First-order magnetic phase transition in layered Sr3YCo4O10.5 + δ-type cobaltites

Abstract

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...

The ferromagnetic and antiferromagnetic phases in anion deficient La0.5−xPrxBa0.5CoO3−δ cobaltites

“…32 It worth to be noting that the antiferromagnet-ferromagnet transitions have been revealed in Sr 3 YCo 4 O 10.5 layered perovskites also. 33 Considering these reports, we have decided to investigate the effect of oxygen deficiency on structural and magnetic properties of La 0.5Àx Pr x Ba 0.5 CoO 3Àd cobaltites with ordinary or layered perovskite structure where oxygen vacancies are disordered. In the present work, it is shown that oxygen deficiency in these cobaltites leads to a concentrational transition from ferromagnetic to pure antiferromagnetic phase through mixed state.…”

The Structure, Magnetic and Magnetotransport Properties of Sr1−xYxCoO3−δ Layered Cobaltites

“…According to, [ 20 ] the Sr 3 LnCo 4 O 10.5+δ composition exhibits a first‐order antiferromagnetic‐ferromagnetic transition below room temperature very similar to that observed in the series of layered LnBaCo 2 O 5.5 cobaltites. [ 2,21 ] The ferromagnetic component disappears with a small substitution of Sr 2+ ions by Ca 2+ ions or Co ions by Fe ions.…”

“…[ 2,21 ] The ferromagnetic component disappears with a small substitution of Sr 2+ ions by Ca 2+ ions or Co ions by Fe ions. [ 18,22,23 ] The appearance of the ferromagnetic component was previously explained by the following reasons: orbital ordering, [ 24 ] ferrimagnetism due to the presence of nonequivalent positions of Co 3+ ions, [ 25 ] noncollinear magnetic structure in the anion‐deficient layers [ 20 ] or formation of ferromagnetic “bags” in oxygen‐enriched CoO 6 layers. [ 26 ]…”

Co-Substitution Effect in Room-Temperature Ferromagnetic Oxide Sr3.1Y0.9Co4O10.5

“…In this material, a spinstate crossover, which is a change from the HS-and IS-states in the CoO6 layer into the LS state, is observed around 150 K. Additionally, it has been reported that the spin-state crossover is enhanced by physical pressure [24] and chemical pressure owing to the pressure-induced enlargement of the crystal-field splitting [25]. In this interesting cobalt oxide, so far, various kinds of investigations [26][27][28][29][30] have been conducted and the A-site substitution has been actively studied in order to reveal and control the spin state of Co ions [25,31,32]. In this study, we substituted nonmagnetic elements of Al and Ga with different ionic radii for the B-site (Co-site) and measured the magnetic properties of Sr3.1Y0.9Co4−xBxO10.5 (B = Al and Ga: x = 0, 0.2, and 0.4).…”