Showing papers on "Magnetic structure published in 1998"

Order-disorder phenomena in new LaBaMn2O6-x CMR perovskites. Crystal and magnetic structure

Abstract: Three new perovskites in the LaBaMn2O6-x family have been synthesized by controlling the oxygen pressure, during both synthesis and postannealing. Structural determination from powder neutron diffraction (PND) data shows that one form of LaBaMn2O6 is cubic (a = 3.906 A), with a disordered distribution of La3+ and Ba2+ cations, whereas a second form of LaBaMn2O6 is tetragonal (a = 3.916 A; c = 7.805 A), with an alternate stacking of lanthanum and barium layers along c. The same La/Ba cation order is observed for the ordered, oxygen-deficient perovskite LaBaMn2O5, which is also tetragonal (a = 5.650 A; c = 7.808 A) and adopts a YBaCuFeO5-related structure. Elucidation of the magnetic structure of LaBaMn2O5, from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn2+/Mn3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn2O6, the PND data show a ferromagne...

Spin-Polarized Vacuum Tunneling into the Exchange-Split Surface State of Gd(0001)

Abstract: Spin-polarized tunneling is demonstrated on Gd(0001) thin films using ferromagnetic probe tips in a low-temperature scanning tunneling microscope. The magnetic field dependent asymmetries in the differential tunneling conductivity are found at bias voltages which correspond to the energies of the spin components of the exchange-split Gd(0001) surface state. Maps of the spatial variation of the asymmetry reveal the magnetic structure of Gd(0001) thin films with a lateral resolution better than 20 nm. It is found that magnetic tip coatings thicker than 100 monolayers Fe may modify the sample domain structure due to the stray field of the tip.

Ternary aluminides LnT2Al10 (Ln=Y, La–Nd, Sm, Gd–Lu andT=Fe, Ru, Os) with YbFe2Al10 type structure and magneticproperties of the iron-containing series

Abstract: Of the title compounds 25 have been prepared for the first time. They crystallize with the orthorhombic YbFe2Al10 type structure (Cmcm, Z=4), which was refined from single-crystal X-ray data of SmFe2Al10 [a=898.9(2), b=1018.6(2), c=904.3(2) pm, R=0.043 for 823 structure factors and 19 variable parameters] and LaOs2Al10 [a=920.5(6), b=1027.4(5), c=916.9(6) pm, R=0.047 for 1475 F values and 21 variables]. Two aluminium sites of LaOs2Al10 show significant deviations from the ideal occupancies, resulting in the exact composition LaOs2Al9.74(2) . The magnetic properties of the iron-containing compounds have been determined with a SQUID magnetometer in the temperature range 2–300 K with magnetic flux densities up to 5.5 T. YFe2Al10 and LaFe2Al10 are Pauli-paramagnetic, indicating that the iron atoms in these isotypic compounds do not carry magnetic moments. The behaviour of the other iron-containing compounds is dominated by the magnetic properties of the rare-earth components. CeFe2Al10 and YbFe2Al10 show mixed-valent behaviour. This can also be concluded from the cell volumes for CeRu2Al10 and YbRu2Al10 . SmFe2Al10 is Van Vleck paramagnetic, and the others follow the Curie–Weiss law with magnetic ordering temperatures of <20 K. The magnetic properties of these compounds were further investigated by magnetization measurements at 2 K.

Structure and Magnetism in λ-MnO2. Geometric Frustration in a Defect Spinel

Abstract: λ-MnO2, a metastable form of manganese dioxide, retains the cubic spinel structure upon lithium removal from LiMn2O4 by soft chemical methods, either electrochemical or acid leaching. The minimum lithium content, achieved by the latter route at pH 1, is Li0.10MnO2, which is in reasonable agreement with previous reports. For lithium contents near the minimum value, long-range antiferromagnetic order sets in below TN = 32 K, and Curie−Weiss susceptibility behavior is found above 125 K, with fitting constants, θc = −104(4) K and C = 1.97(2) emu-K/mol. This value of C is consistent with the lithium content found analytically. The susceptibility is remarkably field dependent in the temperature range near TN for some samples with larger lithium contents, which might be understood in terms of field-induced short-range ferromagnetic correlations. Neutron diffraction studies show a complex magnetic order described by a propagation vector k = (1/21/21/2)(128 Mn moments per magnetic unit cell) and confirm the TN = 3...

Synthesis and Magnetization of New Room-Temperature Molecule-Based Magnets: Effect of Stoichiometry on Local Magnetic Structure by X-ray Magnetic Circular Dichroism

Abstract: The synthesis and the magnetic properties of two new bimetallic chromium−vanadium ferrimagnets, compounds 1 and 2, are reported They belong to a set of nonstoichiometric Prussian blue analogues CIyV[CrIII(CN)6]z·nH2O (C, alkali cation) with TC varying between 295 and 315 K (TC = 310 K for 1 and 315 K for 2) They are obtained through mild chemistry from the molecular precursor [CrIII(CN)6]3- and from vanadium VII salts The synthetic conditions determine (i) the oxidation state of the vanadium (VII, VIII, or VIV), (ii) the stoichiometry, and (iii) hence the magnetic properties The reported systems present a larger magnetization than the first reported V−Cr analogue (036NAβ for 1 and 040NAβ for 2) Furthermore, X-ray magnetic circular dichroism (XMCD) data demonstrate (i) the short-range antiferromagnetic coupling between the spins borne by the chromium(III) and vanadium ions and (ii) the reversal of the local magnetization on vanadium and chromium when changing the stoichiometry

Neutron-Diffraction Studies on the Magnetic Ordering Process in the Layered Mn Perovskite La2-2xSr1+2xMn2O7 (x = 0.40, 0.45 and 0.48).

Abstract: Comprehensive neutron-diffraction studies on La 2-2 x Sr 1+2 x Mn 2 O 7 ( x =0.40,0.45 and 0.48) single crystals have revealed that the low-temperature magnetic phase consists of planar ferromagnetic (FM) and A-type antiferromagnetic (AFM) components, indicating a canted AFM ordering. Upon increasing the hole concentration x , the canting angle between planes changes from 6.3° (nearly planar FM) at x =0.40 to 180° (A-type AFM) at x =0.48, while the ordering temperature of the FM component, T C , decreases from 120 K to 0 K, correspondingly. We have also discovered that the A-type AFM ordering remains above T C and shows an anomalous exponential decrease to T N ∼200 K. This newly found intermediate A-type AFM phase may play a significant role in the enhancement of CMR effects in this layered Mn perovskite system.

Partially Disordered Phase in Frustrated Triangular Lattice Antiferromagnet CuFeO 2

Abstract: We reinvestigated successive magnetic phase transitions ( T N1 ∼14.0 K, T N2 ∼10.5 K) in a frustrated triangular lattice antiferromagnet (TLA) CuFeO 2 by neutron diffraction measurements using single crystals. The magnetic structure of the intermediate-temperature phase between T N1 and T N2 is found to be a quasi -long range ordered sinusoidally amplitude-modulated structure with a temperature dependent propagation wave vector ( q q 0). These features of successive phase transitions are well explained by reinvestigated Monte-Carlo simulation of a 2D Ising TLA with competing exchange interactions up to 3rd neighbors, in spite of the Heisenberg spin character of orbital singlet Fe 3+ magnetic ions.

Intertwined symmetry of the magnetic modulation and the flux-line lattice in the superconducting state of TmNi 2 B 2 C

Abstract: Materials that can in principle exhibit both superconductivity and ferromagnetism are caught in a dilemma: both states represent long-range order, but are in general mutually exclusive. When the material favours a ground state with a large magnetic moment, as is the case for Er4Rh4B (ref. 1), superconductivity is destroyed. For superconductivity to persist, the magnetic structure would need to adopt an antiferromagnetic modulation of short enough wavelength to ensure a small net moment on the length scale of the superconducting coherence length. The intermetallic borocarbide superconductors2,3,4 RNi2B2C (where R is a rare-earth element) have shed new light on this balance between magnetism and superconductivity. The response of these materials in the superconducting state to a magnetic field is dominated by the formation of a flux-line lattice—a regular array of quantized magnetic vortices whose symmetry and degree of order are easily modified and thus can be expected to interact with an underlying magnetic modulation. In TmNi2B2C, superconductivity and antiferromagnetic modulated ordering coexist below 1.5 K (5–7). Here we present the results of a small-angle neutron-scattering study of this compound which show that the structure of the magnetic modulation and the symmetry of the flux-line lattice are intimately coupled, resulting in a complex phase diagram.

Fabrication of submicrometric magnetic structures by electron-beam lithography

Abstract: Submicrometric magnetic structures have been fabricated by electron-beam lithography on Si substrates. High-quality patterns have been obtained with typical length scale of the structures in the range of 100 nm. The designed geometrical configurations are suitable for investigation of their physical properties by transport measurements in a controlled way. In particular, long chains of connected magnetic dots are useful to analyze magnetization reversal processes, whereas ordered arrays of isolated dots can be used to study pinning effects in superconducting films.

Crystal and magnetic structures of Ca4Mn3O10, an n = 3 Ruddlesden-Popper compound

Abstract: The crystal and magnetic structures of the n = 3 Ruddlesden−Popper phase with the ideal composition Ca4Mn3O10 have been studied using X-ray and neutron powder diffraction. The crystal structure at 293 K is relatively insensitive to the partial pressure of oxygen used in sample preparation. A sample prepared in air showed an orthorhombic distortion (space group Pbca, a = 5.26557(12), b = 5.26039(11), c = 26.8276(5) A) from the ideal n = 3 RP structure, as did a sample prepared under 800 atm of O2 pressure (a = 5.26005(4), b = 5.25569(4), c = 26.83543(20) A). Both samples showed a magnetic phase transition at 115 K from a paramagnetic phase with extensive short-range spin ordering to a weakly ferromagnetic (μferro = 2 × 10-3 μB per Mn) low-temperature phase. The antiferromagnetic components of the atomic magnetic moments (2.23(2) μB per Mn) order in a G-type manner within each perovskite block, and the interblock coupling reflects the orthorhombic symmetry of the structure.

Bandwidth-controlled magnetic and electronic transitions in La 0.5 Ca 0.5 − x Sr x MnO 3 ( 0 x 0 . 5 ) distorted perovskite

Abstract: We have investigated the effect of the increase of the one-electron e g bandwidth (W) on the ferromagnetic and antiferromagnetic transitions in charge ordered La 0.5 Ca 0.5 MnO 3 by substituting bigger Sr 2+ ions for Ca 2+ ions through electrical resistivity and low-field ac-susceptibility measurements as a function of temperature. The system La 0.5 Ca 0.5-x Sr x MnO 3 is shown to undergo structural transitions with increasing x. For x=0.0 and 0.1, the structure is orthorhombic with space group Pnma. For x=0.2, 0.3, and 0.4, the structure is consistent with a monoclinic space group I2/a. The end member (x=0.5) has a tetragonal structure (I4/mcm). T C increases with x monotonically, consistent with the fact that the increase of W enhances double-exchange ferromagnetic interaction. On the other hand, in contrast to the expected monotonic suppression, T N increases initially with Sr concentration, followed by a decrease, finally disappearing for x=0.4. The nonmonotonous variation of T N may indicate a possible change in the type of magnetic structure in the antiferromagnetic state.

A model of the formation of stable nonpropagating magnetic structures in the solar wind based on the nonlinear mirror instability

Abstract: A simple model for the formation of stable nonpropagating structures in a magnetized collisionless plasma is presented. The model describes the evolution of an electron-proton plasma from an initially spatially uniform, but unstable, configuration toward a final nonuniform and nonpropagating stable configuration. The model is based on the following hypothesis: (1) one-dimensionality and spatial periodicity, (2) cold electrons, (3) bi-Maxwellian protons as initial condition, (4) conservation of magnetic moment for all protons, (5) conservation of energy for magnetically non trapped protons, (6) spatial pressure balance, (7) evolved structure has a crenellated shape, (8) slow growth of the structure. Given these assumptions all the macroscopic properties of the plasma (density, pressure, and magnetic field) in the saturated state can be computed explicitly. The model shows that a spatially uniform and homogeneous plasma that is unstable against the linear mirror mode can form stable non propagating structures. Thus one can consider the model as a model for the nonlinear mirror instability where the magnetic trapping of protons in the low magnetic field region is the important saturation mechanism. A simple expression for the magnetic field saturation amplitude is found. The pressure balance, between high and low magnetic field regions, which is needed for the evolved structure to be a stable one, is obtained solely through betatron cooling of the trapped protons. Modification of the trapped protons energy due to the Fermi effect seems to be of secondary importance. The model predicts that the evolved structures are characterized by narrow and deep magnetic wells except in the case of very low magnetic pressure (ratio of thermal to magnetic pressure β ≳ 10) where the opposite situation becomes possible. This enforces the idea according to which the proton mirror instability is the driving mechanism for the formation of magnetic holes in high β(≳ 1) plasmas.

Crystal structure, thermal and magnetic properties of La3Co3O8. Phase relations for LaCoO3–δ (0.00≤δ≤0.50) at 673 K

Abstract: Phase relations including reoxidation and decomposition reactions were investigated for samples with nominal composition LaCoO3–δ(0.00≤delta;≤0.50). LaCoO3–δwas synthesized by isothermal reduction at 673 K. Forδ=0.50 and 0.33 distinct phases occur, namely La2Co2O5 and La3Co3O8 . La2Co2O5 oxidizes via La3Co3O8 to LaCoO3 . None of these reduced, vacancy ordered phases show indications for any larger non-stoichiometry. La2Co2O5 and La3Co3O8 are metastable, and undergo irreversible decomposition reactions to CoO and the Ruddlesden–Popper type phases Lam+1ComO3m+1,m=1 and 3, upon heating under inert atmosphere. The crystal structure of La3Co3O8 is determined on the basis of simultaneous refinement of high-resolution powder X-ray diffraction and neutron diffraction data. The space group is P21,a=555.93 pm, b=541.51 pm, c=1177.3 pm, β=90.144° at RT; Rwp(PXD)=4.6%,Rwp(PND)=6.1%, Rp(PXD)=3.6%,Rp(PND)=4.7%. La3Co3O8 adopts a structure which is an intermediate between the brownmillerite and the perovskite type structures. Double layers of corner-sharing CoO6 octahedra are connected by layers with chains of corner-sharing CoO4 tetrahedra running along [010]. La3Co3O8 orders antiferromagnetically at temperatures below TN=35 K. A probable magnetic structure is described on the basis of powder neutron diffraction data. The magnetic moments are µCo(1), tet=3.5±0.1 µB , µCo(2), oct=1.7±0.2 µB and µCo(3), oct=2.0±0.2 µB .

The magnetic structure of

Abstract: We have determined the magnetic structure of the Fe sublattice in by high-resolution neutron powder diffraction. The crystal space group is Cmcm and the magnetic space group is . The Fe modes are and at the 8d, 8e and 8g sites, respectively. The easy direction of magnetization is [100] and the propagation vector is [010].

Magnetic structure of 154 SmMn 2 Ge 2 as a function of temperature and pressure

Abstract: Neutron scattering, ac susceptibility, and NMR measurements have been made on isotopically enriched ${}^{154}{\mathrm{SmMn}}_{2}{\mathrm{Ge}}_{2}.$ All the magnetic states are characterized from much clearer neutron-diffraction spectra than in previous works, in which natural abundance Sm was used. A temperature versus pressure magnetic phase diagram is proposed, consisting of six distinct magnetic states, including previously undetected incommensurate cone structures at low temperatures and pressures. The noncollinear magnetic structures of these magnetic phases are such that a larger antiferromagnetic component appears within (001) planes in the ferromagnetic states than in the antiferromagnetic states. This observation provides an explanation for the anisotropic changes in the lattice constant and the anomalous magnetoresistance of ${\mathrm{SmMn}}_{2}{\mathrm{Ge}}_{2}.$ Previously unexplained susceptibility and NMR measurements are interpreted in terms of changes to the cone structure. Changes in the Mn-Mn spacing and magnetic coupling are consistent with previous observations on related systems.

Neutron powder diffraction study ( T = 4.2-300 K) and polarization analysis of

Abstract: The crystal and magnetic structures of the perovskite have been studied in the temperature range T = 4.2-300 K by powder neutron diffraction. In addition to the antiferromagnetic ordering transition at , a commensurate-incommensurate magnetic transition is detected at . Below this temperature, two sets of satellite peaks surround the (1/2,1/2,1/2) magnetic peak at ?, collapsing into a single set of satellites below 155 K. Polarization analysis has been performed to confirm the magnetic nature of the satellites.

Invar behavior of disordered fcc − Fe x Pt 1 − x alloys

Abstract: Using the tight-binding linearized muffin-tin orbital method combined with the coherent-potential approximation (TB-LMTO-CPA) we have calculated the electronic and magnetic structure of disordered fcc ${\mathrm{Fe}}_{x}{\mathrm{Pt}}_{1\ensuremath{-}x}$ alloys in a broad concentration range. The total energy was determined as a function of the lattice constant and of the magnetic moment (fixed-spin moment method). For iron concentrations between $x=0.10$ and $x=0.85$ the equilibrium lattice constant, the bulk modulus, and the magnetic moment were determined in good agreement with available experimental data. No deviations of the magnetization from the Slater-Pauling curve in the Invar region were found. In that region two minima of the total energy exist, one with a high moment and a large lattice constant and the other with a zero moment and a small lattice constant, which explains qualitatively the Invar effect. Both minima become degenerate at the critical concentration, ${x}_{c}=0.76.$ A nonmagnetic ground state was found for $xg{x}_{c}.$ The energy barrier separating these two minima is two times higher in FePt Invar alloys than in the FeNi system. The relativistic effects were included within the scalar relativistic approximation.

A variable temperature neutron diffraction study of the layered perovskite YBaMn2O5

Abstract: A variable temperature neutron diffraction study has been carried out on the layered perovskite YBaMn2O5 between 100 and 300 K. A broad peak indexed as (1/2, 1/2, 1) on the nuclear cell is consistent with short range MnII /MnIII valence ordering. The magnetic structure below the ferrimagnetic ordering temperature of 167 K has been determined and is in agreement with a previously proposed model. Changes in lattice parameters, bond lengths and angles show evidence of an exchange striction at TC .