Magnetic Properties of Superconducting and Nonsuperconducting (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy

The temperature dependence of the DC susceptibility (T) is measured between 1.7 and 350 K on superconducting and nonsuperconducting bulk melt-processed (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_y by means of SQUID magnetometry. A strong superconducting contribution is found to coexist with a large paramagnetic moment provided by the Nd and Gd ions. The paramagnetic contributions measured on both types of samples follow a Curie–Weiss law; however, an antiferromagnetic ordering is not observed down to 1.7 K.     

SQUID and Magneto-Optic Investigations of Flux Turbulence in the Critical State

Instabilities of the critical state (turbulence) are observed in various high-T _c single crystals by means of magneto-optic (MO) imaging. Using the same samples as in the MO investigations, we observe characteristic steps in the magnetic moment measured by means of a SQUID magnetometer, which allows a continuous temperature sweep. A direct correlation between the steps and the MO images is established. Using these SQUID measurements, a much wider range of field and temperature can be covered. As a result, we find turbulence not only in a narrow temperature window but also in a field window ranging between 0 and 80 mT. Flux turbulence is also observed in magnetization loops. Furthermore, we present tests for turbulence in several different high-T _c samples.     

Room Temperature Ferromagnetism of Ge/MnAs Digital Alloys

The magnetic properties of Ge/MnAs digital alloys on GaAs (001) substrates, grown by molecular beam epitaxy, were investigated using a Quantum Design SQUID magnetometer. The Ge (1 nm)/MnAs (0.15 nm) digital alloy showed ferromagnetism up to 334 K with a coercive field of 576 Oe at room temperature, as determined from temperature-dependent magnetization and hysteresis loop measurements. The ferromagnetic and antiferromagnetic phases of a Ge₇/(Mn_0.5As_0.5)₁ superlattice, investigated using the full-potential linearized augmented plane wave (FLAPW) method yielded a ferromagnetic ground state with a high spin magnetic moment of Mn (3.45 _B) which induces only very small magnetic moments on its neighboring As or Ge atoms.     

A new small nano-Kelvin resolution thermometer for low-temperatureexperiments

A small, high-resolution paramagnetic susceptibility thermometer was developed using a GdCl₃ paramagnetic salt. The device uses a superconducting quantum interference device (SQUID) magnetometer to determine the temperature dependent magnetization of the salt in a magnetic field. The magnetic field is provided by a pair of small samarium cobalt permanent magnet disks situated inside the thermometer housing. This eliminates the need for a heavy, charging solenoid used in a conventional SQUID-based magnetic thermometer system. This thermometer can resolve approximately 10^-9 K near the liquid-gas critical point of ³He (~3.31 K). The drift rate of the thermometer was measured to be <2×10^-13 K/s. This light weight thermometer (~7 g) is a candidate for use in future low-temperature Space Shuttle and International Space Station experiments     

Development of High-T c SQUID System for Nondestructive Evaluation

Nondestructive evaluation (NDE) system with high spatial resolution power based on high-T _c superconducting quantum interference device (SQUID) has been developed. The SQUID is fabricated by YBCO thin film and mounted in vacuum at the end of a cold finger fixed at the inner wall bottom of the dewar, which is filled with liquid nitrogen (77 K) to cool the SQUID sensor. The sensor faces a thin flat sapphire window on the outer wall of the dewar and is thermally isolated from it by a vacuum space at a distance of less than 1 mm. The magnetic field sensitivity of SQUID is about 40 pT/ Hz in magnetically unshielded environment. This NDE system was used to detect the flaws buried into materials. The remnant magnetic field inducted by the local damages in a stainless steel sample was measured. Incorporated a gradiometric double-D excitation coil, the eddy current NDE was carried out. Small cracks in the aluminum below the surface can be detected effectively in unshielded environment.      

Thermomagnetic Flux-Jump Instabilities and Second Magnetization Peak in Bi2Sr2CaCu2O8 High-T c Superconducting Crystals

Measurements of the magnetization hysteresis loop performed on Bi ₂ Sr ₂ CaCu ₂ O ₈ high-T_c superconducting single crystals reveal the occurrence of pronounced jumps of the irreversible magnetization at T > 40 K. In this work we demonstrate their thermomagnetic origin. It is also shown that the low-field hollow in the magnetization hysteresis loops measured at T < 40 K and leading to the so-called second magnetization peak, is the manifestation of a thermomagnetic instability effect. It is suggested that the plastic vortex motion triggers the magnetic instabilities.     

Observation of X-band SQUID signals in a High-Tc superconducting film device

A microwave superconducting magnetometer is described in which a microstrip resonator is coupled to a two-hole high-T _c thin-film SQUID device. Both the microstrip circuit and the thin-film SQUID were fabricated by photolithography techniques. The YBCO thin film was deposited on single-crystal substrate of yttria-stabilized zirconia [YSZ(100)] by an ion beam sputtering technique producing a superconducting transition measured at a critical temperature ofT _c =92 K to within T 3 K. Non linear oscillatory behavior was observed in the microstrip resonator when inductively coupled to the SQUID. This nonlinear behavior yielded a microwave device in which the reflected microwave power varied with applied DC magnetic flux.     

Long-Range Spin-Triplet Superconductivity Induced by Magnetic Field in d Wave Superconductor/Ferromagnet Hybrid System

We theoretically study the long-range spin- triplet superconductivity in d wave superconductor/ ferromagnet/ferromagnet (S/F₁/F₂) trilayer junction, in which the magnetization of F₁ layer could be rotated in the y–z plane by an external magnetic field. The four-component Eilenberger equations were constructed to calculate the superconducting order parameters and density of states (DOS). Near the clean limit, the p wave equal-spin triplet component could be induced when the magnetization directions of F₁/F₂ layers are non-collinear, and the DOS exhibits a split zero-bias conductance peak. The various parameters such as ferromagnetic exchange energy, thickness of ferromagnetic layers, and angles between F₁/F₂ magnetization directions are studied for the effect on inducing triplet superconductivity. By magnetic field controlling the emergence of equal-spin triplet pairings or not, such a tunable S/F₁/F₂ trilayer junction based on long-range spin-triplet superconductivity could be used as a superconducting switch device, which would open up a new view of spintronics.     

Upper Critical Field of Four-CuO2-Layer TlBa2Ca3Cu4O11+δ Single Crystals

Magnetic measurements have been performed on single crystals of TlBa ₂ Ca ₃ Cu ₄ O ₁₁₊ in the magnetic fields up to 5.5 T. No change in the onset temperature of superconducting transition (T _c ) was observed in the magnetic fields between 0.4 and 5.5 T, while the superconducting transitions were broadened significantly in the same field. The linear temperature-dependence of the magnetization was found in a large temperature range below T _c . The slope of the magnetization versus temperature decreased significantly with increasing magnetic field. The experimental findings indicated that the upper critical field (H _C2 ) in this compound was very large and traditional methods of deriving H _C2 value were not applicable. Using the measured magnetization in the reversible region, the H _C2 properties of TlBa ₂ Ca ₃ Cu ₄ O ₁₁₊ single crystals have been studied based on a new model developed by Hao et al.     

Paramagnetic Meissner Effect in Electrochemically Doped Indium-Tin Oxide Films

Transparent conductive indium-tin oxide (ITO) thin films, electrochemically intercalated with alkali (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), alkali earth (Mg⁺², Ca⁺²), or complex NH\(_{4}^{+}\) ions, show tunable superconducting transitions with dome-shaped behavior of T _c versus electron density around the maximum at ～ 5 K. On field cooling, the transition into the superconducting state is accompanied by a paramagnetic response, i.e., an increase of magnetization, rather than the usual diamagnetic Meissner response. We provide an extensive study of this so-called paramagnetic Meissner effect (PME), using DC SQUID, transport measurements and a variety of sample sizes and growth conditions. We show that the PME in electrochemically doped ITO films results from a higher T _c at the sample edges than at the center of disk-shaped samples, causing flux to be expelled towards the center of the disk, following the flux-compression theory of Koshelev and Larkin. Changing to the opposite spatial T _c profile largely removes the paramagnetic response. The paramagnetic magnetization is strongly influenced by sample geometry and flux pinning conditions. The reduction of pinning defects by thermal annealing removes the paramagnetic response. An alternation of the external magnetic field restores the usual Meissner diamagnetism.     

Measurement of absolute intensity of weak magnetic fields using rf biased SQUID

A method to measure the absolute intensity of a weak magnetic field is described. We used a superconducting material as a sensor and detected its magnetization change between the superconducting state and the normal state at the transition temperature. The magnetization change measured by the SQUID system is proportional to the component of the absolute intensity of the magnetic field at the superconductor specimen which in parallel with the axis of the astatic pick up coils. A resolution as good as 10^?5 Oe has been obtained. The practical limit of the resolution of this method is also discussed.     

Phase Diagram of Single Crystal MgB2

Using magnetization, magneto-transport and single-crystal specific heat measurements we have determined the superconducting phase diagram of MgB₂. The superconducting anisotropy changes monotonously from a value of around 2 near T _c to above 4.5 at 22 K. For Hc a pronounced peak effect in the critical current occurs at the upper critical field. We present a strong evidence for a surface superconducting state for Hc which might account for the wide spread in reported values of the superconducting anisotropy .     

Effect of Short-Time Magnetic Relaxation on Hysteresis Curves in Single-Crystal Bi2Sr2CaCu2Ox

We have studied the role of short relaxation time on magnetic hysteresis in a Bi₂Sr₂CaCu₂O _x single crystal. Effects were examined by comparing isothermal magnetization curves obtained over a range of temperatures as a function of the applied magnetic field, H, for magnetic field ramp rates of 3 and 30 Oe/s. At low temperature, a second magnetization peak, H _p, was produced by differences in magnetic relaxation occurring over a short period during the initial stage of relaxation. The data suggested an absence of pinning crossover for H H _p.     

An Operation Circuit of a Micro-SQUID Magnetometer below 1?K

We have been developing a micro superconducting quantum interference device (??-SQUID) magnetometer for the study of quantum effects in ??m- or nm-sized magnetic materials. In the ??-SQUID magnetometer, the sample is placed on the SQUID loop directly and the flux is detected by the loop itself. The close proximity between the sample and the SQUID loop is of great advantage for highly sensitive detection of magnetization from the tiny magnetic materials. However, the heat release during the operation of the SQUID could cause a great problem in experiments below 1?K because of the close proximity of the SQUID and sample. Here we report a digital circuit for the ??-SQUID magnetometer which can reduce the heat release in the low temperature operation and whose parts are commercially available. The circuit consists of a FPGA-digital board with analog-to-digital and digital-to-analog converters, and an independent pre-amplifier. By using this circuit we succeeded in measuring quantum tunneling of magnetization in the single molecule magnet, Mn12Ph at 0.7?K.     

Low-field magnetization curves of YBa2Cu3O7−x and BiSrCaCu2O5.5: Effects of Josephson medium

We present the measurement of dc magnetization curve of three samples of high-T _c superconductors in relatively low magnetic field of ±50 mT and at temperaturesT=4.2 K andT=77 K by a SQUID susceptometer. The samples were polycrystalline YBa₂Cu₃O_7–x, a single crystal of YBa₂Cu₃O_7–x, and polycrystalline BiCaSrCu₂O_5.5. We have especially studied the influence of the Josephson medium inside our samples on their low-field magnetization. We have found closed hysteretic loops in the field ±4 mT with virgin and granular slopes in the sample of polycrystalline YBaCuO and clear flux jumps corresponding to the penetration of low-field (Josephson) vortices. The pinning of these vortices in our case is rather low. In polycrystalline sample of BiCaSrCuO, we have found only reversible magnetization curves corresponding to very low pinning and/or to relatively low number of weak links.     

Anion Disorder and Two-Dimensionality in the Superconducting and SDW States of (TMTSF)2ClO4

The anion-disorder and two-dimensionality effect for the superconducting and SDW phase of (TMTSF) ₂ ClO ₄ was investigated through static magnetization and resistance measurements for various cooling rates through the CIO ₄ ^- ClO ₄ ^- anion ordering temperature. In the slowly cooled phase, we find a strong dependence of the superconducting transition temperature on the a-axis residual resistivity a due to the suppression of anion ordering. This behavior supports models of non-s-wave pairing in (TMTSF) ₂ X. In the rapidly quenched phase, the transition temperature of SDW shows a large increase in the magnetic field. This means that the SDW phase of quenched (TMTSF) ₂ ClO ₄ is strongly suppressed by the two-dimensionality of the system.     

DC SQUIDs based on YBCO step edge junctions

We describe the fabrication and testing of dc SQUIDs (Superconducting QUantum Interference Devices) obtained by photolithographic patterning of YBa₂Cu₃O_7–x thin films deposited both on SrTiO₃ and MgO substrates. The Josephson junctions in the superconducting loop are of the step-edge type, where the weak link is obtained through the growth of grains with different orientations across suitably prepared steps previously etched on the substrate surface. TheI–V characteristics of the devices tested show multiple branches and instabilities having a weak dependence on the external magnetic field, probably due to formation of junction clusters on the substrate step. The SQUID devices showed quantum interference behavior and an easily detectable voltage modulation with the applied magnetic field at a temperature of 77 K.     

The Impact of Nuclear Magnetism on Superconductivity in a Metal with Nuclear Electric Quadrupole Splitting: Indium

We report the first investigation of the impact of nuclear magnetism on superconductivity in the tetragonal metal indium. We have measured the superconducting critical field B_c(T) and in its vicinity the nuclear magnetic heat capacity at ultralow temperatures, 170 KT200 mK. We compare the measured quantities with calculations which consider the nuclear magnetic Zeeman and the dominating nuclear electric quadrupole interaction in indium. The heat capacity data support the occurence of a positive sign of the electrical field gradient at nuclear sites and in consequence the existence of a nuclear low spin ground state. Surprisingly, at lowest investigated temperatures, 170 KT1 mK, the reduction of the critical field B_c(T) clearly exceeds the size of the calculated magnetization ₀ M(B_c, T) which is limited by the nuclear low spin ground state. In all other materials the interplay of nuclear magnetism and superconductivity has been studied so far (Al, AuAl ₂ , AuIn ₂ , Rh, and Sn), the bare nuclear magnetization appeared as an upper limit of the reduction of the critical field.     

Second Magnetization Peak at Low Fields due to Thermomagnetic Flux-Jump Instability

Magnetization measurements performed on a Nb thin film have revealed the existence of a second magnetization peak (SMP) well below the upper critical field boundary H _c2 (T). The results of this work provide clear evidence that the SMP originates from a thermomagnetic flux-jump instability. The similarity of the magnetization loops measured in Bi ₂ Sr ₂ CaCu ₂ O ₈ high-T _c superconducting crystals to those in Nb, as well as their dependence on the crystal size and field step indicate that a common mechanism is responsible for the SMP in these superconductors.     

Physical Proprieties of Ferrites Nanoparticles

Synthesis optical and magnetic of nearly monodisperse Fe₃O₄ nanoparticles through a simple coprecipitation method was studied in this work, Well-defined disperse nanospheres with an average size of 10 nm have been synthesized without any surfactant. Structural optical and magnetic properties were investigated by means of X-ray diffraction, transmission electron microscopy (TEM), UV-visible measurement, and superconducting quantum interference device (SQUID). The critical and blocking temperature, saturation magnetization, magnetic remanence, and coercive field of Fe₃O₄ nanoparticles are obtained at room temperature.