Project

nanomagnetism; magnetism; X-ray Magnetic Circular Dichroism; atomistic scale; synchrotron irradiation; WIEN2k; XMCD; Fe-Cr; ab initio calculations; ferritic steels; beamline techniques

Lead
Lay summary
Magnetism is a fundamental property of ferritic steels that needs to be understood to obtain insight into mechanical and plastic properties. Thus, in this project, synergistic XMCD and ab initio calculations investigate the magnetic properties of the Fe-Cr system.Background:Magnetism is inherent in the nature of ferritic materials. The inclusion of antiferromagnetic Cr in the ferromagnetic Fe matrix leads to anisotropy in the system due to spin-orbit coupling. The Fe-Cr alloy is the fundamental building block of many structural materials design due to superior properties of increased strength and hardness, low oxidation rate, corrosion resistance and retention of strength at high temperatures. However, the influence of the magnetism present in the Fe-Cr alloy has until recently been ignored in its development as a material exposed to extreme conditions. Our recent X-ray magnetic circular dichroism (XMCD) and extended X-ray absorption fine structure (EXAFS) measurements as well as ab initio calculations on two Fe-Cr alloys indicate the significant dependence between magnetism and structure. These results highlight that magnetism affects the properties of the alloy by altering stability, defect mobility and the dynamics of dislocations which in turn influence the material’s lifetime.Aim:This proposal aims to understand the magnetic structure of Fe-Cr alloys and, hence, it’s fundamental mechanical properties change with Cr content as a result of its anisotropic behaviour and in the long term determine optimal Cr concentrations in these alloys. Impact: From XMCD spectra, the orbital and spin moments of Fe and Cr will be determined and directly compared to ab initio calculation results. Trends will track the influence of Cr content, which is important within the search for optimal Fe-Cr alloys. This coupled approach will validate the modelling scheme while bringing forth a better knowledge of the atomistic mechanisms which influence Fe-Cr alloy properties. We expect the insights obtained in the role of magnetism in such mechanisms to impact a broad spectrum of materials users and developers including fission, fusion, aeronautics, space and magnetic storage device technologists.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

Magnetism is inherent in the nature of ferritic materials. The inclusion of antiferromagnetic Cr in the ferromagnetic Fe matrix leads to anisotropy in the system due to spin-orbit coupling. This leads to interesting magnetic configurations in the Fe-Cr alloy phase diagram which can be described as a competitive arrangement between ferromagnetic Fe and antiferromagnetic Cr. The anisotropic magnetoresistance present in multilayered Fe/Cr is the basis for the recent developments in magnetic data storage media. In its alloy form, the Fe-Cr matrix is also the fundamental building block of structural materials design due to its superior properties of increased strength and hardness, low oxidation rate, corrosion resistance and retention of strength at high temperatures. However, the influence of the magnetism present in the Fe-Cr alloy has until recently been ignored in its development as a material exposed to extreme conditions. Indeed, results from neutron scattering measurements and more recently from ab initio calculations indicate that the change from local short range ordering of the Cr to Cr clustering within the alloy is controlled by magnetism. This change is concurrent with an asymmetry in the heat of formation of the alloy which changes from a small negative region of values to a positive one at around 10% Cr. Our recent X-ray magnetic circular dichroism (XMCD) and extended X-ray absorption fine structure (EXAFS) measurements as well as ab initio calculations on Fe-6.2%Cr and Fe-12.7%Cr have furthermore indicated the significant dependence between magnetism and structure in this transition metal alloy. These results highlight that magnetism affects the properties of the alloy by altering stability, defect mobility and the dynamics of dislocations which in turn influence the lifetime of these materials.To obtain a better life-time assessment of Fe-Cr alloys it is, therefore, necessary to understand the alloys’ single defects at the nanoscale. Examining the effects of magnetism in the Fe-Cr alloy system at a fundamental level, thus, becomes essential. The aim of this proposal for two parallel PhD works is to understand how the magnetic structure of the Fe-Cr alloys and, hence, its fundamental mechanical properties change with Cr content as a result of its anisotropic behaviour. The final aim is to determine optimal Cr concentrations in these alloys for use under extreme conditions. These issues will be investigated by synergistic experiments and first principle calculations. Using the element specific XMCD technique, the nanomagnetic properties of Fe and Cr will be studied as a function of Cr concentration. From XMCD spectra, the orbital and spin moments of Fe and Cr will be determined and these results will be directly compared to values obtained by ab initio calculations performed within the Density Function Theory framework. Changes in these values will track the influence of Cr content, which is important within the search for optimal Fe-Cr alloys. Such a coupled approach will validate the modelling of Fe-Cr alloys while, at the same time, bringing forth a better knowledge of the atomistic mechanisms which influence the lifetime of the Fe-Cr alloy matrix. Further, we expect insights into the role that magnetism plays in such mechanisms will impact a broad spectrum of materials users and developers who utilize Fe-Cr as their basis matrix: fission, fusion, aeronautics, space and magnetic storage device technologists.