Showing papers on "Magnetic structure published in 2001"

Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4

Abstract: The compound NiCo2O4, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo2O4 of {Ni3+0.1Co2+0.9}tet[Ni3.5+0.9Co2.5+1.1]octO4. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co3+ and Ni3+. The surface of NiCo2O4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co2+, Co3+, Ni2+, Ni3+ and, probably, Ni4+.

Crystallographic symmetry and magnetic structure of CoO

Abstract: The crystallographic symmetry of antiferromagnetic CoO was studied using high-resolution synchrotron powder diffraction in the temperature range 10--300 K. The high-quality powder patterns unambiguously revealed a monoclinic symmetry (space group $C2/m)$ and allowed the extraction of accurate values for the lattice constants. The temperature dependence of the monoclinic deformation scales with the much stronger tetragonal distortion as determined from laboratory x-ray diffraction. Magnetic ordering is associated with a cubic-to-monoclinic transition that is, thus, of first order. Neutron powder-diffraction data are compatible with a collinear magnetic structure with the moments ordered in the monoclinic ac plane.

Physical properties of RCo2 Laves phases

Abstract: The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

Observation of Field-Induced Transverse Néel Ordering in the Spin Gap System TlCuCl3

Abstract: Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl 3 in magnetic fields parallel to the b -axis. The magnetic Bragg peaks which indicate the field-induced Neel ordering were observed for the magnetic field higher than the gap field H g ≈5.5 T at Q =( h , 0, l ) with odd l in the a * - c * plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced Neel ordering as a Bose-Einstein condensation of magnons.

Evolution of the Magnetic Structure of Hexagonal HoMnO3 from Neutron Powder Diffraction Data

Abstract: Hexagonal, nonperovskite HoMnO3 oxide, containing a triangular arrangement of Mn3+ cations, has been prepared in polycrystalline form by the thermal decomposition of metal citrates. The crystal structure has been refined from neutron powder diffraction data. Magnetic and specific-heat measurements anticipate a complex phase diagram: HoMnO3 becomes magnetically ordered at TN ≈ 72 K, and another two magnetic transitions take place at lower temperatures. Neutron powder diffraction measurements demonstrate that, below the ordering temperature, the moments of the Mn3+ cations adopt a triangular spin arrangement, the magnetic moments lying in the basal plane and parallel to the [100] axis. At T = 44.6 K, the moments suddenly reorientate within the basal plane and become aligned perpendicularly to the initial direction. Below T = 25.4 K, an ordered magnetic moment is observed on the Ho atoms at the 4b sites of the crystal structure, whereas those of the 4a site remain in a paramagnetic state. The Ho atoms adopt...

Ab initio study of disorder effects on the electronic and magnetic structure of Sr 2 FeMoO 6

Abstract: We have investigated the electronic structure of ordered and disordered Sr2FeMoO6 using ab initio bandstructure methods The effect of disorder was simulated within supercell calculations to realize several configurations with mis-site disorders It is found that such disorder effects destroy the half-metallic ferromagnetic state of the ordered compound It also leads to a substantial reduction of the magnetic moments at the Fe sites in the disordered configurations Most interestingly, it is found for the disordered configurations that the magnetic coupling within the Fe sublattice as well as that within the Mo sublattice always remain ferromagnetic, while the two sublattices couple antiferromagnetically, in close analogy to the magnetic structure of the ordered compound, but,in contrast to recent suggestions

Interplay of structural, magnetic and transport properties in thelayered Co-based perovskite LnBaCo 2 O 5 (Ln = Tb, Dy, Ho)

Abstract: The oxygen deficient cobaltites LnBaCo2O5 (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T N ∼340 K and at T CO ∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co2+ and Co3+ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ∼3.7 and ∼2.7 , the electronic configuration of the Co ions in LnBaCo2O5 remains conjectural. Two pictures, with Co3+ ions either in intermediate spin state ( t 5 2g e 1 g ) or in high spin state ( t 4 2g e 2 g ), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo2O5 (Ln = Tb, Dy, Ho).

Spin-density wave in ultrathin Fe films on Cu(100).

Abstract: For Fe films epitaxially grown on Cu(100) at 300 K, the total magnetic moment as a function of film thickness and its temperature dependence have been investigated in situ with a multitechnique approach. The results exclude the collinear type-1 antiferromagnetic configuration as the magnetic structure for face-centered-cubic Fe films on Cu(100). It is proposed that a spin-density-wave state is responsible for the magnetic structure.

Calculated magnetic properties of two-dimensional arrays of nanoparticles at vanishing temperature

Abstract: We calculate the magnetic properties of a monolayer of spherical and uniaxial single domain magnetic nanoparticles, with dipolar interaction, at vanishing temperature. The particles are located on the sites of a lattice of either square or hexagonal structure. We focus on both the magnetic properties (magnetization curve, coercive field) and the orientational structure of the magnetic moments in the layer. We find that the structure of the lattice plays an important role on the orientational structure of the moments, especially in the case of a strong dipolar coupling: an ordered state takes place at a length scale of a few interparticle distances, the nature of which (ferromagnetic or antiferromagnetic) changes with the symmetry of the lattice. However, the magnetic properties are nearly independent of the structure of the lattice.

Spin chirality, berry phase and anomalous Hall effect in frustrated ferromagnets

Abstract: Quantum transport is very sensitive to the interference of the quantal phase. The Aharanov–Bohm effect is one of the most prominent examples of it. In ferromagnets, the non-coplanar spin structure gives rise to the analogous effect due to the spin chirality, i.e. the solid angle subtended by the spins. This contributes to the anomalous Hall effect at low temperatures. We present here the theoretical results on the two-dimensional Kagome lattice, which is a typical example of the flat-band ferromagnet. The Berry phase induced by the tilting of the spins opens the band gap, and quantized Hall conductance σxy=±e2/h is realized without external magnetic field. We also discuss the anomalous Hall effect observed in the three-dimensional metallic pyrochlore ferromagnets R2Mo2O7 (R=Nd, Sm, Gd).

The crystal and magnetic structure of Li-aegirine LiFe 3+ Si 2 O 6 : a temperature-dependent study

Abstract: Single crystals of Li-aegirine LiFe3+Si2O6 were synthesized at 1573 K and 3 GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223 K. LiFe3+Si2O6 is monoclinic, space group C2/c, a=9.6641(2) A, b= 8.6612(3) A, c=5.2924(2) A, β=110.12(1)∘ at 300 K as refined from powder neutron data. At 229 K Li-aegirine undergoes a phase transition from C2/c to P21 /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622 A and 〈SiB–O〉=1.624 A at 100 K are present. The bridging angles of the SiO4 tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100 K in the two independent tetrahedral chains in space group P21 /c, whereas it is 180.83(9)° at 300 K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100 K four Li–O bond lengths lie within 2.072(4)–2.172(3) A, the fifth Li–O bond length is 2.356(4) A, whereas the Li–O3 A bond lengths amount to 2.796(4) A. From 57Fe Mossbauer spectroscopic measurements between 80 and 500 K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5 K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe3+Si2O6. Distinct preordering phenomena can be observed up to 35 K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.

Magnetic properties of the antiferromagnetic double perovskite Ba2PrRuO6

Abstract: The ordered perovskite compound Ba2PrRuO6 is prepared and its magnetic properties are investigated. The Rietveld analysis of the neutron diffraction profiles measured at 150 K shows that the Pr3+ and Ru5+ ions are arranged with regularity over the six-coordinate B sites of the perovskite ABO3 and that Ba2PrRuO6 belongs to space group P21/n, with a = 6.0063(5), b = 5.9863(4), c = 8.4677(7) A and 90.04(2)°. The magnetic susceptibility and the heat capacity measurements show that this compound transforms to an antiferromagnetic state below 117 K. From the neutron diffraction patterns measured at 7 K, the magnetic structure is determined to be of Type I and the magnetic moments of Pr3+ and Ru5+ are estimated to be 2.2(1) and 2.0(2) µB, respectively. Their values are discussed on the basis of theoretical calculations for the crystal field splitting.

In-situ Lorentz TEM cooling study of magnetic domain configurations in Ni/sub 2/MnGa

Abstract: Magnetic domain configurations in the ferromagnetic shape memory alloy Ni/sub 2/MnGa are analyzed by means of Lorentz microscopy and noninterferometric phase reconstruction methods. Domain structures in the cubic phase consist of cross-tie walls in the thinnest portions of the foil, and more complex configurations in thicker regions. At low temperature, the magnetization configurations change as the structure transforms martensitically to a tetragonal phase. A simple model for the magnetization changes is proposed.

Crystal and magnetic structure of YCo4−xFexB

Abstract: YCo4−xFexB compounds have been investigated by means of neutron powder diffraction. Substitution of Co with Fe causes substantial modifications of the crystal structure. Iron occupies preferentially one (2c) of the two cobalt sites (2c and 6i). The Curie temperature of the material increases with Fe content from Tc=380 K for YCo4B, and concurrently a spin reorientation is suppressed. The magnetic phenomenology is explained in terms of the preferential occupation of Fe, which carries in the 2c sites an ordered magnetic moment almost double of that in the 6i sites.

Ab initio density functional study of phase stability and noncollinear magnetism in Mn

Abstract: The crystalline and magnetic structures of all known polymorphs of Mn have been investigated using generalized spin-density functional theory based on an unconstrained vector-field description of the magnetization density. We find that at atomic volumes smaller than 12 A3, the magnetic ground state of α-Mn is collinear with magnetic moments ranging between 0 and 3 µB depending on the local symmetry of the atomic positions. At larger atomic volumes, a metastable collinear configuration coexists with a stable noncollinear state. The noncollinearity of the magnetic structure is driven by the appearance of magnetic moments on sites IV, leading to a frustration of exchange interactions in local triangular configurations. A similar situation is found in β-Mn, with a collinear structure with coexisting magnetic and nonmagnetic sites. The α-phase is found to be stable over a wide range of volumes; under compression a phase transition to hexagonal -Mn is predicted.

MAGNETIC STRUCTURE DETERMINATION FROM POWDER DIFFRACTION USING THE PROGRAM FullProf

Abstract: In this paper the techniques for magnetic structure determination from neutron powder diffraction (NPD) data as implemented in the program FullProf are reviewed. In the general case the magnetic moment of an atom in the crystal is given as a Fourier series. The Fourier coefficients are complex vectors constituting the “unknowns” to be determined. These vectors define the magnetic structure and they correspond to the “atom positions” of an unknown crystal structure. The use of group theoretical methods for the symmetry analysis is needed to find the smallest set of free parameters. In general the Fourier coefficients are linear combinations of the basis functions of the irreducible representations of the wave vector group. The coefficients of the linear combinations can be determined by the simulated annealing (SA) technique comparing the calculated versus the observed magnetic intensities. The SA method has been improved and extended to the case of incommensurate magnetic structures within FullProf.

Short- and long-range ordering in the ilmenite–hematite solid solution

Abstract: Magnetic and cation ordering in the ilmenite–hematite solid solution (FeTiO3–Fe2O3) has been studied using in-situ high-temperature time-of-flight neutron powder diffraction. Synthetic samples containing 60 and 65% FeTiO3 (ilm60 and ilm65, respectively) were heated under vacuum up to 1000 °C and their magnetic structure, crystal structure and cation distribution were determined via Rietveld refinement. The quenched starting materials display diffuse superlattice reflections, indicative of short-range cation order. The short-range ordered structure is interpreted with the aid of statistical simulations to be a fine-scale alternation of ordered and antiordered ilmenite-like twin domains, separated by hematite-like twin-domain boundaries (TDBs). Peak width analysis demonstrates that the twin domains have a pronounced shape anisotropy, with average lengths of 20 ± 1 and 60 ± 2 A along the c-axis, and 100 ± 9 and 100 ± 4 A along [0&1macr;1]* in ilm60 and ilm65, respectively. Long-range order increases initially by a process of domain coarsening as the quenched samples are heated below the cation order-disorder temperature, Tod. The degree of order then decreases as they are heated through the transition. This leads to a kinetic relaxation behaviour, in which the observed rate of ordering is determined by the balance between the rate of coarsening and the rate of disordering within the domains. A phenomenological kinetic model is developed, which provides an excellent description of the observed behaviour in both samples. Once long-range order has been established, the equilibrium degree of order as a function of temperature is well described by a modified Bragg-Williams model, yielding values of Tod=830 ± 20 °C and 911 ± 20 °C for ilm60 and ilm65, respectively. Analysis of the magnetic scattering and spontaneous strain demonstrates that short-range magnetic order remains at temperatures well above the bulk Curie temperatures (Tc=178 °C and 143 °C in ilm60 and ilm65, respectively). This indicates significant magnetic heterogeneity in the samples, which may be related to the presence of a high density of Fe-enriched TDBs in the quenched material.

Magnetic ordering in diluted kagome antiferromagnets

Abstract: Potassium iron jarosite, KFe3(SO4)2(OH)6, provides a model kagome Heisenberg antiferromagnet with which to study the behaviour of this highly frustrated system Magnetic susceptibility, powder neutron diffraction and muon spin relaxation measurements have been performed on samples of protonated and deuterated potassium iron jarosite in which some of the iron is replaced by aluminium ions to study the effect of diamagnetic dilution The two successive magnetic ordering transitions of the pure material drop in temperature on doping In all cases the low temperature phase appears to have long-range ordering of all moments in an array which is coplanar or nearly coplanar with the crystallographic ab axes, while in the intermediate phase there is a component of moment along the c-axis and some degree of dynamic character

Crystal structures and magnetic properties of the 6H-perovskites Ba3LnRu2O9(Ln = Ce, Pr and Tb)

Abstract: Magnetic properties of quaternary oxides Ba3LnRu2O9 (Ln = Ce, Pr and Tb) are reported. These compounds adopt the 6H-perovskite structure with space group P63/mmc, in which the cation sites within the face-sharing octahedra are occupied by Ru ions and those within the corner-sharing octahedra are occupied by Ln ions. The oxidation states of both Ru and Ln ions were found to be tetravalent. These compounds are semiconductors over the temperature range 100–400 K. Measurements of the magnetic susceptibility and specific heat were carried out. The Ln4+ antiferromagnetic transition occurred at 9.5 K for Ln = Tb and 10.5 K for Ln = Pr. Powder neutron diffraction measurements for Ba3TbRu2O9 were performed at 2 and 15 K, and at room temperature. It was found that Ba3TbRu2O9 has a long range antiferromagnetic ordering of Tb4+ ions at 2 K. The moments of the Tb4+ ions order ferromagnetically in the c plane, and these ferromagnetic planes are stacked antiferromagnetically along the c axis. The direction of the ordered moments is parallel to the c axis, and the ordered magnetic moment of the Tb4+ ions is 6.84(4) μB.

Magnetic properties of the S = 1/2 quasi-one-dimensional antiferromagnet CaCu2O3

Abstract: We report single crystal growth and magnetic susceptibility and neutron diffraction studies of the S=1/2 quasi-1D antiferromagnet CaCu2O3. The structure of this material is similar to that of the prototype two-leg spin-ladder compound SrCu2O3. However, the Cu-O-Cu bond angle in the ladder rungs in CaCu2O3 is equal to 123 deg, and therefore the magnetic interaction along the rungs is expected to be much weaker in this material. At high temperatures, the magnetic susceptibility of CaCu2O3 can be decomposed into a contribution from 1D antiferromagnetic chains of finite-size chain segments together with a weak Curie contribution. The intrachain magnetic exchange constant, determined from the magnetic susceptibility measurements, is 2000 K. CaCu2O3 undergoes a Neel transition at T_N=25 K with ordering wavevector of (0.429(5), 0.5, 0.5). The magnetic structure is incommensurate in the direction of the frustrated interchain interaction. Weak commensurate (0.5, 0.5, 0.5) magnetic peaks are also observed below T_N. Application of a magnetic field induces a metamagnetic transition at which the incommensurability of the magnetic structure is substantially reduced. The material possesses only short-range magnetic order above the transition field.

Neutron scattering study of the ferromagnetic superconductor UGe 2

Abstract: Unpolarized and polarized neutron scattering experiments have been performed at ambient pressure on a single crystal of the itinerant electron superconductor ${\mathrm{UGe}}_{2}$ in both the ferromagnetic and the paramagnetic phases. Unpolarized neutrons have confirmed the ${\mathrm{ZrGa}}_{2}$-type orthorhombic crystal structure of ${\mathrm{UGe}}_{2}$ and a ferromagnetic ordering below ${T}_{C}=53 \mathrm{K}$ with the moments aligned along the $a$ axis. No evidence of any modulated component for the magnetic structure has been found. Polarized neutron data have shown a large and almost spherical magnetization distribution at the U sites and no induced moment at the Ge sites. Refinements of the magnetic structure factors within the dipolar approximation allow the magnitude of the orbital and spin uranium moments to be quantified, and a comparison to the measured static magnetization reveals that there is no diffuse contribution.

Synthesis, structural order and magnetic behavior of self-assembled /spl epsi/-Co nanocrystal arrays

Abstract: The synthesis of magnetic nanoparticles with monodispere size distributions, their self assembly into ordered arrays and their magnetic behavior as a function of structural order (ferrofluids and 2D assemblies) are presented. Magnetic colloids of monodispersed, passivated, cobalt nanocrystals were produced by the rapid pyrolysis of cobalt carbonyl in solution. The size, size distribution (std. dev.<5%) and the shape of the nanocrystals were controlled by varying the surfactant, its concentration, the reaction rate and the reaction temperature. The Co particles are defect-free single crystals with a complex cubic structure related to the beta phase of manganese (/spl epsi/-Co). In the 2D assembly, a collective behavior was observed in the low-field susceptibility measurements where the magnetization of the zero field cooled process increases steadily and the magnetization of the field cooling process is independent the temperature. This was different from the observed behavior in a sample comprised of disordered interacting particles. A strong paramagnetic contribution appears at very low temperatures where the magnetization increases drastically after field cooling the sample. This has been attributed to the Co surfactant-particle interface since no magnetic atomic impurities are present in these samples.