SQUID Measurements Research Articles

Dual Coordination Modes of Ethylene-Linked NP2 Ligands in Cobalt(II) and Nickel(II) Iodides

Here we report the syntheses and crystal structures of a series of cobalt(II) and nickel(II) complexes derived from (R)NP2 ligands (where R = OMe(Bz), H(Bz), Br(Bz), Ph) bearing ethylene linkers between a single N and two P donors. The Co(II) complexes generally adopt a tetrahedral configuration of general formula [(NP2)Co(I)(2)], wherein the two phosphorus donors are bound to the metal center but the central N-donor remains unbound. We have found one case of structural isomerism within a single crystal structure. The Co(II) complex derived from (Bz)NP2 displays dual coordination modes: one in the tetrahedral complex [((Bz)NP2)Co(I)(2)]; and the other in a square pyramidal variant, [((Bz)NP2)Co(I)(2)]. In contrast, the Ni(II) complexes adopt a square planar geometry in which the P(Et)N(Et)P donors in the ligand backbone are coordinated to the metal center, resulting in cationic species of formula [((R)NP2)Ni(I)](+) with iodide as counterion. All Ni(II) complexes exhibit sharp (1)H and (31)P spectra in the diamagnetic region. The Co(II) complexes are high-spin (S = 3/2) in the solid state as determined by SQUID measurements from 4 to 300 K. Solution electron paramagnetic resonance (EPR) experiments reveal a high-spin/low-spin Co(II) equilibrium that is dependent on solvent and ligand substituent.

Physical properties of new multiferroic hexagonal YbFeO3 thin film

Temperature dependences of dielectric constant and second-harmonic generation (SHG) intensity of hexagonal (h-) YbFeO3 thin films deposited on yttrium stabilized zirconia (YSZ) (111) substrates were measured from 300 to 470 K. The results reveal that h-YbFeO3 film is ferroelectric below around 350 K. From the SQUID measurements, it is also concluded that this thin film is ferrimagnetic in the low-temperature range.

Ceramic Tl-oxide based superconductors reinvestigated by magnetic separation technique and SQUID measurements

Ceramic samples of Tl 2Ba 2Ca 2Cu 3O 10 (Tl-2223) and TlBa 2Ca 2Cu 3O 9 (Tl-1223) superconductor were resynthesized by using several preparation procedures and investigated by means of magnetic separation technique, allowing evaluation of the superconducting critical temperature of individual grains as small as 25 μm. Although an onset of diamagnetism detected by SQUID measurements was observed around 130 K in Tl-2223 and 134 K in Tl-1223, no grains with T c > 125 K were found for Tl-2223 and with T c > 130 K for Tl-1223. Furthermore, the volume fraction of superconductivity of such ceramics was less than 5% and 3% in best samples of Tl-2223 and Tl-1223, respectively. Here we provide a detailed comparison of results obtained by magnetic separation technique with those by SQUID applied to individual grains and bulk samples.

SYNTHESIS OF COBALT NANOTUBULES AND CHARACTERIZATION OF MAGNETIZATION DYNAMICS

Magnetic nanowire arrays formed by templated electrodeposition are a promising way of making ultra-high density magnetic recording media. We have used both Poly Carbonate Track Etched (PCTE) and Anodic Aluminum Oxide (AAO) membranes to obtain Cobalt nanowires. Also by suitably adjusting the electrodeposition process, hollow Cobalt nanotubules were formed. The hollow cylindrical structure gives a unique capability to tailor their magnetization. The morphology of the samples was investigated using SEM and TEM. Magnetic characterization done using SQUID shows the influence of morphology and that the nanotubules have rather different switching behaviour than solid nanowires. Finite element micromagnetic modeling was performed of a single nanowire, a single nanotubule and also arrays of nanotubules. The calculated static M-H curves were found to match closely the SQUID measurements. The coercivity increases with length while the inner and outer diameters greatly affect rotational switching of the magnetization.

Efficient synthesis and magnetic properties of triphenylamine bearing three nitronylnitroxide radicals

Improved synthesis and reinvestigation of the magnetic properties of triphenylamine bearing three nitronylnitroxide radicals ( 1) were carried out. Single crystal X-ray crystallographic analysis of 1 established that its molecular shape had a C 3-symmetry. Our newly prepared 1 showed different physical properties from those of reported. SQUID measurements of 1 showed that each of the three spins of 1 behaved independently at room temperature. In contrast, the three spins coupled antiferromagnetically at lower temperature as low as ca. 1.8 K. These spin behaviors can be explained by a regular triangular antiferromagnetic model ( J/ k B = −3.3 K) and interpreted as a spin frustration system.

Retention of the Tetragonal to Orthorhombic Structural Transition in F-Substituted SmFeAsO: A New Phase Diagram forSmFeAs(O1−xFx)

In this Letter we propose a new phase diagram for the SmFeAs(O(1-x)F(x)) system, based on careful analysis of synchrotron powder diffraction data, SQUID, and muon spin rotation measurements. The tetragonal to orthorhombic structural transition is slightly affected by F content and is retained for the superconducting samples, even at optimal doping. These findings relate the AFM transition on a different ground with respect to the structural one and suggests that orbital ordering could be the driving force for symmetry breaking.

Studies on Response of Human Hippocampus to Random Somatosensory Stimuli by a SQUID System in a Superconducting Magnetic Shield

An essential role of a superconducting magnetic shield is described for study of higher function of human brains. The present SQUID has a sensitivity of 0.9 fT/(Hz)1/2 at 100 Hz in a superconducting magnetic shield. SQUID sensors are made of SNS (Superconductor/Normal metal/Superconductor) junctions. Recent theories of beamformer and MUSIC (multiple signal classification) emphasize the importance of SNR (signal-to-noise ratio) of data to locate equivalent current dipoles in brains. For instance, MUSIC uses the noise subspace of the covariant matrix practically calculated by measured magnetic fields. Dynamical responses of hippocampus to novelty or unexpectedness are measured by the SQUID system in a superconducting magnetic shield and visualized by swLORETA-an extension of sLORETA (standardized low-resolution electromagnetic tomography). The SQUID system in a magnetic shield of high-Tc superconductor is yielding new information of deep areas of human brains. The sensitivity of neuromagnetic SQUID measurements is proved to be limited by the system noise temperature of a SQUID hardware system but not so-called “brain noises”.

Study of the role of the ligands coordinated at the surface of pure Wüstite nanoparticles prepared following a room temperature organometallic method: Evidence of ferromagnetic – in shell- and antiferromagnetic – in core magnetic behaviors

Wüstite particles (Fe 1− y O) are synthesized using controlled hydrolysis at room temperature of [Fe(N(SiMe 3) 2) 2] and stabilized by amine ligands. This method leads to 5 nm pure wüstite particles. This phase is clearly identified by transmission electron microscopy and wide angle X-ray scattering. Distortion in the crystallographic structure has been demonstrated. Particular attention is paid on the Fe(III) formation. Moreover, a combination of Mössbauer spectroscopy and SQuID measurements, revealed that the particles are composed of an antiferromagnetic core surrounded by a ferromagnetic shell. According to the Néel theory, the Fe(III) and Fe(II) ions present in the particles are ferromagnetically coupled and the proportion of Fe(III) ions varies from 3.9 to 7.1% as a function of the amine ligand.

The Use of Magnetic Dilution To Elucidate the Slow Magnetic Relaxation Effects of a Dy2 Single-Molecule Magnet

The magnetic dilution method was employed in order to elucidate the origin of the slow relaxation of the magnetization in a Dy(2) single-molecule magnet (SMM). The doping effect was studied using SQUID and micro-SQUID measurements on a Dy(2) SMM diluted in a diamagnetic Y(2) matrix. The quantum tunneling of the magnetization that can occur was suppressed by applying optimum dc fields. The dominant single-ion relaxation was found to be entangled with the neighboring Dy(III) ion relaxation within the molecule, greatly influencing the quantum tunneling of the magnetization in this complex.

Magnetisation of bulk Mn 11Si 19 and Mn 4Si 7

The purpose of this paper is to determine by experiment whether Mn 11Si 19 and Mn 4Si 7 in their bulk states have a finite magnetic moment or not. Magnetisation measurements were carried out on these materials using both SQUID system and Kerr rotation system. The high quality samples were grown using the temperature gradient solution growth method. SQUID measurements revealed that Mn 11Si 19 has finite magnetism while Mn 4Si 7 does not in their bulk states. It was also confirmed that Mn 4Si 7 became magnetic and Mn 11Si 19 got to exhibit a distinctive hysteresis in their powdery state. The enhancement of magnetism implied that the surface of the samples was to a great extent linked to its magnetism.

Two-dimensional coordination compounds based on Fe(II) and Co(III) hexacyanometallates with Cu(II)(dien) groups: Structures and magnetic properties

The crystal structures of two copper(II)(dien) coordination compounds with diamagnetic [Fe(II)(CN) 6] 4− or [Co(III)(CN) 6] 3− bridges were determined from single crystal X-ray diffraction and their magnetic properties investigated by SQUID measurements. The first, {[Cu(dien)] 2[Fe(CN) 6]} n ·4 n H 2O ( 1), where dien is diethylenetriamine, consists of linear Cu(II)–Fe(II)–Cu(II) trimers. The Cu 2+ ions are equatorially coordinated by dien and bridged by a hexacyanoferrate anion. A two-dimensional network is formed through longer axial Cu⋯NC–Fe bonds. The second, {{[(Cu(dien)) 2(pz)] 2[Co(CN) 6]}[Co(CN) 6]} n ·6 n H 2O ( 2), where pz − is pyrazolate, contains butterfly-shaped Cu(II) 2–Co(III)–Cu(II) 2 pentamers. The Cu 2+ ions are again equatorially coordinated by dien and bridged by pz − to form a dimer. Two dimers are linked through a [Co(III)(CN) 6] 3− anion to pentanuclear clusters, which are further connected into a layer via the second site of [Co(III)(CN) 6] 3− anions. Cu 2+ has distorted square–pyramidal and octahedral coordination for 1 and 2, respectively, with elongated axial ligand distances due to the Jahn–Teller effect. The magnetic behavior of both compounds is consistent with Cu 2+ S = ½ dimers, although with different bridging groups to give g = 2.157(2) and 2 J/ k B = −1.14(1) K for 1 and g = 2.15(1) and 2 J/ k B = −37.5(2) K for 2.

Resonant soft X‐ray emission and X‐ray absorption studies on Ga1‐xMnxN grown by pulsed laser deposition

AbstractIn this study thin film samples of Ga1‐xMnxN were grown by pulsed laser deposition on Al2O3 (0001) substrates. X‐ray diffraction measurements have confirmed these thin films exhibit hexagonal wurtzite structure. SQUID measurements show room temperature ferromagnetism of these dilute magnetic semiconductors (DMS). The techniques of X‐ray absorption and soft X‐ray emission spectroscopy at the N K‐edge were used to study the changes in the unoccupied and occupied N 2p partial density of states respectively as a function of dopant concentration. These element and site specific spectroscopies allow us to characterise the electronic structure of these doped materials and reveal the influence of the Mn doping on the valence band as measured through the N 2p partial density of states. X‐ray absorption measurements at the Mn L‐edge confirm significant substitutional doping of Mn into Ga‐sites. Finally, measurements of heavily Mn‐doped films using both soft X‐ray absorption and resonant soft X‐ray emission at the N K edge reveal the presence of trapped molecular nitrogen. The trapped molecular nitrogen may be due to the high instantaneous deposition rate in the PLD process for these samples (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis, Molecular Structure, and Physical Properties of the Complexes [{PhB(pz)2(CH2SMe)}2M] (M = MnII, FeII; pz = pyrazol‐1‐yl) Containing a Novel [N,N,S]‐Heteroscorpionate Ligand

AbstractTwo transition metal complexes [L2M] {M = FeII (2), MnII (3)} of the novel [N,N,S] scorpionate ligand L = [PhB(pz)2(CH2SMe)]– have been synthesized and characterized by X‐ray crystallography, magnetic measurements, cyclic voltammetry, and various spectroscopic techniques. In the solid state, both complexes 2 and 3 possess a distorted octahedral ligand sphere, but adopt different configurations (2: cis; 3: trans). According to X‐ray diffraction data, Mössbauer spectroscopy, and SQUID measurements, compound 2 exists as low‐spin complex in the solid state at temperatures T ≤ 20 °C (μeff ≈ 0.5 μB). Upon heating, the magnetic moment μeff increases continuously to a value of 1.8 μB at 90 °C, which indicates a temperature dependent high‐spin ↔ low‐spin transition above room temp. Solutions of 2 in CDCl3 showed paramagnetic behavior at 25 °C (μeff = 3.1 μB; Evans NMR method). Cyclic voltammetry (CH2Cl2, [Bu4N][PF6]; vs. FcH/FcH+) on 2 reveals a reversible FeII/FeIII redox transition at E1/2 = –0.16 V. Density functional theory (B3LYP‐D/def2‐TZVPP//B3LYP‐D/SVP level) has been used to evaluate relative energies of the cis and trans isomers of 2 in their high‐spin and low‐spin states. Theoretical and experimental investigations have been extended to the related literature‐known homoleptic FeII complexes [{PhB(pz)3}2Fe], [{PhB(pz)(CH2SMe)2}2Fe], and [{PhB(CH2SMe)3}2Fe]. The following order of ligand‐field strengths has been established: [PhB(CH2SMe)3]– < [PhB(pz)(CH2SMe)2]– < [PhB(pz)2(CH2SMe)]– < [PhB(pz)3]–. The only significant deviation from this systematic behavior became evident for the unexpectedly low solution magnetic moment of [{PhB(CH2SMe)3}2Fe], which finds no parallel in the solid state. Computed spin‐state splittings for the entire series of complexes, however, confirm the trend in solution magnetic moments and we relate the disaccording experimental observations to the presence of a second stereoisomer in crystals of [{PhB(CH2SMe)3}2Fe] which increases the paramagnetism of solid‐state samples of this complex.

Aliovalent-Ion and Magnetic Field Induced Phase Transition in Multiferroic BiFe<SUB>1−<I>x</I></SUB>Ti<SUB><I>x</I></SUB>O<SUB>3</SUB> System

Multiferroic compounds with general formula BiFe(1-x)Ti(x)O3 (x = 0.1, 0.2, 0.3 and 0.35) have been synthesized by conventional solid state reaction method. The effect of Ti substitution on ferroelectric and magnetic properties is studied. From X-ray diffraction (XRD) analysis, a rhombohedral to orthorhombic phase transition for x > 0.3 was observed. From SQUID measurements, a magnetic field induced phase transition has been observed in the BiFe(1-x)T(x)O3 system for x = 0.3. An anomaly in dielectric constant and dielectric loss in the vicinity of antiferromagnetic Néel temperature (T(N)) and a small enhancement in magnetization have been observed. Magnetization measurements above room temperature showed no systematic variation in antiferromagnetic Néel temperatures on Ti substitution. Further it is seen that this system shows the coupling between electric and magnetic dipoles exhibiting magnetoelectric (ME) effect at room temperature and possess high dielectric constant.

Structural and Magnetic Properties of ZnO Nanocrystals in (Zn, Al)O Films Using Pulse Laser Deposition

In this manuscript, we reported that the room temperature ferromagnetism was observed in (Zn0.70, Al0.30)O film, which was fabricated by a novel physical method (pulse laser deposition (PLD)). The film was deposited from (Zn0.80, Al0.20)O ceramic target onto quartz (110) substrate by PLD at 400 degrees C under an oxygen partial pressure of 10(-4) torr. TEM result shows ZnO NCs with diameter of 4-5 nm and they are quite uniformly embedded into amorphous ZnO-Al2O3 phase. The SAED shows clearly that ZnO NCs possess polycrystalline structure. The SQUID measurement shows that the film has room temperature ferromagnetism (saturation magnetization = 3.6 emu/cm3) with Curie temperature above 300 K. The magnitude of magnetic moment of the films can be changed by tuning ZnO NCs size. Both oxygen partial pressure and film thickness studies show that the origin of ferromagnetism is possibly related to the oxygen defects at the surface of ZnO NCs.

Electronic structure and magnetization correlations in Ni doped ZnO

Effect of hydrogen and vacuum annealing on electronic and magnetic properties of 5% Ni-doped ZnO pellets has been investigated using X-ray diffraction (XRD), magnetization, resistance, and X-ray photoelectron spectroscopy (XPS). Rietveld refinement results confirm that the samples have wurtzite structure with no secondary phase. The SQUID measurements exhibit a paramagnetic state for the as-synthesized Zn 0.95Ni 0.05O, however, the post annealing in H 2 and in vacuum, drives it to a ferromagnetic state at 300 K. Resistance of annealed samples is three orders of magnitude lower than their as-synthesized counterparts. The XPS results confirm bivalency of Ni ions and creation of O vacancies upon annealing, owned to Ni ( 3d)–O ( 2p) hybridization. The ferromagnetism and the consequent electronic changes are retraceable on re-heating the annealed samples in air.

Donor‐Acceptor Couples and Late Transition Metal Complexes of Oxidation‐Labile 1,4,5,8‐Tetrakis(guanidino)naphthalene Superbases

AbstractIn this manuscript the reactivity of the two oxidation‐labile superbases 1,4,5,8‐tetrakis(N,N,N′,N′‐tetramethylguanidino)naphthalene and the newly synthesized 1,4,5,8‐tetrakis(N,N′‐dimethyl‐N,N′‐ethylene‐guanidino)naphthalene(tdmegn) are discussed and compared with that of related organic electron donors. The work includes oxidation with inorganic and organic oxidation reagents, as well as the preparation and characterization of dinuclear CoII, NiII and CuI complexes. Magnetic coupling in the dinuclear CoII and NiII complexes is studied on the basis of SQUID measurements. Finally, the results of experiments on the oxidation of the dinuclear CuI complexes are presented.

Use of long chain amine as a reducing agent for the synthesis of high quality monodisperse iron(0) nanoparticles

This article reports the synthesis of iron(0) nanoparticles at moderate temperature—from 120 °C to 150 °C—using the reduction of the organometallic iron(II) precursor {Fe[N(SiMe3)2]2}2 by hexadecylamine (HDA) in the absence of dihydrogen (H2). The nanoparticles are monodisperse in size and self-assemble into 2D super-lattices suitable for transport measurements. The nanoparticles are stabilized in mesitylene by a mixture of HDA and hexadecylammonium chloride (HDA·HCl). The resulting truncated single-crystalline nanocubes have a narrow size distribution and a high magnetization close to the bulk value. The products are characterized by transmission electronic microscopy (TEM and HRTEM), SQUID measurements, Mössbauer and Infra-Red spectroscopies. Fe(II) reduction is accompanied by oxidation of amines into imines which was detected as a by-product. This reduction occurs at 120 °C and above. The temperature, in conjunction with the reaction time, allows for a fine control of the nano-objects final size. The latter can also be tuned with the HDA·HCl concentration. Finally, this one-pot synthesis produces high-quality magnetic nanoparticles with mean sizes in the range 6 to 10 nm depending on the conditions.

Highly ordered transition metal ferrite nanotube arrays synthesized by template-assisted liquid phase deposition

Highly ordered spinel ferrite MxFe3−xO4 (M = Ni, Co, Zn) nanotube arrays were synthesized in anodic aluminium oxide (AAO) templates with a pore size of 200 nm by combining a liquid phase deposition (LPD) method with a template-assisted route. The morphology of the transition metal ferrite nanotubes was characterized by electron microscopy (FE-SEM; TEM, SAED and HRTEM) and atomic force microscopy (AFM), whereas their chemical composition was determined by inductive coupling plasma (ICP). The phase purity was studied by X-ray diffraction (XRD) and the magnetic properties of the nanotubes were measured by SQUID measurements. Unlike the deposition of thin film structures, nanotube arrays form within the pores of the AAO templates in a much shorter time due to the attractive interactions between the positively charged AAO and the negatively charged metal complex species formed in the treatment solution. The as-deposited nanotubes are amorphous in nature and can be converted into polycrystalline metal ferrites via a post-synthesis heat treatment which induce the dehydroxylation, crystallization and formation of the spinel structure. The resulting nanotubes are uniform with smooth surfaces and open ends and their wall thickness can be varied from 4 to 26 nm by increasing the deposition time from 1 to 4 h. Significant differences in the magnetic properties of the ferrite nanotubes have been observed and these differences seem to result from the chemical composition, the wall thickness and the annealing temperature of the spinel ferrite nanotubes.

Iron silicide nanoparticles in a SiC/C matrix from organometallic polymers: characterization and magnetic properties

Thermolysis of poly(ferrocenylsilane) at 1000 °C or poly[{(dimethylsilyl)ferrocenyl}diacetylene] at 850 °C resulted in ferromagnetic Fe3Si nanoparticles, well distributed in a SiC/C matrix. The average size of the formed nanoparticles, their size distribution and their agglomeration behaviour depend on the choice of polymer and the thermolysis conditions. The size-dependent magnetic properties of the nanoparticles were analyzed by SQUID measurements. The smallest Fe3Si nanoparticles (10–30 nm) are single magnetic domain particles with superparamagnetic behaviour.