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Does materials composition affect the magnetism of Fe-Cr alloys?
Figure 1: Scheme representing the Earth and electron magnets.

The discovery of the mineral magnetite (a naturally occurring magnet) several centuries before Christ, is considered as a first intimation of magnetism. Magnetism is part of many of our nowadays practical applications – e. g. car motors, phones, tape recorders, stereo speakers, computers, etc.

The fundamental building block of magnetism is the electron. The Earth acts as a large  magnetic dipole which has its origin on the motions of the electrons´within the Earth’s Mantle liquid outer core, each electron being a tiny magnet itself. The electron is a rotating electrically charged body, which according to classical electrodynamics, has a magnetic dipole effect creating magnetic poles of equal magnitude but with opposite polarities.
Electrons do not only rotate around themselves, they also “circle” in real space. These combined spinning effects of the electron create, at the atomic level, a spin and orbital magnetic moments which result in a very small but significant magnetic field (Fig. 1).

In most materials, atoms are arranged in such a way that the magnetic orientation of one electron cancels out the orientation of another. The ferromagnetic iron and its binary iron-chromium alloys are different. Their atomic arrangement is such that groups of atoms band together into small areas called Weiss domains, in which the magnetic moments of atoms („elementary magnets“) are uniformly oriented in the same direction, even without an external magnetic field. The so-called Weiss
domains owe their name to the French physicist Pierre-Ernest Weiss (1865– 1940) who discovered these uniformly magnetized regions. The size of these oriented domains is in the range of 10–2 to 10–4 mm including a volume of about 106 to 109 atoms. Photoemission electron microscopy (PEEM) using circularly polarized soft x-rays is a very suitable technique to image these magnetic domains in iron-chromium alloys with a 100 nm spatial resolution. The PEEM technique allows imaging magnetic domains, as well as extracting absorption spectra from single domains, and quantifying the spin and orbital magnetic moments of iron in Fe-Cr alloys]. The experimental determination of these magnetic moments values can be used to directly validate ab initio calculations of magnetic moments performed within the High Temperature Materials project (HTMat, PSI).
It is clear from that the two iron-chromium alloys have different ferromagnetic domain structures. The alloys exhibit dark and bright regions regarded as the fingerprint of magnetic domains with opposite direction of magnetization. Not only the magnetization direction, but also the size and the shape of the magnetic domains are affected by the compositional difference in the alloys.
Consequently, the chromium content of the iron-chromium alloys has a great influence on their local magnetic behavior.

Full scientific article on the theme (PDF download)

XAFS investigations of Cu doped Fe steels: binary and real reactor pressure vessel (RPV) steel alloy study
Figure 1: CU K-edge XANES on FeCu samples. All spectra were collected at 15K, apart from Cu foil that was measured at room temperature. The number of hours in brackets correspond to the annealing time, the annealing temperature was 500°C
Figure 2: KKG and LM3 at the Cu K edge are presented here. Both the samples have the bcc structure as expected typical of iron.

Reactor pressure vessel (RPV) steels samples are complex alloys composed of several elements (Cu, Ni, Mn, C, V, Mo, Si, Cr) with a low concentration (<1.4wt%) balanced by Fe. RPV steels are usually submitted to neutron irradiation and to thermal annealing. The diluted elements, above all Cu, are responsible to cluster formation in the RPV material. Consequently it was important to study how Cu behaves in an Fe matrix. For this reason during this experiment two different alloys were investigated: binary alloy (FeCu) and RPV alloy.

It has been used for measurements a Si(111) crystal to select the energy. The binary alloys were investigated at the Cu K edge and the RPV alloy at the Cu, Ni and Mn K edge. All the Cu K edge analysis was performed at 15 K thanks to a liquid nitrogen cryostat, the Ni and Mn K edges were performed at room temperature.

An ion chamber was used to measure the intensity of the incoming beam. A 5-elements Ge solid state fluorescence detector was applied to record the XAFS spectra in fluorescence mode. Thanks to a setup of the beam line, it was possible to measure a reference foil (Cu, Ni, Mn) by transmission during the acquisition data. In this way it is possible to have a right evaluation of the threshold edge. 9 samples were analyzed: the first 6 are binary alloys (Fe98.5Cu1.5). These samples have been submitted to different annealing treatments. A change of phase is evident in the samples annealed longer (Fig 1).

Three RPV alloys were analyzed (LM3 at the Cu and Ni K edge, KKG at the Cu K edge, and UCSB at the Mn K edge). These samples are used as reference samples to investigate more complex alloys. The complexity of these alloys is due to the many elements diluted in the Fe matrix and eventually to the neutron irradiation and annealing treatments. As far as it is not possible to analyze the active samples at low temperature, it becomes important to perform the analysis of the reference in the best conditions (Fig 2).