Magnetic structure

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

Ab initio density functional study of phase stability and noncollinear magnetism in Mn

Abstract: The crystalline and magnetic structures of all known polymorphs of Mn have been investigated using generalized spin-density functional theory based on an unconstrained vector-field description of the magnetization density. We find that at atomic volumes smaller than 12 A3, the magnetic ground state of α-Mn is collinear with magnetic moments ranging between 0 and 3 µB depending on the local symmetry of the atomic positions. At larger atomic volumes, a metastable collinear configuration coexists with a stable noncollinear state. The noncollinearity of the magnetic structure is driven by the appearance of magnetic moments on sites IV, leading to a frustration of exchange interactions in local triangular configurations. A similar situation is found in β-Mn, with a collinear structure with coexisting magnetic and nonmagnetic sites. The α-phase is found to be stable over a wide range of volumes; under compression a phase transition to hexagonal -Mn is predicted.

MAGNETIC STRUCTURE DETERMINATION FROM POWDER DIFFRACTION USING THE PROGRAM FullProf

Abstract: In this paper the techniques for magnetic structure determination from neutron powder diffraction (NPD) data as implemented in the program FullProf are reviewed. In the general case the magnetic moment of an atom in the crystal is given as a Fourier series. The Fourier coefficients are complex vectors constituting the “unknowns” to be determined. These vectors define the magnetic structure and they correspond to the “atom positions” of an unknown crystal structure. The use of group theoretical methods for the symmetry analysis is needed to find the smallest set of free parameters. In general the Fourier coefficients are linear combinations of the basis functions of the irreducible representations of the wave vector group. The coefficients of the linear combinations can be determined by the simulated annealing (SA) technique comparing the calculated versus the observed magnetic intensities. The SA method has been improved and extended to the case of incommensurate magnetic structures within FullProf.

Magnetoelectric and structural properties of Y 2 CoMn O 6 : The role of antisite defects

Abstract: The finding of new multiferroic materials, where electric and magnetic orders coexist, is a challenging task currently. The double perovskite Y${}_{2}$CoMnO${}_{6}$ shows spontaneous magnetization and electrical polarization at low temperature. Previous investigations of this compound did not reach agreement about the type of magnetic structure present. This study demonstrates that this compound exhibits a collinear ferromagnetic ordering of Co${}^{2+}$ and Mn${}^{4+}$ moments in the $a\phantom{\rule{0}{0ex}}c$ plane with a small antiferromagnetic canting along the $b$ axis. A thorough characterization of the dielectric properties reveals the absence of any related anomaly in the dielectric permittivity and the lack of spontaneous electrical polarization ($P$) in the $P$($E$, electric field) loops. The pyroelectric current is strongly dependent on the number of antisite defects in the Co/Mn arrangement, the heating rate, and the poling field. Thus, the observed electric polarization is due to thermally stimulated depolarization currents ascribed to defect dipoles mainly placed at the antiphase boundaries. No ferroelectric transition occurs in this material, disproving the existence of intrinsic magnetoelectric multiferroicity.

Evidence for magneto-electric and spin–lattice coupling in PbFe0.5Nb0.5O3 through structural and magneto-electric studies

Abstract: Neutron diffraction (ND) studies were carried out on polycrystalline single-phase multiferroic Pb(Fe0.5Nb0.5)O3 (PFN) in the temperature range of 290–2 K to understand the structural and magnetic properties as a function of temperature. ND data were refined using the Rietveld refinement method for both crystallographic and magnetic structures. The structure at room temperature was found to be monoclinic, in Cm space group. No structural transition was observed till 2 K. At low temperatures (i.e., from T < T N; T N = 155 K), an additional peak appears at scattering vector, Q = 1.35 A−1, indicating the onset of antiferromagnetic ordering. The magnetic structure was found to be commensurate with the crystallographic structure and could be refined using the propagation vector, k = [0.125, 0.5, and 0.5]. Magnetization, ferroelectric P–E loops, and dielectric measurements on PFN reveal a strong anomaly at the antiferromagnetic transition temperature (T N) indicating the magneto-electric coupling. The refined temperature-dependent structural parameters such as unit cell volume and monoclinic distortion angle (β) reveal pronounced anomalies at the magnetic ordering temperature (T N), which indicates strong spin–lattice coupling. An anomaly in lattice volume was observed with a small negative thermal expansion below and a large thermal expansion above the T N, respectively. It shows the occurrence of isostructural phase transition accompanying the magnetic ordering below T N ~155 K, leading to significant change in ionic polarization, octahedral tilt angle, and lattice strain around T N. We have used refined atomic positional coordinates from the nuclear and magnetic structures, to obtain ionic polarization. These detailed studies confirm the magneto-electric and spin–lattice coupling in PFN across T N.