Three-Dimensional Spin Fluctuations inNa0.75CoO2

Abstract

We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na0.75CoO2. The data confirm ferromagnetic correlations within the cobalt layers and reveal antiferromagnetic correlations perpendicular to the layers, consistent with an A-type antiferromagnetic ordering. The magnetic modes propagating perpendicular to the layers are sharp, and reach a maximum energy of approximately 12 meV. From a minimal spin-wave model, containing only nearest-neighbor Heisenberg exchange interactions, we estimate the interlayer and intralayer exchange constants to be 12.2+/-0.5 meV and -6+/-2 meV, respectively. We conclude that the magnetic fluctuations in Na0.75CoO2 are highly three dimensional.

X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering inγ-NaxCoO2(x=

“…3,4 The most interesting material in the Pauli metal regime is x ϳ 1 / 3 that exhibits an unexpected superconductivity upon further intercalation with water. 10 For 0.75Յ x Յ 0.84 in the Curie-Weiss metal regime, the onset of a long-range A-type antiferromagnetic ordering ͑Co-intraplane ferromagnetic coupling and Co-interplane antiferromagnetic coupling͒ at ϳ22 K has been indicated, [11][12][13] and the enhanced thermoelectric power due to large spin entropy has been found and explained for ϳ0.84. 14 Intriguingly, the other Curie-Weiss metal, x = 0.71, exhibits no magnetic ordering down to ϳ1.8 K. 3 The exactly half Na filled phase of x = 0.5 is the only compound that shows a metal-to-insulator transition ͑MIT͒ near ϳ51 K. 3,4,15,16 Indeed, the unique transport property for x = 0.5 had stimulated a remarkable number of associated studies, leading to the discovery that the room-temperature crystal structure of x = 0.5 shows an orthorhombic superlattice of ͱ 3a p ϫ 2a p ϫ c p ͑a p and c p , the hexagonal primitive cell parameters͒.…”

“…3,4,17 For 0.75Յ x Յ 0.84 within the ionic context ͑charge localization 15 ͒, it has been suggested that the existence of A-type magnetic ordering near ϳ22 K is rather unexpectedly high considering the dilute spin-half Co 4+ in the lattices. 12,13 The inelastic neutron scattering further indicated that the ferromagnetic intraplane and antiferromagnetic interplane exchange constants are nearly comparable, whereas a much smaller interplane magnitude was expected due to the lamellar structure of ␥-Na x CoO 2 . 12,13 To account for these unexpected magnetic properties, the possible effect of sodium-ordering induced superlattices and itinerant charges deviating from the ionic context has been proposed 12,13 although the detailed superlattices remain to be elaborated.…”

“…12,13 The inelastic neutron scattering further indicated that the ferromagnetic intraplane and antiferromagnetic interplane exchange constants are nearly comparable, whereas a much smaller interplane magnitude was expected due to the lamellar structure of ␥-Na x CoO 2 . 12,13 To account for these unexpected magnetic properties, the possible effect of sodium-ordering induced superlattices and itinerant charges deviating from the ionic context has been proposed 12,13 although the detailed superlattices remain to be elaborated. In x = 0.71, the absence of the A-type magnetic ordering is also puzzling when taking into account that x = 0.71 is compositionally close to the magnetically ordered x = 0.75.…”

59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2 · yH2O

“…The A-type spin fluctuations, i.e. intra-plane ferromagnetic and inter-plane antiferromagnetic fluctuations, are observed in Na 0.7 CoO 2 by inelastic neutron scattering experiments [3,4]. The intercalated water molecules may block the inter-plane antiferromagnetic couplings.…”

“…Except the charge ordered insulator Na x CoO 2 with x ≈ 1/2, Na x CoO 2 and the hydrated compounds are itinerant electronic systems. For x ≈0.7, A−type spin fluctuations (in-plane ferromagnetic and out-of-plane antiferromagnetic spin fluctuations) [3,4] The side views of the crystal structures of a parent Na0.7CoO2, a Na-deintercalated Na0.35CoO2 and a bi layer hydrated Na0.35CoO2 · yH2O. Open arrows indicate soft chemical treatment.…”