SQUID Measurements Research Articles

Magnetic properties and stability of negatively charged doubly bonded C1202− dimers

A new ionic complex {(MDABCO+)·ZnIITMPP}2·(C1202−)·(C6H4Cl2)2.15·(C6H5CN)1.85 (1) (ZnIITMPP: zinc(II) tetrakis(4-methoxyphenyl)porphyrin; MDABCO+: the cation of N-methyldiazabicyclooctane) has been obtained as single crystals. The complex contains negatively charged doubly bonded C1202− dimers whose magnetic properties have been studied. Magnetic moment of the complex is 2.69 μB at 290 K indicating presence of about two spins per dimer. The dimers show narrow doublet-like EPR signal with gM = 2.0014 and the linewidth of 0.315 mT at 290 K. The data of SQUID measurements for 1 can be fitted according to the Heisenberg model for isolated antiferromagnetically interacting spin pairs with exchange interaction of J/kB = −44.7 K. The EPR signal from the dimers and magnetic susceptibility of 1 decrease below 40 K resulting in diamagnetic dilution of the sample and the appearance of triplet features of the signal. Two pairs of lines show peak separation of 5.2 and 2.6 mT and are centered at gT = 2.0008. Averaged separation of 10.2 A between two spins was estimated. Additional narrow doublet-like signal is manifested at 40 K and below. The signal was centered at gI = 1.9984 (20 K) and was attributed to the impurity of non-dimerized C60˙− since its intensity did not exceed 4% from that of the signal from the dimers. Heating of 1 above 380 K resulted in irreversible dissociation of the C1202− dimers to monomeric C60˙− which was accompanied by transition from narrow to broad EPR signal. The degree of dimer-monomer conversion was 97.5% at 446 K.

Characterization of superconducting nanometric multilayer samples for superconducting rf applications: First evidence of magnetic screening effect

The best rf bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field $H\ensuremath{\approx}200\text{ }\text{ }\mathrm{mT}$ close to the thermodynamic critical field ${H}_{c}$. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of ${H}_{c}>H_{c}{}^{\mathrm{Nb}}$ for magnetic shielding of bulk niobium to increase the breakdown magnetic field of superconducting rf cavities. Depositing good quality layers inside a whole cavity is rather difficult, so as a first step, characterization of single layer coating and multilayers was conducted on high quality sputtered samples by applying the technique used for the preparation of superconducting electronics circuits. The samples were characterized by x-ray reflectivity, dc resistivity (PPMS), and dc magnetization (SQUID) measurements. Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating exhibit different behaviors and clearly show an enhancement of the magnetic penetration field. Because SQUID measurements are influenced by edge and shape effects, we propose to develop a specific local magnetic measurement of ${H}_{C1}$ based on ac third harmonic analysis in order to reveal the true screening effect of multilayers.

Biradical Character of Linear π-Conjugated Oligomer Dications Composed of Thiophene, Pyrrole, and Methylthio End-Capping Units

Combined DFT calculations and UV-vis-NIR, ESR, and SQUID measurements revealed that the ground-state electronic structure of a linear π-conjugated oligomer dication composed of two pyrrole and six or seven thiophene rings and methylthio end-capping units is dominated by a singlet biradical character.

High-Temperature DC-Magnetisation Investigation on Undoped and 5 at.% Cobalt Incorporated TiO2 Nanopowders

Nanocrystalline samples of both undoped TiO2 and with 5 at.% Co at cationic sites were prepared by a simple chemical route. X-ray diffraction study of these samples revealed an increase in lattice parameters due to incorporation of Co in the TiO2 lattice and possibly due to formation of oxygen vacancies. No traces of crystalline Co or any other secondary phases could be detected. High-temperature SQUID measurement of these samples exhibited ferromagnetic signature up to 700 K, implying that the Curie temperature of doped and undoped TiO2 exceeds 700 K. Vacuum annealing of both types of samples enhanced the ferromagnetic character. The as-prepared and vacuum-annealed Co incorporated TiO2 samples, in addition to the ferromagnetic character, also exhibited a paramagnetic signal. All these measurements strongly support the role of defects in inducing room-temperature ferromagnetism in undoped and Co doped TiO2 nanopowders, which can be explained using the bound magnetic polaron model.

On-chip SQUID measurements in the presence of high magnetic fields

We report a low temperature measurement technique and the magnetization dataof a quantum molecular spin, by implementing an on-chip SQUID technique.This technique enables SQUID magnetometry in high magnetic fields, upto 7 T. The main challenges and the calibration process are detailed. Themeasurement protocol is used to observe quantum tunneling jumps of theS = 10 molecularmagnet, Mn12–tBuAc. The effect of a transverse field on the tunneling splitting for this molecular systemis addressed as well.

Chromium−Manganese Selenide Carbonyl Complexes: Paramagnetic Clusters and Relevance to C═O Activation of Acetone

The paramagnetic even-electron cluster, [Et(4)N](2)[Se(2)Cr(3)(CO)(10)], was found to react readily with Mn(CO)(5)Br in acetone to produce two unprecedented mixed chromium-manganese selenide carbonyl complexes, [Et(4)N][Me(2)CSe(2){Mn(CO)(4)}{Cr(CO)(5)}(2)] ([Et(4)N][1]) and [Et(4)N](2)[Se(2)Mn(3)(CO)(10){Cr(CO)(5)}(2)] ([Et(4)N](2)[2]). X-ray crystallographic analysis showed that anion 1 consisted of two Se-Cr(CO)(5) moieties, which were further bridged by one isopropylene group and one Mn(CO)(4) moiety. The dianionic cluster 2 was shown to display a Se(2)Mn(3) square-pyramidal core with each Se atom externally coordinated by one Cr(CO)(5) group. The formation of complex 1, presumably via C=O activation of acetone, was further facilitated by acidification of the reaction of [Et(4)N](2)[Se(2)Cr(3)(CO)(10)] with Mn(CO)(5)Br in acetone. Complex 1 readily transformed into 2 upon treatment with Mn(2)(CO)(10) in a KOH/MeOH/MeCN solution. Cluster 2 was a 51-electron species, which readily converted to the known 49-electron cluster [Se(2)Mn(3)(CO)(9)](2-) upon heating and bubbling with CO. Magnetic studies of the even-electron cluster, [Et(4)N](2)[Se(2)Cr(3)(CO)(10)], and the odd-electron species, [Et(4)N](2)[2] and [PPN](2)[Se(2)Mn(3)(CO)(9)], were determined by the SQUID measurement to have 2, 3, and 1 unpaired electrons, respectively. In addition, the nature and formation of complexes 1 and 2 are discussed, and the magnetic properties and electrochemistry of [Se(2)Cr(3)(CO)(10)](2-), 2, and [Se(2)Mn(3)(CO)(9)](2-) were further studied and elucidated by molecular orbital calculations at the PW91 level of density functional theory.

Temperature dependent carrier induced ferromagnetism in Zn(Fe)O and Zn(FeAl)O thin films

Epitaxial films of ZnO doped with magnetic ion Fe and, in some cases, with 1% Al show clear evidence of room temperature ferromagnetic ordering but containing huge amount of paramagnetic moment in it. The total ferromagnetic and paramagnetic contributions have been extracted from the low temperature SQUID measurements. A clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers and number of donors have been found in these films and established the theory of carrier induced ferromagnetism. The experimental data has been best explained through the modification of electronic structure of oxide semiconductors with impurity states.

Biodegradable and Electroactive TEMPO‐Substituted Acrylamide/Lactide Copolymers

Copolymers composed of PLA and PTAm were prepared by a macromonomer approach. The PLA bearing vinyl group at chain end was copolymerized with 2,2,6,6-tetrametylpiperidine-4-ylacrylamide. The resulted copolymers were oxidized by a peroxide to give PTAm-g-PLA. The structures of the copolymers were confirmed by NMR and FTIR spectroscopy. The comparison of (1)H NMR results and SQUID measurements demonstrated that the oxidation of the PTAm fragment proceeded almost to completion. An MTT assay, cell adhesion and spreading evaluation showed that the copolymers exhibited improved cytocompatibility as compared to the PTAm homopolymer due to the introduction of the biocompatible PLA moiety.

LiNi0.1Mn0.1Co0.8O2 electrode material: Structural changes upon lithium electrochemical extraction

Abstract We reported here on the synthesis, the crystal structure and the study of the structural changes during the electrochemical cycling of layered LiNi 0.1 Mn 0.1 Co 0.8 O 2 positive electrode material. Rietveld refinement analysis shows that this material exhibits almost an ideal α-NaFeO 2 structure with practically no lithium–nickel disorder. The SQUID measurements confirm this structural result and evidenced that this material consists of Ni 2+ , Mn 4+ and Co 3+ ions. Unlike LiNiO 2 and LiCoO 2 conventional electrode materials, there was no structural modification upon lithium removal in the whole 0.42 ≤ x ≤1.0 studied composition range. The peaks revealed in the incremental capacity curve were attributed to the successive oxidation of Ni 2+ and Co 3+ while Mn 4+ remains electrochemically inactive.

Organometallic Synthesis of β‐CoAl Nanoparticles and β‐CoAl/Al Nanoparticles and Their Behaviour upon Air Exposure (Eur. J. Inorg. Chem. 11/2010)

The cover picture shows the reaction path followed to produce bimetallic CoAl nanoparticles (background image). Organometallic precursors such as AlCp* and [Co(η3-C8H13)(η4-C8H12) offer the unique advantage of being easily reduced under dihygrogen, thus enabling access to alloyed β-CoAl nanoparticles as evidenced by X-ray absorption and diffusion techniques. As expected, these nanoparticles are highly reactive towards oxygen. The reaction starts with the selective oxidation of Al, with simultaneous segregation of Co. Known in the bulk, this phenomenon is studied for the first time on the nanometer scale. It is clearly evidenced by SQUID measurements: an increase in the ratio of segregated Co atoms is accompanied by an increase in the magnetization of the nanoparticles. To passivate the system, excess aluminum must be decomposed on top of the CoAl nanoparticles. This study evidences the relevance of organometallic chemistry for nanoalloy engineering. Details are discussed in the Short Communication by C. Amiens et al. on p. 1599.

Structural and magnetic properties of Co+ implanted n-GaN dilute magnetic semiconductors

The n-type GaN epilayer was grown on sapphire prepared by metal organic chemical vapour deposition and subsequently Co+ ions implanted. The properties of Co+ ions implanted GaN epilayer were investigated by structural and magnetic measurements. The results of Rutherford backscattering spectrometry and channeling illustrate that an excellent crystalline quality (χmin=1.3%) of as-grown GaN. After the implantation of 150 keV Co+ ions with dose 3×1016 cm−2 into GaN and subsequently annealed at 700, 800 and 900 °C, no secondary phase or metal related-peaks were detected by typical XRD. In addition high-resolution X-ray diffraction (HRXRD) was performed to study structural related properties. The magnetization curves were obtained by SQUID and AGM measurements, a well-defined hysteresis loop was observed even at 300 K. The temperature dependence of magnetization was taken in FC and ZFC conditions showed the highest Curie temperature (TC) ∼370 K recorded for Co+ implanted GaN.

Magnetic susceptibility of magnesium fulleride Mg4C60

Magnesium fulleride Mg4C60 was synthesized by a solid state reaction and assigned a rhombohedral structure with a = 9.99 Å, α = 55.1° from X-ray diffraction measurements. The electrical resistivity measurement of the MgxC60 film showed its non-metallic character. The magnetic susceptibility of Mg4C60 was investigated with ESR and SQUID measurements. From the absence of Pauli paramagnetism in the ESR measurement, Mg4C60 was revealed to behave as a non-magnetic insulator. Large core diamagnetism was detected by SQUID measurements.

Magnetic Behavior of Odd- and Even-Electron Metal Carbonyl Clusters: The Case Study of [Co8Pt4C2(CO)24]n−(n= 1, 2) Carbide Cluster

The reaction of [Co(6)C(CO)(15)](2-) with 2 equiv of PtCl(2)(Et(2)S)(2) affords the new heterobimetallic [Co(8)Pt(4)C(2)(CO)(24)](2-), [1](2-), carbonyl cluster. [1](2-) undergoes reversible chemical and electrochemical oxidation and reduction processes disclosing a complete series of [1](n-) (n = 1-4) clusters. The mono- and dianion of [1](n-) have been isolated as their tetra-substituted ammonium salts and fully characterized by means of IR, (13)C NMR, ESI-MS, and X-ray crystallography. Variable-temperature (VT) solid-state EPR studies on pure crystalline samples indicate that both [1](2-) and [1](-*) are paramagnetic, due to a doublet state of the latter and a triplet state of [1](2-). This conclusion is supported by SQUID measurements on the same crystalline sample of [1](2-). The present study indisputably demonstrates that even-electron transition metal carbonyl clusters (TMCC) can be magnetic.

Controlled Design of Size-Tunable Monodisperse Nickel Nanoparticles

We present here a complete study on a synthesis of nickel nanoparticles involving the reaction of [Ni(acac)2] with oleylamine (OA) and trioctylphosphine (TOP) reactants, whose simultaneous presence and relative amounts are paramount. The role of every reactant in the nucleation and growth of the nanoparticles has been delineated: OA is the reductant and thus controls the nucleation rate, meanwhile TOP provides a tunable surface stabilization through coordination on the Ni(0) surface. This result leads us to a design synthesis providing tailored monodispersed nanoparticles in a wide range (2−30 nm), which allows self-consistent studies of size-induced changes in catalytic and magnetic properties. Additionally, the growth mechanism is demonstrated to include an aggregation step which thus correlates with the polycrystalline feature of the nanoparticles obtained through this process. Moreover, the critical influence of the phosphine in this system was demonstrated a) for the outcome of the ripening mechanisms: defocusing effect and/or reshaping of the particles and b) for the surface properties: binding of the phosphine to the surface and its behavior toward oxidation was investigated by FTIR. Preliminary SQUID measurements show the impact of crystallites size on the magnetic properties.

Oxovanadium(IV) Silsesquioxane Complexes

In the context of a potential modeling of reduced oxovanadium species occurring on the surfaces of silica-supported vanadia catalysts in the course of its turnover, the incompletely condensed silsesquioxane H(3)(c-pentyl)T(7) was reacted with Cl(4)V(THF)(2) (where THF = tetrahydrofuran) in the presence of triethylamine. Precipitation of 3 equiv of HNEt(3)Cl seemed to point to the clean formation of [((c-pentyl)T(7))(V(IV)Cl)] (1), which was supported by electron paramagnetic resonance studies performed for the resulting solutions, but further analytical and spectroscopic investigations showed that the processes occurring at that stage are more complex than that and even include the formation of [((c-pentyl)T(7))(V(V)O)](2) as a side product. Storage of a red-brown hexane solution of this product mixture reproducibly led to the precipitation of blue crystals belonging to the chloride-free compound [((c-pentyl)T(7))(2)(V(IV)=O)(3)(THF)(2)] (2), as revealed by single-crystal X-ray diffraction. Performing the same reaction in the presence of 2 equiv of pyridine leads to an analogous product, where the THF ligands are replaced by pyridine. Subsequent investigations showed that the terminal oxo ligands at the vanadium centers are, on the one hand, due to the presence of adventitious water; on the other hand, the [(c-pentyl)T(7)](3-) ligand also acted as a source of O(2-). The results of SQUID measurements performed for 2 can be interpreted in terms of a ferromagnetic coupling between the vanadyl units. Exposing 2 to a dioxygen atmosphere resulted in its immediate oxidation to yield the V(V) complex [((c-pentyl)T(7))(V(V)O)](2), which may model a fast reoxidation reaction of oxovanadium(IV) trimers on silica surfaces.

TEM Study and Magnetic Measurements of Nano–Scale Particles Formed in Cu–Base Alloys

The precipitation behavior of nano–scale particles formed in Cu–base alloys was studied by means of transmission electron microscopy (TEM) and SQUID measurements. Linear arrangements of two or more nano–scale particles cubic in shape were observed in the <100> orientations of matrices in a Cu–Co alloy. Although the trend was less explicit in a Cu–Fe alloy, Fe precipitates accompanying twin–like lattice modulations were found in the decomposition, when no deformation was applied. The present SQUID measurements revealed several significant influences to magnetic properties were induced during the precipitation in Cu–base alloys. Lorentz electron microscopy confirmed that phase transformation from γ → α occurred at the stage that the Fe particles reach to 40~60nm in size.

A Bioinspired Approach to the Synthesis of Bimetallic CoNi Nanoparticles

Bimetallic CoNi nanoparticles have been prepared within the apoferritin cavity. The protein shell controls size, prevents aggregation, and makes nanoparticles water-soluble. The CoNi series prepared in this way were structurally and magnetically characterized, the resulting magnetic properties varying accordingly with composition (Co(75)/Ni(25), Co(50)/Ni(50), Co(25)/Ni(75)). Co and Ni metals were associated in each nanoparticle, as demonstrated by high-angle annular dark field scanning electron microscopy and electron energy loss spectroscopy (EELS). After intentional oxidation, the CoNi nanoparticles were characterized by EELS, X-ray absorption near edge structure (XANES), and SQUID measurements to evaluate the importance of the oxidation on magnetic properties.

The First Biomagnetic SQUID Measurement: Some Recollections

Event Abstract Back to Event The First Biomagnetic SQUID Measurement: Some Recollections David Cohen1, 2* 1 Mass. General Hospital, Athinoula A. Martinos Center for Biomedical Imaging, United States 2 Massachusetts Institute of Technology, Francis Bitter Magnet Lab, United States I have been asked to say a few words on this 40th anniversary of our paper [1], which introduced the SQUID detector into the area of biomagnetism. In particular, I have been asked to offer some recollections of that work, performed at the Francis Bitter Magnet Lab, at MIT. I find that some of the details seem to be as fresh in my mind today as if they happened yesterday (typical of the aging mind!). A general history of the early biomag work at MIT has already been written [2], but I here concentrate on the first SQUID measurement. I first relate how I happened to arrive at MIT to do this work. Earlier I had been at the University of Illinois at Chicago where I built a modest shielded room to measure the MCG and MEG. I was a young physicist who could gamble on a totally new career, but measuring weak fields of the body was considered really weird by my traditional physics colleagues, and I lacked internal support. Nevertheless, I did complete the measurements that interested me, using the best ac detector then available, which was a room-temperature copper coil, intrinsically very noisy. The next job was to look for a more sympathetic home, and I indeed found it at the MIT Francis Bitter lab, and moved there in 1969. I built a good 5-layer shielded room of roughly spherical shape, but what was needed was a very low-noise detector, to match the low background noise in this new, good room. So, in the fall of 1969, the stage was set for a new detector. I had been hearing about a new superconducting detector, with very low noise. Ed Edelsack, a funding officer who had given me a small grant, told me that he was also funding Jim Zimmerman, who had just co-invented the point-contact SQUID [3]. Ed put me in touch with Jim, and it was arranged that Jim would bring one of his first SQUIDs to MIT, to look for biomagnetic signals in the new room. Jim arrived near the end of December, with his SQUID equipment. After mounting it in the room, we looked at the easiest biomagnetic signal: the human heart. Jim was the subject. The resulting raw MCG signal was almost as clear as a routine ECG and several orders of magnitude better that the raw MCG due to a coil detector. A bit later we wrote our paper [1]. At the time, we had no idea that the use of the SQUID in bio-magnetism would eventually lead to a grand conference in Dubrovnik, 40 years later…

鉄系超伝導多結晶体の微細組織と電流輸送特性

Foreseeing practical applications of recently discovered new series of superconductor —Fe-pnictides, one of the most important parameters is current transport across grain boundaries. We combined SQUID measurements, magneto-optical (MO) imaging, scanning and transmission electron microscope (SEM and TEM) and low temperature laser scanning microscope (LTLSM) in order to understand the relationship between the microstructure and intergrain current transport, so called global current, in a random polycrystalline SmFeAsO0.85 (Sm1111) bulk. Our Sm1111 bulk showed significant global critical current density (Jc) which is more than one magnitude higher compared to random polycrystalline pure YBCO bulks at self field and the same temperature. However there was different temperature dependence of intergrain and intragrain Jc, exhibiting granularity at low temperature which was caused by large difference of Jc on two distinct scales. Strikingly most of intergrain current transport at self field relied on SNS Josephson weak links where supercurrent passed across the conductive impurity phase of FeAs, strongly suggesting the need of eliminating such a wetting phase in order to explore internal blocking effects at grain boundaries which are not fully understood yet.

Synthesis, Characterization and Magnetic Properties of Nickel Ferrite by Soft Chemical Method

Synthesis of nanosized nickel ferrite by a template method where tartaric acid is used as a ligand and nitric acid as an oxidant is described. The synthesized powders were characterized by TGA, XRD, FTIR, TEM, BET, VSM and SQUID studies. The average crystallite size of the phase pure material was of 30 nm and the surface area was found to be 20.37 m2.g−1. Magnetic measurements of the phase pure powder were carried out at room temperature by VSM at a magnetic field of 1 Tesla, and by SQUID at a magnetic field of 6 Tesla. High saturation magnetization of 31.43 emu.g−1 is observed in case of VSM and 43.36 emu.g−1 is observed for SQUID measurements. Comparative analysis of MS values synthesized by different routes are compared.