Crystal and magnetic structure of CoMnGe,CoFeGe, FeMnGe and NiFeGe

The crystal and magnetic properties of CoMnGe, CoFeGe, FeMnGe and NiFeGe compounds are investigated with X-ray, neutron diffraction, magnetometric and Mössbauer effect methods. All compounds have hexagonal Ni₂In-type crystal structure and ferromagnetic properties at low temperatures (except for NiFeGe). Neutron diffraction experiments indicate that the compounds CoMnGe and CoFeGe have a collinear magnetic structure while FeMnGe a noncollinear. The magnetic moments are localized only on Mn and Fe atoms and lie in the basal plane.     

Magnetic properties of RCr2Si2 compounds (R=Tb,Er)

We report on magnetic properties of RCr₂Si₂ compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr₂Si₂-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).     

Diffraction studies of the crystalline and magnetic structures of γ-Fe2O3 iron oxide nanostructured in porous glass

The crystalline and magnetic structures of γ-Fe₂O₃ maghemite synthesized in porous glass in the form of nanoparticles with a mean diameter of 106(2) Å have been determined by the neutrons and synchrotron radiation diffraction methods. Nanostructured maghemite with the spinel structure has vacancies in the octahedral and tetrahedral positions. The magnetic structure corresponds to the usual ferrimagnetic type. The measured magnetic moments are much lower than the moments in a usual sample. Moreover, the moments are strongly different in the octahedral and tetrahedral positions; this difference is explained by the difference in the exchange interaction for moments in different positins.     

Neutron powder diffraction and magnetic studies of mesoporous Co3O4

Samples of mesoporous Co₃O₄, created by using mesoporous silicas KIT-6 and SBA-16 as hard templates to control the growth of Co₃O₄ have been investigated with SQUID magnetometry and neutron powder diffraction, to reveal the effects of high surface area on the magnetic and electronic properties. DC magnetic susceptibility measurements show lower Néel ordering temperatures and lower magnetic moments than in a “bulk” reference. A lower second transition temperature is also observed in the mesoporous samples, associated with the freezing of the surface (shell) magnetic moments. Measurements taken with increasing applied field at constant temperature show the materials to be antiferromagnetic as expected. Complementary parametric neutron powder diffraction studies show similar trends between the two mesoporous samples when looking at their Néel temperatures, and verify long range order within the samples.     

Magnetic properties of the nanocrystalline DyMnO3

We report on the X-ray and neutron diffraction and magnetic measurements of the nanosamples of DyMnO₃ annealed at temperatures of 800 °C, 850 °C and 900 °C. The diffraction data indicate that all the samples crystallize in an orthorhombic crystal structure (space group Pnma). The crystal structure parameters change slightly with preparation. All the samples are antiferromagnets at low temperatures. The Mn magnetic moments order near 40 K, while those of Dy below 8.4 K. The macroscopic magnetic and neutron diffraction data indicate a small difference of properties between the DyMnO₃ samples synthesized at different temperatures. The observed broadening of magnetic peaks connected with the Dy sublattice suggests a cluster-like character of magnetic ordering.     

A neutron diffraction and magnetization study of Heusler alloys containing Co and Zr,Hf, V or Nb

Neutron diffraction, X-ray diffraction and saturation magnetization measurements have been made on a series of ferromagnetic alloys at the composition Co₂YZ, where Y is a group IVA or VA element and Z is a group IIIB or IVB element. The alloys are mainly ordered in the Heusler L2₁ type chemical structure, but some show the presence of a small amount of additional phase, or some preferential disorder.The magnetic results form two distinct groups in which the moments are confined to the Co sites but their magnitude depends on the electron concentration determined by the Y and Z atoms. Although the magnetic moments and Curie temperatures of the alloys in the same group are similar, their lattice parameters differ according to whether Z is a group IIIB or IVB element. Such behaviour indicates that, as in other alloy series with the Heusler structure, a change in lattice parameter has little effect on the magnetic properties in comparison with the effect of a change in electron concentration.     

Non-colinear magnetic structure of U3P4 and U3As4

Neutron diffraction experiments have been performed on single crystal samples of U₃P₄ and U₃As₄. The magnetic ordering is found to be a non-collinear three axial structure in which magnetic moments of U⁴⁺ ions are tilted from the [111] axis by an angle of about twenty degrees within (110) planes.     

Crystal field effect on the magnetic moment's direction of rare earth ions with low point symmetry in r4co3 (r = Tb, Ho, Dy, Er, Tm) compounds

Neutron diffraction measurements, made on powder samples, show that Ho₄Co₃ and Er₄Co₃ intermetallic compounds are ferrimagnetic at 4.2 K. The magnetic moments of the 2 holmium sites are 8.7 and 2.1 μ_B and those of the erbium sites are equal to 8.7 and 8.1μ_B. The cobal⁺ magnetic moment is 0.2μ_B for both compounds. The easy magnetization direction lies on the hexagonal plane for Ho₄Co₃ while for Er₄Co₃ there are 2 anisotropy directions. Exchange interactions between rare-earth ions of both sites are very weak compared with the total crystal field splitting of the ground state multiplet J. The crystal field parameters are calculated and the magnitude and direction of the rare-earth magnetic moments in each site is determined.     

Neutron-diffraction study of lithium ferrite aluminates II. Magnetic moments

The magnetic moments of the tetrahedral (A) and octahedral (B) sublattices are calculated for 0 °K from data on the cation distribution in the Li_0.5Fe_2.5-xAl_xO₄ system. The room-temperature magnetic moments are found by minimizing the discrepancy factor for the magnetic diffraction maxima.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 8, pp. 57–62, August, 1969.In conclusion author thanks S. M. Zilyakov for furnishing the samples, V. A. Somenkov and S. Sh. Shil'shtein for useful discussions, and V. M. Kuchin for assistance in the experiments.     

Study on magnetic properties of nanocrystalline La-, Nd-, or Gd-substituted Ni–Mn ferrite at low temperatures

The effects of rare-earth ions with different radii and magnetic moments on the magnetic properties of Ni–Mn ferrite are investigated. X-ray diffraction pattern has shown the presence of cubic structure of spinel ferrite for all samples. The values of M_s and H_c are decreasing with increasing of testing temperatures for all samples. The H_c value of Ni_0.7Mn_0.3La_0.1Fe_1.9O₄ reaches 1082 Oe at 2 K. Mössbauer spectra tested at 273 K indicate the presence of superparamagnetism for samples calcined at 873 K.     

Ferro-and antiferromagnetic ordering in LaMnO3+δ

A neutron diffraction study of the crystalline structure and magnetic state of LaMnO_3+δ samples with different deviations from oxygen stoichiometry has been made at 4.2 K. It is shown that annealing at reduced oxygen pressure is accompanied by transformation of the magnetic structure from ferromagnetic, with magnetic moments parallel to the b axis, to antiferromagnetic, with the wave vector k=0 and the moments along the a axis (space group Pnma). A comparison of experimental with expected Mn ion moments suggests that magnetic order does not extend throughout the sample volume. Part of the Mn ions form magnetic clusters ～20 Å in size.     

Crystal and magnetic structure of Ho2NiGe6

The crystal and magnetic structure of Ho₂NiGe₆ was studied by powder neutron diffraction. The paramagnetic neutron diffraction data confirmed the Ce₂CuGe₆-type crystal structure reported earlier for this compound. Below the Néel temperature equal to 11 K the Ho magnetic moments form a uniaxial antiferromagnetic ordering. The Ho magnetic moments equal to 8.16(7)μ_B at 1.5 K are parallel to the b-axis. The data are compared with those published for HoNi_0.46(6)Ge₂.     

Magnetic state of intercalation compounds in the Cr<Subscript>x</Subscript>TiTe<Subscript>2</Subscript> system

The temperature and field dependences of the magnetic characteristics of chromium-intercalated titanium ditelluride compounds are investigated over a wide range of chromium concentrations. The Cr_0.5TiTe₂ compound is studied by neutron diffraction. It is revealed that the system under investigation can occur in different magnetic states depending on the chromium concentration. An analysis of the experimental results demonstrates that the interaction between magnetic moments of chromium ions is predominantly ferromagnetic in character. An increase in the chromium concentration leads to ferromagnetic behavior with a pronounced magnetic hysteresis. The magnetic moments of chromium ions in these compounds are estimated.     

Neutron crystal-field spectroscopy of RNi 2 11 B2C (R = Ho,Er, Tm)

Inelastic neutron scattering experiments of the crystal-field (CEF) splitting of the R³⁺ ions (R = Ho, Er, Tm) in the superconductor RNi ₂ ¹¹ B₂C have been performed in the paramagnetic as well as in the magnetically ordered state in order to deduce the CEF parameters which in turn determine the thermodynamic magnetic properties of these systems. The calculated ordered magnetic moments are in agreement with magnetic neutron diffraction results, i.e. both the easy axis and the size of the zero-field moments are correctly reproduced. In addition, the entropy involved in the magnetic ordering obtained from specific heat measurements is in correspondence with the number of low-lying CEF states observed in this study. Our CEF parameter set also reproduces both the behavior and the anisotropy in M/H measured for single crystals.     

POWDER NEUTRON DIFFRACTION STUDIES OF THE CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF Ho2Fe9Ga8 and Ho2Fe9Ga6AI2

The crystallographic and magnetic structures of Ho₂Fe₉Ga₈ and Ho₂Fe₉Ga₆AI₂ were studied by powder neutron diffraction at room temperature. The atom fractional occupancies of Ga and Al and the magnetic moments of Ho and Fe were obtained by using Rietveld analysis program. The magnetic structures of the two samples show an easy-axis anisotropy, with the Fe magnetic moment being ferrimagneticlly coupled to those of Ho.     

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001     

Spin reorientation in Tm2Fe14B

The magnetic structures of Tm₂Fe₁₄B above and below its spin reorientation temperature (approx. 316 K) have been determined from high resolution neutron powder diffraction measurements.Within experimental accuracy all magnetic moments lie in the basal plane at 294 K and along the c-axis at 340 K. The Tm and Fe moments are antiparallel.     

Magnetic structure of V5S8

The magnetic structure of V₅S₈ has been investigated by neutron diffraction at low temperature. Two single crystals (sample 1 and sample 2) grown by the same procedure gave considerably different results. Sample 1 was found to have an incommensurate structure, while sample 2 was found to have simple collinear antiferromagnetic structure consistent with magnetic measurements. Although the positions of the magnetic diffraction peaks of sample 1 were close to those of sample 2, the neutron intensity behavior was quite different and the detailed structure could not be determined in the case of sample 1. In both samples, the magnitude of the moments was evaluated to be much larger than the value estimated from NMR.     

57Fe Mössbauer spectroscopy and X-ray diffraction study of complex metamict minerals. Part II

The magnetic behaviour of the nanocrystalline Ni_0.5Cu_0.5Fe₂O₄, synthesized by the co-precipitation method has been studied. The X-ray diffraction patterns confirm the formation of cubic spinel structure. The dc magnetization measurements show that the samples are superparamagnetic above the blocking temperatures and the blocking temperature increases with particle size. The reduction in saturation magnetization in the case of nanoparticles as compared to their bulk counterpart has been explained on the basis that the magnetic moments in the surface layers of a nanoparticle are in a state of frozen disorder. The hump in ZFC curve progressively shift to a lower temperature with increasing field and broader curves at higher field, suggests the spin-glass nature of the system.