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Showing papers on "Curie–Weiss law published in 2009"


Journal ArticleDOI
TL;DR: In this article, a correlation between increase in hole concentration with decrease in magnetization and Curie temperature in ZnO:Li nanorods was found to explain the observed ferromagnetism.
Abstract: We have observed ferromagnetism in Li-doped ZnO nanorods with Curie temperature up to 554 K. Li forms shallow acceptor states in substitutional zinc sites giving rise to $p$-type conductivity. An explicit correlation emerges between increase in hole concentration with decrease in magnetization and Curie temperature in ZnO:Li. Occurrence of ferromagnetism at room temperature has been established with observed magnetic domain formation in ZnO:Li pellets in magnetic force microscopy and prominent ferromagnetic resonance signal in electron paramagnetic resonance spectrum. Magnetic ZnO:Li nanorods are luminescent, showing strong near UV emission. Substitutional Li atoms can induce local moments on neighboring oxygen atoms, which when considered in a correlated model for oxygen orbitals with random potentials introduced by dopant atom could explain the observed ferromagnetism and high Curie temperature in ZnO:Li nanorods.

142 citations


Journal ArticleDOI
TL;DR: In this article, the temperature dependent dielectric properties of Ba(Zr0.25Ti0.75)O3 were investigated in the frequency range from 1 kHz to 1 MHz.
Abstract: In this work, Ba(Zr0.25Ti0.75)O3 ceramic was prepared by solid-state reaction. This material was characterized by x-ray diffraction and Fourier transform Raman spectroscopy. The temperature dependent dielectric properties were investigated in the frequency range from 1 kHz to 1 MHz. The dielectric measurements indicated a diffuse phase transition. The broadening of the dielectric permittivity in the frequency range as well as its shifting at higher temperatures indicated a relaxor-like behaviour for this material. The diffusivity and the relaxation strength were estimated using the modified Curie–Weiss law. The optical properties were analysed by ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements at room temperature. The UV–vis spectrum indicated that the Ba(Zr0.25Ti0.75)O3 ceramic has an optical band gap of 2.98 eV. A blue PL emission was observed for this compound when excited with 350 nm wavelength. The polarity as well as the PL property of this material was attributed to the presence of polar [TiO6] distorted clusters into a globally cubic matrix. (Some figures in this article are in colour only in the electronic version)

90 citations


Journal ArticleDOI
TL;DR: In this article, a detailed characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics is presented.
Abstract: The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and X-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T. At low temperatures the dielectric constant shows a minor but significant dependence on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.

61 citations


Journal ArticleDOI
TL;DR: In this paper, the dependence of magnetization M on the applied magnetic field H and temperature T was measured carefully, near the Curie temperature TC for the perovskite manganite sample Pr0.6Sr0.4MnO3.

60 citations


Journal ArticleDOI
01 Apr 2009-EPL
TL;DR: In this article, the authors investigated thermally assisted switching analytically and showed that for temperatures close to (but still below) the Curie temperature two reversal modes appear, an elliptical mode and a linear one.
Abstract: For further improvement of magnetic information storage density and writing speed, laser-induced writing procedures have been extensively explored recently. Within the framework of the Landau-Lifshitz-Bloch equation of motion, which does not conserve the length of the magnetization vector, we investigate thermally assisted switching analytically. We show that for temperatures close to (but still below) the Curie temperature two reversal modes appear, an elliptical mode and a linear one. We calculate the coercive fields and energy barriers for both elliptical and linear switching. Investigating the dynamics of linear reversal, which is the more relevant case close to the Curie temperature, we calculate the temperature dependence of the minimal time and field needed for thermally assisted switching below and above the Curie temperature.

56 citations


Journal ArticleDOI
TL;DR: In this article, the influence of demagnetizing factor (N) on the magnetic entropy change (ΔSM) curves is studied for materials with a second order phase transition. And the universal curve for the magnetocaloric effect can be used.
Abstract: The influence of the demagnetizing factor (N) on the magnetic entropy change (ΔSM) curves is studied for materials with a second order phase transition. For this purpose, a soft magnetic amorphous ribbon is measured for different orientations of the magnetic field with respect to the plane of the sample. For temperatures below the Curie temperature (TC), the increase in N causes a decrease in ΔSM, while for temperatures above TC no change in the shape of the curves has been found for the different orientations, as expected. In order to eliminate this influence of N and compare the ΔSM(T) curves for samples with different shapes, the recently proposed universal curve for the magnetocaloric effect can be used.

55 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used spin-density-functional theory in combination with the Heisenberg model to predict an essential independence of the Curie temperature of bcc iron on the external pressure.
Abstract: The pressure dependence of the Curie temperature in bcc iron has been studied employing spin-density-functional theory in combination with the Heisenberg model. We show that the results correctly predict an essential independence of the Curie temperature of bcc iron on the external pressure, in agreement with the experimental findings. This behavior is explained as a result of a competition between the decrease in the local magnetic moments and the increase in the magnetic coupling as function of rising pressure.

47 citations


Journal ArticleDOI
TL;DR: In this article, a series of magnetic amorphous ribbons with different Gd/Co ratios was prepared by melt-spun method, and the maximum values of magnetic entropy changes under a magnetic field of 10 kOe were −3.1, − 3.0, − 2.9, and −2.8
Abstract: A series of magnetic amorphous ribbons with different Gd/Co ratios was prepared by melt-spun method. With the decrease in Gd/Co ratio, Curie temperatures increase gradually from 166 to 193 K. The maximum values of magnetic entropy changes under a magnetic field of 10 kOe are −3.1, −3.0, −2.9, and −2.8 J/kg K for Gd71Co29, Gd68Co32, Gd65Co35, and Gd62Co38 samples, respectively. The approximately constant peak values of ΔSM at different working temperatures indicate that they are advantageous for an Ericsson refrigeration cycle. In addition, these samples have large resistivity and greatly reduced magnetic hysteresis losses, which could increase the refrigeration efficiency. These advantages make the Gd–Co amorphous ribbons good candidates for the practical magnetic refrigeration.

45 citations



Journal ArticleDOI
TL;DR: In this paper, structural, magnetic and electrical properties of the La0.7Ca0.15Sr 0.15Mn1−xMoxO3 (0 x 0.05) compounds have been investigated.
Abstract: Structural, magnetic and electrical properties of the La0.7Ca0.15Sr0.15Mn1−xMoxO3 (0 x 0.05) compounds have been investigated. Powder x-ray analysis reveals that the sample with x = 0 crystallizes in the rhombohedral (R3c) structure, whereas in the Mo doped samples the structure can be indexed by an orthorhombic (Pbnm) structure. The important observations of the magnetic and transport properties are: (i) the Mo substitution induces a distinct suppression of the metal‐insulator (TMI) and ferromagnetic (FM)—paramagnetic transition (TC) and the temperature of TMI was found to be higher than TC in the Mo-doped samples, (ii) the substitution of Mo enhances the magnetoresistance at room temperature, (iii) a large deviation from the Curie‐Weiss law well above TC in the Mo substituted samples indicates the existence of a Griffiths phase and (iv) long-range FM order persists in all samples with a linear decrease of saturation moment as x increases. These results are discussed in terms of the Mn-site disorder and opening of strong FM coupling between Mn 2+ ‐O‐Mn 3+ , due to the Mn 2+ ions induced by Mo 6+ at the expense of Mn 4+ ions in the La0.7Ca0.15Sr0.15Mn1−xMoxO3 system. (Some figures in this article are in colour only in the electronic version)

38 citations


Journal ArticleDOI
TL;DR: The magnetic field-induced entropy change in a ferromagnet undergoing a second-order phase transition at the Curie point is maximum near that point, and the maximum magnetic entropy change, ( - Δ S ) max, is demonstrated to contain, besides the usual term in H 2 / 3, a small negative term independent of H as discussed by the authors.

Journal ArticleDOI
TL;DR: The structural, magnetic and luminescence properties of praseodymium-doped zirconia powders of compositions Pr0.03Zr0.97O2 and Pr 0.05Zr 0.95O2 synthesized by a sol?gel process have been investigated as mentioned in this paper.
Abstract: The structural, magnetic and luminescence properties of praseodymium-doped zirconia powders of compositions Pr0.03Zr0.97O2 and Pr0.05Zr0.95O2 synthesized by a sol?gel process have been investigated. X-ray diffraction patterns indicate that these materials crystallize in a tetragonal fluorite-type structure. Scanning electron microscopy shows that the powders exhibit an agglomerated microcrystalline structure and the grain size may be in the order of 5?20??m. The study of the magnetic properties of these doped metal oxides indicates a Curie?Weiss behaviour in the temperature range (100?300)?K that allow us to estimate an effective magnetic moment of 3.51??B, which indicates the presence of Pr3+ in the grown samples. Cathodoluminescence spectra recorded at temperatures between 85 and 295?K show emission peaks that can be attributed to transitions between different states within the 4f2 configuration of Pr3+ ions incorporated in the zirconia crystal lattice. Thermoluminescence measured at temperatures ranging from 373 to 773?K and at 550?nm wavelength show an intense and broad peak around 653?K for the Pr-doped zirconia which is not observed in the undoped material.

Journal ArticleDOI
TL;DR: In this paper, a single-phased PrCo2B2 compound was prepared by an arc-melting method and the magnetic properties and magnetocaloric effect (MCE) were studied by the magnetization and heat capacity measurement.
Abstract: A single-phased PrCo2B2 compound was prepared by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) were studied by the magnetization and heat capacity measurement. The compound exhibits a single paramagnetic to ferromagnetic transition at the Curie temperature TC of 18 K. A large reversible MCE was observed which is related to a second order magnetic transition from paramagnetic to ferromagnetic state. The values of maximum magnetic entropy change (−ΔSMmax) reach 4.9 and 9.6 J Kg−1 K−1 for the field change of 2 and 7 T with no obvious hysteresis loss in the vicinity of the Curie temperature, respectively. The corresponding maximum adiabatic temperature changes (ΔTadmax) are evaluated to be 4.3 and 9.8 K. The magnetic transition and the origin of large MCE in PrCo2B2 were discussed.

Journal ArticleDOI
TL;DR: In this article, the structure of the glassy system x CuO-65TeO 2 −(35− x )V 2 O 5, 5≤ x ≤ 10% was studied using infrared (IR) and Raman spectroscopy methods as well as magnetic susceptibility measurements.

Journal ArticleDOI
TL;DR: In this article, it was shown that the magnetic susceptibility of YbCu 2 Si 2 follows the Curie-Weiss law with Yb 3+ at high temperatures, but reveals a broad peak around 40 K for H ∥[100], which is due to the formation of a 4 f -itinerant heavy fermion state at lower temperatures.
Abstract: We succeeded in growing high-quality single crystals of a valence fluctuating compound YbCu 2 Si 2 and a divalent compound YbCu 2 Ge 2 . The magnetic susceptibility of YbCu 2 Si 2 follows the Curie–Weiss law with Yb 3+ at high temperatures, but reveals a broad peak around 40 K for H ∥[100], which is due to the formation of a 4 f -itinerant heavy fermion state at lower temperatures. This is also reflected in the temperature dependence of Hall coefficient, thermoelectric power and thermal expansion. The corresponding de Haas–van Alphen (dHvA) branches are approximately explained by the 4 f -itinerant LDA band model, and the 4 f -itinerant LDA+ U model is found to be much applicable to the dHvA data. The cyclotron effective masses of main Fermi surfaces are relatively large, being 30–40 m 0 , which is consistent with the electronic specific heat coefficient γ=150 mJ/(K 2 ·mol). These results indicate that the localized 4 f electrons at high temperatures become itinerant at low temperatures, forming a narrow ...

Journal ArticleDOI
TL;DR: In this article, the Rietveld refinements based on the XRD patterns showed the existence of a continuous solid solution over the whole composition range (0⩽ y ⩽ 1.0) with a continuous evolution of the monoclinic unit cell parameters (S.G. P 2 O 7 ).

Journal ArticleDOI
TL;DR: In this paper, the magnetic and electrical measurements carried out on the R 2 WO 6 tungstates showed a paramagnetic behaviour for samples with R=Nd, Gd, Dy and Ho and more complex one for samples having R=Sm and Eu in the temperature range 4.2-280 k and an insulating state at room temperature.
Abstract: The magnetic and electrical measurements carried out on the R 2 WO 6 tungstates showed a paramagnetic behaviour for samples with R=Nd, Gd, Dy and Ho and more complex one for samples with R=Sm and Eu in the temperature range 4.2–280 K and an insulating state at room temperature. With increasing atomic number of the R element the Curie–Weiss temperature increases from −43.5 K for Nd 2 WO 6 to −2.7 K for Ho 2 WO 6 , excluding Sm 2 WO 6 and Eu 2 WO 6 compounds for that the Curie–Weiss region is not observed and the imaginary part of susceptibility is close to zero. The effective magnetic moment is close to the theoretical one for the free R ion and the magnetic moment measured in magnetic field of 14 T and at temperature of 4.2 K, generally, does not reach the saturation state. The temperature independent residual susceptibility is negative for Nd 2 WO 6 and positive for the remaining compounds suggesting different proportions of the Landau, Pauli and van Vleck contributions to the total susceptibility. An increase of the orbital magnetic contribution to the total magnetic moment is suggested from the fitting of the Lande factor in the compounds under study.

Journal ArticleDOI
TL;DR: In this article, phase relation studies in the Gd 2 O 3 -Nd O 3 system have been performed on (Gd 1− x Nd x ) 2O 3 samples (0⩽ x ⩽1) with the purpose of performing a systematic study of the composition effects on their structural and magnetic properties.

Journal ArticleDOI
TL;DR: In this paper, the effect of dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 (BZT) ceramics are investigated.
Abstract: The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, e−1=em−1[1+(T−Tm)γ/C1](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.

Journal ArticleDOI
TL;DR: In this paper, a photoelectrochemical study of LaCuO2 has been conducted to confirm the p-type conduction of the oxide, which is a direct band-gap semiconductor with a forbidden band of 2.77 eV.

Journal ArticleDOI
TL;DR: In this paper, two novel polyoxometalate derivatives, {XIVWVI10WV2O40[Cu(en)2(H2O)]3} [X=V (1), Si (2); en=ethylenediamine], have been hydrothermally synthesized and characterized by elemental analyses, IR, UV-Vis, XPS, EPR, TG and single crystal X-ray diffraction analyses.

Journal ArticleDOI
13 Nov 2009-Pramana
TL;DR: In this article, the Curie temperature derived from magnetic thermogravimetric analysis shows that the temperature increases with increasing particle size, and the same technique can be extended to study the magnetic phase transition.
Abstract: Mn0.5Zn0.5Fe2O4 ferrite nanoparticles with tunable Curie temperature and saturation magnetization are synthesized using hydrothermal co-precipitation method. Particle size is controlled in the range of 54 to 135 A by pH and incubation time of the reaction. All the particles exhibit super-paramagnetic behaviour at room temperature. Langevin’s theory incorporating the interparticle interaction was used to fit the virgin curve of particle magnetization. The low-temperature magnetization follows Bloch spin wave theory. Curie temperature derived from magnetic thermogravimetric analysis shows that Curie temperature increases with increasing particle size. Using these particles magnetic fluid is synthesized and magnetic characterization is reported. The monolayer coating of surfactant on particle surface is confirmed using thermogravimetric measurement. The same technique can be extended to study the magnetic phase transition. The Curie temperature derived using this measurement complies with the low-temperature magnetic measurement. The room-temperature and high-temperature magnetization measurements are also studied for magnetic fluid systems. The magnetic parameters derived for fluid are in good agreement with those obtained for the particle system.

Journal ArticleDOI
TL;DR: It is shown that, in the case of a well-isolated Kramers doublet with anisotropic g and exchange tensors, the inverse susceptibility for a powder sample does not strictly obey a Curie-Weiss law at low temperature.
Abstract: The local Yb3+ magnetic susceptibility tensor was recently measured in the frustrated pyrochlore compound Yb2Ti2O7 by means of in-field polarized neutron scattering in a single crystal A very anisotropic effective exchange tensor was derived for the Yb3+ ion Using this result, we reinterpret here the data for the powder susceptibility in Yb2Ti2O7 We show that, in the case of a well-isolated Kramers doublet with anisotropic g and exchange tensors, the inverse susceptibility for a powder sample does not strictly obey a Curie–Weiss law at low temperature We discuss the consequences regarding the paramagnetic Curie temperature, usually taken as a measure of the exchange/dipolar interaction, and the exotic 'slow fluctuation' ground state of Yb2Ti2O7

Journal ArticleDOI
TL;DR: In this article, the effect of a magnetic field B along the [111] direction on the quantum spin ice (QSI) state of Tb(2)Ti (2)O(7) pyrochlore magnetic material was investigated.
Abstract: Despite a Curie-Weiss temperature θ(CW)∼-14 K, the Tb(2)Ti(2)O(7) pyrochlore magnetic material lacks long range magnetic order down to at least T(*)≈50 mK. It has recently been proposed that the low temperature collective paramagnetic or spin liquid regime of this material may be akin to a spin ice state subject to both thermal and quantum fluctuations-a quantum spin ice (QSI) of sorts. Here we explore the effect of a magnetic field B along the [111] direction on the QSI state. To do so, we investigate the magnetic properties of a microscopic model of Tb(2)Ti(2)O(7) in an independent tetrahedron approximation in a finite field B along [111]. Such a model describes semi-quantitatively the collective paramagnetic regime where nontrivial spin correlations start to develop at the shortest length scale, that is over a single tetrahedron, but where no long range order is yet present. Our results show that a magnetization plateau develops at low temperatures as the system develops B = 0 ferromagnetic spin ice like 'two-in/two-out' correlations at the shortest length scale. From these results, we are led to propose that the observation of such a [111] magnetization plateau in Tb(2)Ti(2)O(7) would provide compelling evidence for a QSI at B = 0 in this material and help guide the development of a theory for the origin of its spin liquid state.

Journal ArticleDOI
TL;DR: In this article, the complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr 2 Se 4 spinel and an antiferromagnetic order with a Neel temperature T N =20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature θ CW =55.1 K were established.
Abstract: The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr 2 Se 4 spinel. An antiferromagnetic order with a Neel temperature T N =20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie–Weiss temperature θ CW =55.1 K were established. An increasing static magnetic field shifts T N to lower temperatures while a susceptibility peak at T m in the paramagnetic region—to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Neel and Curie–Weiss temperatures. Below T N the magnetic field dependence of susceptibility, χ ac ( H ), shows two peaks at critical fields H c1 and H c2 . The values of H c1 decrease slightly with temperature while the values of H c2 drop rapidly with temperature. The strong changes of H c2 temperature induced are mainly responsible for a spin frustration of the re-entrant type in the spinel under study.

Journal ArticleDOI
TL;DR: In this paper, the dielectric properties of 1.0 mole% Ta 5+ doped BaTiO3 solid solution were investigated and the transition temperature of this solid solution was ∼110 o C.
Abstract: 5+ were investigated. The dielectric properties were analyzed and the relationship between the properties and structure of doped BaTiO3 was established. The fine-grain and high density of the doped BaTiO3 ceramics resulted in excellent dielectric properties. The dielectric properties of this solid solutions were improved by adding a small amount of dopants. The transition temperature of the 1.0 mole% Ta 5+ doped BaTiO3 solid solution was ∼110 o C with a dielectric constant of 3000 at room temperature. At temperatures above the Curie temperatures, the dielectric constant followed the Curie-Weiss law.


Journal ArticleDOI
TL;DR: The ground state properties of CeCoPO, a homologue of the new high temperature superconductors LnFePnO 1 - x F x, were studied by means of susceptibility, specific heat, and resistivity measurements on polycrystals.
Abstract: The ground state properties of CeCoPO, a homologue of the new high temperature superconductors LnFePnO 1 - x F x , were studied by means of susceptibility, specific heat, and resistivity measurements on polycrystals. The observation of a well defined Curie–Weiss behavior above 230 K with μ eff = 2.9 μ B and a ferromagnetic ordering below T C = 75 K is similar to what was observed in LaCoPO and points to magnetism of the Co-3d electrons. However, the Ce-ions are on the border to magnetism with a Kondo scale of T K ~ 40 K and show an enhanced Sommerfeld coefficient of γ ~ 200 mJ / mol K 2 .

Journal ArticleDOI
TL;DR: In this article, XRD and magnetic susceptibility measurements of cubic SmxY2−xO3 solid solutions (x = 0.2, 0.4, 1.7 and 2.0) were analyzed by the Van Vleck equation and the strength of exchange interactions was estimated from low-temperature susceptibility within 2 −20 K.

Journal ArticleDOI
TL;DR: The magnetic properties and magnetocaloric effects of amorphous and crystalline GdCuAl ribbons have been experimentally studied in this article, and the results show that GdCAl exhibits a superparamagnetic behavior, as demonstrated by the appearance of a maximum in the in-phase linear susceptibility χ 1 " and a minimum in the nonlinear susceptibility " at ∼25 K".