Curie-Weiss Paramagnetic Behavior Research Articles

Structural and magnetic study of Yb3+ in the perovskites Sr2YbMO6 (M=Nb, Ta, Sb)

The compounds Sr2YbNbO6, Sr2YbTaO6 and Sr2YbSbO6 have been prepared using solid state methods by heating pelleted reagents in air at temperatures up to 1400°C. Rietveld refinement against room temperature neutron powder diffraction data show that all three compounds crystallise with a cation-ordered variant of the perovskite structure in the P21/n space group. Complete cation ordering occurs between M5+ and Yb3+ over two octahedrally-coordinated sites in the structure and all compounds are stoichiometric in oxygen. The Sb–O bond lengths are similar to related perovskite compounds but differ slightly from those indicated by bond valence sums. Magnetic susceptibility data resemble Curie–Weiss paramagnetic behaviour, but can be better understood as arising from the effect of the octahedral crystal field on the 2F5/2 ground state of Yb3+ leading to a temperature dependent magnetic moment on this ion below 100K.

Synthesis and crystal structures of RE7Zn(21+x)Si(2-x) [RE = Ce, Pr, and Nd; 0.09 (1) < x < 0.95 (1)

The focus of this paper is on the synthesis and crystal structures of three Zn-rich compounds with the general formula RE7Zn(21+x)Si(2-x), where RE = Ce [x = 0.95 (1); heptacerium docosazinc silicon], Pr [x = 0.09 (1); heptapraseodymium henicosazinc disilicon], and Nd [x = 0.53 (1); heptaneodymium docosazinc silicon]. The compounds were obtained by high-temperature reactions, using the respective elements as starting materials. The structures were determined by single-crystal X-ray diffraction. The title compounds crystalize in the orthorhombic space group Pbam (No. 55, Pearson symbol oP60) and are isostructural with about a dozen RE7Zn(21+x)Tt(2-x) (RE = La-Nd; Tt = Ge, Sn, and Pb) compounds previously reported by our group. The results from the present refinements confirm the previously published data on RE7Zn(21+x)Si(2-x) (RE = La and Ce; x ≃ 1.45) [Malik et al. (2013). Intermetallics, 36, 118-126]. Additionally, magnetic susceptibility measurements on the corresponding bulk samples show Curie-Weiss paramagnetic behavior from 5 to 300 K, consistent with RE(3+) ground states and local-moment magnetism due to the core 4f electrons.

Structure and physical properties of EuTa2O6 tungsten bronze polymorph

A tetragonal tungsten bronze (TTB) polymorph of EuTa2O6 was prepared and analyzed. EuTa2O6 crystallizes in the centrosymmetric Pnam space group (with unit cell: a = 12.3693, b = 12.4254, and c = 7.7228 Å) isomorphous with orthorhombic β-SrTa2O6. In contrast to early reports, we see no evidence of deviation from paramagnetic Curie-Weiss behavior among the Eu2+ 4f7spins in EuTa2O6 down to 2 K. Dielectric constant shows a broad peak at ca. 50 K with dielectric dispersion resembling diffuse phase transition. The relaxation time, however, follows a simple (non-freezing) thermally activated process with an activation energy of 92 meV and an attempt frequency of f0 = 5.79 × 1012 Hz. A thermal conductivity of EuTa2O6 shows a low-temperature (T ≈ 30 K) “plateau” region reminiscent of a glass-like behaviour in Nb-based TTB compounds. This behaviour can be attributed to the loosely bound Eu2+ ions occupying large tricapped trigonal prismatic sites in the EuTa2O6 structure.

Antiferromagnetic ordering in NdAuGe compound

The compound NdAuGe was investigated by means of electrical resistivity, ρ(T), magnetic susceptibility, χ(T), magnetization, σ(μ0H), and specific heat, Cp(T), measurements. Powder X-ray diffraction studies confirm a hexagonal LiGaGe-type structure with space group P63mc (No. 186). ρ(T) data show normal metallic behaviour and a tendency toward saturation at higher temperatures. The low temperature ρ(T) data indicate a phase transition around 3.8 K. The low field dc χ(T) data show an antiferromagnetic anomaly associated with a Néel temperature at TN = 3.7 K close to the phase transition observed in ρ(T) results. At higher temperatures, χ(T) follows the paramagnetic Curie-Weiss behaviour with an effective magnetic moment μeff=3.546(4) μB and a paramagnetic Weiss temperature of θp=−6.1(4) K. The value obtained for μeff is close to the value of 3.62 μB expected for the free Nd3+-ion. σ(μ0H) shows a linear behaviour with applied field up to 3 T with an evidence of metamagnetic behaviour above 3 T. Cp(T) confirms the magnetic phase transition at TN = 3.4 K. The 4f-electron specific heat indicates a Schottky-type anomaly around 16.5 K with energy splitting Δ1=25.8(4) K and Δ2=50.7(4) K of the Nd3+ (J = 9/2) multiplet, that are associated with, respectively, the first and second excited states of the Nd3+-ion.

Structural and Magnetic Study of Order–Disorder Behavior in the Double Perovskites Ba2Nd1–xMnxMoO6

The synthesis and structural and magnetic characterization of the site-ordered double perovskites, Ba2Nd1-xMnxMoO6, 0 < x ≤ 1, are reported in order to show the effect of doping Jahn-Teller active, S = 1/2, Mo(5+) into the structure of Ba2MnMoO6, which exhibits anomalous long-range antiferromagnetic order. Rietveld refinements against room temperature neutron powder diffraction data indicate that the tetragonal distortion present in the Ba2NdMoO6 end member persists to x ≤ 0.3. This is predominantly manifested as a tilting of the MO6 octahedra, and there is no evidence of any structural phase transitions on cooling to 1.5 K. For x > 0.3, no deviation from the ideal cubic Fm3̅m symmetry is observed. Furthermore, dc-susceptibility measurements confirm that Mn(2+) is being doped onto the Nd(3+) site, and the associated oxidation of Mo(5+) to Mo(6+). For all compositions, the Curie-Weiss paramagnetic behavior above 150 K indicates negative Weiss constants that range from -24(2) and -85(2) K. This net antiferromagnetic interaction is weakest when x ≈ 0.5, where the disorder in cation site occupancy and competition with ferromagnetic interactions is the greatest. Despite these strong antiferromagnetic interactions, there is no evidence in the dc-susceptibility of a bulk cancellation of spins for x > 0.05. Low-temperature neutron diffraction measurements indicate that there is no long-range magnetic order for 0.1 ≤ x < 0.9. Ba2Nd0.10Mn0.90MoO6 exhibits additional Bragg scattering at 2 K, indicative of long-range antiferromagnetic ordering of the Mn(2+) cations, with a propagation vector k = (1/2, 1/2, 1/2). The scattering intensities can be modeled using a noncollinear magnetic structure with the Mn(2+) moments orientated antiferromagnetically along the four different ⟨111⟩ directions.

Synthesis, properties and phase transitions of pyrochlore- and fluorite-like Ln2RMO7 (Ln = Sm, Ho; R = Lu, Sc; M = Nb, Ta)

We have studied the new compounds with fluorite-like (Ho2RNbO7 (R=Lu, Sc)) and pyrochlore-like (Sm2ScTaO7) structure as potential oxide ion conductors. The phase formation process (from 1200 to 1600°C) and physical properties (electrical, thermo mechanical, and magnetic) for these compounds were investigated. Among the niobate materials the highest bulk conductivity is offered by the fluorite-like Ho2ScNbO7 synthesized at 1600°C: 3.8×10−5S/cm at 750°C, whereas in Sm system the highest bulk conductivity, 7.3×10−6S/cm at 750°C, is offered by the pyrochlore Sm2ScTaO7 synthesized at 1400°C. In Sm2ScTaO7 pyrochlore we have observed the first-order phase transformation at ∼650–700°C is related to rearrangement process in the oxygen sublattice of the pyrochlore structure containing B-site cations in different valence state and actually is absent in the defect fluorites.The two holmium niobates show Curie–Weiss paramagnetic behavior, with the prevalence of antiferromagnetic coupling. The magnetic susceptibility of Sm2ScTaO7 is a weak function of temperature, corresponding to Van Vleck paramagnetism.

Structural and Magnetic Properties of Dilute Magnetic Oxide Based on Nanostructured Co-Doped Anatase TiO2 (Ti1–xCoxO2−δ)

Nanostructured Co-doped anatase TiO2 (Ti1–xCoxO2−δ) samples were prepared and studied with particular emphasis on their compositional, structural, and magnetic properties. A detailed microstructural analysis was carried out to investigate the nature of the Co incorporation into the anatase TiO2 matrix. By combining different techniques, we confirmed the replacement of Ti4+ by Co2+ ions in the anatase TiO2 structure. Neither segregated secondary phases nor Co-rich nanocrystals were detected. Co doping was found to introduce oxygen vacancies into the system by means of a charge-compensation process. Superconducting quantum interference device magnetometry demonstrated paramagnetic Curie–Weiss behavior with antiferromagnetic interactions even in the presence of a high density of oxygen vacancies. The fitting of the M(H) curves in the limits of low and high temperatures enable the fractions of isolated and antiferromagnetically coupled Co ions to be extracted. We discuss the observed magnetic behavior of our ...

Structural, Electrical and Magnetic Investigations of Conductive ZnO: Co Thin Films Deposited by DC Reactive Magnetron Co-Sputtering

Zn1-xCoxO (0 &lt; x &lt; 14.6) diluted magnetic semiconductor coatings have been obtained by reactive DC magnetron sputtering of metallic Zn and Co targets at high pressure and high temperature. The coatings structural properties have been investigated by X-ray diffraction. All as-deposited coatings are crystalized in ZnO structure and neither traces of metallic nor oxide Co-rich clusters were detected in the films. The temperature dependence of the magnetization shows a paramagnetic Curie-Weiss behavior with antiferromagnetic interaction due to the associated spins and simple paramagnetic curie behavior of isolated free spins.

Structure-Property Relationships in Pr&lt;SUB&gt;1&lt;/SUB&gt;&lt;SUB&gt;-&lt;/SUB&gt;&lt;SUB&gt;&lt;i&gt;x&lt;/i&gt;&lt;/SUB&gt;Sr&lt;SUB&gt;1&lt;/SUB&gt;&lt;SUB&gt;+&lt;/SUB&gt;&lt;SUB&gt;&lt;i&gt;x&lt;/i&gt;&lt;/SUB&gt;CoO&lt;SUB&gt;4&lt;/SUB&gt;, (0 ≤ &lt;i&gt;x&lt;/i&gt; ≤ 0.40)

The structural, magnetic and electric properties of Pr1−xSr1+xCoO4 layered perovskite have been investigated systematically over the range of doping, 0 ≤ x ≤ 0.40. The Rietveld refinements of X- ray powder diffraction (XRD) patterns at room temperature indicate that the samples crystallize in the K2NiF4-type structure with group symmetry I4/mmm. The samples of 0 ≤ x < 0.20 show Curie-Weiss paramagnetic behavior. With increasing x (0.20 ≤ x ≤ 0.40), three magnetic transition temperatures can be identified. The ground state exhibits a spin glass (SG) phase below the spin glass transition temperature TSG . Above, there is a mixture of ferromagnetic (FM) and antiferromagnetic (AFM) short-range order for TSG< T < TM with TM denoting the temperature of the corresponding anomaly in the magnetic susceptibility data. The third state can be attributed to a Griffiths phase for TM < T <TG with TG being the transition temperature between the Griffiths phase and the paramagnetic high temperature state. While the SG phase was suppressed with increasing applied field, both TM and TG shifted towards lower temperatures. No indications for a single pure ferromagnetic phase were observed even for applied magnetic fields up to 60 kOe. The electrical resistivity of the samples shows either thermally activated semiconducting behavior or 2D variable range hopping. The results are summarized in (x− T) phase diagram and discussed with respect to structure-property relationships.

Ternary aluminides R0.67Ni2Al6 (R=Sc, Y, Gd–Lu) with partly disordered structures

Ten rare-earth nickel aluminides R0.67Ni2Al6 (R=Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were synthesized by arc melting and their crystal structures studied by X-ray powder diffraction. They crystallize in the hexagonal structure type Yb0.67Ni2Al6: Pearson symbol hP11-2.33, space group P-6m2, a=4.18904(2), c=9.09816(6)Å, which can be decomposed into three kinds of layer. The sequence of layers along [001] is Yb0.67Al–NiAl2–Al–NiAl2, but stacking faults appear due to relative shifts of consecutive Yb0.67Al layers containing two rare-earth metal atoms for one Al-atom triangle. An ordered superstructure is the type ErNi3Al9 (Pearson symbol hR78, space group R32, a=3a0, c=3c0), refined on single crystals.The physical properties of the R0.67Ni2Al6 (R=Tb, Er, Tm, Yb, and Lu) compounds were determined by means of magnetic and electrical transport measurements. All these materials but Lu0.67Ni2Al6 exhibit Curie–Weiss paramagnetic behavior due to the trivalent R ions, and order magnetically at low temperatures. They show good metallic conductivity. The Yb-containing compound is a dense Kondo system with distinct features in the paramagnetic state of the interplay between Kondo and crystalline electric field interactions.

Thermal stability and the magnetic properties of hybrid vanadium oxide-tetradecylamine nanotubes

Vanadium oxide nanotubes (NTs) were synthesized by the sol-gel method followed by a long-term hydrothermal treatment. The obtained nanotubes have a multiwall structure, and 70% of vanadium ions are in the V4+ state. This percentage was derived by evaluating three components of the magnetic susceptibility; namely, (i) the paramagnetic Curie-Weiss behavior, (ii) antiferromagnetic dimers, and (iii) magnetic trimers. The as-made NTs were annealed in situ in the cavity of the electron paramagnetic resonance (EPR) spectrometer. The line shape changes irreversibly at 390 K, and the EPR susceptibility presents an anomaly at 425 K. These changes are interpreted as a partial oxidation of the V4+ ions and consequently a decrease in the concentration of the magnetic species. The quantification of the V4+ ions of the annealed NTs reveals a diminution to 39% of V4+, a weakening of the Curie-Weiss and antiferromagnetic dimers contributions, and the suppression of magnetic trimers. Vibrational studies confirm the decrease of V4+ amount.

Studies on solid solutions based on layered honeycomb-ordered phases P2-Na2M2TeO6 (M=Co, Ni, Zn)

Three complete solid solutions between the layered phases P2-Na2M2TeO6 (M=Co, Ni, Zn) have been prepared by conventional solid state method and investigated through X-ray diffraction, magnetism and optical measurements. All compositions are characterized by a M2+/X6+ honeycomb ordering within the slabs and crystallize in a hexagonal unit cell. However, a structural transition based on a different stacking is observed as nickel (space group P63/mcm) is substituted by zinc or cobalt (space group P6322). All compositions exhibit a paramagnetic Curie–Weiss behavior at high temperatures; and the magnetic moment values confirm the presence of Ni2+ and/or Co2+ cations. The low-temperature antiferromagnetic order of Na2Ni2TeO6 and Na2Co2TeO6 is suppressed by zinc substitution. The color of the obtained compositions varies from pink, to light green and white when M=Co, Ni, Zn, respectively.

Magnetic properties of Co1+yAl2−yO4–SiO2 nanocomposites synthesized by sol–gel method

Nanocomposites made of magnetic spinel oxide Co1+yAl2−yO4 particles dispersed in an amorphous SiO2 matrix were synthesized by a sol–gel technique. Samples with various mass fractions of Co1+yAl2−yO4 in composite x, and with various Co concentrations in spinel y were studied. The grain sizes were below 100 nm with a large size distribution. The paper represents thorough investigation of magnetic properties, but special attention was given to peculiar effects and low temperature magnetic frozen states. Magnetic measurements reveal multiple magnetic ground states at the lowest temperature. The frequency dependent ac susceptibility measurements indicate spin-glass behavior of these ground states at low temperatures. The samples, with higher concentration x of Co1+yAl2−yO4 in SiO2, showed very similar Curie–Weiss paramagnetic behavior with antiferromagnetically interacting Co ions (θ ≈ −100 K). Other samples, with lower x, showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of θ and consequently lower exchange constant.

Magnetic and magnetocaloric properties of Gd3−xTbxGa5O12 (x=0, 1, 2, 3) garnets

The structure and magnetocaloric properties of Tb substituted Gd3Ga5O12 garnets have been investigated. The structural refinement of X-ray diffraction patterns show that Gd3−xTbxGa5O12 (x=0, 1, 2, 3) are isostructural, with the symmetry of a cubic space group Ia3¯d. The temperature dependence of magnetization studies indicate that all the samples follow Curie–Weiss paramagnetic behavior above 25K and the effective magnetic moment increases with Tb concentration. The low temperature magnetization studies suggest that there exists a field dependent antiferromagnetic ordering of rare earth ions in Tb substituted Gd3Ga5O12, which in turn leads to a lower magnetic moment in Gd3−xTbxGa5O12 (x=1, 2, 3), compared to that of Gd3Ga5O12. Magnitude of field corresponding to the antiferromagnetic ordering is found to be dependent on temperature as well as on Tb concentration. Magnetocaloric effect analysis confirms that Tb substitution enhances the magnetic entropy change at higher temperatures and Gd3−xTbxGa5O12 (x=1, 2, 3) are better candidates than Gd3Ga5O12 for low fields and low temperature magnetic refrigeration applications.

Co2P nanostructures by thermal decomposition: phase formation and magnetic properties

We report a controlled synthesis of Co2P nanostructures. The products showing Curie–Weiss paramagnetic behavior have an orthorhombic structure with novel morphology. Nanoneedles with a tip of less than 10 nm homogeneously grew out of the center. The effects of temperature and the morphological evolutions of the intermediates are investigated.

Synthesis and Characterization of CaPd&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; Crystals

A new method for the crystal growth of alkaline-earth palladate CaPd3O4 was developed. The crystals were synthesized on a voltage-applied electrode in a molten chloride solvent. The maximum length of the crystal was about 1.5 mm. The X-ray diffraction data were refined well by assuming a cubic structure of the space group Pm n, and the lattice constant a was 5.7471 (10) ?. The temperature dependence of the resistivity showed semiconductor-like characteristics with a very small activation energy Ea of 0.45 meV at low temperatures, and the resistivity at 300 K was 0.1 cm. The temperature dependence of the molar magnetic susceptibility showed the Curie-Weiss paramagnetic behavior with a small molar Curie constant Cmol of 5.0(1) × 10-3 emu K/(mol Oe), indicating the existence of localized spin moments.

Anomalies of inverse direct current susceptibility in spin-chain compounds Ca3Co2-xMnxO6

Various types of anomalies of inverse dc susceptibility (H/M) have been observed in spin-chain compounds Ca3Co2-xMnxO6. For x = 1.0, the H/M curve exhibits a field-dependent downturn from the paramagnetic Curie-Weiss behavior, signaling the presence of a Griffiths-like cluster phase. A slight decrease of x leads to a rapid suppression of the Griffiths-like singularity, followed by an appearance of field-independent, non-Griffiths-like behavior at x = 0.92. This feature persists until x = 0.33, at and below which the H/M curve presents an unambiguous, field-independent upturn from the Curie-Weiss law. These Mn-doping-dependent anomalies of H/M curves are associated with complex competing magnetic interactions.

Temperature and concentration quenching of mid-IR photoluminescence in iron doped ZnSe and ZnS laser crystals

Concentration dependences of the mid-IR kinetic of luminescence at 5 E↔ 5 T 2 transition in Fe:ZnSe and Fe:ZnS laser samples were studied in 14–300 K temperature ranges. Radiation lifetime in Fe:ZnSe samples measured using low doped samples with iron concentration 0.1×10 18 cm −3 was estimated to be 57 μs. The magnetic susceptibility for higher doped ( C Fe=38 and 112×10 18 cm −3) Fe:ZnSe samples was found to consist of a paramagnetic Curie–Weiss behavior arising from the weakly interacting Fe 2+ ions and a diamagnetic ZnSe contribution plus a temperature-independent, field-dependent contribution possibly arising from very small amounts of aggregated Fe.

Metamagnetism and long range ordering in μ-1,3 bridging transition metal thiocyanato coordination polymers

Reaction of M(SCN) 2 (M = Mn, Fe, Ni) with pyridine (pyr) in aqueous solution at room temperature leads to the formation of the literature known pyridine-rich 1:4 compounds of composition [M(SCN) 2(pyridine) 4] (M = Mn ( 1-Mn), Fe ( 1-Fe), Ni ( 1-Ni)) reported recently. On heating, the 1:4 compounds decompose into their corresponding pyridine-deficient 1:2 compounds of composition [M(SCN) 2(pyridine) 2] n (M = Mn ( 2-Mn), Fe ( 2-Fe), Ni ( 2-Ni)) which decompose on further heating. In the crystal structure of the pyridine-deficient 1:2 compounds the metal cations are coordinated by four N-atoms of two pyridine ligands and two N-bonded thiocyanato anions, each in mutually trans orientation, and by two S-atoms of two adjacent thiocyanato anions in a slightly distorted octahedral geometry. The thiocyanato anions bridge the metal cations into one-dimensional (1D) polymeric chains. IR spectroscopic investigations on the pyridine-deficient 1:2 compounds are in agreement with the presence of μ-1,3 bridging thiocyanato anions. Magnetic measurements of the pyridine-rich 1:4 compounds show only Curie–Weiss paramagnetism whereas for the pyridine-deficient 1:2 compounds an antiferromagnetic ordering for [Mn(NCS) 2(pyridine) 2] n ( 2-Mn) and metamagnetic behavior for [Ni(NCS) 2(pyridine) 2] n ( 2-Ni) is found. For [Cu(NCS) 2(pyridine) 2] n ( 2-Cu) Curie–Weiss paramagnetic behavior is observed. [Fe(NCS) 2(pyridine) 2] n ( 2-Fe) shows metamagnetic behavior, which was already investigated but remeasured for a more detailed characterization.

K 3Ln[OB(OH) 2] 2[HOPO 3] 2 ( Ln=Yb, Lu): Layered rare-earth dihydrogen borate monohydrogen phosphates

Two isotypic layered rare-earth borate phosphates, K 3 Ln[OB(OH) 2] 2[HOPO 3] 2 ( Ln=Yb, Lu), were synthesized hydrothermally and the crystal structures were determined by single-crystal X-ray diffraction ( R3̄, Z=3, Yb: a=5.6809(2) Å, c=36.594(5) Å, V=1022.8(2) Å 3, Lu: a=5.6668(2) Å, c=36.692(2) Å, V=1020.4(1) Å 3). The crystal structure can be described in terms of stacking of Glaserite-type slabs consisting of LnO 6 octahedra interlinked by phosphate tetrahedra and additional layers of [OB(OH) 2] – separated by K + ions. Field and temperature dependent measurements of the magnetic susceptibility of the Yb-compound revealed Curie–Weiss paramagnetic behavior above 120 K ( μ eff=4.7 μ B). Magnetic ordering was not observed down to 1.8 K.