Iron, and specifically iron under extreme conditions of high pressure and temperature, is an element of great importance for its geophysical and practical implications. At room temperature and with increasing pressure, iron shows a phase transition from a bcc () phase to a hcp () phase at a pressure of 130 Kbar. This transition is found to take place in a ~ 30 Kbar pressure interval. Beside the structural phase transition, one finds also an important change in the magnetic properties of the iron atom: namely iron which is ferro-magnetic in the -phase becomes non-magnetic in the -phase. The magnetic properties of iron under pressure and their interplay with the structural phase transitions have been the subject of various theoretical studies.

The investigation of the properties of condensed matter under extreme thermodynamic conditions has received a great impulse since the recent development of very high pressure techniques based on the use of diamond anvil cells (DAC). Using a DAC, it is now possible to bring samples to pressures in the Mbar range. The DAC-based methods have been very successful also when used in combination with intense and small size synchrotron radiation-based X-ray beams, especially those produced at third generation X-ray sources. This has allowed the study of iron's magnetism under high pressure using the 57Fe Mossbauer resonance.

The recent development of high energy resolution X-ray emission spectroscopy (XES) provides an alternative method to Mossbauer to probe the local magnetic properties of atoms. The XES spectra show a series of features which reflect the energies of the different electronic configurations of the excited atom. In particular, we consider an excited atom which has valence electrons with unpaired spins giving rise to a magnetic moment. Here, the exchange interactions between these magnetic electrons and the core electrons in the orbital where the photon emission process has left a core hole are responsible for the appearance of multiplet families in the emission spectrum with a dominant spin character. The energy separation of these families, and therefore the possibility to observe them easily, is related to the strength of this interaction, which is maximum when the hole is in an orbital with the same principal quantum number, n, of the magnetic electrons. In the case of iron, this situation is obtained for the Kß fluorescence line, where the final state hole is in the 3p orbital and the magnetic electrons are in the 3d orbitals.

An experiment was carried out at the inelastic X-ray scattering undulator beamline ID16, aiming to investigate by XES the magnetism in pure Fe under high pressure. The Fe Kß line has been measured across the - phase transition, taking place around Po = 130 Kbar. The disappearance at the phase transition pressure of the satellite peak at the low emission energy side of the Fe Kß line indicates the collapse of the magnetisation in -Fe, confirming the earlier Mossbauer studies. This transition takes place at the same pressure of the structural phase transition, and its width is found as sharp as that of both previous structural and magnetic studies. This observation, reported in Figure 74, demonstrates that the XES method can be used successfully to monitor the magnetic properties of materials as a function of thermodynamic variables as temperature and pressure. Therefore it is an alternative and complementary method to Mossbauer spectroscopy, particularly useful for all those atoms without easily accessible isotopes. This new approach is now being exploited to study the magnetism under high pressure of Invar alloys and other magnetic systems.

Principal Publication and Authors
J.P. Rueff, M. Krisch, Y.Q. Cai, A. Kaprolat, M. Hanfland, M. Lorenzen, C. Masciovecchio, R. Verbeni, F. Sette. Accepted for publication in Physical Review B, Rapid communications.