Magnetism in iron as a function of pressure

Abstract

Magnetism in iron plays a central role in understanding the physical properties of itspolymorphs, including the close-packed high pressure phases. We explore the rich andcomplex magnetic structures of these phases in two ways. We use a first-principlesbased, magnetic tight-binding total energy model to study non-collinear magneticstructures, and an all-electron method to study the collinear state in hcp iron that wepredict in the hcp iron stability range. For the non-collinear study we computethe magnetization energy and moments for various non-collinear ordered spinconfigurations. 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-principlescalculations. In the high pressure stability field of hcp iron we find a stable collinearantiferromagnetic structure (afmII), previously predicted with an all-electron method. Wefurther investigate the afmII structure, computing physical properties from first principlesthat support the notion of antiferromagnetic correlations in hcp iron. We showthat a recently observed anomalous splitting in Raman spectra of hcp iron undercompression can be quantitatively explained by spin–phonon interactions. Toaddress the absence of Mössbauer splitting in experiments on hcp iron we have alsocalculated the hyperfine field of afmII iron and find it to be so small that thepredicted splitting would be smaller than the resolution limit of experiments.