Showing papers on "Magnetic structure published in 1987"

Magnetic structure of Dy-Y superlattices

Abstract: Two samples of Dy-Y superlattices produced by molecular-beam-epitaxy techniques are shown by neutron diffraction to order magnetically in a helix which is incommensurate with the bilayer thickness. One sample consists of 64 bilayers, each bilayer made up of about 15 growth planes (42 A) of Dy atoms followed by 14 planes (38 A) of Y atoms. The second sample has 90 layers, each layer consisting of 9 Dy atomic planes and 8 Dy/sub 0.5/Y/sub 0.5/ alloy planes. The phase coherence of this ordering extends over several bilayers, and is especially striking in the sample where the layers of localized Dy spins are separated by 14 atomic planes of nonmagnetic Y. The fact that the helix chirality propagates across several bilayers rules out a simple scalar Ruderman-Kittel-Kasuya-Yosida coupling between the Dy planes on either side of an Y layer, but suggests instead that a helical spin density wave is induced in the Y conduction electrons. A simple model for the superlattice structure factor demonstrates that observed asymmetries in the magnetic diffraction-peak intensities can be ascribed to the existence of different magnetic modulation wave vectors in each layer type (Dy and Y). In these superlattices the strain clamping by the intervening non-Dymore » layers and the substrate suppresses the first order ferromagnetic transition found in bulk Dy in both zero and finite fields. Although the planar magnetostriction is clamped, it is observed that the application of a magnetic field in the basal plane produces at low temperatures a second order irreversible transition to a metastable ferromagnetic state. At high temperature the magnetization process is initiated by a reduction of the helical coherence length due to a random-field coupling to the uncompensated Dy layer moment.« less

Antiferromagnetism and oxygen deficiency in single-crystal La2CuO4- delta.

Abstract: Two single crystals of undoped La/sub 2/CuO/sub 4/ are studied by neutron diffraction between T = 5 and 300 K. In one crystal oxygen vacancies are created by heat treatment and the magnetic properties are found to depend strongly on this treatment. While the untreated sample is found to order antiferromagnetically at T/sub N/approx. =50 K, the heat-treated crystal orders at T/sub N/approx. =150 K with a somewhat larger moment. The magnetic structure is the same for the two crystals and supports previously reported powder results.

Experimental Study of New Type Phase Transition in Triangular Lattice Antiferromagnet VCl2

Abstract: Critical properties of a Heisenberg antiferromagnet on a triangular lattice are investigated experimentally using a quasi-two-dimensional antiferromagnet VCl 2 by means of neutron scattering. It is found that VCl 2 shows characteristic critical behaviors of two and three dimensional models. A little above T N , the line shape of χ( Q ) is found to be represented by the Ornstein-Zernike form (κ 2 + q 2 ) -1 . This suggests the existence of magnetic point defects which were predicted in the two dimensional model. The measurecl critical exponents are β=0.20±0.02, γ=1.05±0.03, and ν=0.62±0.05, in agreement with the theoretical work for three dimensional SO(3) systems. Successive phase transitions due to a small Ising anisotropy were found at T N1 =35.88±0.01 K and T N2 =35.80±0.01 K. The spin structure and the dispersion relation of the spin wave well below T N are also determined.

Study on amorphous ferrite CaO-Bi2O3-Fe2O3 system

Abstract: Investigation on amorphous ferrite CaO-Bi2O3-Fe2O3 system has been carried out. This is one of new types of amorphous ferrites using antiferromagnetic oxides as starting materials instead of ferrimagnetic oxides. In some compositions ferromagnetic behaviors were observed. Especially, (CaO)20− (Bi2O3)15(Fe2O3)65 glass has magnetization value of 13.2emu/g at room temperature in applied magnetic field of 10 kOe. This value can not be obtained by any pure crystalline calcium ferrite. Mossbauer absorption spectra were measured to clarify the magnetic structure of the glass.

Kohlrausch thermal relaxation in a random magnet.

Abstract: Specific-heat measurements on amorphous Fe Zr&00 (x 90 and 92) by a thermal relaxation method reveal a temperature decay of the form T(t) To+dTexp[ —(t/r)s] below 20 K, where the alloys are asperomagnetically ordered. P=0.5 when To 0. The stretched exponential is a manifestation of nonergodic behavior, due to the inability of the spin system to explore the available random magnetic configurations on the 100-ms time scale of the experiment.

Magnetic, crystallographic and hydrogen absorption properties of YMn2 and ZrMn2 hydrides☆

Abstract: The effect of hydrogen capacity on crystallographic and magnetic properties has been studied on the Laves phase intermetallic of the C15 cubic YMn2 and the C14 hexagonal ZrMn2. It has been found that there exist intermediate metastable β′ phases between the α phase and the fully charged β phase in both compounds. The β′ phase has the same crystal structure as in the hydrogen-free compound and the unit cell volume is smaller than that in the β phase. Magnetization measurements indicate that the β′ phase stabilizes ferromagnetism and, in the β phase, antiferromagnetism or paramagnetism becomes stable. The results obtained are discussed on the assumption that there is a preferential site among the three kinds of tetrahedral hydrogen sites. ZrMn2 exhibited excellent hydrogen absorption properties for use in thermal energy conversion systems operating under high temperatures.

Observation of the Antiferromagnetic Ordering in La2CuO4

Abstract: Neutron diffraction study on La2CuO4, which is the basic material of high-Tc oxide superconductors, revealed the antiferromagnetic ordering in this compound. The value of the magnetic moment is 1.1±0.3 µB per copper atom, and the Neel temperature was found to be 240±10 K from temperature dependence of the intensity of the magnetic reflection.

Occurrence of long‐range helical spin ordering in Dy‐Y multilayers (invited)

Abstract: The magnetic ordering of highly perfect single‐crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 A) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long‐range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 A−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long‐range coupling is a conduction band spin‐density wave in bot...

Magnetic structure of γ-Fe precipitates in a Cu matrix

Abstract: The recent discovery of a periodic distortion of the lattice structure of gamma -Fe precipitates in Cu motivated the authors to re-examine the magnetic structure of these precipitates using a neutron diffraction technique. They find that the spin structure is that of a longitudinal type-1 antiferromagnet as is well known but the spins are within a c plane and modulated in space with a wavevector Q which is the same as that of the displacement wave of the lattice. Several features of the spin structure are discussed.

The magnetic structure of γ-iron-manganese

Abstract: Mossbauer transmission spectra taken, at a temperature of 4.2 K, through a thin single-crystal sample gamma -Fe-Mn, in magnetic fields up to 9 T are presented. These spectra have been fitted to the three energetically favoured magnetic spin models of gamma -Mn. The authors' results are not consistent with the (001) model of a collinear antiferromagnet, whereas fits to the (111) and (110) models are good but indistinguishable.

Neutron diffraction study of artificial CoO‐NiO superlattices

Abstract: Neutron diffraction measurements were made on artificial superlattices of CoO‐NiO, (CoO)m(NiO)n×l prepared on the (0001) surface of α‐Al2O3. (111) layers of CoO and NiO, each less than 2 nm thick (2≤m,n≤8), were stacked alternatively to a total thickness of less than 70 nm (m+n≤15,l≤25). A well‐defined magnetic Bragg peak indexed as (1/2 1/2 1/2) was observed for all the samples, and a magnetic structure of type FCC2 with the spin axis lying in the (111) planes grown was suggested. The temperature dependence of the magnetic peak intensity revealed a sharp, single magnetic transition. The Neel temperature was found to vary as a linear function of n/(m+n).

Magnetic properties of [100] oriented Cu‐Ni superlattices

Abstract: We have fabricated Cu‐Ni superlattices with exclusive [100] modulating orientation by using a dc magnetron sputtering system. The layer thicknesses of both Cu and Ni range from 5 to 125 A. The high quality of the samples is confirmed by the intense satellite peaks in the x‐ray diffraction. The spontaneous magnetization at 5 K, which is found to be inversely proportional to the Ni layer thickness, vanishes at two Ni atomic planes. A very large difference in magnetic surface anisotropy has been found between [100] and [111] textured Cu‐Ni superlattices, displaying the important role of modulating orientation on the magnetic properties of superlattice.

Spin waves in the triangular antiferromagnet CsMnBr3

Abstract: We report neutron scattering measurements of the spin‐wave dispersion relations in the hexagonal triangular antiferromagnet CsMnBr3. The data in the easy plane are described satisfactorily in terms of a model with Heisenberg exchange parameter J2=−0.0019 ±0.0002 meV between a magnetic atom and its six nearest neighbors and a single‐ion anisotropy parameter D=0.014±0.002 meV. In a magnetic field of 3.7 T along a 〈100〉 direction the variation of the staggered magnetization, measured at (1/3 1/3 1), as a function of temperature shows two critical phase transitions separated by about 1.7 K. The triangular antiferromagnetic phase that occurs below 8.6 K in zero field is split by the field into two phases with transitions at 8.9 and 7.2 K. The intermediate phase below 8.9 K is of an unknown nature.

Electric and magnetic structure of an edge plasma in a tokamak with a helical magnetic limiter

Abstract: A strong modification of plasma potential was observed at the plasma surface with the application of a helical field produced by a set of local helical coils. The structure of magnetic field lines was computed by a field line tracing code for a tokamak plasma. Such a field structure was examined in the experiment using electron beam probing. The potential formation and the magnetic structure are discussed referring to the scheme of an ergodic magnetic limiter.

Two‐dimensional neutron diffraction of YFe2O4 and CoCr2O4

Abstract: A new wide‐angle neutron diffraction instrument has been used to study CoCr2O4 and YFe2O4. The instrument is capable of measuring the scattered intensity over a wide region of reciprocal space in a short time, and this feature was used to measure the magnetic diffraction pattern of these materials at temperatures below the magnetic ordering temperature.

Electronic structure of FCC Fe-Mn alloys. II. Spin-density measurements

Abstract: For ptI see ibid, vol17, p1419 (1987) Spin-density measurements for a single crystal of the FCC alloy Fe66Mn34 have been performed using elastic and inelastic neutron scattering Combining the results from elastic scattering with the experimentally determined symmetry of the spin wave spectrum it has been unequivocally deduced that this alloy has a collinear magnetic structure The magnetic form factor of the alloy, the average magnetic moment per atom and the spin-density asphericity have been obtained from a Fourier inversion of the neutron diffraction data Combining these results with a measurement of the magnetic scattering amplitude of a powdered sample of the alloy Fe82Mn13C5 allows the magnetic moments of Fe and Mn in the iron-rich region of the Fe-Mn system to be discussed Comparison of the experimental charge- and spin-density asphericities of Fe66Mn34 with available calculations of the band structure of FCC Fe shows relatively poor agreement The possibility of modifying the band structure of FCC Fe by introducing band splittings corresponding to a state-dependent potential is discussed and shown to lead to better agreement with experimental data