Showing papers on "Magnetic structure published in 1981"

Magnetic structure of small NiFe2O4 particles

Abstract: The magnetic structure of small NiFe2O4 particles has been investigated. Samples (in the few hundred angstrom size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic field applied, unambiguously establish that a non‐collinear structure exists. Further, the magnetic moment at low temperatures is appreciably lower than the value reported for bulk material. A model is proposed in which the NiFe2O4 particles consist of a core with the usual spin arrangement and a surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependence of this structure is also reported.

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Abstract: Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is

Structural and magnetic properties of Y(Mn1−xFex)12

Abstract: The crystallographic and the magnetic structures of Y(Mn1−xFex)12 intermetallic compounds were investigated. They crystallize in the ThMn12 structure type. The Y atoms occupy the 2(a) sites and the transition metals are distributed on 3 nonequivalent sites 8i, 8j, and 8f. We have determined the solid solubility limit (x = 0.67) of Fe in YMn12. Neutron diffraction spectra at different temperatures have been used to study the nuclear and magnetic structure of Y(Mn0.7Fe0.3)12 and Y(Mn0.4Fe0.6)12. The Mn and Fe atoms are found to exhibit strong site preference with the i site favoring Mn atoms and the f site Fe atoms. Accordingly the instability of the RFe12 phase can be explained on the basis of the preferential atomic ordering observed in the ternary compounds. Based on the results of magnetic structure refinements using the Rietveld profiling method, antiferromagnetic, noncollinear structures are proposed for these two compounds.

Crystal and magnetic structure of the NiMnGe1-nSin System

Abstract: Magnetic and crystallographic nature of the compounds NiMnGe1−nSin (0 ≦ n ≦ 1) are studied in the temperature range from 80 to 1000 K by X-ray and neutron diffraction, magnetometric and DTA measurements. On the basis of these experimental data, crystallographic and magnetic phase diagram are constructed. The compounds have NiTiSi type structure at low temperatures and they transform into the Ni2In type structure at high temperatures. Magnetic and neutron diffraction measurement revealed the existence of three regions of different types of magnetic orderings: helicoidal (0 ≦ n ≦ 0.25), noncollinear (0.3 ≦ n ≦ 0.55), and ferromagnetic (0.6 ≦ n ≦ 1.0). The transition from antiferromagnetic to ferromagnetic phase implies the decrease of the MnMn distances. [Russian Text Ignored].

The antiferromagnetic structure of DyMn2O5 at 4.2K

Abstract: Powdered and single crystal specimens of dysprosium manganate, DyMn2O5, have been studied by neutron diffraction at temperatures between ambient and 4.2K. Magnetic transitions are observed at 44K, 18K and 8.4K. Both manganese and dysprosium moments are ordered below 8.4K, where the magnetic structure has been refined. Measurements above 8.4K indicate that this transition is to a second phase in which both moments are again ordered, but the details of the structure of this phase are not yet known. The relationship of the low-temperature phase to the magnetic structure of some isomorphous rare-earth manganates is discussed.

Neutron diffraction studies of the modulated magnetic structures of CeSb and Nd metal (invited)

Abstract: The magnetic phase diagram of CeSb, which has been determined by neutron diffraction and other experimental techniques, is briefly reviewed and related to some of the theoretical models that have been developed to account for it. The results of neutron diffraction studies of Nd metal are also summarized. The data is shown to be consistent with the theoretical prediction that between 9 K and TN there exist two magnetic phases (TN = 19.9 K; T2 = 19.3 K). A discussion is given of some of the difficulties that arise when trying to design a model of the magnetic structure that fully explains all the experimental data. Attention is paid to the prediction of renormalization‐group theory that two dimensionally modulated magnetic structures exist in Nd. It is suggested that although there is no direct experimental evidence for the existence of such structures in Nd, none of the existing experimental data preclude this possibility.

Magnetic and structural investigations on nii2 and coi2

Abstract: Measurements of the magnetic susceptibility of NiI2 show an anisotropic behaviour and a magnetic ordering temperature of 75 K The nuclear and magnetic structures are examined with X-ray and neutron diffraction The nuclear symmetry changes at 60 K from trigonal to monoclinic The magnetic structure of NiI2 is an incommensurate helix of type 1, with propagation vector Q x =01384 a ∗ ; Q y =0; Q z =1457 c ∗ The anisotropy of the magnetic susceptibility of CoI 2 is due to single-ion anisotropy The magnetic structure is a commensurate helix of type 1, with Q x = 1 8 a ∗ ; Q y =0; Q z 1 2 c ∗ 129I-Mossbauer data of NiI2 and CoI2 are in good agreement with the paoposed magnetic structures The stability of magnetic structures in trigonal layer compounds is discussed It is found that the structures of CoI2 and NiI2 are close to the phase boundary between helix-1 type structures and structures with ferromagnetic layers

Neutron diffraction from small numbers of Langmuir-Blodgett monolayers of manganese stearate

Abstract: We report the observation of neutron diffraction from as few as three monolayers of manganese stearate on a plate of about 4-cm/sup 2/ area. These room-temperature (paramagnetic state) measurements could be adequately calculated on a laminar model of the crystal structure. Similar calculations, extended to a hypothetical magnetically ordered structure, indicate that the interlayer magnetic structure would be observable already with three layers of magnetic unit cells.

Spin waves in triple-q--> structures. Application to USb

Abstract: The spin-wave spectrum in a system with triple-q magnetic structure is calculated. The spin waves differ distinctly from those in the corresponding single-q structure, but agree with the excitations observed by Lander and Stirling in uranium antimonide (USb). Their experiments thus directly verify that the spins in USb are ordered in the triple-q structure.

Magnetic ordering in praseodymium at millikelvin temperatures

Abstract: Using thermal neutron scattering techniques, the development of magnetic ordering in single-crystal DHCP praseodymium has been studied over the temperature range 0.03-4.2K. The intensity of the broad elastic peak around the wavevector 0.11 tau 100 (which has been observed in previous studies of Pr) increased steadily as the temperature was reduced. In addition, new satellite reflections originating from a sinusoidally modulated magnetic structure with wavevector 0.13 tau 100 were observed at temperatures well below 1K. The magnetic transition is believed to be driven by an enhancement of the exchange interaction via the hyperfine interaction. No temperature dependence of the magnetic excitation energies between 4.2K and 0.03K was detected.

Magnetic Structure of Cubic FeGe

Abstract: It is shown that in the cubic phase of FeGe a conical spin structure with a cone semi-angle of about 55° with respect to [111] (the direction of the cone axis) is in agreement with data obtained from Mossbauer spectroscopy and neutron diffraction. The temperature dependence of the magnetic hyperfine field close to the ordering temperature of 275 K is described by a critical exponent β = 0.39.

Amorphous magnetic materials

Abstract: Fundamental research on the physics of amorphous magnetic materials is reviewed. The atomic and magnetic structure and the magnetic properties are discussed. Theoretical results on the nature of the magnetic ordering and the spectrum of elementary excitations are reported. Applications are discussed.

Magnetic structure of bornite, Cu5FeS4

Abstract: Neutron powder diffraction techniques have been used to investigate the magnetic structure of bornite, Cu5FeS4. Below the antiferromagnetic ordering temperature of 76 ± 2 K extra peaks appear in the diffraction pattern corresponding to magnetic ordering. The data are consistent with an antiferromagnetic alignment of iron atoms with moments of 4.4 ± 0.3 μB on the iron sites proposed by Koto and Morimoto on the basis of X-ray measurements of the crystal structure. Mossbauer and neutron diffraction data suggest that the second magnetic phase transition at 8 K arises from a spin rotation. Since the magnetic structure gives only superexchange paths between iron atoms through two or more anions, it is difficult to understand why the Neel temperature is so high without invoking small moments on copper atoms

Anisotropy and magnetic phase transitions in the rare earth tetraborides TbB4, HoB4 and ErB4

Abstract: In order to determine the electronic properties of the rare earth tetraborides TbB 4 , HoB 4 and ErB 4 (UB 4 structure type) large single crystals of 50–100 mg were prepared by the Czochralski and the aluminium flux methods. Magnetic and electrical measurements were performed at temperatures ranging from 2 to 300 K and under magnetic fields up to 80 kOe, applied along various crystallographic directions. All these tetraborides are metallic, and they show long-range antiferromagnetic ordering at low temperatures under a weak field. However, their detailed magnetic structure and field-temperature phase diagrams, confirmed by neutron diffraction experiments, are quite different.

Magnetic phase transitions by strong, pulsed magnetic fields at low temperatures in Cu x Zn1−x Cr2Se4

Abstract: The spinels, Cu x Zn1−x Cr2Se4) with x = 0.01, 0.02, 0.025, 0.05, 0.07 have been obtained. For these spinels, as well as for x = 0.0 and 0.1, magnetization measurements were carried out using an induction method in strong, pulsed magnetic fields, in a temperature range of 2.8 K to 30 K. Three magnetic phase transitions were observed. A hypothesis on the magnetic structure of the four phases separated by the three transitions is put forward.

Commensurate turn angle effects in single-crystal antiferromagnetic dysprosium

Abstract: The temperature dependences of the c axis thermal expansion and elastic constant c44 have been measured in single-crystal samples of dysprosium between the Neel temperature TN and Curie temperature TC. Anomalous behaviour is observed at several temperatures in the range TC

Magnetic structures of Y6(Fe1−xMnx)23 compounds

Abstract: Neutron diffraction studies of eleven compounds across the compositional range of the Y6(Fe1−xMnx)23 system reveal the presence of substantial preferential atomic ordering of Fe and Mn atoms on the four transition metal crystallographic sites. Throughout the entire compositional range, Mn atoms prefer to occupy the f2 site and Fe atoms the f1 site. Neutron diffraction profile refinements show no long range magnetic ordering occurring in the compositional range from x = 0.4 to 0.75 as a consequence of the sharply reduced exchange interactions in the ternary compounds. The average Fe moments on each of the sites are reduced in the Fe‐rich ternaries. The manganese atoms are nonmagnetic. In the Mn‐rich ternaries the Fe atoms have no spontaneous moments and the Mn moments are decreased dramatically from Y6Mn23.

Magnetic phase transitions in the Pd2−xCuxMnIn system

Abstract: The Heusler alloys have the L21 structure and occur at the compoistion X2MnZ. The majority, such as Cu2MnIn, are ferromagnetic with a moment of about 4 μB at the Mn sites but some, such as Pd2MnIn, are antiferromagnetic. The Pd2MnIn1−xSnx and Pd2MnIn1−ySby [1,2] series have magnetic structures that change from antiferromagnetic fcc type 2 to type 3A to ferromagnetism as the proportions of the Z site atoms In/Sn/Sb are varied. Recently two models have been proposed to explain the 3d exchange interactions in Heusler alloys. Stearns [3] suggests a model with three interactions and the X atoms playing a dominant role. Price [4] proposes a single interaction dependent only upon the average conduction electron concentration. Results are presented of magnetic and neutron diffraction measurements made on the new series Pd2−xCuxMnIn, in which the conduction electron concentration is varied via the X rather than the Z sites. The changes in magnetic structure observed are similar to those that occur in the series Pd...

Neutron diffraction study of the incommensurate magnetic phase of Ni0.92Zn0.08Br2

Abstract: The magnetic structure of Ni0.92Zn0.08Br2 has been studied by single-crystal neutron diffraction as a function of temperature down to 4.2K and in magnetic fields along (110) up to 3 T. From TN=48.0+or-0.5K to TIC=21.0+or-0.5K the magnetic structure is a collinear easy-plane metamagnetic one like NiCl2. Below TIC it transforms to an incommensurate structure with a propagation length within the basal plane which decreases continuously with decreasing temperature, reaching 71+or-5 AA at 4.2K. Exploration of the scattering in the a*b* plane around (003/2) and (101/2) shows satellite structure best fitted to a ring, i.e. to a completely disordered propagation direction. The magnetic phase diagram has also been determined.

Spin density and cobalt electronic structure in phthalocyaninatocobalt(II): a polarised neutron diffraction study

Abstract: The collection of polarised neutron diffraction data for β-phthalocyaninatocobalt(II), [Co(pc)](CoII, S=½), is described. Flipping ratios for 145 and 337 Bragg reflections were observed at 4.2 K and at magnetic field strengths of 1.49 and 4.6 T respectively. By use of the 4.2 K neutron diffraction nuclear structural data, these were converted into magnetic structure factors FM(hkl). A model of the spin distribution in [Co(pc)] was considered, employing the 3d and diffuse 4s-type orbitals of cobalt, and orbitals of the inner-ring nitrogen and carbon atoms of the ligand macrocycle. The spin populations of these orbitals were refined by a least-squares procedure which fitted calculated to observed magnetic structure factors. The spin populations 3dxz0.40(10), dxz,yz0.17(10), dz20.79(12), dx2–y2–0.21(10), ‘4s’–0.14(6) were found, together with a total populations of –0.17(5) spin on the macrocycle N and C atoms. The cobalt orbital electronic populations deduced are thus 3dxy1.60, dxz1.83, dz21.21, dx2–y20.21 together with either ‘4s’0.14 or ‘4s’1.86. The above orbital populations, except for the dxy case, are in reasonable concordance with knowledge of the electronic structure of the Co2+ ion, particularly when spin–orbit coupling is considered. However, the negative ligand spin densities which must arise through spin polarisation effects are in some conflict with the 1H n.m.r. contact shift deductions. The magnetic moment per cobalt atom, along the b axis, 〈µb〉, at 4.6 T, is determined to be 0.55 B.M. (1 B.M. = 9.274 × 10–24 A m2).

Neutron diffraction study of long-range antiferromagnetic order and crystal structure of uranium (III) tri-iodide

Abstract: Neutron diffraction experiments were performed on polycrystalline UI3 in the temperature range from 1.3 to 293K in order to clarify anomalies which were observed in previous measurements. The present neutron results prove the existence of three-dimensional long-range antiferromagnetic order in UI3 below TN=(2.8+or-0.1)K. The magnetic structure corresponds to k vector (1/2,1/2,1/2), and the magnetic moments of U3+ are oriented parallel to a. Similarly to MnO the U moments of adjacent ferromagnetic (111) layers are aligned in opposite directions. The ordered saturation moment mu U=(1.98+or-0.05) mu B of U3+ (5f3, 4I9.2) at 1.3K indicates a substantial crystal-field reduction. Considerable antiferromagnetic short-range order is observed at temperatures above TN. The parameters of the PuBr3 structure of UI3 were refined by means of the profile method both at 293 and 4.2K.