Types Of Magnetic Structures Research Articles

Negative magnetization and zero-field cooled exchange bias effect in Eu0.9Pr0.1CrO3 ceramics

Interesting magnetic behaviors, including negative magnetization, zero field fooled and field cooled exchange bias effects, have been observed in Eu0.9Pr0.1CrO3. The negative magnetization at low temperature results from the antiparallel coupling between the Pr3+ moment and the canted moment of Cr3+ sublattice. Left shift of zero field cooled M-H loops, and right shift of field cooled M-H loops have been observed, due to the growth and competition of two types of magnetic structures with GxFz (EuCrO3) and GzFx (PrCrO3) orderings under magnetic field. When the cooling field is high enough, the Pr3+ moment is frozen antiparallel with the applied field during the cooling process, giving rise to a positive exchange bias (right shift of M-H). However, when the cooling field is near zero, magnetic clusters with GxFz (EuCrO3) or GzFx (PrCrO3) orderings are formed and distribute randomly during the cooling process, and the net Pr3+ moment get aligned along the applied field during the initial magnetization process to lower the Zeeman energy, and then negative exchange bias (left shift of M-H) appears.

G -type antiferromagnetic order in the metallic oxide LaCu3Cr4O12

The $A$-site-ordered cubic perovskite $\mathrm{LaC}{\mathrm{u}}_{3}\mathrm{C}{\mathrm{r}}_{4}{\mathrm{O}}_{12}$, where a partial Cu-Cr intersite charge transfer transition occurs at ${T}_{\mathrm{CT}}=220\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, was found to be an unconventional metallic and $G$-type antiferromagnetic oxide. Neutron powder diffraction revealed a $G$-type antiferromagnetic ordering at the Cr sites and no ordered moments at the Cu sites. Ab initio electronic structure calculations revealed that the narrowing of the Cr-O-Cr bands due to heavy tilting of the $\mathrm{Cr}{\mathrm{O}}_{6}$ octahedra and the strong hybridization of the $\mathrm{Cu}\text{\ensuremath{-}}3d,\mathrm{Cr}\text{\ensuremath{-}}3d$, and $\mathrm{O}\text{\ensuremath{-}}2p$ orbitals near the Fermi level give an unusual electronic structure in the vicinity of a localized-electron regime. The $G$-type magnetic structure is primarily stabilized by nearest-neighbor antiferromagnetic superexchange interactions of the near-localized Cr spins. The ferromagnetic ${1\phantom{\rule{0.28em}{0ex}}1\phantom{\rule{0.28em}{0ex}}1}$ layers of the $G$-type antiferromagnetic Cr-spin sublattice allow the spin-polarized electron transfer through the strongly hybridized Cu orbitals.

Magnetic nature of surface and exchange bias effect in NiO nanosheets

The Mesoporous NiO nanosheets with an average thickness of ∼30 nm have been synthesized by annealing Ni(OH)2 precursors. Magnetic measurements show that the NiO nanosheets can be considered as a core-shell type magnetic structure, in which the surface shell layer behaves as a coexistence of two-dimensional diluted antiferromagnetic (2D DAFF) and spin glass (SG) behaviors; the core retains its original bulk antiferromagnetic (AFM) phase. Field-cooled hysteresis loop measurements confirm the presence of exchange bias (EB) effect in the NiO nanosheets. The temperature dependence of the exchange bias field indicates that the observed EB effect originates from an interface exchange coupling interaction between the AFM core and the 2D DAFF shell layer. A phenomenological AFM-DAFF-SG magnetic structure model is proposed to interpret the magnetic properties of these NiO nanosheets.

Zigzag type magnetic structure of the spin Jeff = ½ compound α-RuCl3 as determined by neutron powder diffraction

Using high intensity powder neutron diffraction the magnetic structure of a-RuCl3 has been determined. Following the magnetic propagation vector κ = (½, 0, ½) the Jeff = ½ spins of Ru3+ adopt a Zigzag type arrangement on the honeycomb lattice of the layered P3112 structure. The magnetic moments are oriented perpendicular to the trigonal axis. Similarities and differences to previously published single crystal data are discussed. The low value of the magnetic moments, μRu = 0.5(1) μB indicates a possible closeness of α-RuCl3 to the Kitaev spin liquid state.

Incommensurate counterrotating magnetic order stabilized by Kitaev interactions in the layered honeycombα−Li2IrO3

The layered honeycomb magnet $\alpha$-Li$_2$IrO$_3$ has been theoretically proposed as a candidate to display novel magnetic behaviour associated with Kitaev interactions between spin-orbit entangled $j_{\rm eff}=1/2$ magnetic moments on a honeycomb lattice. Here we report single crystal magnetic resonant x-ray diffraction combined with powder magnetic neutron diffraction to reveal an incommensurate magnetic order in the honeycomb layers with Ir magnetic moments counter-rotating on nearest-neighbour sites. This type of magnetic structure has not been reported experimentally before in honeycomb magnets and cannot be explained by a spin Hamiltonian with dominant isotropic (Heisenberg) couplings. The magnetic structure shares many key features with the magnetic order in the structural polytypes $\beta$ and $\gamma$-Li$_2$IrO$_3$, understood theoretically to be stabilized by dominant Kitaev interactions between Ir moments located on the vertices of three-dimensional hyperhoneycomb and stripyhoneycomb lattices, respectively. Based on this analogy and a theoretical soft-spin analysis of magnetic ground states for candidate spin Hamiltonians, we propose that Kitaev interactions also dominate in $\alpha$-Li$_2$IrO$_3$, indicative of universal Kitaev physics across all three members of the harmonic honeycomb family of Li$_2$IrO$_3$ polytypes.

The physical mechanism of magnetic field controlled magnetocaloric effect and magnetoresistance in bulk PrGa compound.

The PrGa compound shows excellent performance on the magnetocaloric effect (MCE) and magnetoresistance (MR). The physical mechanism of MCE and MR in PrGa compound was investigated and elaborated in detail on the basis of magnetic measurement, heat capacity measurement and neutron powder diffraction (NPD) experiment. New types of magnetic structure and magnetic transition are found. The results of the NPD along with the saturation magnetic moment (MS) and magnetic entropy (SM) indicate that the magnetic moments are randomly distributed within the equivalent conical surface in the ferromagnetic (FM) temperature range. PrGa compound undergoes an FM to FM transition and an FM to paramagnetic (PM) transition as temperature increases. The magnetizing process was discussed in detail and the physical mechanism of the magnetic field controlled magnetocaloric effect (MCE) and the magnetoresistance (MR) was studied. The formation of the plateau on MCE curve was explained and MR was calculated in detail on the basis of the magnetic structure and the analysis of the magnetizing process. The experimental results are in excellent agreement with the calculations. Finally, the expression of MR = β(T)X2 and its application conditions were discussed, where X is M(H)/Meff, and Meff is the paramagnetic effective moment.

Experimental evaluation of the ‘transport-of-intensity’ equation for magnetic phase reconstruction in Lorentz transmission electron microscopy

The ‘transport-of-intensity’ equation (TIE) is a general phase reconstruction methodology that can be applied to Lorentz transmission electron microscopy (TEM) through the use of Fresnel-contrast (defocused) images. We present an experimental study to test the application of the TIE for quantitative magnetic mapping in Lorentz TEM without aberration correction by examining sub-micrometer sized Ni80Fe20 (Permalloy) elements. For a JEOL JEM 2100F adapted for Lorentz microscopy, we find that quantitative magnetic phase reconstructions are possible for defoci distances ranging between approximately 200μm and 800μm. The lower limit originates from competing sources of image intensity variations in Fresnel-contrast images, namely structural defects and diffraction contrast. The upper defocus limit is due to a numerical error in the estimation of the intensity derivative based on three images. For magnetic domains, we show quantitative reconstructions of the product of the magnetic induction vector and thickness in element sizes down to approximately 100nm in lateral size and 5nm thick resulting in a minimal detection of 5Tnm. Three types of magnetic structures are tested in terms of phase reconstruction: vortex cores, domain walls, and element edges. We quantify vortex core structures at a diameter of 12nm while the structures of domain walls and element edges are characterized qualitatively. Finally, we show by image simulations that the conclusions of this experimental study are relevant to other Lorentz TEM in which spherical aberration and defocus are dominant aberrations.

Effects of a spin-polarized current assisted Ørsted field in magnetization patterning

A spin-polarized electrical current leads to a variety of periodical magnetic structures in nanostripes. In the presence of the Ørsted field, which always assists an electrical current, the basic types of magnetic structures, i.e., a vortex-antivortex crystal and cross-tie domain walls, survive. The Ørsted field prevents saturation of the nanostripe and a longitudinal domain wall appears instead. Possible magnetization structures in stripes with different geometrical and material properties are studied numerically and analytically.

Temperature and composition phase diagram in the iron-based ladder compoundsBa1−xCsxFe2Se3

We investigated the iron-based ladder compounds (Ba,$\mathrm{Cs}){\mathrm{Fe}}_{2}{\mathrm{Se}}_{3}$. Their parent compounds ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$ have different space groups, formal valences of Fe, and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, x-ray diffraction, and powder neutron diffraction measurements were conducted to obtain a temperature and composition phase diagram of this system. Block magnetism observed in ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$ is not so fragile against Ba doping. A new type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$, but interladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless a finite $T$-linear contribution in specific heat was obtained at low temperatures.

Ferroelectricity and magnetic orderings in Delafossite family compounds

Since discovery of the ferroelectric polarization induced by magnetic field in CuFeO2, delafossite family compounds have attracted much attention, because some theoretical formula, which had been presented, could not explain the ferroelectric polarization of CuFeO2. We have been investigating correlation among their magnetic orderings, lattice symmetry and ferroelectric polarization in AFeO2 (A=Cu, Ag, Na) systems. In CuFeO2, the ferroelectric polarization is induced by chemical substitutions for Fe sites in CuFe1-xBxO2 (B=Al, Ga, Rh, Mn) as well as magnetic field. The magnetic structure in the ferroelectric phase is proper screw type with magnetic point group 21' determined by the neutron diffraction experiments.[1] The ferroelectric polarization parallel to the propagation vector in CuFeO2 can be explained by both extended inverse-Dzyaloshinsky-Moriya(DM) effect and d-p hybridization mechanism. We have also demonstrated that the spin-orbit interaction in Fe ions, coupling spin and orbital orders, plays a crucial role to the ferroelectricity in both of which break the crystal symmetry, by observing incommensurate 2q orbital modulation in the ferroelectric phase of CuFe1-xGaO2 by soft X-ray resonant diffraction.[2] When nonmagnetic Cu ions on A-site are substituted by Ag or Na ions, the magnetic orderings are completely modified from CuFeO2. In AgFeO2, cycloid type magnetic structure with m1' point group is stabilized, which is concomitant with ferroelectric polarization, as the lowest temperature phase in zero-field.[3] Also in alpha-NaFeO2, the other type of cycloidal ordering (m1') appears mainly in magnetic field. In these systems, taking account of extended inverse DM effect, ferroelectric polarization direction is along general direction in the ac plane and does not follow the well-known inverse DM formula. In this presentation, I will discuss the relationship between the magnetic ordering and ferroelectricity in these delafossite family compounds.

125Te Synchrotron-Radiation-Based Mössbauer Spectroscopy of Fe1.1Te and FeTe0.5Se0.5

The 125Te Mossbauer spectra of Fe1.1Te and FeTe0.5Se0.5 were measured using synchrotron-radiation-based Mossbauer spectroscopy. The hyperfine field of Te in the antiferromagnetic phase of Fe1.1Te was evaluated to be 4(1) T at 20 K, and this hyperfine field is considered to be due to the transferred hyperfine field from Fe magnetic moments. The evaluated hyperfine field was comparatively large, suggesting the strong correlation with the magnetic moment of Fe that forms a double-stripe type magnetic structure. For the superconductor FeTe0.5Se0.5, no marked change in its Mossbauer parameters was observed below or above the superconducting transition temperature, indicating that the change in the electronic state of Te was small.

Magnetic properties of LaCo12B6 compound as probed by neutron diffraction and by magnetization study under high pressures

The magnetic properties have been studied by magnetization measurements and the crystal and magnetic structure investigated by neutron diffraction. It is shown that LaCo12B6 compound exhibits an incommensurate conical type magnetic structure. Significantly different magnetic moments have been observed on the two Co crystal sites. The ambient pressure data are compared to that of YCo12B6 isotype and Landau coefficients have been derived from the magnetization curves for both compounds as well as their temperature dependence. The LaCo12B6 compound undergoes a transition from ordered to paramagnetic state at 160K. The volume dependence of intrinsic magnetic properties of the LaCo12B6 compound has been determined by studies under hydrostatic pressure up to 10kbar in temperature range 5–200K. Both the Curie temperature TC and the spontaneous magnetization MS decreases with increasing pressure. The observed decrease of MS with pressure (dMS/dp=−7.4×10−3μB/kbar at 5K) can be attributed to the pressure induced decrease of Co magnetic moments. The decrease of TC with pressure (dTC/dp=−0.49K/kbar) is comparable to the decrease observed on RCo12B6 compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.

Electronic States of Single-Component Molecular Conductors [M(tmdt)2

The electronic states of isostructural single-component molecular conductors [\(M\)(tmdt)2] (\(M\) = Ni, Au, and Cu) are theoretically studied. By considering fragments of molecular orbitals as basis functions, we construct a multiorbital model common for the three materials. The tight-binding parameters are estimated from results of first-principles band calculations, leading to a systematic view of their electronic structures. We find that the interplay between a pπ-type orbital (L) on each of the two ligands and a pdσ-type orbital (Mσ) centered on the metal site plays a crucial role: their energy difference controls the electronic states near the Fermi energy. For the magnetic materials (\(M\) = Au and Cu), we take into account Coulomb interactions on different orbitals, i.e., we consider the multiorbital Hubbard model. Its ground-state properties are calculated within mean-field approximation where various types of magnetic structures with different orbital natures are found. An explanation for the experimental results in [Cu(tmdt)2] is provided: The quasi-degeneracy of the two types of orbitals leads to a dual state where localized Mσ spins appear, and L sites show a nonmagnetic state owing to dimerization. On the other hand, [Au(tmdt)2] locates in the subtle region in terms of the degree of orbital mixing. We propose possible scenarios for its puzzling antiferromagnetic phase transition, involving the Mσ orbital in contrast to previous discussions mostly concentrating on the L sector.

MAGNETIC FIELD STRUCTURES TRIGGERING SOLAR FLARES AND CORONAL MASS EJECTIONS

Solar flares and coronal mass ejections (CMEs), the most catastrophic eruptions in our solar system, have been known to affect terrestrial environments and infrastructure. However, because their triggering mechanism is still not sufficiently understood, our capacity to predict the occurrence of solar eruptions and to forecast space weather is substantially hindered. Even though various models have been proposed to determine the onset of solar eruptions, the types of magnetic structures capable of triggering these eruptions are still unclear. In this study, we solved this problem by systematically surveying the nonlinear dynamics caused by a wide variety of magnetic structures in terms of three-dimensional magnetohydrodynamic simulations. As a result, we determined that two different types of small magnetic structures favor the onset of solar eruptions. These structures, which should appear near the magnetic polarity inversion line (PIL), include magnetic fluxes reversed to the potential component or the nonpotential component of major field on the PIL. In addition, we analyzed two large flares, the X-class flare on December 13, 2006 and the M-class flare on February 13, 2011, using imaging data provided by the Hinode satellite, and we demonstrated that they conform to the simulation predictions. These results suggest that forecasting of solar eruptions is possible with sophisticated observation of a solar magnetic field, although the lead time must be limited by the time scale of changes in the small magnetic structures.

Magnetic order and spin-flop transitions in the cobalt-doped multiferroic Mn1−xCoxWO4

We present a comprehensive single crystal neutron diffraction investigation of the $\rm Mn_{1-x}Co_{x}WO_4$ with $0.02\leq x \leq0.30$. At lower concentration $x \leq 0.05$, the system is quickly driven into the multiferroic phase with spin structure forming an elliptical spiral order similar to the parent compound. The reduction of electric polarization is ascribed to the tilting of the spiral plane. For $x\sim 0.075$, the magnetic structure undergoes a spin flop transition that is characterized by a sudden rotation of the spin helix envelope into the $ac$ plane. This spin structure persists for concentration up to $x=0.15$, where additional competing magnetic orders appear at low temperature. For $0.17 \leq x \leq 0.30$, the system experiences another spin flop transition and recovers the low-$x$ spiral spin configuration. A simple commensurate spin structure with $\vec{q}=(0.5,0,0)$ is found to coexist with the incommensurate spiral order. The complex evolution of magnetic structure in Co doped $\rm MnWO_4$ contrasts sharply with other transition metal ion doped $\rm Mn_{1-x}A_xWO_4$ (A=Zn, Mg, Fe) where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of $\rm Mn_{1-x}Co_{x}WO_4 $ results from the interplay between magnetic frustration and spin anisotropy of the Co ions.

Morphology of the quiet Sun between 150 and 450 MHz as observed with the Nançay radioheliograph

Context. Since its last upgrading in 2004, the Nancay radioheliograph (NRH) is able to produce reliable images of the quiet Sun in the 150–450 MHz range, which corresponds to the low and medium corona. These images are better than those previously obtained with the NRH or other instruments in this range and are suitable for quantitative and systematic exploitation.Aims. We aim to study the radio aspects of the solar atmosphere. We aim to focuss on the description of the morphology and the comparisons with observations in EUV and X-ray ranges and with magnetic structures.Methods. We used the rotational aperture synthesis technique (suitable for non time-varying objects) along with an original self-calibration procedure and a specific deconvolution algorithm (a variant of CLEAN that includes a scale analysis).Results. We present results from radio imaging of the quiet Sun with the NRH. The analysis was carried over about 160 days during the summers (June-August) from 2004 to 2011. We confirms and extend our first results, which were published from a much smaller data sample. We emphasize new aspects of the corona observed in this frequency range, in particular the existence of coronal holes darker than previously reported and dark channels at high frequencies. We give examples that illustrate the complex morphology of coronal structures as revealed by radio imaging, the center-to-limb effects (radio occultation) and the variation with the phase of the solar cycle. We compare our images with large-scale coronal magnetic structures. We show that dark coronal holes and channels seen at high frequencies and bright ribbons seen at low frequencies seem to be associated with particular types of magnetic structures. Conclusions. Detailed radio images in this frequency range are a new tool with high potential for investigating the low and middle corona, since these images can emphasize structures that are not (or much less) visible at other wavelengths. In the near future, there is much to learn from observations with NRH during the ascending part of the cycle (never observed at m/dcm wavelengths ) and from composite images of combined NRH and GMRT data, which will have a much better resolution. In a second step, it will be interesting to obtain circularly polarized images (giving a diagnostic of the coronal magnetic field) and images with new-generation instruments, which will yield detailed images with shorter synthesis or even snapshot images, from which processes evolving on shorter time scales can be studied.

Antiferromagnetic resonance and dielectric properties of rare-earth ferroborates in the submillimeter frequency range

The magnetoresonance and dielectric properties of a number of crystals of a new family of multiferroics, namely, rare-earth ferroborates RFe3(BO3)4 (R = Y, Eu, Pr, Tb, Tb0.25Er0.75), are studied in the sub-millimeter frequency range (ν = 3–20 cm−1). Ferroborates with R = Y, Tb, and Eu exhibit permittivity jumps at temperatures of 375, 198, and 58 K, respectively, which are caused by the R32 → P3121 phase transition. Antiferromagnetic resonance (AFMR) modes in the subsystem of Fe3+ ions are detected in the range of anti-ferromagnetic ordering (T < TN = 30–40 K) in all ferroborates that have either an easy-plane (Y, Eu) or easy-axis (Pr, Tb, Tb0.25Er0.75) magnetic structure. The AFMR frequencies are found to depend strongly on the magnetic anisotropy of a rare-earth ion and its exchange interaction with the Fe subsystem, which determine the type of magnetic structure and the sign and magnitude of an effective anisotropy constant. The basic parameters of the magnetic interactions in these ferroborates are found, and the magnetoelectric contribution to AFMR is analyzed.

Magnetic properties of the [formula omitted] solid solution series

Co 4 Nb 2 O 9 and Mn 4 Nb 2 O 9 order collinear antiferromagnetically with the same magnetic spin structure type below 30 and 125 K, respectively. Magnetization measurements on powder samples of the solid solution series ( Co , Mn ) 4 Nb 2 O 9 prepared by arc melting reveal a linear progression of the Néel-temperature with Co/Mn ratio. Powder neutron diffraction experiments performed for a selected composition confirm the existence of the same magnetic structure type as found for the end members. ( Co , Mn ) 4 Nb 2 O 9 samples prepared by subsolidus reaction and comparably much lower cooling rates after tempering contain very small amounts of additional ( Co , Mn ) 3 O 4 spinel phases with strongly varying transition temperatures as a function of the Co/Mn ratio.

Effect of iron doping on magnetic properties of Sr0.78Y0.22CoO2.625+δ-layered perovskite

The Sr0.78Y0.22Co1−xFexO2.625+δ (x = 0 and 0.12) system has been studied using neutron diffraction technique, magnetization, and elastic properties measurements. Undoped sample exhibits superstructure with 2√2ap * 2√2ap * 4ap metrics and structural phase transition at T = 360 K. Magnetic ordering starts to develop below 360 K. Spontaneous magnetization shows anomalous behavior and reaches maximal value nearly room temperature. A magnetic structure of the both compounds has been described assuming G-type antiferromagnetic ordering. There are two different magnetic moment values for Co ions in CoO6 and CoO4.5 layers. Magnetic moments in CoO4.5 layers are larger than those for octahedrons. The refined values of magnetic moments indicate a mixed low–high spin state of Co3+ for the both layers. There is no evidence for any change of the magnetic structure type with temperature lowering despite the anomalous magnetization behavior. Iron doping (x = 0.12) leads to a suppression of the small ferromagnetic component, disappearance of the 2√2ap * 2√2ap * 4ap metrics, and strong increase of the average magnetic moment for Co4.5 layer. It is suggested that the ferromagnetic component in the undoped samples is a result of non-collinearity of the magnetic moments within CoO4.5 layers.

Spin-Lattice Coupling, Frustration, and Magnetic Order in MultiferroicRMnO3

We have performed high-resolution neutron diffraction and inelastic neutron scattering experiments in the frustrated multiferroic hexagonal compounds RMnO_{3} (R = Ho,Yb,Sc,Y), which provide evidence of a strong magnetoelastic coupling in the whole family. We can correlate the atomic positions, the type of magnetic structure, and the nature of the spin waves whatever the R ion and temperature. The key parameter is the position of the Mn ions in the unit cell with respect to a critical threshold of 1/3, which determines the sign of the coupling between Mn triangular planes.