Static magnetic order inNa0.75CoO2detected by muon spin rotation and relaxation

Abstract

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

Doping effects on the electronic and structural properties ofCoO2: AnLSDA+Ustudy

“…Since the magnetic and/or charge ordering energies are usually very small, it is sometimes difficult to distinguish between different ordering states based only on their energy difference. This is particularly true in magnetically frustrated systems such as CoO 2 , as is evidenced by experiments where no long range magnetic ordering is observed for Na x CoO 2 except for x ∼ 0.75 8,9 . Nevertheless, it was suggested that a short range ferromagnetic (FM) ordering might be preferred in these systems 18 .…”

Magnetic phase transition at88KinNa0.5CoO2Pedrini1, Gavilano2, Felder3 et al. 2005Phys. Rev. B131140View full textAdd to dashboardCite

“…The material with composition x ≈ 0.7 exhibits a large thermoelectric power [3], and indications of a charge instability in the temperature range between 230 and 290 K were observed [4]. A spin-density wave formation at temperatures below 22 K was detected for x = 0.75 [5]. Superconductivity is induced at x = 0.35, if the CoO 2 -layers are intercalated with water [6].…”

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

“…For a comprehensive description of the µSR technique applied to Na x CoO 2 see e.g. Sugiyama et al 20 . While µSR experiments reveal important information on the static magnetism they cannot elucidate on the possible presence of spin fluctuations with a time scale shorter than the order of 10 −10 s. On the other hand, from specific heat we get strong indications of low-energy magnetic excitations as will be discussed in the following.…”