In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

In 2000, a seminal study predicted ferromagnetism above room temperature in diluted magnetic semiconductors and oxides, fuelling tremendous research activity that has lasted for a decade. Tomasz Dietl reviews the progress in understanding these materials over the past ten years, with a view to the future of semiconductor spintronics.

A ten-year perspective on dilute magnetic semiconductors and oxides

Facing the ever-growing demand for data storage will most probably require a new paradigm. Nanoscale magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin ﬁlms in nature. We designed cobalt-based multilayered thin ﬁlms in which the cobalt layer is sandwiched between two heavy metals and so provides additive interfacial Dzyaloshinskii-Moriya interactions (DMIs), which reach a value close to 2 mJ m(-2) in the case of the Ir|Co|Pt asymmetric multilayers. Using a magnetization-sensitive scanning X-ray transmission microscopy technique, we imaged small magnetic domains at very low fields in these multilayers. The study of their behaviour in a perpendicular magnetic ﬁeld allows us to conclude that they are actually magnetic skyrmions stabilized by the large DMI. This discovery of stable sub-100 nm individual skyrmions at room temperature in a technologically relevant material opens the way for device applications in the near future.

https://hal.archives-ouvertes.fr/hal-01344269/document

Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature.

High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre-edge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .

http://www.minsocam.org/msa/ammin/toc/Abstracts/2001_Abstracts/MJ01_Abstracts/Wilke_p714_01.pdf

Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study

This review summarizes recent first-principles investigations of the electronic structure and magnetism of dilute magnetic semiconductors (DMSs), which are interesting for applications in spintronics. Details of the electronic structure of transition-metal-doped III-V and II-VI semiconductors are described, especially how the electronic structure couples to the magnetic properties of an impurity. In addition, the underlying mechanism of the ferromagnetism in DMSs is investigated from the electronic structure point of view in order to establish a unified picture that explains the chemical trend of the magnetism in DMSs. Recent efforts to fabricate high-TC DMSs require accurate materials design and reliable TC predictions for the DMSs. In this connection, a hybrid method (ab initio calculations of effective exchange interactions coupled to Monte Carlo simulations for the thermal properties) is discussed as a practical method for calculating the Curie temperature of DMSs. The calculated ordering temperatures for various DMS systems are discussed, and the usefulness of the method is demonstrated. Moreover, in order to include all the complexity in the fabrication process of DMSs into advanced materials design, spinodal decomposition in DMSs is simulated and we try to assess the effect of inhomogeneity in them. Finally, recent works on first-principles theory of transport properties of DMSs are reviewed. The discussion is mainly based on electronic structure theory within the local-density approximation to density-functional theory.

First-principles theory of dilute magnetic semiconductors

The ESRF beamline ID26 is dedicated to XAFS studies on Ultra-diluted Samples (XAUS beamline). The spectral range 2.3-30 keV is covered. The aim of the beamline is to extract structural and electronic information on dilute samples for which the concentration of the absorbing element ranges from a few ppm up to 10 000 ppm. A wide range of applications is covered in biology, catalysis, chemistry, environmental sciences, solid state physics, etc. The beamline accepted the first users in November 1997. We give herein a brief description of the beamline and report on recent results.

The ESRF beamline ID26: X-ray absorption on ultra dilute sample.

X-Ray Natural Circular Dichroism

Intrinsic ferromagnetism in the high-quality wurtzite ${\mathrm{Ga}}_{0.937}{\mathrm{Mn}}_{0.063}\mathrm{N}$ semiconductor is unambiguously demonstrated by both macroscopic magnetization measurements and x-ray magnetic circular dichroism. The structural quality of the samples grown by plasma-assisted molecular beam epitaxy is confirmed by x-ray diffraction and x-ray linear dichroism. The Curie temperature of a (Ga,Mn)N sample with 6.3% Mn is $\ensuremath{\approx}8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a spontaneous magnetic moment of $2.4{\ensuremath{\mu}}_{B}$ per Mn at $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Intrinsic ferromagnetism in wurtzite (Ga,Mn)N semiconductor

We have measured the ${\mathrm{L}}_{2,3}$ absorption edges of Pd in Pd/Fe multilayers with different Pd interlayer thicknesses. The spectral features observed after the ${\mathrm{L}}_{3}$ and ${\mathrm{L}}_{2}$ white lines indicate that in the atomic layer adjacent to Fe the Pd atoms have a local structure different from fcc. X-ray magnetic circular dichroism (XMCD) at these edges shows that the atoms directly at the interface are strongly polarized with a total moment of about 0.4${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ /atom. The orbital moments are found to be small (0.0--0.04${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ ). The thickness dependence of the XMCD shows that Pd atoms carry a magnetic moment up to four layers from the interface. The magnetic interface appears larger than the crystallographic one.

Structure and magnetism of Pd in Pd/Fe multilayers studied by x-ray magnetic circular dichroism at the Pd L 2,3 sedges

Angle-dependent x-ray magnetic circular dichroism experiments have been performed at both the Co and Pt ${L}_{2,3}$ edges in two epitaxial (111) ${\mathrm{CoPt}}_{3}$ thin films grown at 690 and 800 K. The analysis of the angular variations of the $3d$ orbital magnetic moment shows two different magnetic behaviors: a strong perpendicular magnetocrystalline anisotropy (PMA) for the film grown at 690 K and an almost isotropic behavior for the film grown at higher temperature. The same analysis at the Pt ${L}_{2,3}$ edges suggests that the $5d$ electrons play an important role in the PMA. Our results correlate the appearance of PMA with the existence of anisotropic structural effects induced during the codeposition process.

José Goulon

Papers

The ESRF beamline ID26: X-ray absorption on ultra dilute sample.

X-Ray Natural Circular Dichroism

Intrinsic ferromagnetism in wurtzite (Ga,Mn)N semiconductor

Structure and magnetism of Pd in Pd/Fe multilayers studied by x-ray magnetic circular dichroism at the Pd L 2,3 sedges

Magnetocrystalline anisotropy in (111) CoPt 3 thin films probed by x-ray magnetic circular dichroism