Weiss Behavior Research Articles

Synthesis and magnetic characterization ofTmCo2B2C

A new quaternary intermetallic borocarbideTmCo2B2C has been synthesized via rapid-quench of an arc-melted ingot. Elementaland powder-diffraction analyses established its correct stoichiometry andsingle-phase character. The crystal structure is isomorphous with that ofTmNi2B2C (I4/mmm) and is stable over the studied temperature range. Above 7 K,the paramagnetic state follows modified Curie–Weiss behavior(χ = C/(T−θ)+χ0)wherein χ0 = 0.008(1) emu mol−1 with the temperature-dependent term reflecting the paramagnetism of the Tm subsystem:μeff = 7.6(2) μB (in agreement with the expected value for a freeTm3+ ion)and θ = −4.5(3) K. Long-range ferromagnetic order of the Tm sublattice is observed to develop around∼1 K. No superconductivityis detected in TmCo2B2C down to 20 mK, a feature which is consistent with the general trend in theRCo2B2C series. Finally, the influence of the rapid-quench process on the magnetism (and superconductivity) ofTmNi2B2C will be discussed andcompared to that of TmCo2B2C.

Vibrational, thermal, optical and magnetic investigations of PVA films filled with FeCl3 and CoCl2

Abstract PVA films filled with different mass fractions of X FeCl 3 (15− X ) CoCl 2 were investigated, vibrationally, thermally, optically and magnetically. An assignment of the most notably infrared (IR) peaks was done. Significant vibrational deformations of certain IR peaks with filling were studied. The main characterizing temperatures were recorded, assigned and their FL dependence were studied using differential scanning calorimetric (DSC). The thermal analysis depicts better thermal properties of the filled polymer with increasing X that represents interesting industrial applications. The absorption peaks in the ultraviolet and visible regions were detected and assigned. Electron paramagnetic resonance (EPR) spectra are complicated and characterized by the hyperfine structure. The main features of the EPR investigations are the orbital contribution to the magnetic moment, the important role of the spin–orbit coupling, the localization of the paramagnetic centers as well as the ionic cluster formations. The linear temperature dependence of the reciprocal dc magnetic susceptibility obeys Curie–Weiss behavior characterized by localized magnetic moments. A correlation between vibrational, thermal, optical, and magnetic properties was done.

Magnetic Properties of NpPdSn

A new compound NpPdSn was prepared and studied by X-ray diffraction, magnetization, heat capacity and electrical resistivity measurements, performed in the temperature range 2–300 K and under magnetic field up to 14 T. The crystal structure determined by single-crystal X-ray analysis is hexagonal with ZrNiAl-type (space group P62m). NpPdSn orders antiferromagnetically at 19 K and exhibits a Curie–Weiss behavior with μeff = 2.66 μB and Θp = −47 K. Bulk properties show temperature variations similar to systems with strong electronic correlations with a large negative paramagnetic Curie temperature and an enhanced low-temperature specific heat (γ ≈ 90 mJ/(mol K)). It suggests that NpPdSn may be classified as a new Np-based antiferromagnetic Kondo lattice, one of the very few known amidst transuranium-based intermetallics.

Relaxor behavior and dielectric properties of lead magnesium niobate–lead titanate thick films prepared by electrophoresis deposition

Lead magnesium niobate–lead titanate (0.80Pb(Mg 1/3Nb 2/3)O 3–0.20PbTiO 3, PMN–PT) thick films on platinum metallic foils were successfully fabricated by electrophoretic deposition (EPD). The room temperature relative dielectric constant and the maximum relative dielectric constant were 4645 and 25,640 at 1 kHz, respectively. The relaxor nature of PMN–PT thick films was studied in terms of diffused phase transition along with frequency dispersion of temperature of dielectric maximum ( T m). Deviation from Curie–Weiss behavior and temperature evolution of the local order parameter were found in the films. The degree of relaxation obtained from the modified Curie–Weiss law strongly suggests the relaxor behavior. Vögel–Fulcher relationship was used to analyze the frequency dependence of temperature of dielectric maximum ( T m).

Rare Earth-Transition Metal Indides with Lu5Ni2In4-type Structure

New intermetallic compounds RE5T2In4 (RE = Sc, Y, La-Nd, Sm, Gd-Tm, Lu; T = Rh, Ir) were synthesized by arc-melting of the elements or by induction melting of the elements in tantalum crucibles under flowing argon. The samples were characterized by X-ray powder diffraction. They crystallize with the orthorhombic Lu5Ni2In4-type structure, space group Pbam, Z = 2, a 2 : 1 intergrowth variant of CsCl and AlB2 related slabs of compositions InRE8 (distorted cubes) and RhRE6 (distorted trigonal prisms). Susceptibility measurements of Ce5Ir2In4 have revealed modified Curie- Weiss behavior above 70 K with an experimental magnetic moment of 2.45(1) μB / Ce atom. The cerium magnetic moments order ferri- or ferromagnetically at TC = 7.1(2) K.

Magnetism in REPdSn ( RE = La, Pr, Nd) compounds: A single-crystal study

The lack of data obtained on single crystals of the REPdSn intermetallic compounds ( RE = La, Pr, Nd) motivated us to grow single crystals of these compounds and to study intrinsic magnetic and related electronic and thermal properties of these materials with an emphasis on magnetocrystalline anisotropy. All the three compounds crystallize in the orthorhombic TiNiSi-type crystal structure. The results of specific heat, magnetization and resistivity measurements have revealed that LaPdSn is a paramagnet at temperatures down to 2 K, PrPdSn and NdPdSn order antiferromagnetically below T N = 4.2(1) and 3.4(1) K, respectively, and exhibit considerable magnetocrystalline anisotropy. The latter compound undergoes an order-to-order phase transition at 1.9(3) K. At sufficiently high temperatures the susceptibility of the Pr- and Nd-compounds follows a Curie–Weiss law with the effective magnetic moment almost identical in all the three principal crystallographic directions to the value expected for the corresponding free RE 3+ ion. The values of the paramagnetic Curie temperature manifest the anisotropy of the susceptibility in paramagnetic state, which is particularly pronounced for PrPdSn. As temperature decreases the susceptibility exhibits anisotropic departure from the Curie–Weiss behavior as a consequence of the crystal field (CF) influence on the RE ions. The involvement of the crystal field in the magnetism of the RE ions is also reflected in the Schottky contribution to the specific heat which reflects the splitting of the CF energy levels and the reduced values of the RE ordered magnetic moments due to the unpopulated higher-energy CF energy levels at low temperatures. Interesting features of magnetism in PrPdSn and NdPdSn are reflected also in the electrical resistivity and magnetoresistivity. For closer understanding of magnetism in the studied REPdSn compounds we have also performed first principles electronic structure calculations. We present the calculated density of states (DOS) of REPdSn compounds and we also compare the obtained experimental results with the theoretical predictions from the calculated electronic structure.

Electron spin resonance and quantum critical phenomena in VOx multiwall nanotubes

AbstractBasing on the high frequency (60 GHz) electron spin resonance study of the VOx multiwall nanotubes (VOx ‐NTs) carried out in the temperature range 4.2–200 K we report: (i) the first direct experimental evidence of the presence of the antiferromagnetic dimers in VOx ‐NTs and (ii) the observation of an anomalous low temperature growth of the magnetic susceptibility for quasi‐free spins, which obey the power law χ (T)∼1/T α with the exponent α ≈ 0.6 in a wide temperature range 4.2–50 K. We argue that the observed departures from the Curie–Weiss behaviour manifest the onset of the quantum critical regime and formation of the Griffiths phase as a magnetic ground state of these spin species. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Size effects on thin film ferroelectrics: Experiments on isolated single crystal sheets

Thin lamellae were cut from bulk single crystal BaTiO3 using a focused ion beam microscope. They were then removed and transferred onto single crystal MgO substrates, so that their functional properties could be measured independent of the original host bulk ferroelectric. The temperature dependence of the capacitance of these isolated single crystal films was found to be strongly bulklike, demonstrating a sharp Curie anomaly, as well as Curie–Weiss behavior. In addition, the sudden change in the remanent polarization as a function of temperature at TC was characteristic of a first order phase change. The work represents a dramatic improvement on that previously published by Saad et al. [J. Phys.: Condens. Matter 16, L451 (2004)], as critical shortcomings in the original specimen geometry, involving potential signal contributions from bulk BaTiO3, have now been obviated. That the functional properties of single crystal thin film lamellae are comparable to bulk, and not like those of conventionally deposited heterogeneous thin film systems, has therefore been confirmed.

Crystal structure, spectroscopic properties, and magnetic behavior of the fluoride-derivatized lanthanoid(III) ortho-oxomolybdates(VI) LnF[MoO 4] ( Ln=Sm–Tm)

The fluoride-derivatized lanthanoid(III) ortho-oxomolybdates(VI) LnF[MoO 4] ( Ln=Sm–Tm) crystallize in the monoclinic space group P2 1/ c with four formula units per unit cell ( a=516–528 pm, b=1220–1248 pm, c=659–678 pm, β=112.5–113.1°). The structure contains one crystallographically unique Ln 3+ cation surrounded by two fluoride and six oxide anions in a square antiprism (CN=8). The square antiprisms [ LnF 2O 6] are interconnected via three edges to form layers ∞ 2 { [ Ln F 2 / 2 e O 4 / 2 e O 2 / 1 t ] 6 - } parallel (010), which are cross-linked along [010] by Mo 6+ in tetrahedral oxygen coordination to form the three-dimensional crystal structure. The fluoride anions within this arrangement exhibit a twofold coordination of Ln 3+ cations in the shape of a boomerang, which is connected to another F − anion to form planar [F 2 Ln 2] 4+ rhombuses. Magnetic measurements for GdF[MoO 4], TbF[MoO 4], and DyF[MoO 4] show Curie–Weiss behavior, despite the peculiar arrangement of the lanthanoid(III) cations in layers comparable with those of gray arsenic. Furthermore, Raman, infrared, and diffuse reflectance spectroscopy data for these compounds were recorded and interpreted.

Structural and magnetic properties of wurtzite CoO thin films

Hexagonal CoO thin films on (0 0 0 1) Al2O3 and ZnO are prepared by off-axis and on-axis pulsed laser deposition. The structural properties were studied by in situ reflection high-energy electron diffraction and ex situ x-ray diffraction. The magnetic properties of the films depend on the mode of growth. The films grown in a normal geometry are a mixture of Co3O4 and CoO and show ferromagnetism at room temperature, whereas the films grown off-axis are single phase and present a Curie–Weiss behaviour. Wurtzite CoO does not order magnetically because the antiferromagnetic Co2+–O2−–Co2+ superexchange is geometrically frustrated. It is proposed that defective Co3O4 in thin films is ferrimagnetic, due to the appearance of high spin Co3+ on B sites.

Enhanced Conductivity and Magnetic Ordering in Isostructural Heavy Atom Radicals

Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P42(1)m. The crystal structures consist of slipped pi-stack arrays of undimerized radicals packed about 4 centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E2---E2' contacts. Variable temperature conductivity (sigma) measurements reveal an increase in conductivity with increasing selenium content, particularly so when selenium occupies the E2 position, with sigma(300 K) reaching a maximum (for E1 = E2 = Se) of 3.0 x 10(-4) S cm(-1). Thermal activation energies E(act) follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie-Weiss behavior over the temperature range 20-300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E1 site, undergoes a phase transition at 14 K to a weakly spin-canted (phi = 0.010 degrees) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E2 position, order ferromagnetically, with Curie temperatures of T(c) = 12.8 and 17.0 K, respectively. The coercive fields H(c) at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a-4a are discussed in the light of Extended Hückel Theory band structure calculations.

Synthesis and properties of barium titanate stannate thin films by chemical solution deposition

Barium titanate stannate (BaTi1−xSnxO3, 0 ≤ x ≤ 0.25) thin films were deposited directly on copper foil substrates via a chelate chemical solution process. The films were subsequently crystallized in a reducing atmosphere such that substrate oxidation was avoided and that the 2-valent state of tin could be stabilized. Despite the stabilization of the low-melting temperature SnO oxidation state at high temperatures, the final grain size was smaller with increased tin incorporation similar to other B-site substituted BaTiO3 films. Temperature and field-dependent dielectric measurements revealed a reduction in dielectric constant and dielectric tuning with increasing tin concentration. The reduction in permittivity with reduced grain size is consistent with the well-known trends for ceramic barium titanate and in combination with a defect-dipole model involving Sn acceptors, can be used to explain the experimental trends. Phase transition frequency dependence was studied and for compositions containing up to 25 mole percent tin. No phase transition dispersion was observed and thus no strong evidence of relaxor-like character. The phase transition became increasingly diffuse with deviation from Curie–Weiss behavior, but the observed transition temperatures agreed well with bulk reference data.

Crystal growth and ambient and high pressure study of the re-entrant superconductorTm2Fe3Si5

Tm2Fe3Si5 is known to undergo a transition to the superconducting state (atambient or applied pressure depending on the sample) at a temperatureTc1(∼1.8 K), and at a lowertemperature TN(≈1 K) it undergoes a transition into a long range antiferromagnetically ordered state.Superconductivity is simultaneously destroyed and the sample re-enters the normal state atTc2 = TN. The conditions reported in the literature for the observation of superconductivity inTm2Fe3Si5 are sample dependent, but it is now accepted that stoichiometricTm2Fe3Si5 superconducts only under pressure. Here we report single-crystal growth of stoichiometricTm2Fe3Si5 which does not superconduct at ambient pressure down to 100 mK.Measurements of the anisotropic static magnetic susceptibilityχ(T) and isothermalmagnetization M(H),ac susceptibility χac(T),electrical resistivity ρ(T) and heat capacity C(T) atambient pressure and χac(T) at high pressure are reported. The magnetic susceptibility along thec axis, χc(T), shows a curvature over the whole temperature range and does not followthe Curie–Weiss behavior, while the magnetic susceptibility along thea axis, χa(T), follows a Curie–Weiss behavior between 130 and 300 K with a Weiss temperatureθ and an effectivemagnetic moment μeff which depend on the temperature range of the fit. The easy axis of magnetization is perpendicular to thec axis andχa/χc = 3.2 at 1.8 K. Theambient pressure χac(T) and C(T) measurements confirm bulk antiferromagnetic ordering atTN = 1.1 K. The sharpdrop in χac(T) below the antiferromagnetic transition is suggestive of the existence ofa spin gap. We observe superconductivity only under applied pressuresP≥2 kbar. The temperature–pressure phase diagram showing thenon-monotonic dependence of the superconducting transition temperatureTc onpressure P is presented.

Ferroelectric transitions by Ca substitution in Pb0.7Nd0.2TiO3

Ferroelectric Pb0.7(1−y)Ca0.7yNd0.2TiO3 (PCNT-y) compositions with y ranging from 0 to 0.60, were synthesized via a mechanical activation route. The tetragonality (c/a) of PCNT-y at room temperature decreases from 1.0125 to 1.0000 upon substitution of Pb2+ with smaller sized Ca2+. The composition with Ca-doping y=0.60 demonstrates a unit-cell volume shrinkage of 4.063% as compared to that of Pb0.7Nd0.2TiO3, while the Raman spectra are of the characteristic features of PbTiO3. As the Ca content (y) increases from 0 to 0.60, there is a crossover from a displacive phase transition (y=0 and 0.10) to a diffusive one (0.20≤y≤0.40), accompanied by a decrease in transition temperature (TC or Tmax) at a linear rate of ∼8.2 K/at. %. The compositions with y<0.10 demonstrated the Curie–Weiss behavior in dielectric behavior, while those compositions of 0.20≤y≤0.40 could well be fitted to the Uchino equation with the diffusivity, in agreement with a relaxor ferroelectric behavior. At y≥0.50, ε′ was much reduced and became less sensitive to temperature. Neither εmax′ nor frequency dispersion was detected in PCNT-0.50 and -0.60 in the ε′-T plot, down to 80 K.

Electrical transport, heat capacity, and high-field magnetization study in intermetallic Ni2CeSn compound

Magnetization, heat capacity, and electrical resistivity measurements were performed on a Ni2CeSn compound (orthorhombic structure) in the temperature range of 2–300K. This compound is paramagnetic down to 6K. At higher temperatures above T=150K, the magnetic susceptibility obeys Curie–Weiss behavior yielding an effective magnetic moment μeff=2.56μB∕f.u., which is very close to that of the free Ce3+ ion (2.54μB) with a high negative Curie–Weiss temperature, ΘCW=−170K. As the temperature is decreased, the magnetic moment decreases gradually to 0.43μB∕f.u. at 4.2K. We also infer that, based on the high field (up to 23T) magnetization and the magnetic susceptibility data, a crystal-field splitting of cerium atoms becomes significant at temperatures below 150K. We used heat capacity and resistivity measurements to determine the crystal-field splitting of the Ce3+ magnetic sublevels. It is found that the ground magnetic state for the Ce3+ is a doublet of Jz=±1∕2 states, with a first excited quartet of Jz=±3∕2 and Jz=±5∕2 states separated by ∼107K. The resistivity exhibits a shallow minimum at about 11K, which may be due to the development of partial magnetic order based on these crystal-field-split states.

Characterization, electrical and magnetic properties of PVA films filled with FeCl 3–MnCl 2 mixed fillers

Polyvinyl alcohol (PVA) films filled with different filling levels (FLs) of XFeCL 3(15− X)MnCl 2 were studied. The DSC thermograms exhibited an increase in the melting temperature with filling, indicating better thermal stability of the filled polymer of interesting industrial applications. The amorphous feature of the filled polymer was depicted using XRD scans. Vibrational studies displayed significant structural deformations. The FL dependence of certain IR absorption peaks was discussed. The dc electrical conduction mechanism was interpreted on the basis of the modified interpolaron hopping model. The present results of the dc magnetic susceptibility ( χ) suggested the temperature dependence of Curie–Weiss behavior characterized by localized magnetic moments. The effective paramagnetic moment ( μ eff) was estimated; its dependence on the FL exhibited a non-linear character. The electron spin resonance (ESR) study revealed unresolved broad distorted signals characterized by the hyperfine structure. The ESR parameters were evaluated. A correlation between the above-mentioned studies was discussed to relate the structural, electrical and magnetic properties of the filled PVA polymer.

Structure, microstructure and magnetic properties of Sr 1−xCa xCrO 3 (0⩽ x⩽1)

The effect of the calcium concentration on the structural, microstructural and magnetic properties of Sr (1− x )Ca x CrO 3 with 0⩽ x⩽1 has been studied. The compounds were prepared using high pressure and high temperature synthesis. X-ray diffraction shows that the samples evolve from the cubic Pm-3 m space group for x=0–0.2 to tetragonal I4/ mcm for x=0.4–0.5, then to the orthorhombic Pbnm space group for x=0.6, 0.8 and 1.0. Electron diffraction and high-resolution transmission electron microscopy confirmed the respective cells for the end compositions: a p× a p× a p ( Pm-3 m) for SrCrO 3; and the 2 a p × 2 a p × 2 a p ( Pbnm) for CaCrO 3. For intermediate compositions some extra spots appear in the electron diffraction patterns while the electron micrographs indicate the presence of microdomains. Magnetic measurements show Curie–Weiss behaviour at high temperature for all the samples. A sharp antiferromagnetic (AFM) transition at about 91.5 K appears for x=0.8–1 together with a weak ferromagnetic ordering below T N.

Paramagnetic Properties of NdFeAsO0.94F0.06Polycrystals

The temperature and field dependencies of magnetization of NdFeAsO 0.94 F 0.06 bulk sample are shown to exhibit the Curie–Weiss behaviour, both in the superconducting and normal states. To address the origin of the strong paramagnetic signal, particularly to reveal contributions from antiferromagnetic second phases in sintered NdFeAsO 0.94 F 0.06 , the magnetic response of Nd 2 O 3 was measured and compared with NdFeAsO 0.94 F 0.06 . Our measurements show that paramagnetism of NdFeAsO 0.94 F 0.06 is an intrinsic property dominated by Nd 3+ ions contribution, which cannot be explained by magnetic second phases.

Magnetic susceptibility, spin transition, and electrical conductivity of LaCo1 + x O3 and NdCo1 + x O3

The magnetic susceptibility of Nd2O3, NdCo1 + x O3, and LaCo1 + x O3 (x = 0, 0.05, 0.1, 0.15) has been measured at temperatures from 80 to 950 K, and the electrical conductivity of the neodymium and lanthanum cobaltites (enriched in cobalt relative to neodymium or lanthanum) with the general formulas Nd(La)Co1 + x O3 + 1.5x , or Nd1/(1 + x)(La)1/(1 + x)CoO(3 + 1.5x)/(1 + x), has been measured between 300 and 1050 K. The effective magnetic moments of paramagnetic ions have been determined in the temperature ranges of CurieWeiss behavior and have been used to evaluate the fractions of low-, intermediate-, and high-spin Co3+ ions. Raising the temperature from 320 to 660 K (non-Curie—Weiss behavior) increases the fraction of high-spin Co3+ ions in LaCo1 + x O3 + 1.5x (La1/(1 + x)CoO(3 + 1.5x)/(1 + x) from 27–43 to 56–61%. Moreover, in this temperature range the conductivity of the lanthanum cobaltites rises most steeply. In the range 660–950 K, no spin transition occurs in LaCo1 + x O3 + 1.5x , the slope of the conductivity versus temperature curves gradually decreases, and the conductivity gradually saturates. The conductivity of NdCo1 + x O3 + 1.5x (Nd1/(1 + x)CoO(3 + 1.5x)/(1 + x)) varies considerably in the range 550–950 K, and the spin transition in these cobaltites takes place between 260 and 760 K. Above 760 K, the NdCo1 + x O3 + 1.5x cobaltites with x = 0.05 and 0.10 contain, respectively, 72 and 83% high-spin Co3+ ions and 28 and 17% high-spin Co4+ ions, whereas neodymium cobaltite with x = 0.15 contains 83% high-spin and 17% intermediate-spin Co3+ ions.

The first examples of discotic radicals: columnar mesomorphism in spin-carrying triphenylenes

The synthesis, columnar behaviour and magnetic properties of the first examples of radical discotics are reported. These organic molecular systems with spin sources have been accomplished by covalently linking triphenylene with molecular radicals. The columnar mesomorphism of these new radicals has been established unambiguously using several complementary techniques. Magnetic susceptibility measurements in the fluid columnar, frozen columnar and solid states reveal the antiferromagnetic intermolecular interactions of Curie–Weiss behaviour.