Spin gaps and magnetic structure of NaxCoO2

TL;DR

This paper investigates the magnetic excitations and structure of NaxCoO2 using neutron scattering, revealing spin gaps, anisotropy effects, and a field-induced spin-flop transition, advancing understanding of its magnetic properties.

Contribution

The study provides detailed measurements of spin gaps and magnetic structure in NaxCoO2, including temperature dependence and field-induced transitions, which were not previously characterized.

Findings

Identification of two spin gaps that close above T_m~22 K.

Observation of a magnetic Bragg peak indicating a spin-flop transition.

Correlation of the spin-flop field with the anisotropy gap size.

Abstract

We present two experiments that provide information on spin anisotropy and the magnetic structure of NaxCoO2. First, we report low-energy neutron inelastic scattering measurements of the zone-center magnetic excitations in the magnetically ordered phase of Na0.75CoO2. The energy spectra suggest the existence of two gaps, and are very well fitted by a spin-wave model with both in-plane and out-of-plane anisotropy terms. The gap energies decrease with increasing temperature and both gaps are found to have closed when the temperature exceeds the magnetic ordering temperature T_m~22 K. Secondly, we present neutron diffraction studies of Na0.85CoO2 with a magnetic field applied approximately parallel to the c axis. For fields in excess of ~8T a magnetic Bragg peak was observed at the (0,0,3) position in reciprocal space. We interpret this as a spin-flop transition of the A-type antiferromagnetic structure, and we show that the spin-flop field is consistent with the size of the anisotropy gap.