SQUID Measurements Research Articles

Controlled synthesis and self-assembly of dendrite patterns ofFe3O4 nanoparticles

A method for the growth and self-assembly ofFe3O4 nanoparticles that is template assisted, as well as gas diffusion and surface tensioncontrolled, has been developed at room temperature. Well-defined dendrite patterns ofFe3O4 nanoparticles wereobtained upon ion (Fe3+/Fe2+) entrapment in a polyethylene glycol solution followed byNH3 gas exposure on the surface of an aqueous solution on the glass substrate. During the formation ofFe3O4 nanoparticles, thediffusion of volatile NH3 limits the hydrolysis rate of the molecular precursor and catalyzes slow formation. Thetemplate and surface tension also provided significant driving forces to promotethe formation of dendrite patterns and influence the nature of the pattern. TheFe3+/Fe2+ concentration was varied in order to see the affects on the template molecular weight.Scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-raydiffraction (XRD), and SQUID measurements were used to characterize the final product.The derived patterned structure can be tailored by a simple combination of the physicaland chemical procedure, which provides a new opportunity for obtaining a controllablepattern of nanoparticles.

Synthesis of magnetite nanoparticles via a solvent-free thermal decomposition route

Superparamagnetic nanoparticles have been widely applied in various bio-medical applications. To date, it is still a challenge to synthesize nanosized Fe 3O 4 particles with controlled size and distribution. In this paper, a novel solvent-free thermal decomposition method is reported for synthesizing Fe 3O 4 nanoparticles. Size and morphology of the nanoparticles are determined by TEM while the structure of the nanoparticles is identified by FTIR, XPS and TGA measurements. Magnetic properties of the obtained particles are determined using VSM and SQUID measurement. The particle size of the Fe 3O 4 can be tailored by adjusting either reaction temperature or time. When the reaction temperature is increased to 330 °C and the reaction time is extended to 4 h, the average particle size of the obtained nanoparticles is ∼9 nm, while Ms value reaches ∼76 emu/g. The as synthesized Fe 3O 4 nanoparticles show well-established superparamagnetic properties with the blocking temperature at around 100 K.

Room temperature ferromagnetism in Mn-doped dilute ZnO semiconductor: An electronic structure study using X-ray photoemission

There are several confronting reports of room temperature ferromagnetic (FM) ordering in bulk as well as thin films of dilutely doped or even some undoped semiconductors. We have synthesized and characterized dilute Mn-doped (2 and 4%) ZnO pellets. SQUID measurements confirm that the 2% Mn sample shows the FM ordering above the room temperature and the FM contribution coming mainly from the bulk. However, the ordering gets completely quenched for 4% Mn doping. Upon cooling down, the 2% Mn-doped sample shows further enhancement in the magnetic properties while the 4% sample did not show any FM ordering down to 5 K. The powerful X-ray photoemission spectroscopy (XPS) was employed to compare the electronic structure of these two samples. The XPS results show that the manganese shifts toward the higher valence state upon Mn doping while there is no change in the zinc and oxygen valence. The atomic concentration of divalent Mn state is found to be dominant in the ferromagnetic sample. For the non-ferromagnetic sample, a larger contribution of higher oxidation Mn states is present that is correlated to the suppressed ferromagnetism. Interestingly, the oxygen content is also found to be higher in the 4% Mn sample than that in the 2% Mn sample that has been attributed to the charge neutrality of the samples. The present study provides evidence that the magnetization originates neither from any precipitating secondary phase nor from the oxygen content but the Mn 2+ state plays a significant role for the FM properties in the Mn-doped ZnO system.

Non-destructive assessment of the critical current density in large superconductingpellets

We present a new method of rapidly determining the local critical current density(Jc) in large superconducting pellets without destroying the sample. We show that magnetoscandata averaged over a certain small sample area can be mapped directly onto absolute values ofJc. This conclusion and the corresponding calibration curve are based on results frommagnetoscan and from SQUID measurements of the same sample regions, which were foundto show an excellent correlation. Numerical simulations help to extend the calibrationcurve, to improve the resolution of the method and to reduce possible errors.

On the Question of Ferromagnetism in Proton and He-Irradiated Carbon

Carbon-based materials are the promising candidates for future applications in novel electronics. Carbon nanotube transistors and logic gates, conducting carbon-based polymers have been already demonstrated. In addition some forms of pure carbon should reveal magnetic properties, which in principle might be use in carbon-based spintronics. There are some theoretical predictions about carbon nano-stripes, schwarzite structures, H-doped graphite, in which electron spins on π-orbitals or broken sp 3 -bonds have (in some cases) ferromagnetic alignment. The experiments seeking for ferromagnetism were performed but many of them have failed due to the presence of ferromagnetic impurities (mostly iron). Nevertheless some of the experiments demonstrated ferromagnetism in pure carbon, particularly in carbon nano-foam [1] and proton-irradiated highly oriented pyrolitic graphite (HOPG) [2]. In 2003 Esquinazi used a MeV proton beam and discovered that it trigged ferromagnetism of the HOPG sample. After irradiation the hysteresis loop was observed over the irradiated area using Magnetic Force Microscopy and SQUID measurements. Such magnetic system seems to be very interesting for both: basic science and possible application points of view. However, hardly any other experimental group has confirmed such experimental observation. This situation is embarrassing, because it is not clear if nobody tried to verify the discoveries of Esquinazi et al. or nobody has succeeded in this verification. And since the lack of the phenomenon is difficult to be publish in any paper (vide the rumor of “cold fusion” in 90-ties) there are no information about the efforts of other groups. We have tried to repeat this discovery of the gropup of Esquinazi et al. We have used He + and H + beams focused on graphite sample. The amount of charge deposited in the sample was comparable to the amount of charge used by Esquinazi. Magnetic measurements were performed in SQUID magnetometer. The magnetization of the samples before and after irradiation was compared. Unfortunately we did not observe any ferromagnetic enhancement of magnetization of our irradiated samples. Even if experiment was not the same as Esqinazi’s one, we can exclude some of the mechanisms of ferromagnetism proposed by Esquinazi.

Coupling Agent Effect on Magnetic Properties of Functionalized Magnetite-Based Nanoparticles

The effect of surface inorganic−organic interactions on magnetic and structural properties of iron oxide magnetic nanoparticles functionalized by lipophilic stilbene molecules has been investigated. The molecules have been grafted through either phosphonate or carboxylate coupling agents. Mossbauer spectra recorded at 300 and 77K suggest a global composition of Fe2.82O4 for the two types of functionalization. Complementary in-field Mossbauer and SQUID measurements have demonstrated that the nanoparticles consist in a magnetite core surrounded by an oxidized layer. The oxidized shell exhibits a spin canting in the carboxylate case leading to a decrease of the net magnetization of the oxide nanoparticle. No canting occurs in the phosphonate case, and the magnetic properties are therefore preserved. The magnetic properties thus depend on the coupling agent, e.g., surface interactions. This result is of primary importance to tune the magnetic properties of functionalized nanoparticles for biomedical and high ...

Resonant magnetic scattering of polarized X-rays at the Mn 2p edge from Mn 0.06Ge 0.94 diluted magnetic semiconductor

We have measured the X-ray resonant magnetic scattering at the 2p edge of Mn in diluted magnetic semiconductor Mn 0.06Ge 0.94/Ge(001)2 × 1 thin films. Large magnetic effects in the resonant reflectivity were observed over a wide temperature range. The asymmetry ratio of the magnetic scattering increases with increasing temperature, in accordance to the temperature dependence of the magnetization obtained from SQUID measurements. This is attributed to the contribution of ferromagnetic Mn 5Ge 3 nanoclusters present in the Mn 0.06Ge 0.94 film.

Magnetic properties of nanostructured MnZn ferrite

Mn 0.5Zn 0.5Fe 2O 4 nanoparticles (10–30 nm) have been prepared via mechanochemical processing, using a mixture of two single-phase ferrites, MnFe 2O 4 and ZnFe 2O 4. SQUID measurements (field-cooled magnetization curves and hysteresis loops) were performed to follow the mechanically induced evolution of the MnFe 2O 4/ZnFe 2O 4 mixture submitted to the high-energy milling process. The resulting single MnZn nanoferrite phase was characterized by SQUID ( M– H curve), Faraday balance ( M– T curve) and transmission electron microscopy. The magnetic characteristics of the mechanosynthesized material were compared with those of bulk Mn 0.5Zn 0.5Fe 2O 4. It was found that the saturation magnetization of nanostructured Mn 0.5Zn 0.5Fe 2O 4 (87.2 emu/g) is lower than that of the bulk Mn 0.5Zn 0.5Fe 2O 4, but, the Néel temperature of the sample (583 K) is higher than that of the bulk Mn 0.5Zn 0.5Fe 2O 4.

Estimation of critical current density and grain connectivity in superconducting MgB 2 bulk using Campbell’s method

Many recent reports on the critical current density ( J c) in superconducting MgB 2 bulks indicated that improving the grain connectivity is important, since the obtained J c values were generally much lower than those in other metallic superconductors and it was ascribed to the poor connectivity between grains in polycrystalline MgB 2. In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB 2 bulks prepared by a modified PIT (powder in tube) method. Campbell’s method was applied for the purpose of obtaining the penetrating AC flux profile and the characteristic of AC magnetic field vs. penetration depth from the sample’s surface. A computer simulation on the penetrating AC flux profile in MgB 2 bulks with randomly distributed voids, oxidized grains and other faults was also carried out. J c obtained by Campbell’s method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various faults and the lack of the electrical connectivity. It was verified that the relationship between the global critical current characteristics and the faults contained in MgB 2 samples can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results.

Synthesis and Magnetic Properties of Conjugated Radical Polymer

Abstract An unprecedented conjugated radical polymer, poly-BDPI-2Y, consisting of imidazolyl radical was prepared and its magnetic properties were investigated. The SQUID measurement proves the presence of the ferromagnetic fraction at 300 K.

Synthesis and magnetic properties of the copper(II) complex derived from dimethyl aminomethylphosphine oxide ligand. X-ray crystal structure of DMAO and [Cu(NO 3) 2(POC 3H 10N) 2

The synthesis and properties of the copper(II) complex with dimethylaminomethylphosphine oxide as a ligand will be presented. The complex, with the formula [Cu(NO 3) 2(POC 3H 10N) 2] 1, has been characterized by elemental analysis, IR spectroscopy, SQUID and X-ray measurements. The X-ray structure was determined for the complex 1 and for the ligand dimethylaminomethylphosphine oxide (DMAO) 2. The single crystal X-ray structure of 1 shows that in the crystal the copper ions form distorted octahedral environment, consisting of two oxygen and two nitrogen atoms from the DMAO ligand. Additionally, one oxygen atom of each anion is semi-coordinated to the copper ion. The solid state magnetic measurements show that the complex 1 is paramagnetic with weak antiferromagnetic interactions in low temperatures.

Magnetic moments in Fe–Co/Pt superlattices

The element specific spin and orbital moments for Fe and Co in Fe 40 Co 60 / Pt (0 0 1) superlattices, with layer thicknesses d FeCo = 4 – 10 ML and d Pt = 8 ML , have been studied by X-ray magnetic circular dichroism with focus on the interfacial magnetic moments of the Fe 40 Co 60 layers. The spin moments of Fe and Co showed the same relative decrease at the interfaces with respect to the bulk alloy. The change at the interface was found to be - 0.9 ( 1 ) of the bulk alloy spin moment. The orbital moment for Fe also decreased at the interfaces, whereas the Co orbital moment did not show any appreciable change at the interfaces. In addition, SQUID measurements of the total magnetization allowed the extraction of the induced Pt magnetic moment, which was found to be 0.6 ( 3 ) μ B / atom and is discussed with respect to previous studies.

Characteristic and synthesis mechanism of MgB2 nanoparticles in solid-state reactive sintering

MgB 2 nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994 K, and then directly cooled down at a rate of 40 K/min. The results of X-ray diffraction indicate that MgB 2 superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20 nm are considered to be at the onset for the formation of MgB 2 phase. Transition temperature ( T c ) and critical current density ( J c ) of the compound structures were determined to be 38.5 K and 1.8 × 10 5 A cm −2 by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB 2 nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994 K.

Synthesis and characterization of magnetic Co nanoparticles: A comparison study of three different capping surfactants

This paper compares the performance of three long-chain acids—oleic and elaidic (both olefinic) and stearic (aliphatic)—as a capping agent in the synthesis of magnetic Co nanoparticles. The particles were formed through thermal decomposition of dicobalt octacarbonyl in toluene in the presence of the long-chain acid, and characterized by TEM, high-resolution TEM, and SQUID measurements. Infrared spectra revealed that some of the added olefinic acid was transformed from cis- to trans-configuration (for oleic acid) or from trans- to cis- (for elaidic acid) to facilitate the formation of a densely packed monolayer on the surface of Co nanoparticles. As compared to aliphatic acids, olefinic acids are advantageous for dense packing on small particles with high surface curvatures due to a bent shape of the cis-isomer. The presence of an olefinic acid is able to control particle growth, stabilize the colloidal suspension, and prevent the final product from oxidation by air. Our results indicate that oleic acid, elaidic acid, and a mixture of oleic/stearic acids or elaidic/stearic acids have roughly the same performance in serving as a capping agent for the synthesis of Co nanoparticles with a spherical shape and narrow size distribution.

Magnetic and relaxometric properties of polyethylenimine-coated superparamagnetic MRI contrast agents

Novel systems to be employed as superparamagnetic contrast agents (CA) for magnetic resonance imaging (MRI) have been synthesized. These compounds are composed of an iron oxide magnetic core coated by polyethylenimine (PEI) or carboxylated polyethylenimine (PEI-COOH). The aim of the present work was to prepare and study new nanostructured systems (with better or at least comparable relaxivities, R 1 and R 2, with respect to the commercial ones) with controlled, almost monodisperse average dimensions and shape, as candidates for molecular targeting. By means of atomic force microscopy (AFM) measurements we determined the average diameter, of the order of 200 nm, and the shape of the particles. The superparamagnetic behavior was assessed by SQUID measurements. From X-ray data the estimated average diameters of the magnetic cores were found to be ∼5.8 nm for PEI-COOH60 and ∼20 nm for the compound named PEI25. By NMR-dispersion (NMRD), we found that PEI-COOH60 presents R 1 and R 2 relaxivities slightly lower than Endorem ®. The experimental results suggest that these novel compounds can be used as MRI CA.

Self-assembled ferromagnetic and superparamagnetic structures of hybrid Fe block copolymers

Self-assembled 2D structures on thin films of block copolymers/Fe hybrid materials were fabricated on Si/Ta substrates, either by wet chemistry or laser irradiation at 157 nm. The polymer exhibits micelle-like structures with average dimensions of 5–10 nm and 30–50 nm for light and chemically reduced films respectively. For the laser processed films, SQUID measurements reveal a ferromagnetic response at 5 K, and 100 Oe coercivity was obtained for 2:1 iron concentration. For chemically reduced films, on the other hand, a superparamagnetic response with near zero coercivity at 5 K was obtained.

Secondary Bonding Interactions in Biomimetic [2Fe−2S] Clusters

A series of synthetic [2Fe-2S] complexes with terminal thiophenolate ligands and tethered ether or thioether moieties has been prepared and investigated in order to provide models for the potential interaction of additional donor atoms with the Fe atoms in biological [2Fe-2S] clusters. X-ray crystal structures have been determined for six new complexes that feature appended Et (1(C)), OMe (1(O)), or SMe (1(S)) groups, or with a methylene group (2(C) ), an ether-O (2(O)), or an thioether-S (2(S)) linking two aryl groups. The latter two systems provide a constrained chelate arrangement that induces secondary bonding interactions with the ether-O and thioether-S, which is confirmed by density functional theory (DFT) calculations that also reveal significant spin density on those fifth donor atoms. Structural consequences of the secondary bonding interactions are analyzed in detail, and effects on the spectroscopic and electronic properties are probed by UV-vis, Mössbauer, and (1)H NMR spectroscopy, as well by SQUID measurements and cyclic voltammetry. The potential relevance of the findings for biological [2Fe-2S] sites is considered.

Nanocrystalline Nb-Al-Ge mixtures fabricated using wet mechanical milling

An investigation into Nb-Al-Ge mixtures is presented with special attention to the superconducting compounds Nb3(Al1-xGex) with x = 0, 0.3 and 1, which are reported to provide the highest upper critical field values for Nb-based compounds. Wet mechanical milling using copper milling media and distilled water as a process control agent (PCA) was used with the intention of improving the yield, properties and the performance of these materials. Very high yields of nanocrystalline material were achieved but significant copper contamination occurred - confirmed using inductively-coupled-plasma atomic-emission-spectroscopy. Simultaneous thermogravimetric measurements and differential scanning calorimetry were performed on powders milled for up to 20 h with different PCA content, to quantify the work done on the powders. A typical grain size of a few nm was obtained for the Nb-Al-Ge mixtures after several hours milling. Powder ground for 20 h with 5% PCA was processed using a hot isostatic press (HIP) operating at 2000 atm and temperatures up to 750 °C. The room temperature resistivity decreased as the temperature of the HIPing increased. Unfortunately, despite the nanocrystalline microstructure of the powders and the high HIP temperatures, if superconducting material was formed it was below the detection level of resistivity, Ac. susceptibility and SQUID measurements. We conclude that during milling there was widespread contamination of the powders by the PCA so that milling with distilled water as a PCA is not to be recommended for fabricating nanocrystalline Nb3(Al1-xGex) A15 superconducting compounds.

Colloidal Cobalt-Doped ZnO Nanorods: Synthesis, Structural, and Magnetic Properties

The synthesis of ZnO:Co nanorods in alcoholic solutions and their structural, optical, and magnetic properties are presented. One-dimensional growth is achieved by careful control over the precursor addition and with the aid of ethylene diamine as a ligand. The nanorods are single-crystalline and show growth into the [0001] direction. They exhibit diameters of about 20 nm and lengths up to 150 nm. Using (HR)TEM, EDX, and XANES, we prove the incorporation of Co cations on Zn lattice sites. SQUID and XMCD measurements at cryogenic temperatures show paramagnetic behavior of the doped nanorods. Apparently, there are too few donor electrons to mediate ferromagnetic coupling in the highly crystalline nanorods compared to defect-rich spherical nanoparticles.

λ-MnO2 nanodisks and their magnetic properties

In this work, highly crystalline λ-MnO2 nanodisks are synthesized via a simple microemulsion process by using polyvinylpyrrolidone(PVP) as a capping layer. Detailed information about the morphology, such as size, shapeand crystallinity, was investigated by using x-ray diffraction, field emission scanningelectron microscopy (FESEM), transmission electron microscopy (TEM) as well as highresolution TEM (HRTEM). SQUID measurements were also carried out to explore themagnetic properties on the nanodisks, which may promote their future application innanodevices and nanotechnology.