Ac SQUID Research Articles

Escape rate in ac SQUID on josephson junction based on single- and multi-band superconductors in thermal activation regime

The escape rate of S→R switching (from superconducting S state to unstable resistive R state) in an ac SQUID with Josephson junction based on single- and multi-band superconductors is investigated by taking the frustration effects in a multi-band superconducting state into account. Using the effective critical current approach, it is shown that the escape rate in thermal activation regime can manifest qualitative features caused by the frustration effects in two- and three-band superconductors.

Hybrid Molecular Magnets with Lanthanide- and Countercation-Mediated Interfacial Electron Transfer between Phthalocyanine and Polyoxovanadate.

A series of {V12}-nuclearity polyoxovanadate cages covalently functionalized with one or sandwiched by two phthalocyaninato (Pc) lanthanide (Ln) moieties via V-O-Ln bonds were prepared and fully characterized for paramagnetic Ln = SmIII-ErIII and diamagnetic Ln = LuIII, including YIII. The LnPc-functionalized {V12O32} cages with fully oxidized vanadium centers in the ground state were isolated as (nBu4N)3[HV12O32Cl(LnPc)] and (nBu4N)2[HV12O32Cl(LnPc)2] compounds. As corroborated by a combined experimental (EPR, DC and AC SQUID, laser photolysis transient absorption spectroscopy, and electrochemistry) and computational (DFT, MD, and model Hamiltonian approach) methods, the compounds feature intra- and intermolecular electron transfer that is responsible for a partial reduction at V(3d) centers from VV to VIV in the solid state and at high sample concentrations. The effects are generally Ln dependent and are clearly demonstrated for the (nBu4N)3[HV12O32Cl(LnPc)] representative with Ln = LuIII or DyIII. Intramolecular charge transfer takes place for Ln = LuIII and occurs from a Pc ligand via the Ln center to the {V12O32} core of the same molecule, whereas for Ln = DyIII, only intermolecular charge transfer is allowed, which is realized from Pc in one molecule to the {V12O32} core of another molecule usually via the nBu4N+ countercation. For all Ln but DyIII, two of these phenomena may be present in different proportions. Besides, it is demonstrated that (nBu4N)3[HV12O32Cl(DyPc)] is a field-induced single molecule magnet with a maximal relaxation time of the order 10-3 s. The obtained results open up the way to further exploration and fine-tuning of these three modular molecular nanocomposites regarding tailoring and control of their Ln-dependent charge-separated states (induced by intramolecular transfer) and relaxation dynamics as well as of electron hopping between molecules. This should enable us to realize ultra-sensitive polyoxometalate powered quasi-superconductors, sensors, and data storage/processing materials for quantum technologies and neuromorphic computing.

Effect of unconventional current-phase relation of Josephson junction on escape rate in ac SQUID

The escape rate of S→R switching (from superconducting S state to unstable resistive R state) in an ac SQUID on Josephson junction with unconventional current-phase relation was investigated. We used two types of current-phase relations with additional terms to the first harmonic sinϕ: a case of the second harmonic sin2ϕ and a case of the fractional term sin(ϕ/2). It was shown that escape rate can exhibit qualitative features in the case of junctions with unconventional current-phase relations.

Effect of Unconventional Current-Phase Relation of Josephson Junction on Escape Rate in Ac Squid

Effect of Unconventional Current-Phase Relation of Josephson Junction on Escape Rate in Ac Squid

Filamentization of YBCO Coated Conductors by Microcontact Printing

To reduce ac losses of coated conductors (CCs), microcontact printing was employed as an alternative technology for processing striated tapes. All CCs possessed the same basic cross-sectional architecture: stainless steel/ABAD-YSZ/CeO2/ PLD-YBCO . By magnetron sputtering, thin (100-300 nm) Au layers were deposited on bare (no cover layer on YBCO) tapes. Appropriate SAMs (self-assembled monolayers of alkanethiols) line patterns were printed with elastomeric stamps on the Au layer. A subsequent etching of Au, with ferri/ferrocyanide, and of YBCO, with phosphoric acid, was used to produce the desired filamentary structure. The sample performance was checked by angle-dependent measurements of the critical current density, Jc . Striating the CCs leads to a reduction of hysteretic ac losses of the HTS tapes, which according to E.H. Brandt's calculations depend on the width of the filaments. This was confirmed by AC SQUID measurements of the complex susceptibility and paves the way for long-length processing of low-loss CCs.

Spacing-dependent dipolar interactions in dendronized magnetic iron oxide nanoparticle 2D arrays and powders

Self-assembly of nanoparticles (NPs) into tailored structures is a promising strategy for the production and design of materials with new functions. In this work, 2D arrays of iron oxide NPs with interparticle distances tuned by grafting fatty acids and dendritic molecules at the NPs surface have been obtained over large areas with high density using the Langmuir-Blodgett technique. The anchoring agent of molecules and the Janus structure of NPs are shown to be key parameters driving the deposition. Finally the influence of interparticle distance on the collective magnetic properties in powders and in monolayers is clearly demonstrated by DC and AC SQUID measurements. The blocking temperature T(B) increases as the interparticle distance decreases, which is consistent with the fact that dipolar interactions are responsible for this increase. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples and may be described by using the Vogel Fulcher model.

Ferromagnetic decoration in metal–semiconductor separated and ferrocene functionalized single‐walled carbon nanotubes

AbstractSpin resonance, magnetic measurements, and structural analysis are reported for metal‐semiconductor separated SWCNTs after filling with ferrocene. Raman scattering performed after a heat treatment confirms partial transformation to double‐walled CNTs but results from spin resonance (FMR), X‐ray diffraction, and TEM evidence in addition the growth of ferromagnetic nanoparticles. For the metallic tubes the particles are identified as magnetite (Fe3O4) with full chemical stoichiometry. From the temperature dependence of the FMR response and from measurements of the magnetization by dc and ac SQUID the magnetite crystals are shown to undergo a Verwey transition. The transition temperature from the SQUID experiment is around 125 K as expected but considerably higher than observed from the FMR analysis. Results for semiconducting tubes are similar but magnetic particles are an order of magnitude smaller and exhibit different structures in addition to magnetite.

A mononuclear Fe(III) single molecule magnet with a 3/2↔5/2 spin crossover.

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.

Quantitative nanoscale viscosity measurements using magnetic nanoparticles and SQUID AC susceptibility measurements

The frequency dependent response of suspended thermally blocked magnetic nanoparticles in an oscillating magnetic field was used to quantitatively determine the nanoscale viscosity of a fluid at various temperatures. The nanoscale viscosity was found to be within 7% of the macroscale viscosity determined through rheology, except at low temperatures where crystallization occurs in the fluid. The range of viscosities accessible through this technique and criteria for selection of the magnetic nanoparticle material are discussed.

Modulation SQUID electronics working with high-T c SQUIDs in open space

SQUID electronics optimized for operation in unshielded space with dc high-Tc superconducting quantum interference devices (HTS SQUIDs) are developed, manufactured, and studied. The dynamic characteristics of the SQUID electronics are studied with two magnetic-field sensors based on the HTS SQUIDs: a conventional SQUID sensor with a resolution of 100 fT/Hz1/2 and a supersensitive SQUID sensor with a resolution of 15 fT/Hz1/2 at frequencies exceeding 10 Hz and a resolution of 30 fT/Hz1/2 at a frequency of 1 Hz. Stable operation of the magnetometric channel is demonstrated with both SQUID sensors under urban conditions. On the basis of a complex programmable logic device (CPLD), an ac bias can be realized in the SQUID and the modulation signal can be compensated in the feedback, bias-current, and desired-signal circuits. Such a compensation system is the most appropriate and versatile means of providing stable operation of the magnetometric channel in the presence of the SQUID ac bias, regardless of the type of high-temperature sensor and the configuration of the input contacts in the measurement probe.

Synthesis, structural determination and magnetic properties of layered hybrid organic–inorganic, iron (II) propylphosphonate, Fe[(CH 3(CH 2) 2PO 3)(H 2O)], and iron (II) octadecylphosphonate, Fe[(CH 3(CH 2) 17PO 3)(H 2O)

Fe[(CH 3(CH 2) 2PO 3)(H 2O)] ( 1) and Fe[(CH 3(CH 2) 17PO 3)(H 2O)] ( 2) were synthesized by reaction of FeCl 2·6H 2O and the relevant phosphonic acid in water in presence of urea and under inert atmosphere. The compounds were characterized by elemental and thermogravimetric analyses, UV–visible and IR spectroscopy. The crystal structure of ( 1) was determined from X-ray single crystal diffraction studies at room temperature: monoclinic symmetry, space group P2 1, a = 5.7 0 7 ( 1 ) Å , b = 4.8 1 1 ( 1 ) Å , c = 1 1.8 1 8 ( 2 ) Å , and β = 9 8.6 2 ( 3 ) ° . The compound is lamellar and the structure is hybrid, made of alternating inorganic and organic layers along the c direction. The inorganic layers consist of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from the water molecule, separated by bi-layers of propyl groups. A preliminary structure characterization of compound ( 2) suggests a similar layered structure, but with an interlayer spacing of 40.3 Å. The magnetic properties of the compounds were both studied by a dc and ac SQUID magnetometer. Fe[(CH 3(CH 2) 2PO 3)(H 2O)] ( 1) obeys the Curie–Weiss law at temperatures above 50 K ( C = 3.8 1 cm 3 K mol - 1 , θ = - 6 2 K ), indicating a Fe +II oxidation state, a high-spin d 6 ( S = 2 ) electronic configuration and an antiferromagnetic exchange couplings between the near-neighbouring Fe(II) ions. Below T = 2 2 K , Fe[(CH 3(CH 2) 2PO 3)(H 2O)] exhibits a weak ferromagnetism. The critical temperature of T N = 2 2 K has been determined by ac magnetic susceptibility measurements. Compound ( 2) shows the same paramagnetic behaviour of the iron (II) propyl derivative. The values of C and θ were found to be 3.8 cm 3 K mol - 1 and −44 K, respectively, thus suggesting the presence of Fe +II ion in the S = 2 spin state and antiferromagnetic interactions between Fe(II) ions at low temperatures. Zero-field and field cooled magnetic susceptibility vs. T plots do not overlap below T = 3 0 K , suggesting the presence of an ordered magnetic state. The critical temperature, T N , has been located by the peaks at T N = 2 6 K from the ac susceptibility ( χ′and χ″) vs. T plots. Below T N hysteresis loops recorded in the temperature region 1 6 < T < 2 6 K show an S-shape, while below 15 K assume an ellipsoid form. They reveal that compound ( 2) is a weak ferromagnet. The critical temperature T N in these layered Fe(II) alkylphosphonates is independent of the distance between the inorganic layers.

Search for the chiral superconducting state in Sr2RuO4–Ru eutectic

Sr2RuO4 has become one of the most intensively studied transition metal oxides displaying unconventional superconductivity (Tc=1.5 K). It is known that its eutectic system Sr2RuO4–Ru shows interface superconductivity with an enhanced onset of ∼3 K. It is predicted that in the 3-K phase, a second transition to a chiral state occurs at a temperature between 1.5 and 3 K. The second transition is thought to be observable by ac-susceptibility measurements. We report the low-frequency ac-susceptibility of Sr2RuO4–Ru eutectic measured using a SQUID ac magnetometer.

Evidence for elemental partitioning in gas-atomized Nd 2Fe 14B modified by alloying additions

Measurements of the Curie and spin reorientation temperatures provide information about elemental partitioning as a function of effective quench rate in Nd 2Fe 14B-based powders modified by alloying additions, including Ti and C. The Curie and spin reorientation temperatures of inert gas-atomized (IGA) powders of various size fractions and annealing conditions were measured using differential thermal analysis (DTA) and SQUID AC magnetometry. The Curie temperatures of all size fractions of the Ti/C-containing powders are lower for both the as-quenched and annealed portions than those of the Ti/C-free powders. The spin reorientation temperatures of the Ti/C-modified material are found to be significantly lower than that expected for pure Nd 2Fe 14B. The data suggest that the alloying additions reside in the Nd 2Fe 14B lattice to varying degrees, dependent upon the quench rate experienced by the particle, and are expelled from the compound to the grain boundaries upon extended annealing.

Successive Magnetic Phase Transitions of CucCo1-cCl2-FeCl3 Graphite bi-intercalation Compounds

CucCo1-cCl2-FeCl3 graphite bi-intercalation compounds (GBIC's) have a c-axis stacking sequence of -G-I1-G-I2-G-I1-G-I2-G- (G = graphite layer, I1 = CucCo1-cCl2 layer, and I2 = FeCl3 layer). These compounds undergo magnetic phase transitions at Th, Tcu, Tcl, TSG, and TRSG (Th>Tcl>TRSG=TSG). depending on the Cu concentration. The phase transition at Th is related to a helical spin order. The phase transitions at Tcu and Tcl are associated with a spin order of CucCo1-cCl2 layers. The re-entrant spin glass phase below TRSG for c≤0.4 and the spin glass phase below TSG for c≥0.5 are due to the spin frustration effect occurring in FeCl3 layers. The nature of these phases has been studied using SQUID DC magnetization and SQUID AC magnetic susceptibility.

Spin Frustration Effect in Magnetic Graphite Intercalation Compounds

The magnetic phase transition of stage-2 CucCo1-cCl2 GIC's has been studied using SQUID AC magnetic susceptibility. For 0.4 ≤ c ≤ 0.9 the system undergoes a phase transition at Tcl and a reentrant spin glass transition at TRSG (<Tcl). For 0.9<c ≤ 0.93 the system undergoes a spin glass transition at TSG from the paramagnetic phase to the spin glass phase. No phase transition is observed for c = 1 at least above 0.3 K. The nature of reentrant spin glass phase and spin glass phase is examined from the frequency dependence of absorption X″.

Magnetic phase transition in two-dimensional XY random spin system: stage-2 graphite intercalation compounds

The magnetic phase transition of a two-dimensional (2D) random spin system with competing exchange interactions, stage-2 GICs, has been studied using SQUID AC magnetic susceptibility and SQUID DC magnetization. The magnetic phase diagram of critical temperature versus Cu concentration is determined. For the system undergoes two phase transitions at and . Below a 2D ferromagnetic order is established in each intercalate layer. Below there appears a 3D antiferromagnetic phase with the 2D ferromagnetic layers being antiferromagnetically coupled along the c-axis. For the system undergoes a phase transition at and a reentrant spin glass transition at . For the system undergoes a spin glass transition at from the paramagnetic phase to the spin glass phase. No phase transition is observed for c = 1 at least above 0.3 K because of the fully frustrated nature of the 2D antiferromagnet on the triangular lattice. The enhancement of in the intermediate concentration and the positive sign of the Curie-Weiss temperature for c < 0.83 indicates that the intraplanar interaction between and spins is ferromagnetic. The nature of the ferromagnetic phase, reentrant spin glass phase and spin glass phase is examined from the frequency dependence of absorption .

Dynamic spin fluctuations in stage-2 graphite intercalation compound

Stage-2 GIC is a quasi-two-dimensional XY-like ferromagnet with very weak antiferromagnetic interplanar exchange interaction. This compound undergoes two magnetic phase transitions at and . Between and a two-dimensional long range spin order is established within each island in the intercalate layers. Below there appears a three-dimensional antiferromagnetic ordered phase. The dynamic aspect of spin ordering in this compound has been studied employing the temperature, frequency and field dependence of dispersion and absorption acquired using an AC SQUID magnetometer. The absorption shows a complicated frequency dependence near and , suggesting two kinds of relaxation time associated with intraisland fluctuation and interisland fluctuation is much longer than .

Decanuclear Manganese(III) Complexes with the [Mn10O8]14+ Core: Structural and Magnetochemical Characterization of [Mn10O8(O2CPh)6(chel)8] (chel = pic, dbm)

The preparation and properties are described of two related Mn10 clusters with the same [Mn10O8]14+ (10 MnIII) core. Dissolution of [Mn4O2(O2CPh)6(MeCN)2(pic)2] (1) in MeCONMe2 (DMA) and CH2Cl2 in the presence of picH (picolinic acid) leads to slow crystallization of [Mn10O8(O2CPh)6(pic)8] (2) in 50−55% yield. The same reaction in the presence of dibenzoylmethane (dbmH) instead of picH gives [Mn10O8(O2CPh)6(pic)6(dbm)2] (3). Complex 2·6CH2Cl2 crystallizes in the monoclinic space group P21/n with (at −169 °C) a = 14.987(3) Å, b = 17.177(3) Å, c = 21.250(4) Å, β = 91.48(1)°, and Z = 2. Complex 3·3CH2Cl2 crystallizes in the triclinic space group P1̄ with (at −165 °C) a = 14.892(6) Å, b = 15.537(6) Å, c = 14.697(6) Å, α = 109.82(2)°, β = 97.75(2)°, γ = 111.66(2)°, and Z = 1. The two structures are very similar, differing only in the chelate identity at two positions, and resulting small perturbations in some metric parameters. Variable-temperature magnetic susceptibility data were obtained for 2·3H2O with a dc SQUID in the range 2.00−320 K and in a 10 kG applied field. The effective magnetic moment (μeff) per molecule slowly decreases from 14.28 μB at 320 K to 4.94 μB at 2.00 K. Data were also collected with an ac SQUID in the range 2.00−30.0 K in a 1 G field oscillating at 250 Hz. The ac data essentially superimpose on the dc data except at temperatures ≤6 K. The combined results indicate a ST = 0 ground state for 2 with a very low-lying excited state(s) that is (are) populated even at 2.00 K. The possibility of a ST = 1 (or even ST = 2) ground state cannot be completely ruled out, however, in the absence of data at <2 K.

Multilayer high-temperature superconductors

Abstract Using the BCS approach and a tightbinding description of the band structure, we explore some superconducting properties of layered superconductors. Particular emphasis is devoted to the interlayer pairing interaction g 3 . Negative values of g 3 lead to a staggered order parameter, having alternating signs from layer to layer, while g 3 positive leads to one sign only. Adopting a superconducting loop, interrupted by a multilayer, it is shown that the sign of g 3 can be determined experimentally. The same holds for a DC SQUID configuration, consisting of two multilayer Josephson junctions in a loop, and for an AC SQUID.

Critical behavior in the reentrant phase of (P dFe)Mn

The field and temperature dependence of the nonlinear magnetic response of a reentrant (PdFe)Mn alloy has been investigated within the reentrant phase with an ac SQUID susceptometer operating at 16 Hz. Comparison with numerical simulations based on a mean-field effective-field Ising model with a reentrant phase boundary indicates that the experimental susceptibility isotherms possess several features which may be critical in origin, and suggests a potentially valuable criterion for recognizing critically reentrant behavior.