Magnetism in iron as a function of pressure

Magnetism in iron plays a central role in understanding the physical properties of its polymorphs, including the close-packed high pressure phases. We explore the rich and complex magnetic structures of these phases in two ways. We use a first-principles based, magnetic tight-binding total energy model to study non-collinear magnetic structures, and an all-electron method to study the collinear state in hcp iron that we predict in the hcp iron stability range. For the non-collinear study we compute the magnetization energy and moments for various non-collinear ordered spin configurations. For fcc iron we find non-collinear structures with a wavevector (0, 0, q) with q close to 0.5 to be energetically stable, in agreement with previous first-principles calculations. In the high pressure stability field of hcp iron we find a stable collinear antiferromagnetic structure (afmII), previously predicted with an allelectron method. We further investigate the afmII structure,computing physical properties from first principles that support the notion of antiferromagnetic correlations in hcp iron. We show that a recently observed anomalous splitting in Raman spectra of hcp iron under compression can be quantitatively explained by spin–phonon interactions. To address the absence of Mössbauer splitting in experiments on hcp iron we have also calculated the hyperfine field of afmII iron and find it to be so small that the predicted splitting would be smaller than the resolution limit of experiments.