Spin gaps and magnetic structure of NaxCoO2

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.

arXiv comment9 pages, 7 figures

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• Search for Magnetic Excitation Spectra of NaxCoO2.yD2O -Neutron Scattering-
• From Hubbard bands to spin-polaron excitations in the doped Mott material Na${}_x$CoO${}_2$
• Interplay between magnetism and sodium vacancy ordering in NaxCoO2
• A possibility of high spin hole states in doped CoO${}_2$ layered systems
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