Types Of Magnetic Structures Research Articles

In situ TEM observation of magnetic materials

Magnetic nanostructures and thin films display novel magnetization reversal behavior as a function of size and shape, which makes them appropriate for a range of technological applications. The spatial resolution of in situ transmission electron microscopy techniques such as Lorentz TEM (LTEM) and off-axis electron holography are well suited to analysis of the magnetic domain structure and magnetization behavior of these magnetic nanostructures and thin films. In this article the various techniques that are applicable are described, including the qualitative LTEM imaging modes and the differential phase contrast technique. In addition, quantitative methods for mapping the magnetic induction via phase reconstruction are discussed. In each case the advantages and limitations are presented. Application of the techniques to various types of magnetic structures is then presented, and the article ends with a short summary and a discussion of future developments in this field.

Investigation of a method to calculate spontaneous radiation spectra from relativistic electrons in undulators

Undulators are key devices to produce brilliant synchrotron radiation at the synchrotron radiation facilities. In this paper we present a numerical computing method, including the computing program that has been developed to calculate the spontaneous radiation emitted from relativistic electrons in undulators by simulating the electrons' trajectory. The effects of electron beam emittance and energy spread have also been taken into account. Comparing with other computing methods available at present, this method has a few advantages with respect to several aspects. It can adopt any measured or arbitrarily simulated 3D magnetic field and arbitrary electron beam pattern for the calculation and it's able to analyze undulators of any type of magnetic structure. It's expected to predict precisely the practical radiation spectrum. The calculation results of a short period in-vacuum undulator and an Elliptically Polarized Undulator (EPU) at Shanghai Synchrotron Radiation Facility (SSRF) are presented as examples.

Optical spectroscopy of low-dimensional rare-earth iron borates

The family of RFe 3(BO 3) 4 borates (R=Pr, Nd, Eu–Er, Y) was studied by high-resolution optical absorption and Raman spectroscopies. Structural and magnetic phase transitions were detected and the types of magnetic structure were determined. Energy of crystal-field (CF) levels and exchange splitting of the ground state of the R 3+ ion were obtained from the analysis of optical spectra. CF calculations were carried out. Effective magnetic field at the R 3+ site was found, using the calculated value of the magnetic g-factor. Some peculiarities of modeling the paramagnetic susceptibility of NdFe 3(BO 3) 4 are discussed.

Nutational two-dimensional structures in magnets

New types of magnetic structures in the Heisenberg model are found. Analytical methods are used to describe spiral structures, spiral vortex structures, and their interaction. Methods for obtaining these structures in real systems, including nanomagnets, are discussed.

Melting of magnetic correlations in charge-orbital orderedLa1/2Sr3/2MnO4: Competition of ferromagnetic and antiferromagnetic states

The magnetic correlations in the charge and orbital ordered manganite ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined charge exchange (CE)-type magnetic structure with the corresponding magnons, a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ exhibits a dispersion of ferromagnetic correlations, which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.

The magnetic composition–temperature phase diagram of the kagome mixed system(CoxNi1−x)3V2O8

The magnetic composition–temperature(x–T) phase diagram of themixed kagome system (CoxNi1−x)3V2O8 (CNVO) deduced from neutron powder diffraction and single-crystalheat capacity experiments is presented. CNVO changes its magneticstructure twice below 5.5 K as a function of the compositional parameterx. A samplewith x = 0.98 has been found to be of a critical composition where both ferromagnetic and antiferromagnetic reflectionsof the Co3V2O8 (CVO) type magnetic structure have been observed in the diffraction pattern.Below this composition the ferromagnetic phase is completely suppressed andantiferromagnetic short-range order is present, which is modulated by a propagation vectork1 = (0,δ,0) with δ being temperature and composition dependent. Below 4.2 Kδ has a constantvalue of 0.4 for 0.76<x<0.92. In the region 0.71<x<0.76 a change of the magnetic structure into the long-range orderedNi3V2O8 (NVO) typewith k2 = (δ,0,0), whereδ is only dependenton x, takes place.Finally, for x<0.035 the positions of the magnetic reflections become temperature dependent again.

The magnetic structure of convection-driven numerical dynamos

The generation of a magnetic field in numerical simulations of the geodynamo is an intrinsically 3-D and time-dependent phenomenon. The concept of magnetic field lines and the frozen-flux approximation can provide insight into such systems, but a suitable visualization method is required. This paper presents results obtained using the Dynamical Magnetic Field line Imaging (DMFI) technique, which is a representation of magnetic field lines accounting for their local magnetic energy, together with an algorithm for the time evolution of their anchor points. The DMFI illustrations are consistent with previously published dynamo mechanisms, and allow further investigation of spatially and temporally complex systems. We highlight three types of magnetic structures: (i) magnetic cyclones and (ii) magnetic anticyclones are expelled by, but corotate with axial flow vortices; (iii) magnetic upwellings are amplified by stretching and advection within flow upwellings, and show structural similarity with helical plumes found in rotating hydrodynamic experiments. While magnetic anticyclones are responsible for the regeneration of a stable axial dipole, herewe showthat excursions and reversals of the dipole axis are caused by the emergence of magnetic upwellings, which amplify and transport a generally multipolar magnetic field from the inner to the outer boundary of the models. Geomagnetic observations suggest the presence of magnetic structures similar to those found in our models; thus,we discuss howour results may pertain to Earth's core dynamo processes. In order to make DMFI a standard tool for numerical dynamo studies, a public software package is available upon request to the authors (supplementary material is available at: http://www.ipgp.jussieu. fr/∼aubert/DMFI.html).

Spin, charge, and orbital ordering inRBaMn2O5+δ(R=Y, La;0⩽δ⩽1) and their dependence on oxygen content and size of theRconstituent

The effect of oxygen content on spin, charge, and orbital ordering in $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$ ($R=\mathrm{Y}$, La; $0\ensuremath{\leqslant}\ensuremath{\delta}\ensuremath{\leqslant}1$) is studied by density-functional-theory-based calculations as implemented in the full-potential linearized-augmented plane-wave method. Structural optimizations using the projector augmented wave method have been performed for all the phases and the calculated structural parameters are found to be in good agreement with experimental values. Total-energy calculations have systematically been performed including force as well as stress minimization for paramagnetic, ferromagnetic, and antiferromagnetic configurations. For $\ensuremath{\delta}=0$, the ground state is found to be ferrimagnetic, whereas the variants with oxygen contents $\ensuremath{\delta}=1∕2$ and 1 give rise to an antiferromagnetic ground state, all in perfect agreement with experimental findings. The electronic-band characteristics are analyzed using total and site- and orbital-projected densities of states and the examination shows that the electronic structure undergoes a gradual change from semiconductor-to-metal behavior on going from $\ensuremath{\delta}=0$ to 1. Even the $\mathrm{GGA}+U$ approach failed to reproduce the insulating state for the phases with $\ensuremath{\delta}=1$, indicating that introduction of the experimental $CE$-type magnetic structure may be important. The charge and orbital ordering are analyzed with the help of the energy-projected-density matrices of the $d$ electrons. Very different ordering patterns have emerged for the different phases under investigation, indicating that both cation radii and oxygen stoichiometry play an important role in deciding spin, charge, and orbital ordering.

Magnetic anisotropy of the Nd2Fe14B compound and its hydride Nd2Fe14BH4

Magnetization curves of single crystals of Nd2Fe14B and its hydride Nd2Fe14BH4 have been measured along their principal crystallographic directions in a temperature range of 4.2–280 K. The magnetic anisotropy constants, which allow one to describe experimental magnetization curves as well as the low-temperature “easy-cone”-type magnetic structure and field-induced first-order magnetic phase transitions, have been determined in terms of a collinear-ferrimagnetic-ordering model. The anisotropy constants were shown to decrease in magnitude upon hydrogenation. In this case, the ratios of the effective fourth-and sixth-order constants to the second-order constant increase, whereas the constants responsible for the basal-plane anisotropy exhibit a more than threefold decrease. At the same time, the spin-reorientation temperature and opening of the magnetization cone at 4.2 K remain virtually unchanged. The results obtained are discussed in terms of the single-ion-anisotropy theory.

Preparation of nanocrystalline Mn–Al–C magnets by melt spinning and subsequent heat treatments

Mn 54Al 44C 2 ribbon samples have been prepared by melt spinning in a single-phase ɛ hexagonal disordered state and its exothermal transformation at around 500 °C into a tetragonal L1 0 type magnetic structure (τ-phase) was followed by DTA, DSC, X-ray diffraction and thermomagnetic measurements. The metastable τ-phase stabilized by carbon addition could be transformed reversibly into the ɛ-phase at around 800 °C, without decomposition into the stable Al 8Mn 5 and β manganese phases. The almost constant Curie temperature obtained for partially transformed two-phase samples indicated a compositionally invariant transformation. By grinding the thin (25 μm) ribbon into sub-millimeter flakes, a bulk anisotropic magnet could be compacted by magnetic field oriented powder technology.

Atomic structure of soft magnetic Fe-Si alloys with induced magnetic anisotropy

The atomic structure of iron-based alloys with 5–6 at. % Si has been studied using X-ray diffraction. It has been shown that the average size of B2-ordered clusters existing in these alloys depends on the sample prehistory (on the preparation method). Thermomagnetic and thermomechanical treatments were used to produce states with a desired type of magnetic structure. The largest size of B2 clusters was obtained in the direction of the magnetic anisotropy after it was induced. Based on the results of simulation of diffraction patterns, the microstructure of clusters was interpreted as consisting of adjacent pairs of B2 cells surrounded by distorted cells of the bcc matrix.

Damping of magnetohydrodynamic waves by resonant absorption in the solar atmosphere

In the last decade we have been overwhelmed by an avalanche of discoveries of magnetohydrodynamic (MHD) waves by the Solar and Heliospheric Observatory and Transition Region and Coronal Explorer observatories. Both standing and propagating versions of fast magnetoacoustic and slow magnetoacoustic MHD waves have been detected. Information on the damping times and damping distances of these waves is less detailed and less accurate than that on periods and amplitudes. Nevertheless, observations show the damping times and damping lengths are often short. Also, different types of MHD waves in different types of magnetic structures likely require different damping mechanisms. The phenomenon of fast damping is well documented for the standing fast magnetosonic kink waves in coronal loops. This paper concentrates on standing fast magnetosonic waves. It reports on results on periods and damping times due to resonant absorption in one-dimensional and two-dimensional models of coronal loops. Special attention is given to multiple modes.

Magnetic properties of selected Mn-based transition metal compounds withβ−Mnstructure: Experiments and theory

Two compounds, ${\mathrm{Mn}}_{3}\mathrm{CoSi}$ and ${\mathrm{Mn}}_{3}\mathrm{CoGe}$ have been synthesized and found to crystallize in the ${\mathrm{AlAu}}_{4}$ type structure, an ordered form of the $\ensuremath{\beta}\ensuremath{-}\mathrm{Mn}$ structure. The magnetic structure and properties have been studied by magnetometry and neutron powder diffraction and the theoretical work is based on first principles total energy calculations. Comparison is made with the magnetic properties of the isostructural compounds ${\mathrm{Mn}}_{3}\mathrm{IrGe}$ and ${\mathrm{Mn}}_{3}\mathrm{IrSi}$. The solid solutions ${\mathrm{Mn}}_{3}{\mathrm{Ir}}_{1\ensuremath{-}y}{\mathrm{Co}}_{y}\mathrm{Si}$ $(0\ensuremath{\leqslant}y\ensuremath{\leqslant}1)$ and ${\mathrm{Mn}}_{3}{\mathrm{CoSi}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ are also studied. A noncollinear antiferromagnetic structure is experimentally observed for $y=0.20$ as well as for $x=0.50$ and 1.0, similar to that of ${\mathrm{Mn}}_{3}\mathrm{IrSi}$ and ${\mathrm{Mn}}_{3}\mathrm{IrGe}$, with 120\ifmmode^\circ\else\textdegree\fi{} angles between magnetic moments on a triangular network of Mn atoms, and this finding is corroborated by theoretical calculations. For $y=0.80\text{--}1.0$ a transformation to a new type of magnetic structure takes place. The magnetic transition temperature decreases on decreasing unit cell dimension, with good qualitative agreement with the decay of the calculated interatomic exchange energy. Both theory and experiments find the magnitude of the Mn magnetic moment to decrease on decreasing unit cell volume, the same trend is found in calculations for $\ensuremath{\beta}\ensuremath{-}\mathrm{Mn}$.

Reinvestigation of Magnetic Structures for the Thermally Induced States of CuFe1-xAlxO2(x=0.00, 0.02 and 0.05) Using a Four-Circle Neutron Diffractometer

We have reinvestigated the effect of non-magnetic impurity on thermally-induced magnetic orderings in geometrically frustrated triangular lattice antiferromagnets CuFe 1- x Al x O 2 with x =0.00, 0.02 and 0.05, using a four-circle neutron diffractometer. Although we inferred in our previous work with a three-axis diffractometer that a proper screw type magnetic structure is realized in the intermediate-temperature (IM) phase of the samples with x =0.02 and x =0.05, in the present work we found that a sinusoidally amplitude-modulated magnetic structure with the magnetic moments canted by about 50° from the c axis toward the hexagonal direction is realized. These experimental findings suggest that the quasi-Ising magnetic orderings in CuFeO 2 are stabilized not with the strong Ising anisotropy expected from the Ising-like successive magnetic phase transitions in CuFeO 2 but with the small Ising anisotropy added to strongly competing exchange interactions.

Magnetism in single crystal Ce 2Fe 17 with two types of magnetic ground states

Magnetization and neutron diffraction measurements have been performed on two Ce 2Fe 17 single crystals that have different magnetic ground states. Two types of magnetic structure in the ground states for Ce 2Fe 17 are confirmed. From the above results, it is deduced that the hybridization between the Fe 3d- and Ce 4f-electrons plays an important role on the magnetism of Ce 2Fe 17 in the ground states.

Anisotropy of magnetic properties in LaMnO 3 single crystals

The magnetization M and the magnetic susceptibility χ of single crystals LaMnO 3 have been studied as functions of external magnetic field H (up to 50 kOe), angle φ between the field direction and a crystallographic axis in the LaMnO 3 lattice, and temperature T. At T=2 K the spontaneous magnetization M 0 and the magnetic susceptibility along the main directions a, b, c in the orthorhombic lattice (P nma) of LaMnO 3 are 6.5 emu/g and χ b ≈ χ c ≈2 χ a ≅0.00013 cm 3/g, respectively. The results support a canted A a F b -type magnetic structure, however the spin–flop transition at H<50 kOe was not observed.

Magnetic Structure of K2NiF4-type Iron(III) Oxide Halides

ABSTRACTMaterials with the K2NiF4 structure and a magnetic atom on the B site are examples of a pseudo-2D square lattice. The nearest neighbour interactions are very strong and, with iron(III), usually lead to xy antiferromagnetic coupling. We have used powder neutron diffraction at 2 K to study the magnetic structures of A2FeO3X where A is Ca or Sr and X is Cl or Br. There are two common magnetic structures for K2NiF4-type xy antiferromagnets, those of La2NiO4 and La2CuO4, here we find the latter. In Sr2FeO3F we find a more complex situation. The reflections corresponding to the La2CuO4 magnetic structure are present from 2 K to ambient temperature, but a further set of peaks decays from 2 K to around 60 K. These correspond to a magnetic structure with a doubled c-axis. This is due to a magnetic structure in which La2NiO4-and La2CuO4-type stacking alternate. To our knowledge this is the only example of this magnetic structure type.

Incommensurate magnetic structure ofβ−MnO2

$\ensuremath{\beta}\ensuremath{-}{\mathrm{MnO}}_{2}$ has a well-known screw type magnetic structure. According to Yoshimori [J. Phys. Soc. Jpn. 14, 807 (1959)] the pitch of this screw is exactly $\frac{7}{2}c.$ We performed neutron powder diffraction measurements which show that the pitch of the screw is about 4% shorter than $\frac{7}{2}c$ so $\ensuremath{\beta}\ensuremath{-}{\mathrm{MnO}}_{2}$ has an incommensurate magnetic ordering. This result has important consequences on the calculation of the values of the exchange integrals in $\ensuremath{\beta}\ensuremath{-}{\mathrm{MnO}}_{2}$ by Yoshimori, and Ohama and Hamaguchi [J. Phys. Soc. Jpn. 30, 1311 (1971)]. The present results are compared with the neutron powder diffraction data measured on $\ensuremath{\beta}\ensuremath{-}{\mathrm{MnO}}_{2}$ by Gonzalo and Cox [An. Fis. 66, 407 (1970)] and recent magnetic x-ray diffraction by Sato et al. [J. Phys. Soc. Jpn. 70, 37 (2001)]. The values of the magnetic moments of ${\mathrm{Mn}}^{+4}$ ions are estimated.

NMR excitation by an electric field as a dynamic manifestation of magnetoelectric and antiferroelectric interactions

The possibility of excitation of NMR signals by an ac electric field in magnetically ordered crystals is discussed. Such signals can be recorded using the time-dependent component of the electric polarization vector. It is assumed that the electric and magnetic characteristics are coupled to each other through magnetoelectric and antiferroelectric interactions. Several types of magnetic structures are analyzed in which these interactions are not forbidden by symmetry. Such structures include two-sublattice single-position ferro-and antiferromagnetic phases in centrosymmetrical crystals, two-sublattice magnetic crystals without a center of symmetry (such as KNiPO4), and four-sublattice antiferromagnetic crystals with three types of antiferromagnetism vectors (such as Cr2O3 and α-Fe2O3).

Theoretical study of the field-induced pattern formation in magnetic liquids.

When a thin layer of magnetic fluid confined with an immiscible nonmagnetic liquid is subjected to a perpendicular field, the formation of hexagonal and labyrinthine patterns is observed experimentally. To develop a coherent theoretical description of this phenomenon, the free energy functionals of both types of magnetic structures are derived. Both energy functionals have the same form, which explains that the theoretical results found in this paper for hexagonal and labyrinthlike striped patterns are analogous. The size of the patterns is determined by minimizing the free energy. The influence of the method for computing the magnetic energy on the theoretical results is studied. An accurate computation of the magnetic energy proves important in predicting the experimental pattern size as a function of external field and of layer height. How the results change, when a constant magnetization is assumed during the pattern formation is also investigated. The transition between hexagonal and striped structures is studied by a comparison of their free energies. The ratio of the magnetic to the nonmagnetic liquid is found to be an important factor for the relative stability of the patterns. In agreement with experiments, striped structures are observed at large phase ratios, whereas at small phase ratios hexagonal patterns predominate.