Modification of the magnetic structure of UAs — Appearance of an incommensurate phase below TN in UAs0.97Se0.03, studied by neutron diffraction from a single crystal

The magnetic structure of UAs_0.97Se_0.03 has been studied by neutron diffraction from a single crystal in zero applied magnetic field. It is found to be antiferromagnetic, of type-IA (++--) below T_o = 75.6 K and of type-I (+-+-) above T_o. The type-I persists till T_IC = 113.5 K, while above it and up to T_N = 122 K an incommensurate phase appears, thereby modifying the magnetic structure in pure UAs. The k-value of the wavevector $\text{K}$ (along cubic axes) is changing from 0.642 at T_N to 0.652 at T_IC. The transitions at T_o and T_IC are first-order while the transition at T_N is second-order. The ordered magnetic moment is 2.15 μ_B at T = 4.2 K, it varies smoothly to 1.95 μ_B at T = 75.4 K and drops drastically to 1.47 μ_B at T = 76 K.     

Thermal and magnetic studies of the nearly one-dimensional antiferromagnetic system CsNiBr3

Magnetic susceptibility, specific heat and ¹³³Cs magnetic resonance measurements in a single crystal of CsNiBr₃ are reported. The data reveal two magnetic transitions separating the paramagnetic phase from the antiferromagnetic ground state. At the higher transition temperature T_N2 = (14.25 ± 0.05)K a net magnetic moment is observed only along the hexagonal c-axis, while only below the lower transition temperature T_N1 = (11.75 ± 0.05)K a perpendicular component of the magnetic moment appears also. Above T_N2 CsNiBr₃ can be described as a one-dimensional antiferromagnet with intrachain exchange interaction $\text{J}\text{k}{}_{\mathrm{B}}\text{= ?(17.0 ± 0.2)}\text{K}$ and single-ion anisotropy constant $\text{D}\text{k}{}_{\mathrm{B}}\text{? ?1.5}\text{K}$. Below T_N1, the data are consistent with the non-colinear triangular structure of the Ni²⁺ moments proposed previously for the isomorphic crystal CsNiCl₃. A reduced value of the zero-temperature susceptibility over the classical value is found and atrributed to the zero point deviations.     

Neutron diffraction study of the crystal and magnetic structure of the ionic ferromagnet Cs2CrCl4

Neutron powder diffraction has been used to investigate the crystal and magnetic structure of the ionic ferromagnet Cs₂CrCl₄, T_c ～ 58K. Data taken at ambient temperature, 78 and 4.2K indicate the structure remains tetragonal $\text{I4}\text{mmm}$ at low temperatures. A ferromagnetic alignment of the spins is confirmed. The ordered moment at 4.2K is found to be 4.1 ± 0.2μ_B, and the best agreement between the calculated profile and the data suggests its direction lies at an angle of 56 ± 6° to the unique axis.     

Magnetism and structural chemistry of ternary silicides: (RE,Th, U)Pt2Si2 (RE = rare earth)

Ternary silicides (RE, U, Th)Pt₂Si₂ have been prepared from the elements. All the compounds (RE= Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and U, Th) were found to be isotypic and crystallize with the primitive tetragonal CePt₂Si₂-type structure closely related to the CaBe₂Ge₂-type. The magnetic properties of these alloys were studied in the temperature range 1.5 K < T < 1100 K and in fields up to 1.3 T revealing a typical Van Vleck paramagnetism of free RE³⁺-ions for temperatures T > 200 K. A nonmagnetic ground state is reflected from the magnetic susceptibility data of CePt₂Si₂, which are interpreted in terms of interconfiguration fluctuations (ICF). The magnetic results of SmPt₂Si₂ (μ_eff = 0.7 BM) compare well with the ideal Van Vleck behavior of Sm₃₊ ions with a $\text{J =}\text{5}\text{2}$ ground state and a low-lying excited first level $\text{J =}\text{7}\text{2}$. At temperatures below 40 K antiferromagnetic ordering is found for (Gd, Tb, U)Pt₂Si₂; whereas in case of (Dy, Ho, Er, Tm)Pt₂Si₂ the onset of ferromagnetism is indicated below 4 K. None of the samples exhibited a superconducting transition above 1.8 K.     

Magnetic structure of BaCaFe4O8-analysis of neutron diffraction measurements

The compound BaCaFe₄O₈ crystallizes in the trigonal space group P$\text{31}$m with one formula unit per unit cell with lattice constants $\text{a = 5.4059}\text{A}$ and $\text{c = 7.7023}\text{A}$ Neutron diffraction measurements carried out on a powder sample over the temperature range 300–900 K showed that the compound undergoes a magnetic transition to an antiferromagnetic state at a Néel temperature T_N = 680 ± 5 K. Analysis of the room temperature neutron diffraction pattern gave a magnetic unit cell that has the same periodicty as the crystallographic one. An antiferromagnetic model is proposed with the iron spin magnetic moments parallel to the c-axis of the unit cell. The magnetic moment of the Fe³⁺ ion was found to be (4.5 ± 0.1)μ_B     

Specific heat of (C6H11NH3) CuCl3 (CHAC), a system of ferromagnetic chains

The heat capacity of (C₆H₁₁NH₃) CuCl₃ (CHAC) has been measured for 0.45 < T < 60 K. Three-dimensional ordering is observed at T = 2.214 K. The data in the paramagnetic region can be described by a ferromagnetic $\text{S =}\text{1}\text{2}$ Heisenberg linear chain model system with J/k = +45 ± 5K.     

Crystal field parameters and phase transitions in ErSb

The crystal field levels of the Er ($\text{J =}\text{15}\text{2}$) ion in a single crystal of ErSb have been measured by inelastic neutron scattering. The crystal field parameters obtained by a least squares fit to the spectra at several temperatures are: B₄ = (0·473 ± 0·005) × 10^?2°K and B₆ = (0·59 ± 0·06) × 10^?5°K, which differ considerably from the values o by interpolation from measurements on other compounds. In addition the temperature dependence of the magnetic scattering in the vicinity of the Néel temperature (T_N = 3·55°K) clearly demonstrates that the transition is second order in contrast to the first order behavior suggested by specific heat measurements. Also, any lattice distortion accompanying the magnetic ordering is less than 0.1 per cent, the resolution of the present experiment.     

The antiferromagnetic structure of ErCo2Si2 by neutron diffraction

The magnetic structure of the tetragonal ErCo₂Si₂ compound is determined by neutron diffraction on powder sample at 4.2 K. The magnetic ordering is connected with a symmetry lowering, magnetic space group P_2s$\text{1}$ (Sh⁷₂)k = 000. The structure is collinear antiferromagnetic with the erbium magnetic moments making an angle of 56.2° with the c axis. The magnetic moment value for erbium is 6.75μ_B.     

Magnetic ordering in (La,Gd) Al2 alloys

The detailed dependence of the magnetic ordering temperature θ on Gd concentration n below ～ 15 at.% Gd substitution for La is reported for the system ($\text{La}$Gd) Al₂. In this concentration range, the θ vs.n curve lies markedly below the extrapolation of the linear behavior previously observed for concentrations ? 20 at.% Gd, and apparantely goes to zero at a finite concentration. The effective moment of Gd is enhanced by 1 μ_B over the free ion value.     

Magnetic properties of RFe6Al6 alloys with R=Y,Dy

Intermetallics crystallizing in ThMn₁₂ type structure were investigated. Magnetostatic measurements showed that the magnetic ordering temperature and the magnetic moment of YFe₆Al₆ samples depend strongly on thermal and mechanical treatment. These measurements for a powdered sample of YFe₆Al₆ showed that the alloy was a ferromagnet with a Curie temperatureT _C =265 K and a magnetic moment μ=5.1 μ_B/f.u. at 77.4 K. From X-ray, magnetostatic and Mössbauer effect measurements it appears that the Fe atoms prefer the 8j and 8f crystallographic positions. Magnetostatic measurements for a powdered sample of DyFe₆Al₆ showed that this alloy was a ferrimagnet with the ordering temperatureT ₀=311 K and magnetic moment μ=1.1 μ_B/f.u. at 77.4 K.     

Magnetic studies in amorphous GdxY1−x alloys

We report here our magnetic study on amorphous Gd_xY_1?x alloys with x = 0.17 and 0.70. The alloy with x = 0.17 is paramagnetic down to 4.2 K. For x = 0.70, magnetization shows a peak in low magnetic fields (H < 0.3 T). Magnetic saturation is difficult to obtain even for fields up to 15 T. An extrapolated value gives a moment of $\text{6.2 μ}{}_{\mathrm{B}}\text{Gd at}$. T_c is about 70 K. This led us to the conclusion that ferromagnetic and antiferromagnetic clusters are present in this alloy. The results are discussed by comparing them with crystalline Gd-Y alloys and amorphous Gd-Al, Gd-Au, etc. The effect of Y seems to be specific.     

Accumulating evidence for the two-step magnetic ordering in the borocarbides DyNi2B2C and DyCo2B2C

Accumulating evidence for a two-step magnetic ordering in the borocarbide DyNi₂B₂C is summarized, including earlier overlooked evidence for the initial magnetic transition and a recent magnetization study of polycrystalline samples. The two-step ordering involves initial two-dimensional ferromagnetic ordering in the DyC (basal) planes at T_N (=16.3 K), gradual build-up of the three-dimensional (3D) alternate stacking of ferromagnetic planes, and a final 3D ordering in the AF–I-related structure at a lower temperature T_o (=10.4 K), depicting a first-order transition. Supporting evidence for the two-step magnetic ordering in DyNi₂B₂C comes from point-contact spectroscopy measurements in the normal state for DyNi₂B₂C–Ag contact, and from similar behaviour of PrNi₂B₂C and (Pr_0.91Dy_0.09)Ni₂B₂C. In the isostructural borocarbide DyCo₂B₂C the two magnetic transitions (at 7.8 and 2.6 K) deduced from the specific-heat measurements are also attributed to a two-step magnetic ordering.     

Theorie semimicroscopique des solutions solides Cr1−xVxN

The solid solutions Cr_1?xV_xN show at low temperatures a progressive establishment of an orthorhombic distorsion and an antiferromagnetic order (of the fourth kind) for $\text{x <}\text{1}\text{3}$ accompagnied by a (cubic) fraction which remains disordered at the lowest temperatures : No ordering occurs for $\text{x >}\text{1}\text{3}$. Assuming a Gaussian distribution of local concentrations in otherwise homogeneous cells (cellular model), the mean magnetic moment, measured by neutron diffraction can be related to a constant moment of Cr provided the local vanadium concentration is smaller than 0.26. The cell size contains 20 metal atoms.     

Structure and orientational ordering of nitrogen molecules physisorbed on graphite

The structure and orientational ordering of nitrogen molecules physisorbed on graphite have been studied by low-energy diffraction (LEED). A two-sublattice in-plane herringbone structure with glide lines along two perpendicular directions is inferred from LEED patterns at T < 30 K from the monolayer where the molecular centers have the commensurate $\text{(}\text{3}\text{×}\text{3}\text{)}$ 30° structure. The orientational order-disorder transition of this commensurate phase was examined by superlattice spot intensity and angular profile measurements for 20 < T < 38 K. A rapid drop in superlattice intensity is observed near 27 K. The persistence of some intensity to 38 K. is suggestive of residual short-range orientational ordering and perhaps finite size or heterogeneity effects. For increasing coverage at T = 15 K, there is first a transition to a previously unobserved uniaxial incommensurate phase and then a transition to an apparently triangular incommensurate phase. The orientational superlattice spots are clearly present in the uniaxial phase, but are much weaker in the triangular incommensurate phase. At 31 < T < 35 K, an apparently triangular incommensurate phase with no detectable orientational superlattice spots is observed. The lattice constant versus equilibrium vapor pressure curve has been determined in the latter case assuming a continuous transition. The lattice constants of the incommensurate phases are used to place limits on the extent of possible phase-coexistence regions between the commensurate, uniaxial incommensurate, and triangular incommensurate phases. The LEED patterns from the bilayer at T = 15 K indicate a double-period superlattice structure of the triangular incommensurate phase which does not have the glide line symmetries of the commensurate monolayer. Some effects of heterogeneity on these phase transitions are discussed. A phase diagram for 10 < T < 40 K is proposed.     

Neutron scattering study of the 1-D short range and 3-D long range magnetic order in TMNB

Quasielastic neutron scattering experiments on TMNB show for T > 2.8 K a diffraction pattern characteristic for a 1-D ferromagnet with an exchange energy along the chain J/k_B ? 10 K. For T < T_N =2.7 K the 3-D magnetic ordering was found to be of a simple antiferromagnetic type. A lattice distortion was observed as well below 200 K.     

Electrical transport properties of a quasi-one-dimensional Nb3Te4 single crystal

The electrical resistance of a linear chain metal Nb₃Te₄ were measured from 1.3 to 320 K. The residual resistance ratio $\text{R(300}\text{K}\text{)}\text{R(4.2}\text{K}\text{)}$ is about 3. Nb₃Te₄ shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature T_c, the critical magnetic field H_c2 is proportional to δT=T_c?T. The angular dependence of H_c2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.     

Polarized neutron study of covalency effects in yttrium iron garnet

Magnetic structure amplitudes of 170 reflections up to sin $\text{θ}\text{λ}\text{= 1.0}\text{A}\text{?}{}^{\mathrm{?1}}$ were measured by polarized neutron diffraction at T = 295K on yttrium iron garnet. The data set was completed by model calculations and the magnetization density was determined. The magnetic moments obtained by integration and refinement are considerably reduced on both iron sites due to charge density transferred from intervening ligand ions. A residual moment of uncancelled spin of (0.032 ±0.004) μ_B is observed on the oxygen ion. Evidence for magnetization density on the oxygen atom and between oxygen and tetrahedral iron was found. A qualitative discussion in terms of a molecular orbital model is given. A further data set collected at 4.2K showed equally a magnetic moment lower than expected for the free ion.     

Theory of localized magnetic moments in metals I finite cluster approach

The origin of localized magnetic moments formation in metals is investigated theoretically using a self-consistent local spin density molecular cluster approach. Clusters with up to 55 atoms are employed to describe isolated impurity local moment behavior in the cases of Fe$\text{Ag}$ and Fe$\text{Pd}$. Densities of states and spin magnetic moments were determined and compared with results of spectroscopic (notably photoemission) and magnetization measurements, respectively. In the case of a noble metal host, the spin magnetization density is found to be highly localized around the Fe site; the iron moment is ≈ 3.9μ_B and the polarization of the host Ag atoms is small. In the case of a transition metal host, the iron moment is ≈ 3.2 μ_B but here the strong hybridization of the Fe-3d and Pd-4d states results in a large induced magnetic moment in the host PD metal — in essential agreement with experiment for this giant moment system.     

Neutron diffraction study of UAs in high magnetic fields

Magnetization experiments have shown that at low temperature and in an applied magnetic field of ～90 kOe antiferromagnetic (AF) uranium arsenide transforms to a new ferrimagnetic state. The high-field transition produces a state with magnetic components $\text{parallel}$ to the applied field. A small ferromagnetic component (0.4 μ_B) is seen in magnetization experiments but with neutron diffraction we have observed directly the large AF component (～ 1.8 μ_B) which has a $\text{q}$ value of 0.58 ± 0.01 $\text{c}$¹ in contrast to the zero field type IA structure with $\text{q}\text{= (0,0,0.50)}$. This transition is quite different from that occuring in the conventional spin-flop transition.     

Magnetic properties of CuCr2Se4 single crystals

The magnetic properties of CuCr₂Se₄ single crystals, which were grown by chemical transport reactions using iodine as a carrier, have been investigated. The magnetic moment at 0 K is found to be 5.07 μ_B per mole. The susceptibility at high temperatures follows a Curie-Weiss law with an asymptotic Curie temperature 430 K and a Curie constant 2.55 emu · $\text{deg}\text{mol}$. The magnetocrystalline anisotropy constants K₁ and K₂ are ? 6.9x 10⁵ and ? 0.9x 10⁵$\text{erg}\text{cm}{}^3$, respectively, at 5.1 K.In order to examine the effects of annealing on the magnetocrystalline anisotropy, the ferromagnetic resonance at room temperature was measured after annealing in vacuum and subsequently in an atmosphere of Se. It is found that the absolute values of K₁ and K₂ decrease after annealing in vacuum and increase to the initial values after annealing in an atmosphere of Se.