SQUID Measurements Research Articles

Structural, electrical and magnetic properties of nickel manganite obtained by a complex polymerization method

Structural, electrical and magnetic properties of nickel manganite ceramics obtained by sintering fine powders prepared by a complex polymerization method are given in this work. The phase composition of the synthesized material was examined by x-ray powder diffraction (XRPD). Field-emission scanning electron microscopy (FE-SEM) was used to analyze the obtained powder morphology. Scanning electron microscopy (SEM) was used to analyze the microstructure of sintered ceramics. The activation energy of conduction Ea and the coefficient of temperature sensitivity B25/100 were calculated from direct current (DC) resistivity measurements. The magnetization dependence of temperature M(T) and alternating current (AC) susceptibility data obtained from SQUID measurements clearly demonstrate that quadruple magnetic phase transitions can be readily detected at TM1~115K, TM2~105K, TM3~38K and TM4~7K. These findings suggest a novel magnetic transition for nickel manganite at low temperature TM4.

A series of 3D metal organic frameworks based on [24-MC-6] metallacrown clusters: structure, magnetic and luminescence properties.

Four isostructural metal organic frameworks (MOFs), namely [Co6(HipO)6]·6H2O (1), [Mn6(HipO)6]·6H2O (2), [Cd6(HipO)6]·6H2O (3) and [Zn6(HipO)6]·7H2O (4) (H3ipO = 2-hydroxyisophthalic acid), were synthesized and structurally characterized. They have a 3D (4,6)-connected framework based on [24-MC-6] metallacrown clusters ([24-MC-6]-based MOFs). The arrangements of the 24-MC-6 metallacrown SBUs show a regular change indicated by the orientation of their symmetry axes, resulting in a special dense packing mode different from other [24-MC-6]-based MOFs. The analysis of SQUID measurements reveal that compound 1 displays the dominant antiferromagnetic exchanges in 300-10 K between the adjacent Co(II) ions and a ferromagnetic-like behavior at lower temperatures, whereas compound 2 shows an antiferromagnetic interaction between the adjacent Mn(II) ions. Compound 1 exhibits a magnetocaloric effect (MCE) with the resulting entropy change (-ΔS(m)) of 15.20 J kg(-1) K(-1) for ΔH = 50 kG at 6 K, which is the highest value among the cobalt-based MOFs with MCE reported so far. The luminescence properties of compounds 3 and 4 were studied, both of them exhibit photoluminescence in the solid state at room temperature which can be ascribed to intraligand π→π* transitions.

A cubic Fe4Mo4 oxo framework and its reversible four-electron redox chemistry.

The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron-molybdenum oxo compounds. In this context, [(Me3TACN)Fe](OTf)2 was reacted with (nBu4N)2[MoO4], which led to an oxo cluster, [[(Me3TACN)Fe][μ-(MoO4-κ(3)O,O',O″)]]4 (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me3TACN)Fe(2+) and [Mo═O](4+) units in an alternating fashion, being bridged by oxido ligands. The cyclic voltammogram revealed four reversible oxidation waves that are assigned to four consecutive Fe(II) → Fe(III) transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a Fe(II)2Fe(III)2Mo(VI)4 constellation, [Fe4Mo4](TCNQ)2 (2), could be isolated after treatment with TCNQ. The Fe(II)Fe(III)3Mo(VI)4 stage could be reached via oxidation with DDQ or 3 equiv of thianthrenium hexafluorophosphate (ThPF6), giving [Fe4Mo4](DDQ)3 (4) or [Fe4Mo4](PF6)3 (5), respectively. The fully oxidized Fe(III)4Mo(VI)4 state was generated through oxidation with 4 equiv of ThPF6, leading to [Fe4Mo4](PF6)4, which showed a unique behavior: upon storage, one of the [Mo═O](4+) corners inverts, so that the terminal oxido ligand is located in the interior of the cage, leading to the formation of [[(Me3TACN)Fe]4[μ-([MoO4]3[MoO4(MeCN-κN)])-κ(3)O,O',O″)](PF6)4 (7). In this form, the compound could no longer be employed to enter the cyclic voltammogram recorded for 1, 3, and 5 from the oxidized side; no discrete redox events were observed. Compounds 1-3 and 7 were characterized structurally and 1, 3, and 7 additionally by SQUID measurements and Mössbauer spectroscopy. The data reveal a high degree of charge delocalization. (16)O/(18)O exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water-(18)O exchanges its label with all of the oxido ligands (bridging and terminal). This property relates to the ion mobility being held responsible for the activity of iron molybdate catalysts compared to neat MoO3 or Fe2O3.

Excellent piezoelectric and electrical properties of lithium-doped ZnO nanowires for nanogenerator applications

The well-aligned Li-doped zinc oxide nanowires (ZnO NWs) were successfully grown on Si substrates with seed layer by the hydrothermal method. The presence of Li in ZnO NWs is confirmed through EELS spectrum analysis. The acceptor energy level of the Li dopant is estimated to be 93.6meV from the temperature-dependent PL spectra. From the SQUID measurements, the coercivity and saturation magnetization of the Li-doped ZnO NWs at room temperature are found to be 166Oe and 9.64memu/g, respectively. Additionally, the Li-doped ZnO NWFET confirms that the NWs were p-type with a high carrier mobility of 14.59cm2/Vs and an effective hole carrier concentration of 1.13×1017cm−3. The piezoelectric response of the Li-doped ZnO NWs produces much higher piezoelectric output current, voltage, and power than pure ZnO NWs. These results indicated that p-type ZnO NWs are a promising candidate for nanogenerator devices.

Synthesis of Microcrystals of the [Fe(L)(bipy)] Spin Crossover Coordination Polymer in a Poly‐4‐vinylpyridine Matrix

Microcrystals of the spin-crossover coordination polymer [FeL(bipy)] (L=[3,3']-[1,2-phenylenebis(iminoethylidyne)]bis-(2,4-pentanedionato)(2-), bipy=4,4'-bipyridine) have been prepared in a poly(4-vinylpyridine) (P4VP) matrix. This was done by sequential addition of the iron(II) precursor complex and the bridging ligand bipy to a P4VP matrix, and by repetition of this cycle. The obtained composite material was characterized using TEM, SEM, XRPD, and SQUID measurements, and Mössbauer spectroscopy. With repeating cycles, the size of the [FeL(bipy)] crystals in the P4VP matrix increases from submicrometer to micrometer dimensions. A strong dependence on the number of cycles is observed. Above a critical size and concentration, the microcrystals show the same cooperative spin transition as the bulk material. No indication for a gradual spin transition is observed, but the remaining iron centers are either high-spin or low-spin depending on the coordination environment.

Study of the structural and thermal stability of Li0.3Co2/3Ni1/6Mn1/6O2

Thermal and structural stabilities of the delithiated positive electrode material LixCo2/3Ni1/6Mn1/6O2were studied by X-ray diffraction, magnetic and thermogravimetric analysis. In the opposite to the classical electrode materials LiNiO2 and LiCoO2, the structural symmetry (S.G. R-3m) of the starting material LiCo2/3Ni1/6Mn1/6O2 is preserved during the electrochemical cycling with a small variation of the unit cell parameters. Squid measurements evidenced that practically no Ni2+ ions were present in the lithium slab even after the lithium extraction process. For the thermal stability, the highly oxidized phase Li0.3Co2/3Ni1/6Mn1/6O2 was tested. This delithiated phase undergoes only 5.16% weight loss after heating up to 600°C. This weight loss has no effect on the structure symmetry as the starting α-NaFeO2 type structure was preserved during the thermal treatment. The obtained results coupled to the excellent electrochemical features of LiCo2/3Ni1/6Mn1/6O2clearly showits ability to compete with the commercialized cathode materials

Correlation Between Temperature Dependent Photoluminescence and Origin of Room Temperature Ferromagnetism in Li Doped ZnO Nano-Flowers

Nanostructured ZnO is one of the most important dilute magnetic semiconductor materials (DMS) where room temperature ferromagnetism has been achieved by the doping of transition metals. However, transition metal doped ZnO are predominantly n-type materials. ZnO nanomaterial based DMS with p-type majority charge carrier are of great research interest as one can make use of both the magnetic as well as the optical proprieties for opto-electronic and spintronics applications. Recently, it has been reported that nonmagnetic Li doping in ZnO thin films show stable p-type characteristics with room temperature ferromagnetism. Thus there is a great need to understand both the optical and the magnetic properties in Li doped ZnO nanomaterials and a correlation between them. Here we report on the synthesis and studies of room temperature ferromagnetism and optical properties of Li doped ZnO nano-flowers (NFs). The room and low temperature SQUID measurement confirms the ferromagnetism in Li doped ZnO NFs. The temperature dependent photoluminescence measurements shed light on the origin of ferromagnetism. The comparison of the low temperature PL spectra with those of Li doped ZnO NFs shows appearance of distinct new peaks related to acceptor bound exciton and Zn vacancy in Li doped ZnO. Based on these findings from the optical studies we have correlated the origin of room temperature ferromagnetism with stable Zn vacancy in Li doped ZnO NFs. We also discuss the role of Li in stabilizing the Zn vacancy in particular lowering of the formation energy of the latter.

Ferromagnetism and transport properties of the Kondo system Ce4Sb1.5Ge1.5

Ferromagnetic ordering with a small magnetic moment is found below 14K from SQUID measurements for the compound Ce4Sb1.5Ge1.5. The transport characteristics of a number of Ce4Sb3−xTx (T=Ge, Si, Sn, Pb, Al) systems were measured at room temperature, Ce4Sb1.5Ge1.5 demonstrating the highest Seebeck coefficient. Its transport properties (resistivity, thermal conductivity, and thermoelectric power) are experimentally investigated in detail up to low temperatures. The Kondo-lattice behavior of resistivity and its anomaly at the ordering point are found. A comparison with the Wiedemann–Franz law is performed.

Hyperfine interactions in R x Gd1− x Fe3 intermetallics R = Tb, Y

The structural and magnetic properties of rare earth iron intermetallic compounds TbxGd1−xFe3 and YxGd1−xFe3 (x = 0. 0, 0. 1, 0. 2, 0. 4, 0. 5, 0. 6, 0. 8, 1. 0) was studied by X-ray diffraction, the 57Fe Mossbauer effect and SQUID measurements. All investigated compounds crystallize in the rhombohedral PuNi3-type of crystal structure. The investigation of magnetic properties of RxGd1−xFe3 proved their ferrimagnetic behavior. The Curie temperature of the investigated compounds decreases with the increase of R concentration from 721K (GdFe3) to 655K (TbFe3) and 533K (YFe3). The saturation magnetic moment MS in the RxGd 1−xFe3 system increase with x parameter. The Mossbauer spectra are analyzed using four sextets, corresponding to three crystallographically (b, c, h) and four magnetically (b, c, h1, h2) inequivalent sites for iron. The mean hyperfine magnetic field increases with increase of the Gd concentration

Tuning Ground States of Bis(triarylamine) Dications: From a Closed‐Shell Singlet to a Diradicaloid with an Excited Triplet State

Three bis(triarylamine) dications were isolated by using weakly coordinating anions. Their electronic structures in the ground state were investigated by various experiments in conjunction with theoretical calculations. The ground-state electronic structures of these species were tunable by substituent effects, with two of them as closed-shell singlets and one of them as an open-shell singlet in the solid state. The excited state of the latter is thermally accessible, indicated by EPR and SQUID measurements. The work provides a new and stable diradicaloid structure motif with an excited triplet sate.

Tuning Ground States of Bis(triarylamine) Dications: From a Closed‐Shell Singlet to a Diradicaloid with an Excited Triplet State

AbstractThree bis(triarylamine) dications were isolated by using weakly coordinating anions. Their electronic structures in the ground state were investigated by various experiments in conjunction with theoretical calculations. The ground‐state electronic structures of these species were tunable by substituent effects, with two of them as closed‐shell singlets and one of them as an open‐shell singlet in the solid state. The excited state of the latter is thermally accessible, indicated by EPR and SQUID measurements. The work provides a new and stable diradicaloid structure motif with an excited triplet sate.

Magnetic, Magnetocapacitance And Dielectric Properties Of BiFeO3 Nanoceramics

Single phase Bismuth ferrite (BiFeO3, BFO) nanoparticles (particle size ~ 50-100 nm) were synthesized by a novel chemical sol-gel technique. The detailed microstructural analysis has been performed through HRXRD, HRTEM and FESEM techniques. The nanoparticles are found to crystallize with distorted rhombohedral structure having R3c space group. The dielectric constant and tan δ loss are found to vary monotonically with temperature measured at different frequency ranging from 1 kHz to 1 MHz. The M (H) hysteresis behavior at 5 K reveals weak ferromagnetic nature of the BiFeO3 nanoparticles having coercivity (Hc) ~ 720 Oe and magnetization (Ms) ~ 1 emu/g at 5 T from SQUID measurements. The multiferroic character of BFO nanoparticles is confirmed through magnetoelectric response. The typical value of magnetodielectric response is observed to be 0.4% at 4300 Oe at room temperature at a frequency of 1 kHz. All the results suggest that the BFO nanoparticles are technologically very promising as far as magnetoelectric properties are concerned.

Magnetic response of polycrystalline YBaCo4O7+δ synthesized through the physical and chemical route: The role of phase inhomogeneities

Polycrystalline YBaCo4O7+δ samples were obtained through a standard solid state reaction, and their structural, morphological, electrical, and magnetic properties are carefully studied. The X-ray powder diffraction (XRD) patterns showed reflections of a pure hexagonal structure (space group P63mc) with lattice parameters being very close to those reported in the literature. Although XRD analysis showed that the main phase present is 114, the presence of secondary phases could not be ruled out based solely on the XRD characterization. Indeed, sensitive SQUID magnetic measurements showed that the samples were affected by very small quantities of the 112 phase (YBaCo2O5.5), which typically manifests itself through a conspicuous increase in the magnetization at~300K. The results achieved corroborated the predictions concerning the difficulty of stabilizing the 114 phase when synthesized via the standard solid-state reaction. With this in mind, we next attempted to obtain the compound with improved phase purity. In so doing, the YBaCo4O7+δ compound was synthesized through a wet chemistry method based on a citrates route. The XRD patterns recorded for these samples revealed well-defined peaks corresponding to a pure hexagonal structure. More interestingly, SQUID measurements show no sign of features in the M(T) curve at temperatures as low as~80K. This result was consistent with the magnetic behavior observed in YBaCo4O7+δ single-crystals. At temperatures below~80K, a clear feature was observed which seemed to correlate with a transition into an antiferromagnetic state. Isothermal magnetization recorded at 70K showed that field-induced effects manifested themselves through the appearance of a ferromagnetic-like component. This ferromagnetic component may arise from spin canting of the underlying antiferromagnetic state or through field-induced structural transition (at least at local scale). Although a definitive interpretation of the in-field behavior from magnetization data alone is difficult because of the unknown role of the yttrium ion, the results achieved suggest that the magnetic behavior observed in members of the R-114 family is not necessarily linked to the moment of the rare-earth ion, as in case of YBaCo4O7+δ, since the yttrium ion is not magnetic. Beyond this important finding, the experimental results reported in the present paper demonstrate that the tested chemical route is suitable for synthesizing complex, single-phase oxides, such as the YBaCo4O7+δ cobaltate. The success in synthesizing high-purity YBaCo4O7+δ allows one to subtract parasitic effects from the intrinsic magnetic behavior of this challenging system.

Grain size effects on the magnetic properties of ZnxMn1−xFe2O4 nanoferrites

Abstract Single phase nanoferrites bearing the chemical formula ZnxMn1−xFe2O4 (0≤x≤1.0) have been produced under low reaction temperature of 200 °C. The compounds were characterized by X-ray diffraction, Mossbauer and SQUID measurements. The particle size varies between 5 nm and 14 nm. The particle size dependence of the magnetic properties is investigated. Our results indicate spin-glass behavior at temperatures between 100 K and 200 K. The variation of Zn concentration has also significant effects on the structural and magnetic properties. The complex variation of lattice parameter with x is explained on the basis of redistribution of Zn atoms in both tetrahedral (A) and octahedral (B) sites. The 57Fe Mossbauer effect spectra show paramagnetic spin state in all the compounds.

Tuning of the ground state in electron doped anthracene.

High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation.

Structural rearrangements induced by acid–base reactions in metal carbonyl clusters: the case of [H3−nCo15Pd9C3(CO)38]n−(n = 0–3)

The new bimetallic [HCo15Pd9C3(CO)38](2-) tri-carbide carbonyl cluster has been obtained from the reaction of [H2Co20Pd16C4(CO)48](4-) with an excess of acid in CH2Cl2 solution. The mono-hydride di-anion can be reversibly protonated and deprotonated by means of acid-base reactions leading to closely related [H(3-n)Co15Pd9C3(CO)38](n-) (n = 0-3) clusters. The crystal structures of the three anionic and the neutral clusters have been determined as their H3Co15Pd9C3(CO)38·2thf, [NEt4][H2Co15Pd9C3(CO)38]·0.5C6H14, [NMe3(CH2Ph)]2[HCo15Pd9C3(CO)38]·C6H14 and [NEt4]3[Co15Pd9C3(CO)38]·thf salts. They are composed of a Pd9(μ3-CO)2 core stabilised by three Co5C(CO)12 organometallic fragments. The poly-hydride nature of these clusters has been indirectly inferred via chemical, electrochemical and magnetic measurements. Besides, cyclic voltammetry shows that the [H(3-n)Co15Pd9C3(CO)38](n-) (n = 1-3) anions are multivalent, since they undergo two or three reversible oxidations. SQUID measurements of [HCo15Pd9C3(CO)38](2-) indicate that this even electron cluster is paramagnetic with two unpaired electrons, giving further support to its hydride nature. Finally, structural studies show that the Pd9 core of [H(3-n)Co15Pd9C3(CO)38](n-) (n = 0,1) is a tri-capped octahedron, which becomes a tri-capped trigonal prism in the more charged [H(3-n)Co15Pd9C3(CO)38](n-) (n = 2,3) anions. Such a significant structural rearrangement of the metal core of a large carbonyl cluster upon protonation-deprotonation reactions is unprecedented in cluster chemistry, and suggests that interstitial hydrides may have relevant stereochemical effects even in large carbonyl clusters.

Effects of surfactants on the magnetic properties of iron oxide colloids

Iron oxide nanoparticles are having been extensively investigated for several biomedical applications such as hyperthermia and magnetic resonance imaging. However, one of the biggest problems of these nanoparticles is their aggregation.Taking this into account, in this study the influence of three different surfactants (oleic acid, sodium citrate and Triton X-100) each one with various concentrations in the colloidal solutions stability was analyzed by using a rapid and facile method, the variation in the optical absorbance along time.The synthesized nanoparticles through chemical precipitation showed an average size of 9nm and a narrow size distribution. X-ray diffraction pattern and Fourier Transform Infrared analysis confirmed the presence of pure magnetite. SQUID measurements showed superparamagnetic properties with a blocking temperature around 155K. In addition it was observed that neither sodium citrate nor Triton X-100 influences the magnetic properties of the nanoparticles. On the other hand, oleic acid in a concentration of 64mM decreases the saturation magnetization from 67 to 45emu/g. Oleic acid exhibits a good performance as stabilizer of the iron oxide nanoparticles in an aqueous solution for 24h, for concentrations that lead to the formation of the double layer.

Dispersion Force Stabilized Two-Coordinate Transition Metal–Amido Complexes of the −N(SiMe3)Dipp (Dipp = C6H3-2,6-Pri2) Ligand: Structural, Spectroscopic, Magnetic, and Computational Studies

A series of high spin, two-coordinate first row transition metal-amido complexes, M{N(SiMe3)Dipp}2 {M = Fe (1), Co (2), or Ni (3); Dipp = C6H3-2,6-Pr(i)2} and a tetranuclear C-H activated chromium amide, [Cr{N(SiMe2CH2)Dipp}2Cr]2(THF) (4), were synthesized by reaction of their respective metal dihalides with 2 equiv of the lithium amide salt. They were characterized by X-ray crystallography, electronic and infrared spectroscopy, SQUID magnetic measurements, and computational methods. Contrary to steric considerations, the structures of 1-3 display planar eclipsed M{NSiC(ipso)}2 arrays and short M-N distances. DFT calculations, corrected for dispersion effects, show that dispersion interactions involving C-H-H-C moieties likely stabilize the structures by 21.1-29.4 kcal mol(-1), depending on the level of the calculations employed. SQUID measurements confirm high spin electron configurations for all the complexes and substantial orbital contributions for 1 and 2.

On the mechanism of the interactions of neutrons and gamma radiation with nuclear graphite—Implications to HTGRs

Nuclear-grade varieties of graphite being considered for use in high-temperature gas reactors were exposed to gamma radiation (up to 87MGy) and to fast neutrons (up to 5.4×1017n/cm2 in air, 8.8×1011n/cm2 in water-saturated helium). Results of XPS measurements indicated that gamma or mixed-field irradiation initially enhances the oxygen content in the surface region of the graphite, but this content decreases at higher doses, probably due to decomposition of surface CO complexes. Results of EPR measurements showed that gamma irradiation at low doses causes a decrease in the concentration of unpaired spins, but higher doses cause this concentration to rise. SQUID measurements of magnetic susceptibility support the EPR findings. At the dose range explored in this study, no structural changes were observed using XRD and Raman spectroscopy. In general, no significant differences were observed among the five varieties of graphite with respect to the effects of irradiation. Impurity analysis by means of GDMS and ICP-AES showed that impurity concentrations that concentrations of impurities, particularly those of neutron-absorbing impurities, were within the range specified for high-purity nuclear graphite. Activation relevant impurity concentrations, too, were very low.

Pentanuclear Heterometallic {Ni2Ln3} (Ln = Gd, Dy, Tb, Ho) Assemblies. Single-Molecule Magnet Behavior and Multistep Relaxation in the Dysprosium Derivative

The reaction between Ln(III) chloride and NiCl2·4H2O salts in presence of a multidentate sterically unencumbered ligand, (E)-2,2'-(2-hydroxy-3-((2-hydroxyphenylimino)methyl)-5-methylbenzylazanediyl)diethanol (LH4) leads to the synthesis of four isostructural pentanuclear hetereometallic complexes [Ni2Dy3(LH)4]Cl (1), [Ni2Gd3(LH)4]Cl (2), [Ni2Tb3(LH)3(LH2)]Cl2 (3), [Ni2 Ho3 (LH)3 (LH2)]Cl2 (4) with unprecedented topology. Here the two compounds 1 are 2 are monocationic and crystallize in chiral space group, P2(1)2(1)2(1) whereas compounds 3 and 4 are dicationic and crystallize in achiral space group P2(1)/n. The total metal framework, {Ni2Ln3} unit is held by four triply deprotonated ligands [LH](3-) in 1 and 2 whereas in case of 3 and 4 three triply deprotonated [LH](3-) and one doubly deprotonated [LH2](2-) ligands are involved. In these complexes both the lanthanide ions and the nickel(II) ions are doubly bridged and the bridging is composed of oxygen atoms derived from either phenolate or ethoxide groups. The analysis of SQUID measurements reveal a high magnetic ground state and a slow relaxation of the magnetization with two relaxation regimes for 1. For the thermally activated regime we found an effective energy barrier of U(eff) = 85 K. Micro Hall probe loop measurements directly proof the single-molecule magnet (SMM) nature of 1 with a blocking temperature of T(B) = 3 K and an open hysteresis for sweep rates faster than 50 mT/s.