Here we report an unconventional magnetic and transport phenomenon in a layered cobalt oxide, NaxCoO2. Only for x = 0.75, a magnetic transition of the second order was clearly detected at Tm ∼ 22 K where an apparent specific-heat jump, an onset of extremely small spontaneous magnetization, and a kink in resistivity came in. Moreover large positive magnetoresistance effect was observed below Tm. These features of the transition strongly indicate the appearance of an unusual electronic state that may be attributed to the strongly-correlated electrons in Na0.75CoO2.

An uncommon oxygen absorption/desorption behavior is reported for the cation-stoichiometric cobalt oxide, YBaCo 4 O 7 , structurally composed of two kinds of layers of corner-sharing CoO 4 tetrahedra. We have found that YBaCo 4 O 7+δ absorbs and desorbs oxygen up to δ ≈ 1.5 in a narrow temperature range below 400 °C. The oxygen uptake/release process is highly reversible, being controlled by both temperature and oxygen partial pressure. Such a large low-temperature oxygen-content tunability is of great promise in regard to applications related to, for example, oxygen storage. Materials with similar characteristics are, to a large degree, lacking today.

In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by Fe57 Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.

Electrical resistivity (ρ) and thermoelectric power (S) were measured for NaxCo2O4 samples with various Na contents (x). A highly precise control of Na nonstoichiometry was facilitated by optimizing the synthesis procedure. With increasing x, the absolute value of ρ monotonically decreased, while the value of S increased. For the x=1.5 sample, S reached a value as high as ∼120 μV/K (at 300 K).

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The nature of the magnetic transition of the Na-rich thermoelectric Na0.75CoO2 at 22K was studied by positive muon-spin-rotation and relaxation (µ + SR) spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero field µSR measurements indicated the existence of a static internal magnetic field at temperatures below 22 K (= Tm). The observed muon spin precession signal below Tm consisted of three components with different precession frequencies, corresponding to three inequivalent muon + sites in the Na0.75CoO2 lattice. The total volume fraction of the three components was estimated as ∼21% at 2.5 K; thus, this magnetic transition was not induced by impurities but is an intrinsic change in the magnetism of the sample, although the sample was magnetically inhomogeneous otherwise. On the other hand, a similar experiment on a Na0.65CoO2 sample exhibited no magnetic transition down to 2.5 K; which indicates that the average valence of the Co ions is responsible for inducing the magnetic transition at 22 K. PACS numbers: 76.75.+i, 72.15.Jf

The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1222͒ have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magnetosuperconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O 2 planes below T c ϳ40 K. The exact nature of Ru spins magnetic ordering is still being debated, and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac vs T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin-glass with magnetically interacting clusters. Such an interpretation is supported by thermoremanent magnetization ͑TRM͒ measurements at Tϭ60 K. TRM relaxations are well described by a stretched exponential relation, and present significant aging effects.