Pressure-induced electron spin transition in the paramagnetic phase of the GdFe3(BO3)4 Heisenberg magnet

The HS -> LS spin crossover effect (high-spin -> low-spin transition) induced by high pressure in the range 45-53 GPa is observed in trivalent Fe3+ ions in the paramagnetic phase of a (GdFe3)-Fe-57(BO3)(4) gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K-beta high-resolution x-ray emission spectroscopy (YES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.