The magnetic field and pressure dependence of the magnetic ordering transition in NaxCoO2 (0.6≤x≤0.72)

We have measured the magnetic field (H≤90 kOe) and pressure (P≤10 kbar) dependence of the magnetic ordering temperature, Tmag, in single crystal samples of NaxCoO2 for a range of Na concentrations (0.60≤x≤0.72). We show that in zero field Tmag remains constant with decreasing x before magnetic order disappears at x = 0.65. Heat capacity and magnetization data show that for x = 0.70 Tmag is unchanged in an applied field. In contrast, magnetization data collected under hydrostatic pressure show that Tmag increases from 22.0 K at 1 bar to 25.4 K at 10 kbar. This rise is at odds with the behaviour expected for model spin density wave systems.