SQUID Susceptometry Research Articles

Flux focusing with a superconducting nanoneedle for scanning SQUID susceptometry

A nanofabricated superconducting quantum interference device (nano-SQUID) is a direct and sensitive flux probe used for magnetic imaging of quantum materials and mesoscopic devices. Due to the functionalities of superconductive integrated circuits, nano-SQUIDs fabricated on chips are particularly versatile, but their spatial resolution has been limited by their planar geometries. Here, we use femtosecond laser 3-dimensional (3D) lithography to print a needle onto a nano-SQUID susceptometer to overcome the limits of the planar structure. The nanoneedle coated with a superconducting shell focused the flux from both the field coil and the sample. We performed scanning imaging with such a needle-on-SQUID (NoS) device on superconducting test patterns with topographic feedback. The NoS showed improved spatial resolution in both magnetometry and susceptometry relative to the planarized counterpart. This work serves as a proof-of-principle for integration and inductive coupling between superconducting 3D nanostructures and on-chip Josephson nanodevices.

Modulation of magnetic properties at room temperature: coordination-induced valence tautomerism in a cobalt dioxolene complex.

The valence-tautomeric six-coordinate complex [Co(tbdiox)2(4-papy)2] (1; tbdiox = redox-active 3,5-di-tert-butyl-o-dioxolene, 4-papy = 4-phenylazopyridine) was synthesized and its electronic structure examined. Whereas 1 shows regular thermally driven valence tautomerism in the solid state, it partially dissociates in solution to form the five-coordinate species [Co(tbdiox)2(4-papy)] (2) and free 4-papy. Species 1 and 2 exhibit different electronic structures-low-spin (ls) Co(III) and high-spin (hs) Co(II), respectively-in solution at room temperature and therefore different magnetic properties. Since 1 and 2 are in an equilibrium that is 4-papy-dependent, the magnetic moment of the solution species can be tuned by means of the ligand content. Thus, the concept of coordination-induced valence tautomerism (CIVT) has been introduced. The electronic structures of 1 and 2 as well as their CIVT were elucidated by X-ray crystallography, electrochemistry, titration experiments, and all variable-temperature SQUID susceptometry, NMR, EPR, and electronic absorption spectroscopy. The experimental findings are strongly supported by broken-symmetry DFT calculations. The magnetic exchange interactions in different types of valence-tautomeric cobalt complexes were explored computationally.

Agreement between local and global measurements of the London penetration depth

Recent measurements of the superconducting penetration depth in Ba(Fe1−xCox)2As2 appeared to disagree on the magnitude and curvature of Δλab(T), even near optimal doping. These measurements were carried out on different samples grown by different groups. To understand the discrepancy, we use scanning SQUID susceptometry and a tunnel diode resonator to measure the penetration depth in a single sample. The penetration depth observed by the two techniques is identical with no adjustments. We conclude that any discrepancies arise from differences between samples, either in growth or crystal preparation.

Capped-Tetrahedrally Coordinated Fe(II) and Co(II) Complexes Using a “Click”-Derived Tripodal Ligand: Geometric and Electronic Structures

The 'Click'-derived tripodal ligand tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine, tbta, was used to synthesize the complexes [Fe(tbta)Cl]BF(4), 1, and [Co(tbta)Cl]BF(4), 2. Both complexes were characterized by (1)H NMR spectroscopy and elemental analysis. Single-crystal X-ray structural determination of 2 shows a 4 + 1 coordination around the cobalt(II) center with a rather long bond between Co(II) and the central amine nitrogen atom of tbta. Such a coordination geometry is best described as capped tetrahedral. 1 and 2 are thus the first examples of pseudotetrahedral coordinated Fe(II) and Co(II) complexes with tbta. A combination of SQUID susceptometry, EPR spectroscopy, Mössbauer spectroscopy, and DFT calculations was used to elucidate the electronic structures of these complexes and determine the spin state of the metal center. Comparisons are made between the complexes presented here with related complexes of other ligands such as tris(2-pyridylmethyl)amine, tmpa, hydrotris(pyrazolyl) borate, Tp, and tris(2-(1-pyrazolyl)methyl)amine, amtp. 1 and 2 were tested as precatalysts for the homopolymerization of ethylene, and both complexes delivered distinctly different products in this reaction. Blind catalyst runs were carried out with the metal salts to prove the importance of the tripodal ligand for product formation.

Scanning SQUID susceptometry of a paramagnetic superconductor

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth $\ensuremath{\lambda}$ on a nonsuperconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: (1) $\ensuremath{\delta}$-doped SrTiO${}_{3}$, which has a predominantly diamagnetic response, (2) a thin film of LaNiO${}_{3}$, which has a predominantly paramagnetic response, and (3) the two-dimensional electron layer at a SrTiO${}_{3}$/LaAlO${}_{3}$ interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements.

Local measurement of the penetration depth in the pnictide superconductorBa(Fe0.95Co0.05)2As2

We use magnetic force microscopy (MFM) and scanning SQUID susceptometry to measure the local superfluid density $\rho_{s}$ in Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$ single crystals from 0.4 K to the critical temperature $T_c=18.5$ K. We observe that the penetration depth $\lambda$ varies about ten times more slowly with temperature than previously published, with a dependence that can be well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of $\lambda$, as well as obtain its temperature dependence and spatial homogeneity. We find $\rho_{s}$ to be uniform despite the highly disordered vortex pinning.

Evidence for a Nodal Energy Gap in the Iron-Pnictide Superconductor LaFePO from Penetration Depth Measurements by Scanning SQUID Susceptometry

We measure changes in the penetration depth lambda of the T_{c} approximately 6 K superconductor LaFePO. In the process, scanning SQUID susceptometry is demonstrated as a technique for accurately measuring local temperature-dependent changes in lambda, ideal for studying early or difficult-to-grow materials. lambda is found to vary linearly with temperatures from 0.36 to approximately 2 K, with a slope of 143 +/- 15 A/K, suggesting line nodes in the superconducting order parameter. The linear dependence up to approximately T_{c}/3, similar to the cuprate superconductors, indicates well-developed nodes.

Whole Liver Iron Overload Measurement by Magnetic Iron Detector (MID). A Non Cryogenic Bio-Susceptometer.

Accurate assessment of body-iron accumulation is essential for diagnosis and therapy of iron-overload in diseases such as thalassemia, hereditary hemochromatosis and other forms of severe congenital or acquired anemias. At present, the gold standard to determine liver-iron concentration (LIC) is the invasive liver needle biopsy. This technique might lead to large error, in assessing iron burden, due to the heterogeneous distribution of iron deposition in the liver. SQUID bio-susceptometer and MRI are currently the only non-invasive validated methods for LIC measurements. The susceptometer presented herein, named Magnetic Iron Detector (MID), measures directly the iron overload in the whole liver. All of its components operate at room temperature. Since February 2005 about 150 patients and 90 healthy volunteers have been measured and the measures were obtained in blind. The local Ethics Committee approved the study and all subjects gave informed consent. The result of correlations with the LIC measurements by SQUID susceptometry (Dr. A. Piga, Turin) in 43 patients showed a R 0.86 (Fig 1). In 2 patients, affected by Congenital Hemocromatosis, we correlated the LIC measurement by MID with the assessment of the expected iron depletion obtained with the phlebotomy therapy R 0.94 (Fig 2). All the measurements were correlated with the serum-ferritin concentration values R 0.72. We obtained correlation with the LIC measurement by liver biopsy in 7 patients R 0.89, further measures are in progress. The reproducibility of the iron overload of the same patients, measured after a relatively short lapse of time, is better than 0.5g. In conclusion the data obtained shows that MID is a reliable instrument for the diagnosis of the liver iron overload and for the follow-up of the chelation therapy. It is simpler to operate being manageable directly in the Clinical Center and more affordable than competing techniques. [Display omitted] [Display omitted]

Preliminary Results of Full Body Iron Overload Measurement by a Magnetic Susceptometer.

Accurate assessment of body-iron accumulation is essential for managing therapy of iron-chelating diseases characterized by iron overload such as thalassemia, hereditary hemochromatosis, myelodysplasia and other forms of severe anemia. At present, the gold standard to determine liver-iron concentration (LIC) is liver needle biopsy. In this work, we present an alternative non-invasive technique to measure LIC based on a room-temperature susceptometer. SQUID biosusceptometers and MRI are currently the only validated non-invasive methods for LIC measurements. However, SQUIDs are liquid helium-cooled superconducting devices, therefore costly and resource intensive. Furthermore, SQUIDs are only sensitive to a fraction of the liver volume because of their magnetic configuration. MRI requires large magnets with dedicated software and hardware, trained operators, and is accurate only at low iron concentration. The susceptometer presented herein measures iron overload in the whole liver, as the entire human torso fits within its region of sensitivity. Since all of its components operate at room temperature, this susceptometer is more affordable then competing techniques and can reach a wider hospital base. The study was approved by the local Ethics Committee and all subjects gave informed consent. Since February 2005, 40 patients (30 thalassemia major or intermedia, 5 hereditary hemochromatosis, 5 other severe anemia) and 68 healthy volunteers have been measured. The signal picked up by the susceptometer has two sources: an overall magnetic background of the torso and an eventual contribution from liver iron excess. After measuring the magnetic signature of a patient, statistical analysis methods and neural-network simulations (trained using the control data) are employed to estimate the background signal, given the patient anthropometric data. Liver-iron overload is then determined by subtraction of the estimated background from the total measured signal. The refinement of the methodology is in progress and, at present, the error in liver iron is about 1g (SD), corresponding to typical concentrations of 0.5 mg/cm^3. A correlation study between iron overload and blood serum-ferritin concentration in the patient population attained a correlation coefficient R~0.73. Comparison with measurements of LIC via SQUID susceptometry on a subset of 30 patients participating in the present study (carried out by Dr. A. Piga at Ospedale S. Anna, Torino, Italy) yields a correlation coefficient R~0.77. Four patients (3 thalassemia major, 1 hereditary hemochromatosis) under intensive iron depletive therapy have been measured at least twice; our estimate of liver iron reduction is compatible with the clinical data (R~0.76). Comparison with LIC measured via biopsy is in progress. All comparison were blinded. These preliminary results indicate that possible applications of this non-invasive, full-body susceptometer include monitoring the efficacy of the therapy as well as improving the diagnosis and care management of patients with iron overload. [Display omitted]

Synthesis and magnetic properties of novel nitronyl nitroxides: racemic 2-phenyl-4-alkyl-4,5,5-trimethyl-4,5-dihydro-1 H-imidazolyl-1-oxyl-3-oxide

Novel chiral nitronyl nitroxides having various substituents such as alkyl, alkoxymethyl, silyloxymethyl and acyloxymethyl groups at C(4) position of the imidazole ring were synthesized as the racemates. Magnetic properties of the obtained nitroxides were characterized by means of EPR spectroscopy and SQUID susceptometry. All the obtained nitroxides obeyed Curie–Weiss law and showed very weak anti-ferromagnetic interactions at low temperatures. Their Weiss constants ( θ) were estimated to be −1 to 0 K.

4,6-Bis(trifluoromethyl)- N, N′-di- tert-butyl-1,3-phenylenebis(aminoxyl) and its bis(hexafluoroacetylacetonato)manganese(II) complex: synthesis, X-ray crystallography, and magnetism

Synthesis, crystallography, and magnetic characterization of bis(aminoxyl) 1 and its 2:1 complex with Mn(hfac) 2 ( 2) are described. X-ray structure of 2 (at 20 K) provides one of the rare examples of discrete molecules found for the complexes of Mn(hfac) 2 and bis(aminoxyls). The differences in conformations of the bis(aminoxyl) moieties in 1 and 2 lead to ferromagnetic and antiferromagnetic exchange coupling through the 1,3-phenylene moiety, in 1 and 2, respectively, as determined by SQUID susceptometry. DFT calculations (at the crystallographic geometries) give the triplet and the singlet ground states for 1 and for bis(aminoxyl) in 2, respectively. Two analogous conformations of the bis(aminoxyl) are proposed to be responsible for the unprecedented S-shaped time dependence for the conversion from the singlet to the triplet state of 1 in 2-methyltetrahydrofuran.

NO Disproportionation Reactivity of Fe Tropocoronand Complexes

The synthesis and characterization of divalent [Fe(TC-5,5)] (1) and trivalent [Fe(OTf)(TC-5,5)] (2) tropocoronand complexes are described. Compound 1 reacts with 1 equiv of NO to form the {FeNO}7 complex 3. A single-crystal X-ray structure determination of 3 reveals a trigonal bipyramidal geometry with a linearly coordinated nitrosyl (Fe−N−O = 174.3(4)°) having a short Fe−N distance of 1.670(4) A. EPR and Mossbauer spectroscopy, SQUID susceptometry, and normal coordinate analysis indicate 3 to be a low-spin {FeIII(NO-)}2+ species. In the presence of excess NO, 3 converts to a metastable nitrosyl−nitrito complex that decomposes by losing NO2, which subsequently nitrates the aromatic tropolone rings of the ligand. The final products of the NO disproportionation reaction are N2O and [Fe(NO)(TC-5,5-NO2)] (4). The νNO stretching band of 4 is increased to 1716 cm-1 from its value of 1692 cm-1 in 3, owing to the electron-withdrawing nitro groups on the ligand, and the compound no longer promotes the disproportio...

Formation of Fe(III)Fe(IV) Species from the Reaction between a Diiron(II) Complex and Dioxygen: Relevance to Ribonucleotide Reductase Intermediate X

Formation of Fe(III)Fe(IV) Species from the Reaction between a Diiron(II) Complex and Dioxygen: Relevance to Ribonucleotide Reductase Intermediate X

An Internal Hyperfine Field of 62.4 T in Ferromagnetically Ordered α-Iron(II) Octaethyl-Tetraazaporphyrin

We report on the magnetic properties of the molecular solids derived from square-planar, neutral iron(II) octaethyltetraazaporphyrin, FeOETAP. This compound exists as two polymorphs, α and β. dc SQUID and ac susceptometry measurements (1–1000 Hz) show that the α polymorph orders as a soft molecular ferromagnet below TCurie = 2.8 K, whereas the β form, with the same number of unpaired spins and a similar g value, exhibits zero-field splitting to a non-magnetic ground state. 57Fe-Mössbauer spectroscopy indicates that α-FeOETAP exhibits a remarkably large internal hyperfine field, Hint, of 62.4 T. This, to our knowledge, is the highest value of Hint ever reported for Fe, regardless of its spin state.

Disproportionation of Nitric Oxide Promoted by a Mn Tropocoronand

The synthesis and characterization of the divalent tropocoronand, [Mn(THF)(TC-5,5)] (1), are described. Compound 1 reacts with NO to afford a discrete mononitrosyl [Mn(NO)(TC-5,5)] (2), which was isolated and shown formally to contain the {MnIIINO-}2+ fragment by infrared spectroscopy, normal coordinate analysis, X-ray crystallography, and SQUID susceptometry. The Mn center in 2 has idealized trigonal bipyramidal geometry with the NO ligand occupying a coordination site in the equatorial plane. The Mn−N−O bond angle is nearly linear, 174.1(3)°. Compounds 1 and 2 react with excess NO to afford N2O and [Mn(NO2)(TC-5,5)] (3). The X-ray structure of 3 reveals coordination of the nitrito ligand through one oxygen atom, the Mn−O bond length being 2.09(4) A. The other oxygen atom interacts weakly with the manganese center at a distance of 2.61(5) A. Quantitation by gas chromatographic analysis of released N2O confirms that 1 equiv forms for each molecule of 2 which converts to 3. Infrared spectroscopic experimen...

Paramagnetism of Tetranuclear Complexes between TCNX Ligands (TCNE, TCNQ, TCNB) and Four Pentaammineruthenium or Dicarbonyl(pentamethylcyclopentadienyl)manganese Fragments.

The tetranuclear complexes {(&mgr;(4)-TCNX)[Ru(NH(3))(5)](4)}(A)(8) and (&mgr;(4)-TCNX)[Mn(CO)(2)(C(5)Me(5))](4) [A = PF(6) or CF(3)SO(3); TCNX = TCNE (tetracyanoethene), TCNQ (7,7,8,8-tetracyano-p-quinodimethane), or TCNB (1,2,4,5-tetracyanobenzene)] were studied by variable-temperature (2-300 K) SQUID susceptometry. Mono- and dinuclear species [(PhCN)Ru(NH(3))(5)](PF(6))(2) (PhCN = benzonitrile) and {(&mgr;-L)[Ru(NH(3))(5)](2)}(PF(6))(4) (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also investigated for comparison and were found to be essentially diamagnetic. Despite the even electron count, both the ruthenium and manganese tetranuclear complexes are paramagnetic, albeit with different spin-spin exchange coupling patterns. The manganese systems are characterized by exchange-coupled S = 1 states at the individual metal centers, whereas the magnetic behavior of the tetranuclear ruthenium compounds results from an exchange-coupling interaction between two S = (1)/(2) sites, identified as Ru(III)/Ru(II) mixed-valence pairs.

Metamagnetic domains and dynamic fluctuations in FeBr2

The mixed phase and the regime of non-critical fluctuations of the magnetic phase diagram of FeBr2 is investigated by SQUID susceptometry and light diffraction techniques. The experiments seem to evidence instability of the tricritical point as conjectured recently. The observation of stripe domains and light diffraction below and above Tm=4.6 K and the virtual continuation of the phase transition line to above Tm are in agreement with the occurrence of a critical endpoint at Tm and of a bicritical endpoint at T>Tm.

SQUID magnetocalorimetry of disordered and artificially structured magnetic semiconductors

Thermal-wave SQUID susceptometry (TWS) is applied to determine the thermally modulated magnetic moment of a magnetic semiconductor Pb1-x-ySnxMnyTe (x=0.72, y=0.03) and of a magnetic semiconductor superlattice [EuTe(5)/PbTe(15)]400 together with their magnetic heat capacities within one working cycle. The former system is a ferromagnet with reentrance into a spin-glass behavior, the latter is an antiferromagnet-(EuTe)/paramagnet-(PbTe) superlattice grown by molecular beam epitaxy with few atomic layers (5/15) and 400 repeated periods. The surprisingly strong ferromagnetism in the superlattice EuTe/PbTe is believed to arise from the formation of coupled 2D polarons at the hetero-interfaces stabilized by dipole-dipole interaction.

Structural and magnetic properties of diode radio-frequency sputtered Cr/Co multilayers

Structural properties of diode radio-frequency (rf)-sputtered Cr/Co multilayers have been investigated using in situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, and nuclear magnetic resonance. Results have been correlated with magnetic characterizations obtained by SQUID susceptometry. Interdiffusion along ≂10 Å occurs at the ‘‘Cr on Co’’ interface. The Co on Cr interface appears sharper. X-ray diffraction and nuclear magnetic resonance show that Co layers grow with bcc structure when the Co layer thickness dCo is lower than ≂15 Å. In this case, the films are strongly textured with the Cr bcc (100) direction perpendicular to the plane of the substrate. A better structural coherence is observed for the thinner layers. When dCo is thicker than ≂15 Å, a mixture of hcp and fcc Co phases appears by x-ray diffraction and nuclear magnetic resonance. The good structural quality of the films is confirmed by the occurrence of satellite peaks in the x-ray diffraction patterns. The Co magnetic moment extracted from the hysteresis loops measurements is coherent with these changes of the cobalt structure with layer thickness. In plane magnetization measurements show a antiferromagnetic coupling of the Co layers through the Cr layers.

Determination of the crossover critical exponent in a YBaCuO superconductor glass by SQUID susceptometry

Using a SQUID susceptometer we have recorded the ultra low field (3mOe - 6.5 Oe) ZFC dc magnetization of a YBaCuO sintered sample. All the curves show two well defined steps that correspond respectively a) to the transition of the superconducting grains at a field independent temperature T C ≈ 91.5 K and b) to the percolation transition of the Josephson Junction system at a lower and field dependent temperature, T c(H) (identified with the inflection point of the ZFC curves). T c(H) scales as H 1/ψ with 1/ψ = 0.58± 0.03. We show also that the scaling behaviour can be extended to a rather large region around the critical temperature T c(H). This result demonstrates the relevance of the magnetic field for the critical transition of the J-J system.