Magnetic structure

About: Magnetic structure is a research topic. Over the lifetime, 10787 publications have been published within this topic receiving 207143 citations.

Crystalline, electronic, and magnetic structures of θ-Fe3C, χ-Fe5C2, and η-Fe2C from first principle calculation

Abstract: First principle calculations have been performed to study the crystalline, electronic, and magnetic structures of three iron-carbide systems: θ-Fe3C, χ-Fe5C2, and η-Fe2C. The Kohn-Sham equations were solved by applying the full-potential linearized augmented plane wave method. The generalized gradient approximation in the Perdew-Wang formalism was used to the exchange and correlation energy functional. The internal positions of atoms within the unit cell were optimized and the ground state properties such as lattice parameter and bulk modulus were calculated. The results are compared with experimental data when available. Comparison of the two metastable systems χ-Fe5C2 and η-Fe2C shows that the last one has lower formation energy; this is corroborated by the formation sequence observed during tempering. The electronic structures of the three carbides were then studied and the magnetic moments calculated by means of electronic spin-resolved density of state calculations at their equilibrium lattice consta...

Magnetic properties of the CrMnFeCoNi high-entropy alloy

Abstract: We present experimental data showing that the equiatomic CrMnFeCoNi high-entropy alloy undergoes two magnetic transformations at temperatures below 100 K while maintaining its fcc structure down to 3 K. The first transition, paramagnetic to spin glass, was detected at 93 K and the second transition of the ferromagnetic type occurred at 38 K. Field-assisted cooling below 38 K resulted in a systematic vertical shift of the hysteresis curves. Strength and direction of the associated magnetization bias was proportional to the strength and direction of the cooling field and shows a linear dependence with a slope of $0.006\ifmmode\pm\else\textpm\fi{}0.001 \mathrm{emu}\mathrm{T}$. The local magnetic moments of individual atoms in the CrMnFeCoNi quinary fcc random solid solution were investigated by ab initio (electronic density functional theory) calculations. Results of the numerical analysis suggest that, irrespective of the initial configuration of local magnetic moments, the magnetic moments associated with Cr atoms align antiferromagnetically with respect to a cumulative magnetic moment of their first coordination shell. The ab initio calculations further showed that the magnetic moments of Fe and Mn atoms remain strong (between 1.5 and $2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$), while the local moments of Ni atoms effectively vanish. These results indicate that interactions of Mn- and/or Fe-located moments with the surrounding magnetic structure account for the observed macroscopic magnetization bias.

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Abstract: Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is

CuO: X-ray single-crystal structure determination at 196 K and room temperature

Abstract: An X-ray single-crystal determination of the CuO structure has been made at 196 K, i.e. below the Neel temperature 230 K, and, as a check, the crystal structure at room temperature was also determined. The correct space group for the structure at both temperature was found to be Cc. Earlier results of magnetic and neutron diffraction measurements can be explained as antiferromagnetic coupling between copper atoms via oxygen (superexchange) in chains running in the (1 0-1) direction. The structural results show changes with temperature in Cu-O distance in these chains: in each -Cu-O-Cu-group the longer is increased and the shorter decreased when passing from 196 K to room temperature. This implies a weaker antiferromagnetic coupling at room temperature. The refinement of CuO-structure at room temperature reported earlier by Asbrink and Norrby (1970) showed the symmetry to be C2/c. An attempt to refine CuO in the space group Cc with the old data was not successful. The different results obtained with different crystals are tentatively explained from published observations regarding valence fluctuations in CuO and non-stoichiometry caused by cation vacancies.

Magnetic structure of erbium.

Abstract: We present a synchrotron x-ray scattering study of the magnetic phases of erbium. In addition to the magnetic scattering located at the fundamental wave vector tau/sub m/ we also observe scattering from magnetoelastically induced charge modulations at the fundamental wave vector, at twice the fundamental, and at positions split symmetrically about the fundamental. As the temperature is lowered below 52 K the charge and magnetic scattering display a sequence of lock-in transitions to rational wave vectors. A spin-slip description of the magnetic structure is presented which explains the wave vectors of the additional charge scattering.