Superconductivity and Unusual Magnetic Features in Amorphous Carbon and in Other Unrelated Materials

Traces of superconductivity (SC) were observed in three different inhomogeneous sulfur-doped amorphous carbon (a-C) systems: (i) in old commercial powder, (ii) in fabricated (by pyrolytic decomposition of sucrose) powder, and (iii) in a-C thin films. (i) The commercial a-C powder contains 0.21 at% sulfur and reveals traces of two superconducting phases around T _C ?34 and T _C～65 K. (ii) The fabricated a-C powder is diamagnetic but not superconducting. However, mixtures of this powder with sulfur (a-CS) which were heated to 250 °C also show traces of SC up to T _C～42 K. (iii) Non-superconducting a-C thin films were grown by electron-beam induced deposition. SC at T _C～34 K emerged only after heat treatment with sulfur. It is proposed that the high SC states in a-CS (T _C～65 K at ambient pressure) and in H₃S materials (T _C = 203 K under >200 GPa) has the same origin. In both sulfur-containing systems, SC is induced by the interaction between electrons and the high frequencies of low mass atom (H or C) vibrations a-CS products obtained by heating commercial and fabricated a-C powders at 400 °C show unusual magnetic features: (a) pronounced irreversible peaks around 55–80 K which appear in the first zero-field-cooled (ZFC) runs only. This temperature range is close to the highest T _C observed. (b) These peaks are totally suppressed in the second ZFC sweeps measured a few minutes later. (c) The peaks reappear after 18 months. (d) Around the peak position, the field-cooled (FC) curves cross the ZFC plots (ZFC > FC). All these phenomena are intrinsic properties of the amorphous carbon materials and were not observed in crystalline graphite. The ZFC > FC state was also observed in chiral-based magnetic memory device and unexpectedly in the liver taken from a patient with mantle cell lymphoma. This peculiarity will be discussed.     

Structure and Magnetic Properties of La0.66Sr0.33MnO3 Thin Films Derived Using Sol-Gel Technique

In this paper the results of a study of the structural and magnetic properties of La_0.66Sr_0.33MnO₃ (LSMO) polycrystalline films grown on glass substrate using Sol-Gel technique are presented. The samples were structurally characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. The average grain size range of 30 nm has been obtained from XRD investigations of granular LSMO samples. Zero Field Cooling (ZFC) and Field Cooling (FC) magnetization measurements have been performed and magnetic hysteresis loops of LSMO were recorded at various temperatures. The temperature dependences of the magnetization of LSMO films recorded in ZFC and FC regimes exhibited considerable difference between the curves. The blocking and the ferromagnetic phase transition temperatures of about 262?K and 300?K, respectively, were observed from magnetization measurements.     

Effects of Bismuth Doping on the Physical Properties of La0.6−x Bi x Sr0.4CoO3 (0≤x≤0.15) Cobaltites

The structural and magnetic properties of cobaltites with the nominal composition La_0.6?x Bi _x Sr_0.4CoO₃ (0≤x≤0.15) have been investigated. X-ray diffraction analysis using Rietveld refinement show that all our samples crystallize in the rhombohedral structure. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT exhibits thermomagnetic irreversibility signature of short range ferromagnetic interactions in our samples. The ferromagnetic–paramagnetic transition temperature decreases with increasing Bi amount. In the paramagnetic phase, all our synthesized samples obey to the Curie–Weiss law. The Curie–Weiss analysis suggests that the spin state of Co³⁺ is IS while Co⁴⁺ ions are in IS for x=0 and LS for x≥0.05. The Bi doping also leads to a weakening in the magnetic moment. The magnetic entropy change exhibits a maximum value which decreases from 2.28 J/kg?K for x=0 to 1.24 J/kg?K for x=0.15 upon a magnetic field change of 5 T.     

Effect of Nickel Doping on the Magnetotransport Properties of?Sm0.55Sr0.45MnO3 Manganites

We studied the effects of nickel (Ni) doping on the magneto-transport properties of Sm_0.55Sr_0.45MnO₃ manganites near the metal-insulator transition. Various concentrations of Ni-doped Sm_0.55Sr_0.45MnO₃ samples up to 10% were prepared (Ni was partially substituted at the Mn-site). The temperature dependence of resistivity and magnetoresistance were measured as a function of Ni concentrations at various applied magnetic fields. We observed a nonlinear reduction of the metal-insulator transition temperature (MIT) with increasing concentration of Ni, 5% of Ni was sufficient to completely suppress the insulator-metal transition. Moreover, we observed dramatic increases of the resistance of the doped material with an increasing Ni-doping (5% of Ni increases R by more than 1000 times). The resistivity peaks at various magnetic fields collapses on themselves at the high temperature ends above the MIT. We also performed magnetization versus temperature measurements on both Ni-free the Ni-doped samples for FC and ZFC states. The FC and ZFC curves rapidly decrease to paramagnetic state at 175 K and 130 K for ZFC and FC states, respectively. For other Ni-doped samples, we observed a reduction in the paramagnetic transition temperature with increasing Ni concentration.     

Magnetic study of single domain ?-Fe3N nanoparticles synthesized by precursor technique

Nanostructured single domain ?-Fe₃N particles have been prepared by chemical route. Magnetic dipolar interaction among particles has been observed. The FC and ZFC magnetization curves show the superparamagnetic blocking temperature around 125 K, at low applied field of 0.005 T. At higher applied field, these ZFC and FC magnetization curves merge because of increase in magnetization energy that overcomes the magnetic anisotropy and dipolar interaction energies. Mössbauer spectroscopy further corroborates the superparamagnetic nature at 300 K.     

Structural, Transport, and Magnetic Properties of (Ru0.9Nb0.1)Sr2(Gd0.67R0.67Ce0.66)Cu2O z (R = Nd, Gd, and Tb)

Samples with nominal compositions of (Ru_0.9Nb_0.1)Sr₂(Gd_0.67R_0.67Ce_0.66)Cu₂O _z (R = Nd, Gd, Tb) were prepared, and the influence of doped rare-earth element on the structural, magnetic and superconducting properties of these samples has been investigated. Resistivity and room-temperature thermoelectric power measurements showed that the superconductivity is mainly affected by the change in the hole concentration induced by doping of the rare-earth element. All of these samples exhibited weak ferromagnetic behavior at temperatures below approximately 90 K with the branching of zero-field-cooled (ZFC) and field-cooled (FC) magnetization. In the ZFC magnetization measurements, diamagnetic transition was observed at temperatures below approximately 15 K for the R=Gd and Nd samples. Magnetization measurements also revealed that the partial substitution of Tb for Gd results in a significant enhancement of weak-ferromagnetic component of the FC magnetization, as well as an increase in the magnetic ordering temperature up to 20 K. A quite opposite behavior was observed in the FC magnetization for the case of the R=Nd sample. The experimental results are discussed in conjunction with the local structural changes in the Ru sublattice, which are induced by doping of the rare-earth element, based on the results of Rietveld refinement of the X-ray diffraction data.     

Effects of Lu Doping on the Magnetic Behavior of YBCO Superconductors Prepared by MPMG Method

We report on magnetic behavior of Y _1?xLu _xBa ₂ Ca ₃ O _7?δ (x= 0, 0.5) samples synthesized by a modified melt powder melt growth (MPMG) method at three different sintering temperatures (920, 930, 940 ^°C). Isothermal magnetization measurements were done as a function of magnetic field at 20 and 77 K upon zero-field cooling (ZFC) process. The critical current densities as a function of magnetic field have been estimated from M–H hysteresis loop. Partial Lu substitution for Y on YBCO superconductors improved significantly the bulk critical current density, which is confirmed by field-cooled magnetization and AC susceptibility measurements. Magnetic relaxation measurement has been carried out in remanent magnetization regime. AC susceptibility measurements have been performed as a function of temperature to determine the transition temperature.     

Magnetic, Electrical and Thermodynamic Properties of UCuT x Al11−x Alloys Where T = Mn, Fe and x=4 and 5

The structure, magnetic, electrical, and thermodynamic properties of UCuT _x Al_11?x alloys, where T = Mn or Fe and x=4 or 5 are presented. The behavior of the Fe alloys is ferromagnetic-like with the Curie points amounting to 180 and 230 K, and the saturation magnetic moments under magnetic field of 5 T equal to 4.75 and 6.02 μ_B/f.u., respectively, whereas under a magnetic field of about 34 T the magnetic moments amount to 6.9 and 9.0 μ_B/f.u. for the alloys with x=4 and 5, respectively. The Curie points are reflected in the temperature dependence of the specific heat in which the anomalies are found at 180–200 and 230 K for alloys with x=4 and 5, respectively, however, it shows no reflection in the temperature dependence of the electrical resistivity. The field dependence of the magnetization at T=1.9 K for both compounds exhibits considerable hysteresis. There is a pronounced difference between ZFC and FC magnetization in its temperature dependence below the Curie point for materials with x=4 and 5. The Mn alloys exhibit ferrimagnetic-like character for which, supposedly, the interplay of the uranium and manganize sublattices is responsible. Magnetic transitions are determined at T _N =300 (x=4) and 380 K (x=5). However, those anomalies do not find confirmation in the temperature dependence of the specific heat and the electrical resistivity. Magnetic moments determined at T=1.9 K and in a magnetic field of 5 T are very low and in both cases amount to about 0.35 μ_B/f.u. and these values are slightly higher in a magnetic field of 34 T reaching a value of about 1.5 μ_B/f.u. Also for the Mn alloys the clear difference between ZFC and FC magnetization in its temperature dependence below the Curie point is observed.     

Temperature evolution of structure and magnetic properties in the perovskite Sr2MnSbO6

The structural and magnetic properties of the perovskite Sr₂MnSbO₆ have been studied. Combining neutron and X-ray powder diffraction data, the temperature evolution of the structural parameters was investigated with the Rietveld method between 2 and 1000 K. The crystal structure is tetragonal (space group I4/m) within the temperature interval of 2-750 K and cubic (space group Fm-3m) above 750 K. Both octahedral B-site positions were found to be partially occupied by Mn and Sb, but with different Mn/Sb ratios. The magnetic susceptibilities showed irreversibility between field cooled and zero-field cooled (ZFC) conditions and spin glass like magnetic dynamics including aging and memory phenomena at temperatures below 30 K; all appearing well above a broad maximum at 13 K in the ZFC susceptibility curves. This suggests that the material reaches an unconventional spin-glass state at low temperatures, possibly arising from a competitive situation between the double exchange (ferromagnetism) and the super-exchange (antiferromagnetism). Neutron diffraction patterns showed no evidence of a long-range magnetic ordering at 2 K which is consistent with spin glass behavior. The factors governing the observed structural and magnetic properties of Sr₂MnSbO₆ are discussed and compared with those of other quaternary Mn- and Sb-containing perovskites. Graphs of the temperature of magnetic phase transitions as functions of the cation size were constructed and are discussed for the AB³⁺_1/2B⁵⁺_1/2O₃ series with isomorphous substitution of B³⁺ and B⁵⁺ cations. Possible influence of the A-cation sublattice on magnetic properties is also shortly considered.     

Superconductivity of a Calcium-Doped Graphite CaC30

We report the results of a dc magnetization study of the graphite doped with Ca atoms (CaC₃₀). A Ca-doped graphite was prepared in an ultra-high vacuum (UHV) chamber without lithium atoms used for making bulk CaC₆. The superconducting transition temperature (T _SC) was about 9.2 K, which is lower than that of CaC₆ (11.5 K). Further, at a 50 mT, the second transition occurs at higher temperatures in ZFC and FC, viz. 250 K in ZFC and 130 K in FC.     

Study of magnetic inhomogeneity in β-Cu3Fe4V6O24

The temperature dependence of dc magnetization and electron paramagnetic resonance (EPR) spectra of the ??-Cu₃Fe₄V₆O₂₄ multicomponent vanadate were investigated. Dc magnetic measurements showed the presence of strong antiferromagnetic interactions (Curie-Weiss temperature, ?? ?? 80 K) at high temperatures, while zero-field-cooled (ZFC) magnetization revealed a cusp-like maximum in low fields at T _f1 = 4.4 K, which coincides with the splitting of the ZFC and FC curves. Another maximum was registered at T _f2 = 3.0 K. These two temperatures (T _f1 and T _f2) could be regarded as freezing temperatures in the spin glass state of two magnetic sublattices of Fe1 and Fe2 ions. The EPR spectrum of ??-Cu₃Fe₄V₆O₂₄ is dominated by a nearly symmetrical, very intense and broad resonance line centered at g _eff ?? 2.0 that could be attributed to iron ions. Below 10 K, an additional EPR spectrum with g ₁ = 2.018(1) and g ₂ = 2.175(1) appears, as well as a very weak line at g_eff = 1.99(1). The former spectrum is probably is due to divalent copper ions, and the latter line due to vanadium V⁴⁺ complexes. The temperature dependence of EPR parameters (g-factor, linewidth, integrated intensity) was determined in the range of 3?C300 K. Two low-temperature maxima in the temperature dependence of the integrated intensity (at 40 and 6 K) were fitted with a function suitable for pairs of exchange-coupled Fe³⁺ ions. A comparison of dc magnetic susceptibility and EPR integrated intensity indicates the presence of spin clusters, which play an important role in determining the low-temperature magnetic response of _??-Cu₃Fe₄V₆O₂₄.     

The hydrothermal synthesis of super-paramagnetic barium hexaferrite particles

Superparamagnetic Ba-hexaferrite nanoparticles were prepared using modified hydrothermal synthesis. The precursor and hydroxide [OH⁻] concentrations were optimized and the synthesis temperature and time were drastically reduced. The size and the morphology of synthesized nanoparticles was confirmed by transmission electron microscopy (TEM) images. The crystal structure of the nanoparticles was characterized by X-ray diffraction data. Powders synthesized at 160 °C exhibit a bimodal particle size distribution while those synthesized at T_S = 150 °C show a monomodal particle size distribution. Zero-field-cooling (ZFC) and field-cooling (FC) magnetization measurements were performed using a superconducting quantum interference device magnetometer from 2 to 300 K to investigate the magnetic properties of nanoparticles. The FC/ZFC magnetization measurements showed a typical superparamagnetic behavior. The synthesized superparamagnetic particles exhibit a disc-like shape, in average 11 nm wide and 3 nm thick with a room temperature magnetization of approximately 10 Am²/kg at 5 T.     

Magnetic Properties of LCMO/LSMO Thin Films on LAO and ALO Substrates

Two La_0.7Ca_0.3MnO₃(35?nm)/La_0.7Sr_0.3MnO₃(30?nm) manganite bilayers were grown on LaAlO₃(LAO) and Al₂O₃(ALO) substrates (i.e., the LCMO/LSMO/LAO and LCMO/LSMO/ALO samples) using a magnetron sputtering technique. The individual Curie temperatures of the LSMO and LCMO sublayers were 325?K, 225?K for the LCMO/LSMO/LAO sample and 325?K, 210?K for the LCMO/LSMO/ALO sample. The zero-field-cooled (ZFC) magnetization of the samples demonstrated maximums at ??150?K and ??170?K, respectively. The difference between the ZFC and field-cooled (FC) magnetizations was greater for the bilayer deposited on ALO substrate due to its polycrystalline nature. The coercive field of the LCMO/LSMO/LAO sample was greater at room temperature 298?K than at 150?K: This was interpreted as due to a change of the magnetic state of the LCMO sublayer in this temperature interval.     

New results on Magnetic Properties of Tin-Ferrite Nanoparticles

This paper presents new physical properties of nanocrystalline SnFe₂O₄ ferrites, synthesized by co-precipitation method. Magnetization measurement indicates superparamagnetic behavior; the blocking temperature is about 300?K. We performed X-ray diffraction, infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), Energy Dispersive Spectroscopy (EDS), UV-visible measurement, superconducting quantum interference device (SQUID) and zero-field-cooled (ZFC)/field-cooled (FC) measurements. TEM images show the high crystallinity and grain size of ferrite nanocrystals. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is a partially inverse spinel. The obtained UV?Cvis data were used to calculate the energy band gap (3.82?eV) of nanocrystalline SnFe₂O₄. On the other hand the magnetic properties of the samples; saturation magnetization (M _s ) and coercive field (H _c ) were determined using a superconducting quantum interference device (SQUID).     

Structural and Magnetic Studies of Ca2?x Sm x MnO Compounds (x=0–0.4)

The Ca_2?x Sm _x MnO₄ (x=0?C0.4) compounds were synthesized by a solid?Csolid method and characterized by XRD at room temperature. The XRD profiles were indexed with a tetragonal structure (I/4mmm space group) for x??0.3 and orthorhombic one (Pnma space group) for x=0.4. The magnetic measurements were studied as a function of temperature (T=2?C300?K) and applied field (?? ₀ H=0?C10?T). When the temperature decreases, each compound has shown first a ferromagnetic?Cparamagnetic (FM?CPM) transition and then an antiferromagnetic?Cferromagnetic (AFM?CFM) one. The transition temperatures are found to be Sm-doping dependent. For all compounds, a spin-glass phenomenon is evidenced by FC/ZFC magnetization curves.     

Synthesis and Physical Properties of La0.53Ca0.26Ba0.21Mn0.77Ti0.21Zr0.02O3 Multiferroic Material

This work reports the synthesis, structural characterization, ferroelectric and magnetic behavior of the La_0.53Ca_0.26 Ba_0.21Mn_0.77Ti_0.21Zr_0.02O₃ (LCBMTZ) complex perovskite. The material is obtained by the solid state reaction method. Rietveld refinement of X-ray diffraction experiments shows that the material crystallizes in a tetragonal perovskite structure, space group P4mm (#99). Impedance spectroscopy measurements give evidence of ferroelectricity with high values of electric permittivity (ε′=119.2) and electrical polarization curves confirm it. Measurements of magnetic hysteresis loops at 10 and 150 K reveal a ferromagnetic behavior. On the other hand, the comparison of the FC and ZFC magnetization and resistivity measurements shows different magnetic regimes associated with changes in the resistivity. The analysis of these curves suggests the existence of magnetoelectric coupling in temperature regimes below 228.09 K.     

Influence of Ag-doping in La0.5Sr0.5CoO3 on Its Structural and Magnetic Properties

The structural and magnetic properties of the mixed valent cobaltites La_0.5Sr_0.5?x Ag _x CoO₃ (0≤x≤0.15) with perovskite structure have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement shows that all our samples crystallize in the orthorhombic structure with $R\overline{3}c$ space group. Although the unit cell volume remains almost constant, the Co–O bond length and the Co–O–Co bond angle are sensitive to the Ag addition. All our studied samples undergo a paramagnetic-ferromagnetic transition with decreasing temperature. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT of La_0.5Sr_0.5CoO₃ (x=0) sample exhibits thermomagnetic irreversibility indicating frustration and competition of both antiferromagnetic and ferromagnetic interactions. It is suggested that Co³⁺ ions are in the intermediate spin state but Co⁴⁺ ions stay in a mixture of intermediate and high spin states. Ag addition leads to dramatic changes in magnetic properties. An interesting result has been obtained for La_0.5Sr_0.4Ag_0.1CoO₃, where the ZFC and FC curves coincide. For La_0.5Sr_0.35Ag_0.15CoO₃ (x=0.15) sample, the observed thermomagnetic irreversibility is much higher than that observed in x=0 sample. Hysteresis loops were recorded for x=0, 0.05, 0.1, and 0.15. The coercitive field is found to increase with Ag content reaching 0.26 T for x=0.15 whereas the remanent magnetization decreases.     

Magnetic and Mössbauer Studies of Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Magnetization and ⁵⁷Fe Mössbauer effect spectroscopy (MS) studies of Ba(Fe_1?x Ni _x )₂As₂ single crystals (x=0 to 0.054) at temperatures (5 K to 300 K) have been performed. Magnetic measurements show that for BaFe₂As₂ the magnetic moment decreases below T _N=136 K. T _N is suppressed monotonically by Ni doping. On the other hand, for higher x values the magnetic moment increases below T _N. Unexpectedly for x=0.024 (T _N=67 K), the virgin zero-field-cooled (ZFC) curve is higher than that of field-cooled (FC) one below 48 K. The magnetic MS spectra of this sample are composed of a superposition of two subspectra, corresponding to commensurate and incommensurate field distributions. The average magnetic hyperfine field H _eff decreases with T and becomes zero at 80 K. For higher x values, the samples become superconducting at T _C=15.5 and 19 K for x=0.046 and 0.054, respectively. For both samples below T _C, the FC curves are positive (the paramagnetic Meissner effect) up to applied field of H～15 Oe and the susceptibility is inversely proportional to H. The MS spectra below and above T _C are almost identical, indicating that the MS parameters are not sensitive enough to detect the superconducting state. The peculiar phenomena observed are attributed to disorder induced by the presence of Ni atoms in the Fe sublattice.     

M?ssbauer and Magnetic Studies of Co0.5Mn0.5Fe2O4 and Mn0.1Mg0.2Co0.7 Fe2O4 Nanoferrites

We report the magnetic properties of Mn_0.5Co_0.5Fe₂O₄ and Mn_0.1Mg_0.2Co_0.7Fe₂O₄ nanoferrites. The compounds were synthesized by a glycol-thermal method with average particle sizes of about 13?nm and 8?nm, respectively. The M?ssbauer measurements were done at 300?K. The distribution of cations between tetrahedral?(A) and octahedral?(B) sites is investigated. The M?ssbauer spectra indicate ferrimagnetic behavior of the compound. Field cooled (FC) and zero field cooled (ZFC) magnetizations were performed by a Superconducting Quantum Interference Device (SQUID) magnetometer from 4?C380?K. Variation of the magnetizations with the applied fields (up to 50 kOe) were recorded at isothermal temperatures 4, 50, 100, 200 and 300?K. An increase in FC magnetization is observed with increasing applied field. This is explained based on superparamagnetic behavior of the particles.     

Size Effects on the Magnetic Properties of ZnFe2O4 Nanoparticles

We report structural and magnetic measurements on ZnFe₂O₄ nanoparticles obtained through coprecipitation chemical method. The Rietveld analysis of X-ray patterns reveals that (i) our samples are single phase, (ii) the average particle size increases with synthesis temperature, and (iii) the cationic disorder increases with decreasing of the mean particle size. The Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetization measurements show that the blocking temperature increases with increasing of the particle size and, to the sample grown at T=850?°C, it is possible to observe both Néel temperature to larger particles and blocking effects to smaller particles. Finally, we have observed that the coercive field does not decay with the square root of temperature following the Néel relaxation and Bean–Livingston approaches.