Magnetism in dense hexagonal iron

Abstract

The magnetic state of hexagonal close-packed iron has been the subject of debate for more than three decades. Although Mö ssbauer measurements find no evidence of the hyperfine splitting that can signal the presence of magnetic moments, density functional theory predicts an antiferromagnetic (afm) ground state. This discrepancy between theory and experiment is now particularly important because of recent experimental findings of anomalous splitting in the Raman spectra and the presence of superconductivity in hexagonal close-packed iron, which may be caused by magnetic correlations. Here, we report results from first principles calculations on the previously predicted theoretical collinear afm ground state that strongly support the presence of afm correlations in hexagonal close-packed iron. We show that anomalous splitting of the Raman mode can be explained by spinphonon interactions. Moreover, we find that the calculated hyperfine field is very weak and would lead to hyperfine splitting below the resolution of Mö ssbauer experiments. P hysical properties of iron are of great importance to many fields in the sciences, as iron is one of the most abundant and stable elements in the universe and the very basis for the steel industry. Hexagonal close-packed (hcp) iron, the form stable at high pressure, plays a central role in geophysics, as the Earth's inner core is thought to be primarily composed of this phase (1, 2), and in our understanding of impact and explosive phenomena in iron and steel (3). The magnetic state of iron has a major influence on the physics of iron and iron alloys, including the relative stability of the iron polymorphs (4, 5). The magnetic structure of the hcp phase has been the subject of a scientific debate for three decades (6), leading to contradictory results from experiments and theory. Although experiments are interpreted to show the absence of magnetism in hcp iron (6-10), computations based on density functional theory find an antiferromagnetic (afm) ground state stable to Ϸ50 GPa (11), similar to magnetism in the double hcp phase (5). The possible presence of magnetism in hcp iron as further substantiated in this article has important implications. In geophysics many experiments are carried out on potentially magnetic hcp iron and are then extrapolated to pressures of Earth's core, into the nonmagnetic region of the hcp stability field at high pressure, and may consequently not be valid. The possible presence of magnetism in hcp iron also plays an important role in the discussion of the recently observed superconductivity of hcp iron (12, 13): Magnetic correlations in hcp iron appear to be necessary to explain the observed pressure dependence of its superconductivity (14-16).The two lower-pressure polymorphs of iron are both magnetic. The phase stable around ambient conditions, body-centered cubic (bcc), owes its stability entirely to the presence of ferromagnetism (4). Heating above the Curie temperature causes the spins to disorder and the net magnetization to vanish, but the ind...

First-principles thermal equation of state and thermoelasticity of hcp Fe at high pressures

“…At low pressures, the antiferromagnetic nature of the ground state hcp Fe leads to splitting of the Raman frequencies, 21 and substantial temperature and compositional dependence. 55 All these account for some of the discrepancies between theory and experiment.…”

“…[12][13][14][15][16][17][18][19] Other major problems include possible subsolidus phase transitions 2,4,5,11,20 and the magnetic structure of the dense hexagonal iron. [21][22][23] First-principles calculations predicted that hexagonal close-packed iron has antiferromagnetic ground state up to 50…”

“…24,25 Recent antiferromagnetic calculations explained the anomalous splitting of the Raman mode and the absence of hyperfine splitting in Mössbauer measurements in hpc iron. 21 Knowledge about the elasticity of hcp iron and its pressure and temperature dependences plays a crucial role in understanding the seismological observations of the inner core, such as the low shear velocity and the elastic anisotropy. Several sets of firstprinciples elastic moduli have been reported for hcp Fe at high pressures [26][27][28][29][30][31] , most of which are zero-temperature calculations.…”

The effect of silicon impurities on the phase diagram of iron and possible implications for the Earth's core structure

“…shown to be the stable hcp phase at low pressures [16,17]. were then allowed to relax and the total energy was calculated.…”

Fe–Ni Alloys at High Pressures and Temperatures: Statistical Thermodynamics and Kinetics of the $L1_{2}$ or $D0_{19}$ Atomic Order

“…3.8, а [69], а на рис. 3,8, б наведено теоретично прогнозовану за-лежність від тиску магнетних моментів ОЦК-й ГЩП-Fe [69].…”