Showing papers on "Magnetic structure published in 1997"

Large Low-Field Magnetoresistance in La0.7Ca0.3Mno3 Induced by Artificial Grain-Boundaries

Abstract: A number of different compounds, such as those derived from LaMnO3, have recently been shown to exhibit very large changes (up to 106%) in electrical resistance when a magnetic field is applied1–4—a phenomenon known as colossal magnetoresistance (CMR). But magnetic fields of several tesla are typically required to obtain such a large magnetoresistive effect, thus limiting the potential for applications. Nevertheless the complex and intimate link between magnetic structure, crystallographic structure and electrical resistivity in CMR materials, in addition to being of fundamental scientific interest, appears to provide some scope for engineering a more sensitive magnetoresistive response. Here we elucidate the effect of specific structural defects on the CMR behaviour of the compound La0.7Ca0.3MnO3. We have made thin film devices that isolate the contribution of a single grain boundary that was introduced into an epitaxial film of the material by growing it on a bicrystal substrate. These devices display sharp resistance switching in magnetic fields orders of magnitude less than those normally associated with CMR. These results both provide insight into the role of grain boundaries, and demonstrate the potential for developing sub-micrometre magnetic field sensors based on the CMR effect.

Patterned magnetic nanostructures and quantized magnetic disks

Abstract: Nanofabrication, offering unprecedented capabilities in the manipulation of material structures and properties, opens up new opportunities for engineering innovative magnetic materials and devices, developing ultra-high-density magnetic storage, and understanding micromagnetics. This paper reviews the recent advances in patterned magnetic nanostructures, a fast-emerging field, including (1) state-of-the-art technology for patterning of magnetic nanostructures as small as 10 nm; (2) engineering of unique magnetic properties (such as domain structures, domain switching, and magnetoresistance) by patterning and controlling the size, shape, spacing, orientation, and compositions of magnetic materials; (3) quantized magnetic disks-a new paradigm for ultra-high-density magnetic storage based on patterned single-domain elements that have demonstrated a storage density of 65 Gb/in/sup 2/ (nearly two orders of magnitude higher than that in current commercial magnetic disks) and a capability of 400 Gb/in/sup 2/; (4) novel magnetoresistance sensors based on unique properties of magnetic nanostructures; (5) other applications of nanoscale patterning in magnetics such as the quantification of magnetic force microscopy (MFM) and a new ultra-high-resolution MFM tip; and (6) sub-10-nm imprint lithography-a new low-cost, high-throughput technology for manufacturing magnetic nanostructures.

Molecular-Based Magnetism in Bimetallic Two-Dimensional Oxalate-Bridged Networks. An X-ray and Neutron Diffraction Study.

Abstract: Bimetallic, oxalate-bridged compounds with bi- and trivalent transition metals comprise a class of layered materials which express a large variety in their molecular-based magnetic behavior. Because of this, the availability of the corresponding single-crystal structural data is essential to the successful interpretation of the experimental magnetic results. We report in this paper the crystal structure and magnetic properties of the ferromagnetic compound {[N(n-C3H7)4][MnIICrIII(C2O4)3]}n (1), the crystal structure of the antiferromagnetic compound {[N(n-C4H9)4][MnIIFeIII(C2O4)3]}n (2), and the results of a neutron diffraction study of a polycrystalline sample of the ferromagnetic compound {[P(C6D5)4][MnIICrIII(C2O4)3]}n (3). Crystal data: 1, rhombohedral, R3c, a = 9.363(3) A, c = 49.207(27) A, Z = 6; 2, hexagonal, P63, a = 9.482(2) A, c = 17.827(8) A, Z = 2. The structures consist of anionic, two-dimensional, honeycomb networks formed by the oxalate-bridged metal ions, interleaved by the templating cations. Single-crystal field dependent magnetization measurements as well as elastic neutron scattering experiments on the manganese(II)−chromium(III) samples show the existence of long-range ferromagnetic ordering behavior below Tc = 6 K. The magnetic structure corresponds to an alignment of the spins perpendicular to the network layers. In contrast, the manganese(II)−iron(III) compound expresses a two-dimensional antiferromagnetic ordering.

Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors

Abstract: The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF) 2 X and the spin density wave phase in (TMTSF) 2 X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments by Nakamura et al on (TMTTF) 2 Br and (TMTTF) 2 SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI) 2 Ag is predicted to have a similar antiferromagnetic spin structure.

Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors

Abstract: The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments on (TMTTF)$_2$Br and (TMTTF)$_2$SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)$_2$Ag is predicted to have a similar antiferromagnetic spin structure.

High-Temperature magnetic ordering in a new organic magnet

Abstract: Magnetization, heat capacity, and neutron diffraction experiments on the \ensuremath{\beta}-phase of the dithiadiazolyl radical, p-NC.${\mathrm{C}}_{6}$${\mathrm{F}}_{4}$.CNSSN., provide conclusive evidence that this system exhibits noncollinear antiferromagnetism at 35.5 K, an unprecedented temperature for an organic radical. On the basis of magnetization and powder neutron diffraction results, coupled with theoretical calculations of the spin distribution within the molecule, a magnetic structure for this compound is proposed in which the interactions propagate through $\mathrm{S}\dots{}\mathrm{N}$ contacts.

New Inorganic Spin-Peierls Compound NaV 2O 5 Evidenced by X-Ray and Neutron Scattering

Abstract: In this letter we report lattice dimerization and spin-gap formation of NaV 2 O 5 below its spin-Peierls transition temperature T sp =35.3 K, in which there are V 4+ ( S =1/2) linear chains along the b axis of its orthorhombic structure. A single-crystal X-ray scattering experiment shows superlattice reflections with a lattice modulation vector q =(1/2, 1/2, 1/4) . Inelastic neutron scattering from a powder sample at T =7 K clearly demonstrates a gap energy of 9.8 meV at a scattering vector | Q |=1.0 A -1 corresponding to the reciprocal lattice point (1, 1/2, 0).

Evolution of Magnetic and Superconducting Fluctuations with Doping of High-Tc Superconductors

Abstract: Electronic Raman scattering from high- and low-energy excitations was studied as a function of temperature, extent of hole doping, and energy of the incident photons in Bi2Sr2CaCu2O8±δsuperconductors. For underdoped superconductors, short-range antiferromagnetic (AF) correlations were found to persist with hole doping, and doped single holes were found to be incoherent in the AF environment. Above the superconducting (SC) transition temperatureTc, the system exhibited a sharp Raman resonance of B1g symmetry and energy of 75 millielectron-volts and a pseudogap for electron-hole excitations below 75 millielectron-volts, a manifestation of a partially coherent state forming from doped incoherent quasi particles. The occupancy of the coherent state increases with cooling until phase ordering atTc produces a global SC state.

Continuous and Reversible Change of the Magnetic Coupling in an Fe/Nb Multilayer Induced by Hydrogen Charging

Abstract: We show in this Letter that the magnetic coupling of a [26{Angstrom}Fe/15{Angstrom}Nb]{times}18 multilayer is changed in a continuous and reversible way by introducing hydrogen into the sample. The magnetic structure and its change during hydrogen charging (and the following decharging) is measured {ital in situ} by neutron reflectivity. The alteration of the magnetic coupling upon hydrogenation is confirmed by SQUID magnetization measurements. We attribute the change of the coupling to a change of the effective Fermi wave vector in Nb due to hydrogen uptake. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic Structure of Cr in Exchange Coupled Fe/Cr(001) Superlattices

Abstract: We demonstrate how the noncollinear exchange coupling between the Fe layers in Fe/Cr(001) superlattices is caused by a frustrated spiral modulation of the Cr moments not observed in bulk. The noncollinear coupling vanishes above the N\'eel temperature of this commensurate antiferromagnetic Cr order. This clarifies the essential contribution of Cr ordering to the coupling in the regime of smallest thicknesses where no incommensurate Cr spin density wave can form. For larger Cr thicknesses we observe a predicted incommensurate to commensurate transition with temperature.

Magnetic dipole-dipole interactions and single-ion anisotropy : Revisiting a classical approach to magnets

Abstract: The weak classical through-space magnetic dipolar interactions have, in general, not been an important component in stabilizing bulk magnets, as the strong quantum-mechanical through-bond exchange couplings dominate. We show that for materials composed of strongly exchange-coupled magnetic chains with very weak interchain exchange, the interchain dipolar interactions may dominate, leading to sizable ordering temperatures. The single-ion anisotropy together with the anisotropy of the dipole interaction determine the magnetic ordering directions. The family of [Mn(porphyrin)]+[cyanocarbon]- magnets is shown to order due to classical interchain dipolar interaction in combination with the single-ion anisotropy, resulting in canted magnetic structures.

Magnetic phase diagrams of

Abstract: For the compound of wolframite structure three antiferromagnetic phases are known to exist in the absence of an external field. In this contribution we present the magnetic phase diagrams up to a field strength of 20 T. The plane spanned by the easy direction and the orientation of the twofold screw axes is exposed for the application of external magnetic fields, and the change in the topology of the phase diagrams with the variation of the specific field direction within this plane is described in detail. The application of an external magnetic field perpendicular to this plane hardly affects the stability ranges of any of the phases.

Neutron diffraction study of the influence of structural disorderon the magnetic properties of Sr2FeMO6 (M=Ta,Sb)

Abstract: The crystal structure of the perovskite Sr 2 FeTaO 6 has been refined by simultaneous analysis of X-ray and neutron powder diffraction data collected at 280 K; space group Pbnm, a=5.6204(3), b=5.6161(3), c=7.9266(3) A. The structure is of the GdFeO 3 type, with a disordered distribution of Fe and Ta over the six-coordinate cation sites. The structure of Sr 2 FeSbO 6 has been refined in a similar manner; space group P2 1 /n, a=5.6132(5), b=5.5973(5), c=7.9036(7) A, β=90.01(1)°. The two crystallographically distinct six-coordinate sites in Sr 2 FeSbO 6 are occupied in a partially ordered manner [0.795(6):0.205(6)] by Fe and Sb atoms. Neutron diffraction data collected from Sr 2 FeTaO 6 at 1.5 K show no evidence of long-range magnetic ordering and, in the light of previous susceptibility and Mossbauer measurements, it is concluded that Sr 2 FeTaO 6 is a spin glass below 23 K. Neutron diffraction data collected from Sr 2 FeSbO 6 at 1.5 K include magnetic Bragg peaks characteristic of a type I magnetic structure with an average ordered moment of 3.06(9) µ B per Fe atom on the Fe-dominated octahedral site, and no significant ordered moment on the second site. The magnetic Bragg scattering decreases to zero in the temperature interval 1.5≤T/K≤37(2). It is concluded that the partial cation ordering leads to the coexistence of a magnetically ordered spin system and a spin-glass system.

Antiferromagnetism, Ferromagnetism, and Phase Separation in the GMR System Sr2-xLa1+xMn2O7

Abstract: Neutron and synchrotron X-ray powder diffraction techniques have been used to refine the crystal and magnetic structures of the n = 2 Ruddlesden-Popper (RP) system Sr2LaMn2O7. The sample is shown to be biphasic, although both phases are of the RP type and have similar structural parameters. The majority phase (81%) adopts a collinear antiferromagnetic structure below â?¼210 K whereas the minority phase is ferromagnetically ordered below â?¼125 K. The ordered magnetic moments lie in the xy plane in both phases. The behavior observed is discussed in terms of the interplay between structural and electronic factors. Comparison with data obtained previously by other workers leads to the conclusion that our results have some general significance in the study of n = 2 RP systems.

Study of Spin-Gap Formation in Quasi-Two-Dimensional S= 1/2 System CaV 4O 9: Neutron Scattering and NMR

Abstract: Neutron inelastic scattering study has been performed on aligned single crystals of quasi-two-dimensional spin-gap system CaV 4 O 9 . By analyzing the dispersion curves and the structure factor of the triplet excitation, a physical picture of the present spin system has been constructed: The plaquette singlet units of four S =1/2 V-atoms within four “corner-linked” VO 5 pyramids form the two-dimensional linkage of the spins by the interactions between the V-spins within the “edge-linked” VO 5 pyramids. We have remeasured the NMR relaxation rates and obtained the spin-gap behavior consistent with the neutron results.

Coherent waves of magnetic polarons propagating in La{sub 1{minus}x}Ca{sub x}MnO{sub 3}: An inelastic-neutron-scattering study

Abstract: Inelastic neutron scattering has been performed on semiconducting La{sub 1{minus}x}Ca{sub x}MnO{sub 3} single crystals, which exhibit a slightly canted magnetic structure at a low doping rate (x=0.05,0.08). The dynamic study reveals the coexistence of two excitation branches with very different characteristics, and local modes for x=0.05. The first branch is anisotropically dispersed and is readily attributed to spin waves; the Mn spins are coupled by exchange forces, indicating a strong decrease of the antiferromagnetic interplane coupling by doping. The second branch is weakly and isotropically dispersed, which suggests a coupling induced by the hole hopping. Its unusual q-dependent intensity reveals that this excitation is associated with a very short ferromagnetic correlation length ({approx}8{endash}10 {Angstrom}), reminiscent of a picture of mobile {open_quotes}magnetic polarons.{close_quotes} {copyright} {ital 1997} {ital The American Physical Society}

A structural and magnetic study of the defect perovskite ? from high-resolution neutron diffraction data

Abstract: The fine details of the crystal structure of have been determined from a high-resolution neutron diffraction study at room temperature. The powder sample, showing an excellent crystallinity, was prepared by topotactic reduction of in the presence of Zr. The unit cell can be described as a monoclinic superstructure of perovskite ( is the edge of the ideal perovskite), and contains one-dimensionally infinite chains of flattened octahedra running parallel to the c axis, with square planar units connecting the octahedra chains. Bond valence calculations are consistent with divalent Ni cations in both coordination polyhedra. Neutron diffraction experiments reveal 3D magnetic ordering below K. A magnetic structure with S = 1 moments antiferromagnetically coupled along the chains parallel to the c axis is proposed, and the absence of magnetic moments at the units is also discussed.

Off-axis electron holography of epitaxial FePt films

Abstract: Off-axis electron holography at the nanometer level has been used to investigate the magnetic microstructure of thin epitaxial FexPt1−x (x∼0.5) ordered alloy films. High-resolution electron microscopy in cross section showed high quality epitaxial growth but also revealed some widely spaced regions with structural defects. Lorentz microscopy and off-axis electron holography in field-free conditions established conclusively that similar defective areas were associated with local perturbations of the in-plane magnetic field within the thin films. Further holographic observations with the FePt[001] axis parallel to the film normal revealed variations in phase shifts in the vacuum outside the sample which indicated flux leakage along the film normal. Overall, the results demonstrate that off-axis electron holography is a highly useful technique for mapping local variations of the in-plane magnetic structure associated with defective thin films.

Noncollinear magnetic structure in Ni0.35Fe0.65

Abstract: Magnetic structure of NicFe1−c alloys in the INVAR region has long been a matter of great scientific interest and controversy. Using the locally self-consistent multiple scattering method, which has recently been extended to treat noncollinear magnetic systems, we studied the magnetic structure of Ni0.35Fe0.65 alloys. To simulate the alloys, we constructed a large fcc based sample which contains 256 sites occupied randomly by Ni and Fe atoms. The ground state magnetic structure is found to consist of noncollinear configurations associated with Fe-rich regions. In particular, Fe sites surrounded completely by other Fe atoms have antiferromagnetic alignments, while Fe sites having less than three Ni nearest-neighbors have a variety of noncollinear arrangements.

Spin structures with frustrated moments in RNiAl intermetallic compounds

Abstract: Magnetic order in the ternary equiatomic intermetallic compounds PrNiAl, DyNiAl and HoNiAl is investigated by means of neutron powder diffraction and dc- and ac-susceptibility measurements. DyNiAl shows two magnetic phases: It is ferromagnetic between T1 =15 K and Tc = 31 K and a tilted antiferromagnet below T1. HoNiAl is a ferromagnet in a narrow temperature region just below Tc = 14.5 K, but furthermore exhibits two tilted antiferromagnetic phases (T1 =5.5 K, T2 = 12.5 K). The corresponding magnetic structures differ in the ordering of some of the magnetic moments which are coupled by frustrated bonds. The existence of those frustrated moments, which is induced by the topology is a common phenomenon in the hexagonal RNiAl series. They are manifested with reduced mean values of their moments in the elastic neutron spectra. Another interesting observation is the scaling of the second temperature T1 at which the frustrated spins change their order: For all heavy rare earths investigated so far (R = Tb, Dy, Ho) we find T1: TN,C ≈ 1: 2. PrNiAl displays an incommensurate sinusoidal (TSW) structure with XY spin behaviour below TN = 6.9 K, and only one ordered phase is found. The observed ordering temperatures roughly follow the de Gennes scaling factor.

Interplay of magnetic and hydrogen orders in the laves hydride YMn 2 H 4.3

Abstract: We have performed a neutron-diffraction study of the Laves hydride ${\mathrm{YMn}}_{2}{\mathrm{H}}_{4.3}$ to investigate the interplay between the atomic order in the hydrogen sublattice and the magnetic structure. We find that structural order in the H sublattice and antiferromagnetic order in the Mn sublattice have the same rhombohedral symmetry. At ambient pressure we observe a first-order magnetostructural transition, where the magnetic sublattice transforms from a paramagnetic state to an antiferromagnetic one, as the hydrogen atoms form the ordered superstructure. An applied pressure decouples the two types of order, which then occur at different temperatures. It allows us to separate the effects of structural order from those related to the average Mn-Mn distance.

Charge ordering at room temperature in

Abstract: has been studied by means of a.c. magnetic susceptibility, x-ray and neutron diffraction from 1.5 up to 330 K. This compound shows a structural transition at 299.7 K. Below this temperature a charge ordering state was observed by both x-ray and neutron diffraction measurements. The structure is orthorhombic at temperatures higher than 300 K, but at lower temperatures the patterns show new reflection peaks that can be accounted for in a monoclinic unit cell with stripes of and in the ac-plane. Moreover, the system orders antiferromagnetically at around 120 K giving rise to a magnetic structure of CE type. The magnetic susceptibility measurement clearly shows an anomalous behaviour at both transition temperatures. The charge ordering temperature for the compound is the highest reported in the series (RE = rare earth). This is argued to be a consequence of the larger orthorhombic distortion produced in the unit cell by the lower size of ion.

Magnetic, structural, and spin dynamical properties of La1−xCaxMnO3

Abstract: Neutron scattering has been used to study the magnetic order, spin dynamics, and structural properties of La1−xCaxMnO3 in the ferromagnetic regime (0