Showing papers on "Magnetic structure published in 2004"

Structural Anomalies and Multiferroic Behavior in Magnetically Frustrated TbMn2O5

Abstract: We have studied the magnetostructural phase diagram of multiferroic TbMn2O5 as a function of temperature and magnetic field by neutron diffraction. Dielectric and magnetic anomalies are found to be associated with steps in the magnetic propagation vector, including a rare example of a commensurate-incommensurate transition on cooling below 24 K, and in the structural parameters. The geometrically frustrated magnetic structure is stabilized by "canted antiferroelectric" displacements of the Mn3+ ions, an example of the magnetic Jahn-Teller effect. The Tb moments order ferromagnetically at low temperatures in an applied field, while the Mn magnetic structure is largely unchanged.

Thermal expansion and crystal structure of cementite, Fe3C, between 4 and 600 K determined by time-of-flight neutron powder diffraction

Abstract: The cementite phase of Fe3C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with Tc ≃ 480 K, greatly affects the thermal expansion coefficient of the material. The average volumetric coefficient of thermal expansion above Tc was found to be 4.1 (1) × 10−5 K−1; below Tc it is considerably lower (< 1.8 × 10−5 K−1) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a third-order Gruneisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-field theory.

Magnetic Doppler imaging of 53 Camelopardalis in all four Stokes parameters

Abstract: We present the first investigation of the structure of the stellar surface magnetic field using line profiles in all four Stokes parameters. We extract the information about the magnetic field geometry and abundance distributions of the chemically peculiar star 53 Cam by modelling time-series of high-resolution spectropolarimetric observations with the help of a new magnetic Doppler imaging code. This combination of the unique four Stokes parameter data and state-of-the-art magnetic imaging technique makes it possible to infer the stellar magnetic field topology directly from the rotational variability of the Stokes spectra. In the magnetic imaging of 53 Cam we discard the traditional multipolar assumptions about the structure of magnetic fields in Ap stars and explore the stellar magnetic topology without introducing any global a priori constraints on the field structure. The complex magnetic model of 53 Cam derived with our magnetic Doppler imaging method achieves a good fit to the observed intensity, circular and linear polarization profiles of strong magnetically sensitive Fe  spectral lines. Such an agreement between observations and model predictions was not possible with any earlier multipolar magnetic models, based on modelling Stokes I spectra and fitting surface averaged magnetic observables (e.g., longitudinal field, magnetic field modulus, etc.). Furthermore, we demonstrate that even the direct inversion of the four Stokes parameters of 53 Cam assuming a low- order multipolar magnetic geometry is incapable of achieving an adequate fit to our spectropolarimetric observations. Thus, as a main result of our investigation, we discover that the magnetic field topology of 53 Cam is considerably more complex than any low-order multipolar expansion, raising a general question about the validity of the multipolar assumption in the studies of magnetic field structures of Ap stars. In addition to the analysis of the magnetic field of 53 Cam, we reconstruct surface abundance distributions of Si, Ca, Ti, Fe and Nd. These abundance maps confirm results of the previous studies of 53 Cam, in particular dramatic antiphase variation of Ca and Ti abundances.

Creation of an antiferromagnetic exchange spring.

Abstract: We present evidence for the creation of an exchange spring in an antiferromagnet due to exchange coupling to a ferromagnet. X-ray magnetic linear dichroism spectroscopy on single crystal Co/NiO(001) shows that a partial domain wall is wound up at the surface of the antiferromagnet when the adjacent ferromagnet is rotated by a magnetic field. We determine the interface exchange stiffness and the antiferromagnetic domain wall energy from the field dependence of the direction of the antiferromagnetic axis, the antiferromagnetic pendant to a ferromagnetic hysteresis loop. The existence of a planar antiferromagnetic domain wall, proven by our measurement, is a key assumption of most exchange bias models.

Solar magnetic elements at 0".1 resolution. General appearance and magnetic structure

Abstract: New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were obtained at the recently commissioned Swedish l-m Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.1) structures and find new forms of magnetic structures in this particular region. A cotemporal Call H-line image is used to examine the low-chromosphere of network elements. A cotemporal Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.18) FWHM with a flux sensitivity of approximately 130 Mx cm -2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s -1 . We find that magnetic flux in this region as seen in both the magnetngram and the G-band image is typically structured into larger, amorphous, ribbons which are not resolved into individual flux tubes. The measured magnetic flux density in the ribbon structures ranges from 300 to 1500 Mx cm -2 , the higher values occurring at localized concentrations embedded within the ribbons. The Dopplergram indicates relative down-flows associated with all magnetic elements with some indication that higher downflows occur adjacent to the peak magnetic flux location. The mean absolute magnetic flux density of the remnant plage network is about 130 Mx cm -2 ; in the lowest flux regions of the field-of-view, the mean absolute flux density is approximately 60 Mx cm -2 . Within these quiet regions we do not find evidence of pervasive kilo-gauss strength magnetic elements as seen in recent high resolution internetwork studies. In general, the observations confirm recent 3-dimensional numerical simulations which show that the magnetic field in high-density regions such as plage is concentrated in complex structures that are not generally composed of discrete magnetic flux tubes.

Magnetic structure and electric-field gradients of uranium dioxide: An ab initio study

Abstract: The magnetic structure of uranium dioxide has been investigated using linear augmented plane wave ab initio calculations, taking into account spin-orbit coupling, strong Coulomb correlation (using the $\mathrm{LDA}+U$ approach) and noncollinear magnetism. The collinear 1-k antiferromagnetic type-I structure and the noncollinear antiferromagnetic 2-k and 3-k orderings have been tested. The 1-k and 2-k structures can be excluded by a comparison between the calculated and experimental uranium electric field gradients (EFG). It is shown that the measured EFG agrees with theory only in the 3-k structure and an additional deformation of the oxygen cage with an oxygen displacement of ${\ensuremath{\Delta}}_{O}=0.16\mathrm{a}.\mathrm{u}.$ Also the magnetic moments and the corresponding hyperfine fields agree reasonably well with experiment.

Electronic structures and ferromagnetism of Cu- and Mn-doped ZnO

Abstract: The electronic structures of magnetic orderings of Zn1?xTMxO (TM = Cu, Mn) have been studied with the B3LYP hybrid density functional. The corrections for energy band dispersions with respect to the local density approximation (LDA) are similar to GW results, but lead to an improved energy gap. Cu and Mn are close to +2 valence in Zn1?xTMxO (TM = Cu, Mn), but Cu1+ would be realized in n-type ZnO. Cu-?and Mn-doped ZnO have ferromagnetic and antiferromagnetic ground states, respectively. Magnetic couplings between transition metal ions depend sensitively on the interatomic distance.

Incommensurate antiferromagnetic order in the quantum chain compound LiCuVO4

Abstract: The linear spin chain (S= 1 2 ) compound LiCuVO4 was investigated by elastic neutron diffraction, susceptibility and heat-capacity measurements on single crystals. Long-range incommensurate magnetic ordering with a propagation vector k=(0,0.532,0) is observed below 2.1(1) K by elastic neutron diffraction. The refinement of the magnetic structure indicates that the Cu2+ moments lie within the ab-plane, to enclose an angle of ∼90° to each other along the chain, and have a magnitude of 0.31(1)μB. These findings are attributed to the competition of nearest- and next-nearest-neighbour exchange coupling in the edge-sharing CuO2 chains.

First-principles characterization of ferromagnetic Mn 5 Ge 3 for spintronic applications

Abstract: In the active search for potentially promising candidates for spintronic applications, we focus on the intermetallic ferromagnetic ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$ compound and perform accurate first-principles full-potential linearized augmented plane wave calculations within density functional theory. Through a careful investigation of the bulk electronic and magnetic structure, our results for the total magnetization, atomic magnetic moments, metallic conducting character, and hyperfine fields are found to be in good agreement with experiments, and are elucidated in terms of a hybridization mechanism and exchange interaction. In order to assess the potential of this compound for spin-injection purposes, we calculate Fermi velocities and degree of spin polarization; our results predict a rather high spin-injection efficiency in the diffusive regime along the hexagonal $c$ axis. Magneto-optical properties, such as ${L}_{2,3}$ x-ray magnetic circular dichroism, are also reported and await comparison with experimental data.

Magnetic turbulence in the plasma sheet

Abstract: [1] Small-scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investigated by means of a wavelet estimator suitable for detecting distinct scaling characteristics even in noisy measurements. The spectral estimators used for this purpose are affected by a frequency-dependent bias. The variances of the wavelet coefficients, however, match the power-law shaped spectra, which makes the wavelet estimator essentially unbiased. These scaling characteristics of the magnetic field data appear to be essentially nonsteady and intermittent. The scaling properties of bursty bulk flow (BBF) and non-BBF associated magnetic fluctuations are analyzed with the aim of understanding processes of energy transfer between scales. Small-scale (∼0.08-0.3 s) magnetic fluctuations having the same scaling index α ∼ 2.6 as the large-scale (∼0.7-5 s) magnetic fluctuations occur during BBF-associated periods. During non-BBF associated periods the energy transfer to small scales is absent, and the large-scale scaling index a ∼ 1.7 is closer to Kraichnan or Iroshnikov-Kraichnan scalings. The anisotropy characteristics of magnetic fluctuations show both scale-dependent and scale-independent behavior. The former can be partly explained in terms of the Goldreich-Sridhar model of MHD turbulence, which leads to the picture of Alfvenic turbulence parallel and of eddy turbulence perpendicular to the mean magnetic field direction. Nonetheless, other physical mechanisms, such as transverse magnetic structures, velocity shears, or boundary effects can contribute to the anisotropy characteristics of plasma sheet turbulence. The scale-independent features are related to anisotropy characteristics which occur during a period of magnetic reconnection and fast tailward flow.

Giant exchange bias and the vertical shifts of hysteresis loops in γ-Fe2O3-coated Fe nanoparticles

Abstract: We fabricated core/shell-structured Fe nanoparticles, in which the α-Fe core is about 5 nm in diameter and the γ-Fe2O3 shell is about 3 nm thick, and systematically studied their structural and magnetic properties. The magnetic hysteresis (M–H) loops, measured at low temperatures, after the particles were cooled from 350 K in a 50 kOe field, show significant shifts in both horizontal and vertical directions. It has been found that the exchange-bias field can be as large as 6.3 kOe at 2 K, and that the coercive field is also enhanced greatly in the field-cooled (FC) loops. The large exchange bias and vertical shifts of the FC loops at low temperatures may be ascribed to the frozen spins in the shells. A simple model is proposed to interpret the observations.

Phase transitions, partial disorder and multi-k structures in Gd2Ti2O7

Abstract: The geometrically frustrated antiferromagnet Gd2Ti2O7 exhibits magnetic behaviour of such complexity that it poses a challenge to both experiment and theory. Magnetic ordering commences at TN = 1.1 K and there is a further magnetic phase transition at K. Here we use neutron diffraction to definitively establish the nature of the phase transition at and the magnetic structure adopted below this temperature. Between and TN the structure is partly ordered, as previously reported. Below the remaining spins order, but only weakly. The magnetic structure in this temperature range is shown to be a 4-k structure, closely related to the 1-k structure previously suggested. The 4-k and 1-k variants of the structure are distinguished by analysis of the diffuse scattering, which we believe represents a new method of solving the 'multi-k' problem of magnetic structure determination.

Crystal structures and magnetic properties of rare earth tantalates RE3TaO7 (RE = rare earths)

Abstract: The crystal structures and magnetic properties of rare earth tantalates RE3TaO7 (RE = rare earths) are reported. Their crystal structures were grouped into three types: La3NbO7-type, Y3TaO7-type, and defect-fluorite-type. For the Ho3TaO7 compound, two different phases (Y3TaO7-type and defect-fluorite-type) were prepared. At around room temperature, Nd3TaO7 was found to transform from the Y3TaO7-type phase to La3NbO7-type phase with increasing temperature. Temperature dependences of the magnetic susceptibilities and specific heats indicated that the Nd and Tb compounds undergo a 'two-step' antiferromagnetic transition with separate ordering of ions on different crystallographic sites below 2.6 and 3.6 K, respectively. The Dy3+ ion was found to be in an antiferromagnetic state below 2.3 K. The Ho3TaO7 with the Y3TaO7-type structure showed an antiferromagnetic-like transition, while no magnetic ordering was observed down to 1.8 K for the defect-fluorite-type Ho3TaO7.

Raman studies on spintronics materials based on wide bandgap semiconductors

Abstract: Structural properties of GaN and ZnO layers doped with magnetic impurities were investigated by Raman scattering. Long-range lattice ordering and local atomic arrangement around magnetic impurities were analysed, and their solubility limit was considered. For this study, GaN layers doped with Mn and Cr, and ZnO layers doped with Co and V were prepared by molecular beam epitaxy and pulsed laser deposition, respectively. ZnO layers codoped with Ga and N were also studied for the purpose of p-type activation. These samples were observed using a Raman microprobe using visible and deep UV lasers for excitation. The main results are as follows: In Ga1−xMnxN layers, a uniform solid solution was formed for Mn concentration up to x = 1–2%. An impurity mode was observed at 585 cm−1 and assigned to a local vibrational mode of Mn substituting the Ga site. At higher Mn concentrations, rapid deterioration in lattice ordering occurred. Ga1−xCrxN layers showed good lattice ordering up to x = 3–5%. The samples showed resonance enhancement of LO-phonon signals when excited by a UV laser at 266 nm (4.7 eV). This indicates the photo-injection of free carriers to a diluted magnetic semiconductor. ZnO layers codoped with Ga and N showed many impurity modes due to host lattice defects. A strong signal at 580 cm−1 showed a characteristic broadening at high concentrations of N and Ga. This suggested the formation of complex centres with N or related defects. Zn1−xCoxO and Zn1−xVxO layers formed uniform solid solutions up to , but precipitation of the secondary phase was observed at . These samples presented common defect modes as observed in codoped samples. Our result suggests that the impurity modes in ZnO-based materials can be used as a sensitive probe of host lattice defects induced by the impurity incorporation process.

Magnetic structure of CeRhIn5 as a function of pressure and temperature

Abstract: We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn 5 at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ceand the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and nuclear quadrupole resonance measurements confirm the presence of bulk superconductivity. This work places important constraints on an interpretation of the relationship between antiferromagnetism and unconventional superconductivity in CeRhIn 5 .

Various magnetic domain structures in a Ni–Mn–Ga martensite exhibiting magnetic shape memory effect

Abstract: Magnetic domain structures of the Ni–Mn–Ga martensite were observed by means of type I and type II magnetic contrast in scanning electron microscope. The different configuration of magnetic domain patterns coupled together with the twin structures were studied in multivariant, two-variant, and single-variant martensite. The martensitic band contains broad stripelike magnetic domains following the easy axis of magnetization, i.e., the crystallographic c axis. These stripe domains are connected by 90° domain walls creating a staircaselike structure in the adjoining bands. It is found that the internal twins, substructures of the martensite twin domains, are distorted into a zig–zag shape in order to accommodate the main band magnetization. Furthermore, the dagger-shaped stripe domains occur only when the internal twins are present. When the sample exhibits the single-variant state, the internal twins disappear totally and the stripe magnetic domains spread over the whole specimen. The configuration observed here for the magnetic microstructure together with the crystallographic microstructure can help in understanding the magnetic shape memory effect.

Crystal and magnetic structure of the Ca3Mn2O7 Ruddlesden–Popper phase: neutron and synchrotron x-ray diffraction study

Abstract: The crystallographic and magnetic structures of Ca3Mn2O7 Ruddlesden–Popper phase have been determined by a combination of neutron and synchrotron x-ray diffraction. Two-phase behaviour observed at room temperature is attributed to an incomplete structural phase transition. The magnetic structure was solved in the Shubnikov group with dominant G-type antiferromagnetic order in the perovskite bilayers. The temperature evolution of the structural and magnetic parameters is presented.

Magnetic structure of NiCr2O4 studied by neutron scattering and magnetization measurements

Abstract: The magnetic ordering of the normal spinel ferrimagnet NiCr 2 O 4 below T C = 74 K was reinvestigated by neutron scattering and magnetization measurements on a powder specimen. We found another magnetic transition at T S = 31 K besides T C in both experiments. The ordering of a ferrimagnetic (longitudinal) component and that of an antiferromagnetic (transverse) component occur at T C and T S , separately. A new magnetic structure model of NiCr 2 O 4 below T S with a spontaneous magnetization of about 0.3 µ B /formula is proposed based on experimental neutron scattering intensity.

Stability-enhancing underlayer for exchange-coupled magnetic structures, magnetoresistive sensors, and magnetic disk drive systems

Abstract: An exchange-coupled magnetic structure includes a ferromagnetic layer, a coercive ferrite layer, such as cobalt-ferrite, for biasing the magnetization of the ferromagnetic layer, and an oxide underlayer, such as cobalt-oxide, in proximity to the coercive ferrite layer. The oxide underlayer has a lattice structure of either rock salt or a spinel and exhibits no magnetic moment at room temperature. The underlayer affects the structure of the coercive ferrite layer and therefore its magnetic properties, providing increased coercivity and enhanced thermal stability. As a result, the coercive ferrite layer is thermally stable at much smaller thicknesses than without the underlayer. The exchange-coupled structure is used in spin valve and magnetic tunnel junction magnetoresistive sensors in read heads of magnetic disk drive systems. Because the coercive ferrite layer can be made as thin as 1 nm while remaining thermally stable, the sensor satisfies the narrow gap requirements of high recording density systems.

Commensurate-incommensurate magnetic phase transition in magnetoelectric single crystal LiNiPO(4).

Abstract: Neutron scattering studies of single crystal ${\mathrm{LiNiPO}}_{4}$ reveal a spontaneous first-order commensurate-incommensurate magnetic phase transition. Short- and long-range incommensurate phases are intermediate between the high temperature paramagnetic and the low temperature antiferromagnetic phases. The modulated structure has a predominant antiferromagnetic component, giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg reflection, and a ferromagnetic component, giving rise to peaks at small momentum transfers around the origin at $(0,\ifmmode\pm\else\textpm\fi{}Q,0)$. The wavelength of the modulated magnetic structure varies continuously with temperature. It is argued that the incommensurate short- and long-range phases are due to spin-dimensionality crossover from a continuous to the discrete Ising state.

Interplay between reversible and irreversible magnetic phase transitions in polycrystalline Gd 5 Ge 4

Abstract: Temperature and magnetic field dependent magnetization and heat capacity of polycrystalline ${\mathrm{Gd}}_{5}{\mathrm{Ge}}_{4}$ have been measured. In addition to the antiferromagnetic ordering observed at the N\'eel temperature, ${T}_{N}=128\mathrm{K},$ there is a cusp at $\ensuremath{\sim}17.5\mathrm{K}$ in the low-field zero-field cooled (zfc) $M(T)$ curves, below which the zfc and field-cooled (fc) magnetic data exhibit irreversibility. The zfc and fc magnetization data show a complex mixture of reversible and irreversible behaviors at fields between $\ensuremath{\sim}10$ and $\ensuremath{\sim}18\mathrm{kOe},$ which is correlated to the magnetic field induced transitions between the antiferromagnetic (AFM) and the ferromagnetic (FM) states. The initial zfc $M(H)$ data below a certain temperature exhibit two transitions: a discontinuous metamagnetic-like transition and a continuous magnetic moment rotation process. The anomalies in the isofield and isothermal magnetization data indicate a complex magnetic structure at low temperatures, e.g., a complex canted AFM structure. In addition, magnetic field or temperature induced $\mathrm{AFM}\ensuremath{\leftrightarrow}\mathrm{FM}$ transitions occur under certain conditions. The unusual magnetic behavior is discussed in terms of a possible complex magnetic structure at low temperatures and a martensitic-like structural change induced by the magnetic field

Noncollinear magnetic order in quasicrystals.

Abstract: Based on Monte Carlo simulations, the stable magnetization configurations of an antiferromagnet on a quasiperiodic tiling are derived theoretically. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that the superposition of geometric frustration with the quasiperiodic ordering leads to a three-dimensional noncollinear antiferromagnetic spin structure. The structure can be divided into several ordered interpenetrating magnetic supertilings of different energy and characteristic wave vector. The number and the symmetry of subtilings depend on the quasiperiodic ordering of atoms.

Dynamics of magnetically retained supraparticle structures in a liquid flow

Abstract: We study the dynamic motion of magnetic structures composed of micron-size ferromagnetic particles. The supraparticle structures are retained in a microfluidic channel by a local oscillating magnetic field while subjected to a viscous liquid flow. We analyze the position of the magnetic structures as a function of the flow velocity and of the frequency and amplitude of the magnetic field. We also study the conditions for release of the magnetic structures from the localized magnetic field region.

High pressure effects on the crystal and magnetic structure of La0.7Sr0.3MnO3

Abstract: The crystal and magnetic structure of manganite La07Sr03MnO3 has been studied in the pressure range 0–75 GPa and the temperature range 4–300 K The ferromagnetic state of La07Sr03MnO3 remains stable in the whole studied pressure range The Curie temperature increases as d TC/d P = 43 K GPa−1 Unlike the manganites with orthorhombic crystal structure, the pressure-induced increase of TC in La07Sr03MnO3 having the rhombohedral crystal structure of symmetry may be explained by modification of structural parameters only The difference between the properties of manganites with rhombohedral and orthorhombic structures under high pressure is discussed in terms of the symmetry of MnO6 octahedra

Surface effects in α-Fe2O3 nanoparticles

Abstract: The magnetic properties of 5 nm $\alpha $ -Fe2O3 nanoparticles have been investigated by magnetization measurements on a sample consisting of homogeneously dispersed, non-interacting, nanoparticles in a polymer matrix. The results indicate that the magnetic properties are mainly determined by surface effects, which manifest themselves in high coercive field, high irreversibility field and shifted hysteresis loop, after field cooling. These effects come from surface anisotropy and exchange anisotropy, due to the coupling between the disordered surface magnetic structure, with multiple spin configurations, and the core antiferromagnetically ordered structure.