Showing papers on "Curie–Weiss law published in 2008"

Coherence-incoherence crossover in the normal state of iron-oxypnictides and importance of the Hund's rule coupling

Abstract: A new class of high temperature superconductors based on iron and arsenic was recently discovered, with superconducting transition temperature as high as 55 K. Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a Curie Weiss susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T*, the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence-incoherence crossover temperature is a very strong function of the strength of the Hund's rule coupling J_Hund. On the basis of the normal state properties, we estimate J_Hund to be 0.35-0.4 eV. In the atomic limit, this value of J_Hund leads to the critical ratio of the exchange constants J_1/J_2~2. While normal state incoherence is in common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron-oxypnictides, is unique due to its multiorbital electronic structure.

Evolution from Itinerant Antiferromagnet to Unconventional Superconductor with Fluorine Doping in LaFeAs(O1-xFx) Revealed by 75As and 139La Nuclear Magnetic Resonance

Abstract: We report experimental results of 75 As and 139 La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1- x F x ) ( x = 0.0, 0.04, and 0.11). In the undoped LaFeAsO, 1/ T 1 of 139 La exhibits a distinct peak at T N ∼142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1/ T 1 T exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1/ T 1 T both at the 75 As and 139 La site with a gap value of Δ PG ∼172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1/ T 1 in both compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures, which suggests unconventional super...

A comparative study of the magnetic properties of bulk and nanocrystalline Co3O4

Abstract: A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co3O4 nanoparticles (NP), synthesized by a sol–gel process, is presented. Both samples possess the cubic phase with a slightly lower (by 0.34%) lattice parameter for the Co3O4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co3O4 NP is quite consistent with the electron microscopic observations. The bulk Co3O4 has particle size in the 1–2 µm range. A Neel temperature of TN = 30 K (lower than the 40 K usually quoted in the literature) is determined from the analysis of the magnetic susceptibility versus temperature data for bulk Co3O4. This TN = 30 K is in excellent agreement with the TN = 29.92 K reported from specific heat measurements. The Co3O4 NP powder exhibits a still lower TN = 26 K, possibly due to the associated finite size effects. The values of coercivity, Hc = 250 Oe, and exchange bias, He = −350 Oe, together with the training effect have been observed in the Co3O4 NP sample (cooled in 20 kOe). Both Hc and He approach zero as . For T>TN, the χ versus T data for both samples fit the modified Curie–Weiss law (χ = χ0+C/(T+θ)). The magnitudes of C, θ and TN are used to determine the following: exchange constants J1ex = 11.7 K, J2ex = 2.3 K, and magnetic moment per Co2+ ion μ = 4.27 μB for bulk Co3O4; and J1ex = 11.5 K, J2ex = 2.3 K and μ = 4.09 μB for Co3O4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth ΔH that is larger for the Co3O4 NP. Details of the temperature dependence of ΔH, line intensity I0, and disappearance of the EPR on approach to TN are different for the two samples. These effects are discussed in terms of spin–phonon interaction and additional surface anisotropy present in Co3O4 NP.

Carrier-induced ferromagnetism in Ge0.92Mn0.08Te epilayers with a Curie temperature up to 190 K

Abstract: IV-VI diluted magnetic semiconductor Ge0.92Mn0.08Te epilayers are grown on BaF2 substrates by molecular beam epitaxy. The ferromagnetic behaviors, such as the spontaneous magnetization, the coercive field, and the Curie temperature TC, are altered by the hole concentration p. In the Ge0.92Mn0.08Te layer with high p, strong magnetic anisotropy and the temperature dependence of the magnetization expected for homogeneous ferromagnets are observed, implying that long-range ordering is induced by the holes. The maximum TC reaches 190 K for 1.57×1021 cm−3.

Anisotropy in magnetic properties and electronic structure of single-crystal LiFePO4

Abstract: crystals The values of the in-plane nearest- and next-nearest-neighbor spin exchange J1 and J2, interplane spin exchange J, and single-ion anisotropy D, obtained recently from neutron scattering measurements, are used for calculating the Curie temperatures with the formulas derived from the mean-field Hamiltonian It is found that the calculated Curie temperatures match well with that obtained by fitting the magnetic susceptibility curves to the modified Curie-Weiss law For the polarized Fe K-edge x-ray absorption spectra of single-crystal LiFePO4, a different feature assignment for the 1s →4p transition features is proposed and the anisotropy in the intensities of the 1s →3d transition features is explained adequately by a one-electron theory calculation of the electric quadrupole transition terms in the absorption coefficient

Magnetocaloric response of FeCrB amorphous alloys: Predicting the magnetic entropy change from the Arrott-Noakes equation of state

Abstract: The magnetic entropy change in Fe92−xCr8Bx (x=12,15) amorphous alloys has been studied. Increasing the B content, both the peak entropy change and the Curie temperature of the alloy increase. This is in agreement with an increase in the average magnetic moment per iron atom. The thermal and field dependences of the magnetic entropy change curves have been analyzed with the use of the Arrott–Noakes equation of state. It is shown that determining the parameters in this equation of state (through fitting the magnetization data) allows prediction of the field and temperature dependences of the magnetic entropy change curves in a broad temperature range around the Curie temperature.

Phase transition hysteresis and anomalous Curie-Weiss behavior of ferroelectric tetragonal tungsten bronzes Ba2RETi2Nb3O15: RE = Nd, Sm

Abstract: The ferroelectric tetragonal tungsten bronze (TTB) phases, Ba2RETi2Nb3O15:RE=Nd,Sm, were prepared by low temperature solvothermal synthesis. The permittivity versus temperature data of sintered ceramics show two unusual features: first, a hysteresis of 50–100 °C between values of the Curie temperature Tc on heat-cool cycles and second: a huge depression in the Curie–Weiss temperature T0. Both effects are attributed to the complex nature of their TTB-related crystal structures with different superstructures above and below Tc and the difficulty in nucleating ferroelectric domains on cooling through Tc. Several factors may contribute to the latter difficulty: first, the structures contain two sets of crystallographic sites for the “active” Ti, Nb ions; second, the distribution of Ti and Nb over these two sets of sites is not random but partially ordered; and third, below Tc a weak commensurate superstructure perpendicular to the polar c axis is present, but above Tc a weak incommensurate superstructure in ...

Effect of sintering optimization on the electrical properties of bulk BaxSr1−xTiO3 ceramics

Abstract: BaxSr1−xTiO3 (x=0.6, 0.75, 0.80, 0.85 and 0.9) compositions are prepared by solid-state reaction route using controlled heating and cooling. Density optimization by varying sintering temperature was achieved. X-ray diffraction (XRD) analysis shows the phase pure materials. The lattice constant decreases from 3.9868 A (x=0.90) to 3.9449 A (x=0.60) with increasing Sr2+; the tetragonal distortion also decreases. Dielectric constant show sharp peaks for samples having low strontium content (0.10, 0.15) and gets smeared out as the strontium content is increased (0.20, 0.25). For further higher Sr2+ composition (0.40), the dielectric peak could not be observed in the measured temperature range. The peak broadening in Sr2+ rich compositions indicates that diffused transitions and is attributed to the disorder in the arrangement of cations at A-site. A modified Curie–Weiss law using exponent γ and diffuseness parameter δ fits temperature dependence of dielectric constant in paraelectric phase. Both the parameters increase with increasing Sr2+ content showing that with strontium substitution, the material becomes disordered and may be correlated with the compositional fluctuation in solid solution. Frequency dependence of dielectric behavior is analyzed in terms of Debye formalism and nature of dielectric relaxation processes is explored using Cole–Cole formalism. Ferroelectric hysterisis loops reveal spontaneous polarization and coercive field increases with deceasing Sr content.

Near-zero-moment ferromagnetism in the semiconductor SmN

Abstract: The magnetic behavior of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T-C) of 27 3 K, evidenced by the occurrence of hysteresis below T-C. The ferromagnetic state is characterized by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetization at 2 K, measured after cooling in the maximum field, is 0.035 mu(B) per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for Sm+3 in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field. The susceptibility in the paramagnetic phase shows temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the J = 7/2 excited state and the 2 ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of 0.4 mu B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm+3 ions.

Magnetic properties of fcc Ni-based transition metal alloys

Abstract: Electronic properties and finite-temperature magnetism of Ni-based transition metal alloys with the face-centered cubic structure are studied theoretically by ab initio calculations. While the calculated total and local magnetic moments agree well with the experiment, the evaluation of the Curie temperature from first principles represents a much more delicate problem. The mean-field approximation and the random-phase approximation (RPA), as well as the renormalized RPA by Bruno that was extended to random alloys, were tested: the latter giving the most satisfactory agreement with the experiment for a broad class of Ni-based alloys of the type ${\text{Ni}}_{1\ensuremath{-}x}{\text{M}}_{x}$ ($M=\text{Cu}$, Pd, Co, Fe, and Mn) over the whole concentration range.

Lattice anomalies and magnetic excitations of the spin web compound Cu3TeO6

Abstract: We report on the magnetic susceptibility and Raman scattering measurements of the S = 1/2 three-dimensional (3D) spin web compound Cu3TeO6. The magnetic susceptibility shows an antiferromagnetic ordering at T-N similar to 61 K and a deviation from the Curie-Weisslaw around 150 K. Raman spectra show the emergence of a new mode at 132 cm(-1) for temperatures below T * similar to 50 K (approximate to 0.8 T-N). At the same temperature, phonon anomalies in intensity and frequency show up. This gives evidence of pronounced magneto-elastic effects. In addition, we observe a two-magnon Raman spectrum around 214 cm(-1). Compared to typical 3D spin systems, its robust temperature dependence suggests that a frustrated spin topology with a reduced spin coordination number also enhances spin-phonon couplings.

Nature of magnetic coupling between Mn ions in As-grown Ga1-xMnxAs studied by X-ray magnetic circular dichroism.

Abstract: The magnetic properties of as-grown Ga1-xMnxAs have been investigated by the systematic measurements of temperature and magnetic field dependent soft x-ray magnetic circular dichroism (XMCD). The intrinsic XMCD intensity at high temperatures obeys the Curie-Weiss law, but a residual spin magnetic moment appears already around 100 K, significantly above the Curie temperature (T_{C}), suggesting that short-range ferromagnetic correlations are developed above T_{C}. The present results also suggest that the antiferromagnetic interaction between the substitutional and interstitial Mn (Mn_{int}) ions exists and that the amount of the Mn_{int} affects T_{C}.

Volume dependence of the magnetic coupling in LaFe13−xSix based compounds

Abstract: The effects of pressure and hydrogenation, the former causes a lattice contraction while the latter causes an expansion of the sample, have been systematically studied for LaFe13−xSix (x=1.3–2.1). It is found that the typical change of the Curie temperature is ∼150K when ∼1.6H∕f.u. is absorbed and ∼−106K as the pressure sweeps from 0to1GPa. One of the most remarkable results of the present work is the presence of a universal relation between Curie temperature and phase volume. The former linearly grows with the increase of lattice constant (∼1510K∕A), irrespective of how the phase volume is modified. This result implies the exclusive dependence of the magnetic coupling in LaFe13−xSix on the Fe–Fe distances and the interstitial hydrogen does not affect the electronic structure of the compounds.

Magnetic properties of bulk amorphous TbxFe1−x

Abstract: Magnetization measurements have been made on four bulk samples (compositions x=018, 118, 167 and 25) prepared by direct current rapid sputtering Neutron diffraction measurements show these samples to be amorphous All four samples are ferrimagnetic; however, the 17% sample is almost completely compensated at temperatures below 100K With decreasing Tb‐content, the Curie temperatures fall from a maximum of 405K for x=25 to the value of 245K for x=018 Anomalously large coercive fields and time dependence magnetizations are present at low temperatures

Magnetic properties of RMn2X2 compounds (R=Rare earth, Y or Th and X=Ge, Si)

Abstract: Magnetic properties of RMn2Ged2 and RMn2Si2 compounds crystallizing in th BaAl4 type structure have been investigated over a wide temperature range (4.2 K to 1200 K). LaMn2Ge2, CeMn2Ge2, PrMn2Ge2, NdMn2Ge2 and LaMn2Si2 all possess Curie temperatures above 300 K and the easy direction of magnetization have been found to be along the c‐axis. When R is a heavy rare earth antiferromagnetic coupling was obtained at 4.2 K between the rare earth and manganese moments whereas when R is a light rare earth a ferromagnetic coupling of moments was obtained. Ymn2Si2, ThMn2Si2 and ThMn2Ge2 possess a small moment <⋅2μB at 4.2 K. High temperature susceptibilities on RMn2Si2 compounds indicate a Curie‐Weiss behavior.

Modification of the spin state in Sm0.52Sr0.48MnO3 by external magnetic field

Abstract: The effect of applied magnetic field (H) on the magnetic properties of Sm052Sr048MnO3 single crystal in the paramagnetic (PM) state has been investigated We observe a field induced steplike jump in magnetization (M) above TC (110K) The temperature and magnetic field dependence of susceptibility reveal that the PM phase of this system is quite complicated due to the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interactions The nature of magnetic interaction changes abruptly from AFM to FM at around 35T, above which M ceases the steplike jump

Metamagnetic transition and extremely large low field magnetocaloric effect in La0.7Ca0.3MnO3 manganite

Abstract: Metamagnetic transition was observed in low magnetic field for La07Ca03MnO3−δ lanthanum manganite prepared by solid-state technology Magnetization shape causes giant low field magnetocaloric effect at Curie temperature TC=242K Magnetic entropy changes ΔSm, deduced with isothermal magnetization curves, are equal to 70 and 80J∕kgK in the field change from 0 to 10 and 20T, respectively The obtained ΔSm are essentially higher than that reported before for lanthanum manganites with similar TC The observation shows the way for the construction the magnetic refrigerants, working in the field of permanent magnets, which is beneficial for household and automotive applications

Magnetic studies on Mn-doped TiO2 bulk samples

Abstract: Mn-doped TiO2 (Ti1−xMnxO2) bulk samples with nominal composition x = 0.02, 0.04, 0.08 have been prepared by a solid-state reaction and sintered at different temperatures ranging from 450 °C to 900 °C. For samples with x = 0.02, magnetic investigations show that room temperature ferromagnetism can be obtained and the magnetization of samples decreases monotonically with the increase in the sintering temperature. For the samples sintered at 600 °C with different doping content, the temperature dependence of magnetic susceptibility shows that a magnetic transition appears near room temperature, besides a magnetic transition at 43 K perhaps caused by Mn3O4. From the extrapolation of the inverse magnetic susceptibility curves at high temperatures, a positive Curie–Weiss temperature is obtained and it shifts to a low temperature with Mn doping content, revealing that ferromagnetic coupling decreases monotonically with the increase in Mn doping content. In addition, an exchange bias is clearly observed below 60 K, which also provides strong evidence that Ti1−xMnxO2 is ferromagnetic.

Structural and magnetic properties of wurtzite CoO thin films

Abstract: 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.

Magnetic susceptibility of YbRh2Si2 and YbIr2Si2 on the basis of a localized 4f electron approach

Abstract: We consider the local properties of the Yb 3+ ion in the crystal electric field in the Kondo lattice compounds YbRh2Si2 and YbIr2Si2. On this basis we have calculated the magnetic susceptibility, taking into account the Kondo interaction in the simplest molecular field approximation. The resulting Curie‐Weiss law and Van Vleck susceptibilities could be excellently fitted to experimental results over a wide temperature interval where thermodynamic and transport properties show non-Fermi-liquid behavior for these materials. (Some figures in this article are in colour only in the electronic version)

Supramolecular Approach to the Synthesis of [60]Fullerene−Metal Dithiocarbamate Complexes, {(MII(R2dtc)2)x·L}·C60 (M = Zn, Cd, Hg, Fe, and Mn; x = 1 and 2). The Study of Magnetic Properties and Photoconductivity

Abstract: Coordination assemblies formed by metal(II) dithiocarbamates MII(R2dtc)2 (M = Zn, Cd, Hg, Mn, and Fe; R = Me, Et, iPr, nPr, and nBu) and nitrogen-containing ligands (L = DABCO, DMP, and HMTA) are shown to be sterically well consistent with the spherical shape of fullerenes and are effectively cocrystallized with them to form complexes {(MII(R2dtc)2)x·L}·C60 (x = 1 and 2) with layered or three-dimensional (3D) packing of fullerene molecules (1−11). For the first time the molecular structures of the MII(R2dtc)2 assemblies in the complexes with C60 are presented (including those with M = Mn, Fe, and Hg). According to optical and electron paramagnetic resonance (EPR) spectroscopy 1−11 have a neutral ground state. Magnetic susceptibilities of {[MII(Et2dtc)2]2·DABCO}·C60·(DABCO)2 (M = Mn and Fe) follow the Curie–Weiss law in the 50–300 K range with the Weiss constants Θ = 0.35 and 1.70 K, respectively. Magnetic moments of the complexes equal to 8.23 and 6.88 μB at 300 K correspond to the high-spin state of the ...

Magnetic properties of La0.7Sr0.3MnO3 nanopowders

Abstract: The peculiarities of the magnetization of a nanopowder (particle size 12nm) of the manganite La07Sr03MnO3 are investigated It is shown that for temperatures below the Curie point the magnetization of the powder is of the ferromagnetic type with a relatively small value of the remanent magnetization The minimum of the derivative of the spontaneous magnetization with respect to temperature lies 20K below the Curie temperature determined by extrapolation of the temperature dependence of the coercivity These features of the magnetization are a consequence of a large variance of the anisotropy fields of the particles, the distribution function for which was determined from incremental hysteresis loops It is also found that the Curie temperature of the powder has a strong variance, with an rms deviation of at least 20K The mean value of the Curie temperature of the particles is 325K The spontaneous magnetization of the powder particles does not have (even in the low-temperature region) a saturation region This temperature dependence of the spontaneous magnetization influences the trend of the temperature dependence of the coercive field, leading to a deviation from the Neel–Brown law It is also shown that for the given ensemble of particles the blocking temperature corresponds to the temperature of the maximum of the ratio of the magnetization measured after cooling in the zero-field regime to the spontaneous magnetization

Magnetic properties of Nd1−xAgxMnO3 compounds

Abstract: Polycrystalline Nd1−xAgxMnO3 compounds were prepared in single-phase form up to 20% of Ag doping. They are found to crystallize in the Pbnm space group with an increase in lattice parameters with doping. The magnetic properties were studied by measuring dc magnetization and ac susceptibility. A paramagnetic to ferromagnetic transition along with the signature of the presence of competing antiferromagnetic interaction has been observed. The saturation magnetic moment up to 2.2 μB per formula unit has been observed at 78 K. The measured magnetization has been analyzed by using the Brillouin function and by taking into account the ferromagnetic interaction. The measured magnetization has been explained on the basis of spin canting with a ferromagnetic component along a unique axis and with an antiferromagnetic interaction in a plane perpendicular to the axis. The average magnetic moment estimated from the Curie–Weiss law fit in the paramagnetic region is comparable to the theoretical magnetic moment due to Mn3+/Mn4+ and Nd3+ ions. The analyses of frequency variation of ac susceptibility and harmonic susceptibility indicate the presence of spin-glass-like behavior below the ferromagnetic transition temperature.