Showing papers on "Magnetic structure published in 1999"
TL;DR: A review of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors, is given in this paper.
Abstract: Mixed-valence manganese oxides (R1-χAχ)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, ...
1,757 citations
TL;DR: In this paper, the volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random face-centred cubic iron-nickel alloy, in which they allow for non-collinear spin alignments, that is, spins that may be canted with respect to the average magnetization direction.
Abstract: In 1897 Guillaume1 discovered that face-centred cubic alloys of iron and nickel with a nickel concentration of around 35 atomic per cent exhibit anomalously low (almost zero) thermal expansion over a wide temperature range. This effect, known as the Invar effect, has since been found in various ordered and random alloys and even in amorphous materials2. Other physical properties of Invar systems, such as atomic volume, elastic modulus, heat capacity, magnetization and Curie (or Neel) temperature, also show anomalous behaviour. Invar alloys are used in instrumentation, for example as hair springs in watches. It has long been realized that the effect is related to magnetism2,3; but a full understanding is still lacking. Here we present ab initio calculations of the volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random face-centred cubic iron–nickel alloy, in which we allow for non-collinear spin alignments—that is, spins that may be canted with respect to the average magnetization direction. We find that the magnetic structure is characterized, even at zero temperature, by a continuous transition from the ferromagnetic state at high volumes to a disordered non-collinear configuration at low volumes. There is an additional, comparable contribution to the net magnetization from the changes in the amplitudes of the local magnetic moments. The non-collinearity gives rise to an anomalous volume dependence of the binding energy, and explains other peculiarities of Invar systems.
449 citations
TL;DR: In this article, the magnetic and structural phase diagram of iron employing the full potential linear augmented plane-wave method within the generalized gradient approximation was investigated, and total energy calculations have been performed together with investigations with varying $c/a$ ratio to check the phase stability.
Abstract: We have investigated the magnetic and structural phase diagram of iron employing the full potential linear augmented plane-wave method within the generalized gradient approximation. Therefore, total-energy calculations have been performed together with investigations with varying $c/a$ ratio to check the phase stability. This study focuses on the structural and magnetic properties relevant to Invar and anti-Invar and structural phase transitions occurring in these materials. We show that the properties of antiferromagnetic fcc iron can be understood by collinear full potential calculations. In order to do this, the antiferromagnetic structure has been distorted by short-range ferromagnetic nearest-neighbor coupling. From this we can conclude that the classical low-spin behavior can be replaced by antiferromagnetic ordering. Additionally, the thermal properties of iron, especially the free-energy and thermal-expansion coefficient $\ensuremath{\alpha}(T)$ have been analyzed, which is important for the understanding of the anti-Invar effect. The free energy and $\ensuremath{\alpha}(T)$ were estimated from a Debye scheme for which ab initio results were given as input. Besides the more common cubic phases we have investigated hcp Fe at large volumes in view of its magnetic structure.
305 citations
TL;DR: Inelastic neutron scattering was used to study the wave vector- and frequency-dependent magnetic fluctuations in single crystals of superconducting YBa2Cu3O6+x, and the appearance of the pseudogap determined from transport and nuclear resonance coincides with formation of the resonance in the magnetic excitations.
Abstract: Inelastic neutron scattering was used to study the wave vector– and frequency-dependent magnetic fluctuations in single crystals of superconducting YBa2Cu3O6+x. The spectra contain several important features, including a gap in the superconducting state, a pseudogap in the normal state, and the much-discussed resonance peak. The appearance of the pseudogap determined from transport and nuclear resonance coincides with formation of the resonance in the magnetic excitations. The exchange energy associated with the resonance has the temperature and doping dependences as well as the magnitude to describe approximately the electronic specific heat near the superconducting transition temperature (Tc).
254 citations
TL;DR: In this paper, the geometrical and electronic structures for the magnetic structure of the Heusler alloys with composition were calculated and the structural trends with varying X and Y were explained by a d-occupation model, while a rigid-band model can account for the trends with changing M.
Abstract: Magnetically driven actuator materials, such as the ternary and intermetallic Heusler alloys with composition , are studied within the density-functional theory (DFT) with the generalized gradient approximation (GGA) for the electronic exchange and correlation. The geometrical and electronic structures for the magnetic structure are calculated. The structures and magnetic moments at equilibrium are in good agreement with the experimental values. The structural trends with varying X and Y are explained by a d-occupation model, while a rigid-band model can account for the trends with changing M.
241 citations
TL;DR: In this paper, the existence of a ferrimagnetic state at low temperatures has been shown to be disproved by electron spin-down t 2g electrons using magnetic and structural characterisation.
178 citations
TL;DR: The lower coordination of surface spins is responsible for anomalous magnetic properties of ferrimagnetic NiFe2O4 and antiferromagnetic NiO4 in both cases as discussed by the authors.
171 citations
TL;DR: The phase boundaries between the paramagnetic and ordered states were determined for and. The phase boundaries for these field directions coincide when normalized by the g-factor as discussed by the authors, and the excitation gap at zero temperature was re-evaluated as.
Abstract: The temperature and field variations of the magnetization have been measured for which has a singlet ground state with an excitation gap. It is found that undergoes three-dimensional ordering in magnetic fields. The phase boundaries between the paramagnetic and ordered states are determined for and . The phase boundaries for these field directions coincide when normalized by the g-factor. The excitation gap at zero temperature is re-evaluated as .
168 citations
TL;DR: In this paper, a single-crystalline DyB 2 C 2 compound with the tetragonal structure was investigated and the magnetic properties were investigated on a single crystalized DyB2C 2 compound, and the results were well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one.
Abstract: Magnetic properties were investigated on a single-crystalline DyB 2 C 2 compound with the tetragonal structure. Spontaneous magnetizations appear along the a- and [1 1 0]-directions below T C =15.3 K. A very small shoulder is observed only along the c -axis at T Q =24.7 K, although large λ-type anomalous specific heats are observed at T C and T Q . There exists a large magnetic anisotropy in the tetragonal basal plane which is observed in magnetization processes below T C . Neutron powder diffraction experiments reveal that magnetic reflections are observed only below T C . In the magnetically ordered phase, the Dy moments are arranged to be perpendicular to neighbors along the c -axis. The results are well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one. DyB 2 C 2 is a novel compound with a high antiferroquadrupolar ordering temperature of 24.7 K.
146 citations
TL;DR: Polarized-neutron specular reflectometry (PNR) was developed in the 1980s as a means of measuring depth-resolved magnetization in flat films with characteristic thicknesses from 2 to 5000 A as mentioned in this paper.
137 citations
TL;DR: The crystal structure of Ca2Fe2O5 has been studied as a function of temperature up to 1000°C as discussed by the authors, and the magnetic structure was refined in space group Pcm′n′.
TL;DR: The magnetic structures of the HfFe6Ge6-type structure RMn6Sn6 (R=Gd-Er) compounds have been investigated by neutron diffraction and 155 Gd and 166 Er Mossbauer spectroscopy as mentioned in this paper.
TL;DR: In this paper, a large cell tetragonal structure in space group I41/amd was proposed for the spinel LiMn2O4 at 10 K and 333 K. While complete segregation of the Mn3+ and Mn4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn3 + and Mn 4 + ion is not random and that there is a degree of charge segregation.
Abstract: Powder neutron diffraction has been used to study the nature of the structural transition away from the Fd3m cubic structure upon cooling below ∼285 K in the spinel LiMn2O4. We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I41/amd for the material at 100 K. While complete segregation of the Mn3+ and Mn4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn3+ and Mn4+ ions is not random and that there is a degree of charge segregation. Further, LiMn2O4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Direct current magnetic susceptibility measurements show field-cooled, zero-field-cooled irreversibility at ∼65 K and a maximum in zero-field-cooled data at ∼40 K. Neutron diffraction shows magnetic scattering in the form of a broad peak assigned to short-range order which develops above 100 K. Upon cooling to 60 K additional Bragg peaks are seen, signaling ...
TL;DR: In this paper, the superconductivity of RuSr2GdCu2O8 was shown to coexist with the ferromagnetic phase of the material by using the inhomogeneous (layered) crystal, electronic, and magnetic structure.
Abstract: Relying on the inhomogeneous (layered) crystal, electronic, and magnetic structure, we show how superconductivity can coexist with the ferromagnetic phase of RuSr2GdCu2O8 as observed by Tallon and coworkers. Since the Cu d_{x^2-y^2} orbitals couple only to apical O p_x, p_y orbitals (and only weakly), which also couple only weakly to the magnetic Ru t_{2g} orbitals, there is sufficiently weak exchange splitting, especially of the symmetric CuO2 bilayer Fermi surface, to allow singlet pairing. The exchange splitting is calculated to be large enough that the superconducting order parameter may be of the Fulde-Ferrell-Larkin-Ovchinnikov type. We also note that \pi-phase formation is preferred by the magnetic characteristics of RuSr2GdCu2O8.
TL;DR: In this article, a ferromagnetic spin structure with magnetic moments oriented perpendicular to the c-axis of the hexagonal nuclear structure is observed with magnetic saturation moment 2.2 μB at 4.2 K from neutron diffraction and 2.0 μB from magnetic measurements and decreases in a Brillouinlike manner on heating to TC=575 K.
TL;DR: The non superconducting RSr 2 RuCu 2 O 8+ δ (R=Eu and Gd) compounds are magnetically ordered below T N =168 and 185 K, respectively.
Abstract: The non superconducting RSr 2 RuCu 2 O 8+ δ (R=Eu and Gd) compounds are magnetically ordered below T N =168 and 185 K, respectively. Magnetic susceptibility (ac and dc) studies indicate that the magnetic ordering is due to the Ru sublattice. The Gd sublattice in GdSr 2 RuCu 2 O 8 is antiferromagnetically ordered at 2.8 K. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields, originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The magnetic behavior of this system and that of the well-known itinerant ferromagnet SrRuO 3 are compared.
TL;DR: In this paper, the magnetic properties of the particle shell and grain boundaries, generally different to these of the core, and responsible for phenomena such as interphase exchange penetration, Curie temperature enhancement and magnetic coupling are reviewed.
Abstract: The more interesting features in magnetism of systems of nanoparticles are reviewed. Tailoring of soft and hard magnetic materials as well as basic studies on magnetic interactions are discussed. Particular emphasis is given to the magnetic properties of the particle shell and grain boundaries, generally different to these of the core, and responsible for phenomena such as interphase exchange penetration, Curie temperature enhancement and magnetic coupling. The magnetic behaviour of different nanocrystalline systems has been described. Spin disorder has been found to be a general trend for the magnetic ground state of the outer shell of magnetic particles. Disorder at the surface can be due to competing interactions with different signs originating from broken bonds or topological disorder (grain boundaries), random surface anisotropy, surface magnetostriction, compositional gradients and in general to the enhanced gradient of different properties at the surface. The spin-glass-like ground state of the surface only affects the macroscopic properties in nanocrystalline samples for which the ratio between the number of atoms at the interface and the number of atoms in the core can be enormous, actually as large as 30%.
TL;DR: In this article, the results of an LDA and LDA+U band structure study of the monoclinic and the corundum phases of V2O3 were presented.
Abstract: We present the results of an LDA and LDA+U band structure study of the monoclinic and the corundum phases of V2O3 and argue that the most prominent (spin 1/2) models used to describe the semiconductor metal transition are not valid. Contrary to the generally accepted assumptions we find that the large on site Coulomb and exchange interactions result in a total local spin of 1 rather than 1/2 and especially an orbital occupation which removes the orbital degeneracies and the freedom for orbital ordering. The calculated exchange interaction parameters lead to a magnetic structure consistent with experiment again without the need of orbital ordering. While the low-temperature monoclinic distortion of the corundum crystal structure produces a very small effect on electronic structure of v2o3, the change of magnetic order leads to drastic differences in band widths and band gaps. The low temperature monoclinic phase clearly favors the experimentally observed magnetic structure, but calculations for corundum crystal structure gave two consistent sets of exchange interaction parameters with nearly degenerate total energies suggesting a kind of frustration in the paramagnetic phase. These results strongly suggest that the phase transitions in V2O3 which is so often quoted as the example of a S=1/2 Mott Hubbard system have a different origin. So back to the drawing board!
TL;DR: In this paper, a change of magnetic structure from an antiferromagnetic to a ferrimagnetic (or ferromagnetic) structure occurs in the milled samples, correlated with the redistribution of the cations, Zn and Fe, in the A and B sublattices.
TL;DR: The usefulness of Mossbauer spectroscopy for the investigation of magnetic multilayer systems is described in this article, where position-specified information is available by preparing samples in which monatomic 57Fe probe layers are placed at specific vertical positions, e.g. at interfaces or at the surface.
TL;DR: A new reverse Monte Carlo (RMC) method for modelling both lattice and magnetic disorder in powder crystalline materials by direct calculation of the structure factor has been developed.
Abstract: A new reverse Monte Carlo (RMC) method for modelling both lattice and magnetic disorder in powder crystalline materials by direct calculation of the structure factor has been developed. The method, the program and the basic theory are described in some detail. Initial results from modelling the lattice and magnetic structure of MnO around the Neel temperature are also presented.
TL;DR: In this article, the magnetic structures of a multilayer multi-layer were determined by resonant x-ray magnetic scattering using circular polarized light of energies tuned close to the Gd L and the Fe K absorption edges.
Abstract: The magnetic structures of a $({\mathrm{F}\mathrm{e}/\mathrm{G}\mathrm{d})}_{15}$ multilayer are determined by resonant x-ray magnetic scattering using circular polarized light of energies tuned close to the Gd L and the Fe K absorption edges. Difference superlattice Bragg peaks observed by flipping the photon helicity show that the magnetic moments of the Gd layers are directed antiparallel to the in-plane applied field at temperatures higher than 180 K, and are twisted below. The local Gd magnetizations in each 5.4-nm-thick layer are highly nonuniform in both magnitude and twist angle in the out-of-plane direction: the interface sublayers nearly fully magnetize at room and low temperatures under the influence of the adjacent Fe magnetizations, whereas the central sublayers show measurable spontaneous magnetizations at 200 K and below. An application of the $1\ensuremath{-}{(T/T}_{c})$ law shows a reduced Curie temperature ${(T}_{c}=214\mathrm{K})$ compared with bulk Gd for the central sublayers, while ${T}_{c}=1023\mathrm{K}$ for the interface sublayers. The interface and central sublayers exhibit distinct twist behaviors as a function of temperature below the compensation temperature, indicating the short-range nature of the Fe-Gd interaction. The element-specific resonant x-ray scattering confirmed the antiferromagnetic arrangement of the Gd and Fe moments at room temperature.
TL;DR: In contrast to the isostructural ordered Mn3Pt alloy, these observations indicate that an AF-AF phase transition is absent in the ordered Mn 3Ir alloy as discussed by the authors.
Abstract: Neutron-diffraction measurements were made on an ordered Mn3Ir single crystal in a wide temperature range up to 1030 K The ordered Mn3Ir alloy was found to maintain an antiferromagnetic (AF) triangular spin structure up to the Neel temperature TN=960±10 K The lattice parameter a shows a continuous change in the temperature range including TN In contrast to the isostructural ordered Mn3Pt alloy, these observations indicate that an AF–AF phase transition is absent in the ordered Mn3Ir alloy
TL;DR: For non-collinear magnetic alignment is somewhat unusual for equilibrium, and those systems exhibiting such ground states are frequently 'victims' of competing forces as discussed by the authors, however, magnetic alignment for non-equilibrium, or dynamical systems is universal.
TL;DR: In this paper, the magnetic properties of the single-crystalline tetragonal HoB 2 C 2 compound were investigated and it was shown that the phase below 5.0 K is the one with competitive coexistence of the antiferromagnetic and antiferroquadrupolar order.
Abstract: Magnetic properties of the single-crystalline tetragonal HoB 2 C 2 compound were investigated. An antiferromagnetic transition occurs at 5.8 K. At 5.0 K, a spin-reorientation transition of the first order occurs, and spontaneous magnetizations appear along the a - and [1 1 0]-axes. Magnetization processes below 5.0 K are well scaled down from those of the antiferroquadrupolar ordering compound DyB 2 C 2 and are well interpreted by postulating the magnetic structure similar to that of DyB 2 C 2 . The phase below 5.0 K is the one with competitive coexistence of the antiferromagnetic and antiferroquadrupolar order. The result implies that the antiferroquadrupolar ordering transition occurs below the antiferromagnetic transition temperature and that a pure antiferroquadrupolar ordered phase only in the magnetic fields exists along the [1 1 0]-axis.
TL;DR: In this paper, the structure and phase composition as well as fine magnetic structure of mechanically alloyed (MA) Fe 100− x Ni x compositions (x =10, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 50, 60, 70, 80, 90 ) were investigated by Mossbauer spectroscopy and X-ray diffractometry.
TL;DR: In this paper, the magnetic structure of Co(6 nm)/Cu (6 nm) multilayers with weak coupling was determined by combining magnetic reflectivity and scanning electron microscopy with polarization analysis.
Abstract: Polarized neutron reflectivity and scanning electron microscopy with polarization analysis are combined to determine the magnetic structure of Co(6 nm)/Cu(6 nm) multilayers. These data resolve a controversy regarding the low-field state of giant-magnetoresistive (GMR) multilayers with weak coupling. As-prepared samples show a strong antiparallel correlation of in-plane ferromagnetic Co domains across the Cu. At the coercive field, the Co domains are uncorrelated. This irreversible transition explains the decrease in magnetoresistance from the as-prepared to the coercive state. For both states, the Co moments reside in domains with in-plane sizes of $\ensuremath{\approx}0.5--1.5\ensuremath{\mu}\mathrm{m}$.
TL;DR: The magnetic behavior of the ternary intermetallics, R 2 PdSi 3 (R=Ce, Nd, Tb-Er), has been investigated by magnetisation, electrical-resistivity ( ρ ) and neutron-diffraction measurements as mentioned in this paper.
TL;DR: In this article, the intermediate and final phases during mechanosynthesis of ZnFe2O4 were studied and a model of particles with a magnetic ordered core coupled to a spin-disordered surface layer was described.