Mesoscopic phase separation inNaxCoO2(0.65⩽x

Abstract

Nuclear magnetic resonace ͑NMR͒, EPR and magnetization measurements in Na x CoO 2 for 0.65ഛ x ഛ 0.75 are presented. While the EPR signal arises from Co 4+ magnetic moments ordering at T c Ӎ 26 K, 59 Co NMR signal originates from cobalt nuclei in metallic regions with no long range magnetic order and characterized by a generalized susceptibility typical of strongly correlated metallic systems. This phase separation in metallic and magnetic insulating regions is argued to occur below T * ͑x͒ ͑220-270 K͒. Above T * an anomalous decrease in the intensity of the EPR signal is observed and associated with the delocalization of the electrons which for T Ͻ T * were localized on Co 4+ d z 2 orbitals. It is pointed out that the in-plane antiferromagnetic coupling J Ӷ T * cannot be the driving force for the phase separation. PACS number(s): 76.60. Ϫk, 76.30.Ϫv, 71.27.ϩa II. EXPERIMENTAL RESULTS A. Sample preparationNa x CoO 2 (x = 0.65, 0.70, and 0.75) samples were prepared following the "rapid heat-up" method. 12,13 A stoichiometric mixture of 99.99% purity Co 3 O 4 and Na 2 CO 3 was thoroughly ground, and placed in a furnace and preheated at 750°C for 12 hours. The obtained samples were reground and annealed for 15 hours at 850°C in air, followed by slow cooling to room temperature. X-ray powder diffraction measurements confirmed all samples to be single phase of hex-PHYSICAL REVIEW B

Heat capacity studies of the magnetic phase transition in sodium-rich NaxCoO2 (0.73⩽x⩽0.87)

“…This is in agreement with the previous evidence of the intrinsically inhomogeneous magnetic state related to the Na doping [21]. Even though there are strong indications (EPR and NMR measurements [22], neutron scattering results [23,24] and STM investigations [25]) that this inhomogeneity is of mesoscopic scale, the observed coexistence of two magnetic phases, ordering at different temperatures [17], represents an example when the system is macroscopically phase separated. Thus, the intense debate about the character of inhomogeneities in cobaltates has still not come to the end.…”

Evidence of a single nonmagneticCo3+state in theNa1CoO2cobaltate

“…In Na x CoO 2 , sodium nuclei are coupled by large hyperfine couplings to neighboring cobalts through oxygen orbitals. 7,21 These hyperfine couplings turn sodium nuclei into sensitive probes of the electronic spin susceptibility 59 χ spin of the nearby Co layers through…”

Localized versus itinerant magnetic moments inNa0.7CoO2

“…In particular, the postulated [22,23,24] itinerant 3d electron system seems at odds with the claims of previous 23 Na-NMR studies, which reported that between 40 and 200 K the 23 Na-NMR response can be understood by assuming the presence of localized 3d moments below room temperature. [9,14] This seemed justified from observing that the Knight shifts 23 K of the NMR signals from different Na sites scale with the magnetic susceptibility χ(T ). The latter exhibits a clear Curie-Weiss type behavior and, in the same temperature range, the spinlattice relaxation rate T…”

“…[9,10,11,12,13,14,15,16,17,18] The Co ions are enclosed in edge sharing O-octahedra which in turn are separated by insulating Na layers. [19,20] The latter mainly act as a charge reservoir.…”