partially substitute for strontium ions and vice versa [12][13][14][15]. Its crystal structure is built by alternating anion deficient CoO 4 + δ layers and lay ers formed by CoO 6 octahedra with sharing vertices. This class of compounds is characterized by high mag netic ordering temperature (up to 360 K) [16][17][18]. Spontaneous magnetization appears below 360 K, attains its maximum value near room temperature, and then decreases gradually down to liquid helium temperatures [16][17][18][19][20]. Several conjectures explaining the anomalous temperature dependence of the mag netization have been put forward. In [21], it was sug gested that a certain fraction of Co 3+ ions undergo the transition from the high spin to low spin state with a decrease in temperature. However, the neutron dif fraction studies do not reveal any anomalous decrease in the magnetic moment when the temperature is decreased [19,20,22]. In the whole temperature range below the magnetic ordering point, the antiferromag netic G type structure was observed, whereas it was impossible to reliably detect the ferromagnetic contri bution since it turned out to be quite small. For this reason, in [19,20], the ano...