Direct Link between Low-Temperature Magnetism and High-Temperature Sodium Order inNaxCoO2

Staging model of the ordered stacking of vacancy layers and phase separation in layeredNaxCoO2(xShu1, Huang2, Chu3 et al. 2009Phys. Rev. B1413402View full textAdd to dashboardCite

“…Although crude magnetic-phase mappings in this range before suggest that T N varies between 22-27 K, 3,14 Schulze et al recently found an additional 8 K phase for x ϳ 0.80 and 0.85, which can be obtained only after a slow cooling process. 10 With carefully tuned single crystals in the narrow range of 0.82-0.86, we are able to revisit the magnetic-phase diagram and untangle the mystery of T N variation. Magnetic-susceptibility measurement results are shown in Fig.…”

“…[7][8][9] Recent studies of x ϳ 0.80 and 0.85 by Schulze et al conclude that Na ordering is highly dependent on the cooling rate, where an additional magnetic ordering below 8 K appears only after the sample is slowly cooled through the 300-200 K range. 10 However, the real impact of the successive Na rearrangement processes remains to be clarified and the phase diagram must be revisited.…”

Ionic Hubbard model on a triangular lattice forNa0.5CoO2,Rb0.5Powell1, Merino2, McKenzie3 2009Phys. Rev. B9020View full textAdd to dashboardCite

“…This prediction is consistent with the experimental measurements of the charge disproportionation. 26,29,40 Further, these calculations predict a gap of the same order of magnitude as seen in the ARPES ͑Ref. 6͒ and the resistivity, 4 predict the large moment seen in neutron scattering, 34,35 and reproduce the main features seen in the optical conductivity.…”

“…Furthermore, cooling a material at different rates found that the presence of sodium ordering can drive an additional magnetic phase transition at x = 0.8 and 0.85. 40 B. Theories of Na 0.5 CoO 2 Choy et al 41 considered an extended Hubbard model that included the Coulomb interaction between electrons on neighboring sites, V, but neglected the Na ordering and the "ionic" term in Eq.…”

Spin fluctuations, magnetic long-range order, and Fermi surface gapping inNaxCoO2

“…For x = 0.75, only 21% magnetically ordered volume fraction has been reported 20 which was interpreted as being the result of a phase separation into one magnetically ordered and one non-ordered phase 21 to explain the large non-magnetic volume fraction. In single crystals the actual phase separation and thus coexistance of two distinct magnetic phases at low temperatures has been directly observed recently: In addition to the well-known 22 K AFM transition 6,7,8,26 there is another transition at 8 K associated with a distinct second phase whose formation is dependent on the sample's cooling protocol 27 and can be completely suppressed by thermal quenching. This points to sodium order as a driving force of magnetic order due to the patterned sodium Coulomb potential breaking the crystal symmetry and highlights the importance to account for multiple phases when interpreting low-T data.…”