Two dimensionality in quasi-one-dimensional cobalt oxides (original) (raw)
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Evidence of Two Dimensionality in Quasi-One-Dimensional Cobalt Oxides
Physical Review Letters, 2006
The quasi-one-dimensional (Q1D) cobalt oxides A n2 Co n1 O 3n3 (A Ca, Sr, and Ba, n 1 ÿ 1) were investigated by muon-spin spectroscopy under applied pressures of up to 1.1 GPa. The relationship between the onset Néel temperature T on N and the interchain distance (d ic ), which increases monotonically with n, is well fitted by the formula T N =T N;0 1 ÿ d ic =d ic;0 ; here T on N 100 K for Ca 3 Co 2 O 6 (n 1) and 15 K for BaCoO 3 (n 1) at ambient P. The T on N ÿ d ic curve also predicts a large P dependence of T N for the compounds with n 5, i.e., in the vicinity of d ic;0 , while the n 1-4 compounds show only a very small effect. Indeed, our high-pressure SR results show that T N of BaCoO 3 is enhanced by P with a slope of 2:2 K=GPa, whereas no detectable changes by P for both Ca 3 Co 2 O 6 and Sr 4 Co 3 O 9 (n 2). This clearly confirms the role of the 2D-antiferromagnetic interaction on T on N in the Q1D cobalt oxides.
Two dimensionality in quasi-one-dimensional cobalt oxides confirmed by muon-spin spectroscopy
Journal of Magnetism and Magnetic Materials, 2007
The quasi-one-dimensional (Q1D) cobalt oxides, A nþ2 Co nþ1 O 3nþ3 (A ¼ Ca, Sr and Ba, n ¼ 1À1), were investigated by muon-spin spectroscopy under applied pressures of up to 1.1 GPa. The relationship between the onset Ne´el temperature T on N and the inter-chain distance ðd ic Þ, which increases monotonically with n, is well fitted by the formula T N =T N;0 ¼ ð1 À d ic =d ic;0 Þ b . The T on N 2d ic curve also predicts a large P dependence of T N for the compounds with nX5, i.e., in the vicinity of d ic;0 , while the n ¼ 124 compounds show only a very small effect. Indeed, our high-pressure m þ SR results show that T N of BaCoO 3 ðn ¼ 1Þ is enhanced by P, with a slope of 2.1 K/GPa, whereas no detectable changes between ambient pressure and 1.0 GPa for both Ca 3 Co 2 O 6 ðn ¼ 1Þ and Sr 4 Co 3 O 9 ðn ¼ 2Þ. This clearly confirms the role of the 2D-AF interaction on T on N in the Q1D cobalt oxides. r
Magnetism of layered cobalt oxides investigated by muon spin rotation and relaxation
Physical Review B, 2002
Muon spin rotation-relaxation (SR) spectroscopy has been used to investigate the magnetic properties of polycrystalline Ca 3Ϫx M x Co 4 O 9 (xр0.5,M ϭSr,Y, and Bi͒ and Na 0.7 CoO 2 samples in the temperature range between 2.5 and 300 K. It was found that Ca 3 Co 4 O 9 exhibits a magnetic transition at around T c ϭ100 K; at lower temperatures, two types of relaxation were observed using a weak transverse field SR technique with Hϭ104 Oe: one with a fast relaxation rate F ϳ10 s Ϫ1 and the other with a slow S ϳ0.1 s Ϫ1 . Zero-field SR measurements suggest the existence of an incommensurate spin-density-wave ͑SDW͒ state below T c ͑i.e., T c ϭT SDW ), although a ferrimagnetic M -H loop was observed by a dc susceptibility measurement below 19 K. The substitution of Y or Bi for Ca increased T SDW , while the substitution of Sr for Ca did not affect T SDW . This indicates that the SDW transition depends strongly on the average valence of the Co ions. The related material Na 0.7 CoO 2 showed no magnetic transitions below 30 K. Considering the difference between the crystal structures of Ca 3 Co 4 O 9 and Na 0.7 CoO 2 , we suggest that Co ions in the rocksalt-type layers of Ca 3 Co 4 O 9 are likely to play a significant role in inducing the SDW transition around 100 K.
Muon spin relaxation study of misfit-layered cobalt dioxide
Solid State Communications, 2010
In order to clarify the nature of a possible magnetic phase transition at T m = 13 K for the misfit layered cobalt dioxide with a double rocksalt (RS-) type block, [Ca 0.85 OH] RS 1.16 [CoO 2 ], we have measured the muon spin rotation and relaxation (µ + SR) spectra in the temperature (T ) range between 1.8 and 20 K. All µ + SR parameters, such as the dipolar interaction frequency (f µOH ) due to the formation of µ + -OH, its asymmetry (A µOH ) and relaxation rate (σ µOH ), are found to be independent of T both above and below T m . This is a clear indication that the transition at T m is not due to magnetic order but has a structural and/or electronic origin. In addition, we have been able to determine the muon stopping site for this misfit layered compound. It is found to be located within the CaOH blocking layers, where the muon and OH-group form a water-like Hµ + O unit as indicated by a very slow oscillation in the zero-field relaxation spectrum.
Physical Review B, 2008
Thanks to the unique power of muon-spin spectroscopy, we found that the quasi-one-dimensional Co-Rh oxides A n+2 CoRh n O 3n+3 ͑A = Ca, Sr; n = 1, 2, and 3͒ exhibit a two-dimensional antiferromagnetic transition that ranges from T N on = 185 K for n = 1 to 125 K for n = 3 with a transition width ͑⌬T͒ of about 80 K. The variation of T N on with n is explained by the increase in the distance between the neighboring CoRh n O 3n+3 chains. Static magnetic order is observed below the end point of T N ͑=T N on − ⌬T͒ for each of the three samples. The existence of the two-frequency components in the zero field spectrum indicates the appearance of ferrimagnetic order for Ca 3 CoRhO 6 below 20 K.
Physical Review B Condensed Matter and Materials Physics, 2008
Thanks to the unique power of muon-spin spectroscopy, we found that the quasi-one-dimensional Co-Rh oxides A n+2 CoRh n O 3n+3 ͑A = Ca, Sr; n = 1, 2, and 3͒ exhibit a two-dimensional antiferromagnetic transition that ranges from T N on = 185 K for n = 1 to 125 K for n = 3 with a transition width ͑⌬T͒ of about 80 K. The variation of T N on with n is explained by the increase in the distance between the neighboring CoRh n O 3n+3 chains. Static magnetic order is observed below the end point of T N ͑=T N on − ⌬T͒ for each of the three samples. The existence of the two-frequency components in the zero field spectrum indicates the appearance of ferrimagnetic order for Ca 3 CoRhO 6 below 20 K.
Magnetic behaviour of quasi-one-dimensional oxides, Ca3Co1+xMn1−xO6
Solid State Communications, 2003
The results of ac and dc magnetization and heat capacity measurements on the oxides, Ca3Co1+xMn1−xO6, forming in a K4CdCl6-derived rhombohedral quasi-one-dimensional crystal structure, are reported. As far as Ca3Co2O6 is concerned, the results reveal truly complex nature of the two magnetic transitions, identified to set in at 24 and 12 K in the previous literature. However, partial replacement of Co by Mn apparently results in a long magnetic ordering of an antiferromagnetic type (below 13 and 18 K for x= 0.0 and 0.25 respectively), instead of spin-glass freezing in spite of the fact that there is Co-Mn disorder; in addition, interestingly there are hysteretic spin reorientation effects as revealed by isothermal magnetization behavior.
Static magnetic order and anisotropy of the layered cobalt dioxides and
Physica B: Condensed Matter, 2009
The magnetism of a Pb-doped Bi 2 Sr 2 Co 2 O y (BSCO) crystal has been investigated by positive muon-spin rotation and relaxation (m þ SR) spectroscopy. Weak transverse field (wTF)-m þ SR measurements show that the whole sample enters into a magnetic state below 4:5K.Combiningtheresultsofzero−field(ZF)−mþSRexperimentwithsusceptibilitymeasurements,itisclarifiedthatthesampleisinaferromagneticorderedphasewithaCurietemperature(TC)of4.7Kandwiththeorderedinternalmagneticfieldparalleltothec−axiswithin4:5 K. Combining the results of zero-field (ZF)-m þ SR experiment with susceptibility measurements, it is clarified that the sample is in a ferromagnetic ordered phase with a Curie temperature (T C ) of 4.7 K and with the ordered internal magnetic field parallel to the c-axis within 4:5K.Combiningtheresultsofzero−field(ZF)−mþSRexperimentwithsusceptibilitymeasurements,itisclarifiedthatthesampleisinaferromagneticorderedphasewithaCurietemperature(TC)of4.7Kandwiththeorderedinternalmagneticfieldparalleltothec−axiswithin AE 15 . On the other hand, a pure BSCO crystal is also found to exhibit a bulk magnetic transition at 1.0 K by m þ SR. Since the relationship between the reduced T and reduced internal magnetic field for BSCO is almost equivalent to that for Pb-doped BSCO, the origin of the magnetic transition for both crystals is thought to be explained by common physics.
Static magnetic order in Na 0.75 CoO 2 detected by muon spin rotation and relaxation
Physical Review B, 2003
The nature of the magnetic transition of the Na-rich thermoelectric Na 0.75 CoO 2 at 22 K was studied by positive muon-spin-rotation and relaxation ( ϩ SR) spectroscopy, using a polycrystalline sample in the temperature range between 300 and 2.5 K. Zero-field SR measurements indicated the existence of a static internal magnetic field at temperatures below 22 K (ϭT m ). The observed muon-spin precession signal below T m consisted of three components with different precession frequencies, corresponding to three inequivalent ϩ sites in the Na 0.75 CoO 2 lattice. The total volume fraction of the three components was estimated as ϳ21% 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 Na 0.65 CoO 2 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.
Cascade of Bulk Magnetic Phase Transitions in NaxCoO2 as Studied by Muon Spin Rotation
Physical Review Letters, 2005
Using muon spin rotation, well-defined bulk ∼ 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x = 0.85 with the highest transition temperature ever observed for x ≥ 0.75. This stresses the diversity of x ≥ 0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x = 0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 K and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.