Fig. 4: Magnetism evolution of magnetite to megabar pressures.
a Schematic diagram of the evolution of magnetic domains and their stray magnetic fields in magnetite with pressure. b, c Pressure dependence of the magnetite magnetic field \({B}_{z}^{{{{\rm{mag}}}}}\). In (b), the black, red, and blue lines indicate the magnetic field \({B}_{z}^{{{{\rm{mag}}}}}\) with an external magnetic field of ~240 G at positions NVo1, NVo2, and NVo3 in exp2, respectively, in the inset. The dashed blue line in the inset marks the Fe3O4 sample. c Black and red lines show the magnetic field \({B}_{z}^{{{{\rm{mag}}}}}\) in exp1 (at positions NVo in Fig. 3a) and exp2 (at positions NVo in Fig. 3b) with an external magnetic field of ~130 G and ~570 G along NV axis respectively. The external magnetic fields used in exp1 and exp2 are to ensure the \({\Delta }_{B}\gg {\Delta }_{{{{{\rm{\sigma }}}}}_{\perp }}\) condition in 80 GPa and 120 GPa, respectively. (See Supplementary Note 4, section 6.4 for details of measurement deviation) (d) Pressure dependence of the linewidth broadening \({\varGamma }^{{{{\rm{mag}}}}}\). The black and red dots show the data from exp1 (at positions NVi in Fig. 3a) and exp2 (at positions NVi in Fig. 3b), respectively. The dotted lines are guide lines describing the trend of changes. The error bars in (b), (c), and (d) correspond to a 95% confidence interval. e The phase diagram of Fe3O430. FM stands for ferromagnetic, FiM stands for ferrimagnetic, and PM stands for paramagnetic. f–j Magnetic field imaging of the surface of Fe3O4 in exp2 with an external magnetic field of ~240 G at pressures of 38.2, 45.5, 50.4, 58.4 and 65.0 GPa, respectively. The dashed blue line in (e) marks the Fe3O4 sample and dashed gray lines mark the magnetic domain wall. (The details of error analysis are in Supplementary Note 4).
