Combined effect of substrate temperature and laser energy density on laser ablated BiFeO3 thin films

Bismuth ferrite (BiFeO₃) thin films were prepared on Pt(111)/Ti/SiO₂/Si substrates by laser ablation deposition at various substrate temperature (T _sub) and laser energy density (D _L). The combined effect of T _sub and D _L on crystalline phase, preferential orientation and surface morphology of the films were investigated. The microstructure and morphology of the films showed a strong dependence on T _sub and D _L simultaneously. BiFeO₃ thin films with single phase could be prepared at T _sub-D _L of 893 K?1 J/cm², 943 K?2 J/cm² and 963 K?3 J/cm², respectively. Film deposited at T _sub = 963 K and D _L = 3 J/cm² exhibited better ferroelectric property due to the combination of high (111) preferential orientation, dense surface morphology and less Fe²⁺ content.     

Pulsed laser deposition of BiFeO3 thin films with large polarization on Pt(111)/Ti/SiO2/Si by controlling substrate temperature

Bismuth ferrite (BiFeO₃) thin films with large polarization were grown on Pt(111)/Ti/SiO₂/Si by optimizing the substrate temperatures (T _sub). The BiFeO₃ thin films were prepared by pulsed laser deposition and the effects of T _sub (T _sub = 853–913 K) on crystallization orientation, surface morphology and properties of the films were investigated. The microstructure and morphology of the films showed a strong dependence on T _sub. The film prepared at T _sub = 893 K had a relatively high degree of (111) preferential orientation and densely packed morphology. A large polarization with the maximum remanent polarization of 108 μC/cm² was obtained, which was due to the high degree of preferential orientation and the dense surface morphology at the optimum substrate temperature.     

Ag5Pb2O6/CuO Composite, an Approach to Ambient Temperature Superconductivity

Byström?CEvers compound Ag₅Pb₂O₆ is highly conducting oxide, and it was prepared from the components Ag₂O and PbO₂ fired in a 300 bar O₂ atmosphere and 623 K. Compound was milled, and particles were dispersed in a powdered CuO matrix. For low concentrations of metallic particles (<38 w/w percents), the composite is semiconductor or insulator, while for concentrations >48 w/w percents material is metallic down to liquid helium temperatures. For intermediate concentrations, the composite exhibits properties characteristic for superconductors with transition temperatures 178 K<T _c<356 K, dependent on the size of dissolved metallic particles and their mutual distance. Magnetic data support a presumption of novel superconductivity. Samples were prepared with SC transition temperature T _c=138 K, and in the temperature range 145?C800 K electric resistance obeys the temperature dependence ln?R??(T ₀/T)^1/2, and this indicates a possible 1-dimensional conduction mechanism.     

Vibrational spectrum and X-ray diffraction studies of CeZrO4 phase with an ordered arrangement of Ce and Zr ions prepared by graphite reduction

A series of binary CeO₂–ZrO₂ compounds with the composition of Ce/Zr = 1:1 (molar ratio) were reduced by graphite powders at various reducing temperatures (1173 ≤ T _red. ≤ 1573 K) for 24 h in a self-made reducing device. The reduced products obtained at various T _red. were characterized by X-ray diffraction and Vibration Spectrum. In the case of 1223 ≤ T _red. ≤ 1573 K, the reduced phase is identified as the k-CeZrO₄ phase with the space group of P2₁3, in which Ce and Zr ions are in an ordered arrangement similar to that of the pyrochlore Ce₂Zr₂O_7, and the ordering level between Ce and Zr ions in the pyrochlore-type k-CeZrO₄ phase decreased with the decreasing T _red.. The phase is stabilized without decomposition down to room temperature; the reason is attributed to the reducing atmosphere retention caused by graphite until the end of experiment. In the case of T _red. < 1223 K, a phase transition from k-CeZrO₄ phase to t′-Ce_0.5Zr_0.5O₂ phase was observed. The formation of t′ phase is due to the fact that the reducing atmosphere is too weak to severe enough to yield Zr⁴⁺/Ce⁴⁺ cation ordered configurations. The graphite reducing mechanism is discussed in the paper.     

Copper-containing glass polyalkenoate cements based on SiO<Subscript>2</Subscript>–ZnO–CaO–SrO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses: glass characterization,physical and antibacterial properties

A series of copper (Cu)-containing glasses were synthesized and incorporated into a SiO₂–ZnO–CaO–SrO–P₂O₅-based glass system. Additions of 6 and 12 mol% CuO retained the amorphous character, and glasses were processed to possess similar particle sizes and surface areas. Glass characterization using X-ray photoelectron spectroscopy and magic angle spinning nuclear magnetic resonance determined that the addition of 12 mol% CuO increased the fraction of Q⁴-speciation and the concentration of bridging oxygens. Each glass presented solubility profiles for the release of Si⁴⁺ (18–31 mg/L), Ca²⁺ (13–16 mg/L), Zn²⁺ (<3 mg/L) and Sr²⁺(2–10 mg/L); however, no Cu²⁺ or P⁵⁺ were released. Cu-GPCs were formulated, and the working time (T _w) and setting times (T _s) were found to be dependent on both polyacrylic acid concentration and CuO addition. The mechanical properties, i.e. the compressive strength (18–30 MPa) and the adhesive bond strength (0.79–1.32 MPa), were relative low which is likely due to the glass structure. Antibacterial properties were evaluated in E. coli (4 mm), S. epidermidis (10 mm), S. aureus (UMAS-1) and vancomycin resistant S. aureus (2 mm) and presented antibacterial effects in each microbe tested.     

Multiple Superconducting Gaps and Anisotropic Spin Fluctuations in Hole-Doped and Electron-Doped Iron-Pnictides: NMR Studies

We present extensive ⁷⁵As-NMR data on the electron-doped pnictides PrFeAsO_0.89F_0.11 (T _c=45 K), LaFeAsO_0.92F_0.08 (T _c=23 K), and the hole-doped Ba_0.72K_0.28Fe₂As₂ (T _c=31.5 K) single crystal. We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T ₁ decreases below T _c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T _c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps.     

Atomic Motion in Low-Coverage Helium Films Adsorbed in FSM Nanochannels

⁴He and ³He films adsorbed in nanoporous silicates have shown similar heat capacities until the quantum-fluid layer appears at coverages over the first-layer completion n ₁. To obtain information on dynamics of adatoms at low coverages below the quantum fluid region, we have done pulsed-NMR experiment at 3.3 MHz for ³He films adsorbed in straight 2.4 nm nanochannels of FSM silicates. The spin-lattice and spin-spin relaxation times T ₁ and T ₂ observed at 0.54–7 K were well described by the two-dimensional version of the Bloembergen-Purcell-Pound model. At coverages 0.4–1.7n ₁, minima of T ₁, indicating the spin correlation time τ _c of 4.8×10^?8 sec, were observed at temperatures between 6 and 3 K. With decreasing temperature, changes in T ₁ and T ₂ become small below about 1.5 K, suggesting crossover from thermally-activated motion to quantum tunneling. In contrast to large variations below n ₁, both relaxation times above n ₁ are almost independent of coverage, which is likely to indicate that τ _c is determined by interlayer exchange of adatoms. Below n ₁, onsets for localization of adatoms were suggested by a decrease in T ₂.     

Superconducting Properties of Ag and Sb Substitution on Low-Density YBa2Cu3O δ Superconductor

The effects of silver (Ag) and antimony (Sb) substitution on low-density YBa₂Cu₃O _δ (YBCO) superconductor were investigated. Two series of sample with a nominal composition of YBa_2?x Ag _x Cu₃O _δ and YBa_2?x Sb _x Cu₃O _δ where x=0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and 0.50 were synthesized and characterized. All Ag-doped samples showed metallic behavior at the normal state and T _C?onset was found at 90 K. T _C?zero decreased as the Ag concentration increased. Optimum Ag concentration was achieved at x=0.20 where T _C?zero has the highest value of 87 K and J _C at 70 K is 16.50 A/cm². For Sb-doped case with x≤0.30, the samples showed metallic behavior above T _C?onset while semiconducting behavior was shown for x≥0.40. The optimum Sb concentration was achieved at x=0.15 where T _C?zero is 85 K and J _C value measured at 70 K is 2.75 A/cm². T _C?onset and T _C?zero were found to decrease toward higher Sb concentration. The crystallographic structure transformed to tetragonal in Sb-doped samples of x≥0.30 while other samples remain orthorhombic.     

Magnetization Study in CuCr2O4 Spinel Oxide

We report magnetization and ac susceptibility as functions of the temperature and frequency for CuCr₂O₄ spinel oxide from 2 K to 300 K. Bulk CuCr₂O₄ crystallizes at room temperature in a tetragonal distorted spinel and above 865 K its structure is cubic spinel; distortion is caused by Jahn–Teller Cu²⁺ ions. The magnetization data of the polycrystalline sample indicates ferrimagnetic order below T _C =122 K. Magnetization isotherm resulted in an average magnetic moment of 0.08 μ_B/f.u. at 2 K values lowest to expected value. This discrepancy can be explained assuming a triangular configuration of spins Cr³⁺ and Cu²⁺. The ferromagnetic phase of the sample does not show glassy behavior. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferrimagnet.     

Hyperfine Induced Magnetic Ordering in a S=1/2 Quantum Magnets on Kagomé Lattice

Susceptibility and specific heat measurements have been carried out down to 23 mK on polycrystalline [Cu₃(titmb)₂(OCOCH₃)₆]?H₂O {titmb = 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6 trimethylbenzene} with S=1/2 antiferromagnet on Kagomé lattice. The results show an unexpected ferromagnetic long range ordering at 56 mK. The entropy change from 10 K to 23 mK is much larger than the entropy corresponding to electron spin S=1/2 of Cu²⁺. The large excess entropy reduction strongly suggests a novel magnetic ordering of the coupled Cu electron and nuclear spin (I=3/2) system. We propose that a frustrated ground state changes to the complex ferromagnetic ground state with help of the enhanced indirect nuclear spin interaction through the hyperfine interaction.     

High Field and Low Temperature Magnetization of ErCu2Si2 Single Crystal

The magnetic measurements have been performed for high fields up to 18 T and low temperatures down to 0.5 K on the single crystal compound ErCu₂Si₂. In the temperature dependence of magnetic susceptibility, two anomalies are seen at T _N =1.51 K and T _t =0.97 K which is corresponding to an antiferromagnetic ordering temperature and may come from a change of magnetic structure. Low temperature magnetization shows a metamagnetic transition around 0.4 T along the easy magnetization direction of [001]. The magnetization is almost saturated above 3 T and reaches 8.8μ _B /f.u. at 18 T. A metamagnetic transition appears around 0.5 T in the basal plane magnetization processes as well. The anisotropy within the basal plane is fairly large for high fields. These behaviors have been explained from crystalline electric field effects and magnetic interaction in terms of molecular field.     

3He-19F interaction at the surface of fluorocarbon polymers

The interaction between¹⁹F nuclear spins in Teflon and³He spins on its surface has been studied in magnetic fields up to 6 Tesla. At temperatures below 1 K, a monolayer of³He is sufficient to increase the NMR relaxation rate of¹⁹F more than an order of magnitude over the value with vacuum or with a coverage of pure⁴He. With a DLX 6000 substrate, the coupling between the two nuclear species weakens towards 6 T. With an Aldrich substrate, the coupling is still active, but it was not observed that the magnetization of a³He film follows that of¹⁹F substrate spins. In monolayer experiments, the field dependence of the fluorine nuclear relaxation time T_1F is stronger than a quadratic law above 2.5 T, while with liquid³He this field dependence is reduced to a linear law. Most of the data are discussed in terms of a coupling model, in which the¹⁹F-³He nuclear dipolar interaction is modulated by motion of³He atoms. The temperature dependence for T_1F points to mechanisms involving vacancies, mass fluctuations or exchange inside the³He surface layer or, in one case, paramagnetic centers, in the³He-¹⁹F coupling. The relaxation time of the fluorine dipolar reservoir in the Aldrich substrate is of the order of minutes at 3 K and 2.5 T, even in the presence of³He.     

Specific heat and magnetocaloric effect of Pr1−x Ag x MnO3 manganites

This paper presents experimental results of the specific heat and magnetocaloric effect in Pr_1?x Ag _x MnO₃ (x = 0.05–0.25) manganites. A maximum value of magnetic entropy change for the sample with x = 0.15 was observed in magnetic field of 18 kOe at T _C = 125 K and was ΔS _max ≈ 2.96 J/kg K. In Pr_0.95Ag_0.05MnO₃ sample, a sign inversion of the magnetocaloric effect was observed at low temperatures (~80 K) in low magnetic fields of 750 Oe. The coexistence of ferromagnetic and canted antiferromagnetic phases with closely spaced critical temperatures was found to force the magnetocaloric effect into abrupt sign inversion.     

Studies on Low-Field and Room-Temperature Magnetoresistance in La2/3(Ca1−x Sr x )1/3MnO3 Perovskites

The polycrystalline perovskites La_2/3(Ca_1?x Sr _x )_1/3MnO₃ (x=0,0.2,0.4,0.6,0.8,1) were successfully prepared by a modified method of solid-state reactions, in which the ingredient mixture with ethanol as a liquid milling medium to form suspension was milled by high-energy ball milling for 10 h and sintered in air at 1400 °C for 10 h. The microstructure, electrical transport, and low-field magnetoresistance (LFMR) of the perovskites were investigated to study the room-temperature magnetoresistance (RTMR) behavior. The results reveal that the metal-to-insulator transition temperature (T _MI) increased with increasing doping level x, and the peak values of the magnetoresistance (MR) near T _MI dropped with the more Ca²⁺ substituted. A single-phase La_2/3Ca_1/3MnO₃ showed T _MI at 263 K and the peak MR of 23 % in the applied field of 3 kOe near T _MI. The LFMR effect at room temperature could be obtained by controlling the doping Sr²⁺-substituted Ca²⁺ level. When x=0.29, transition temperature (T _MI) was 305.30 K, and the MR effect was recorded up to 12.9 % at 298.55 K and 3 kOe. Finally, the possible mechanism is discussed.     

Impact of Mn Substitution at Ru Site in?RuSr2(Eu1.4Ce0.6)Cu2O10?�� Magneto-Superconductor

We report the effect of Mn ion substitution on the structural, superconducting and magnetic properties of polycrystalline Ru_1?x Mn _x Sr₂(Eu_1.4Ce_0.6)Cu₂O_10??? system with x=0.0 to 0.50. All the samples crystallizes with tetragonal structure in I4/mmm space group. RuSr₂(Eu_1.4Ce_0.6)Cu₂O_10??? (EuRu-1222) is a reported magneto-superconductor with magnetic ordering at 100 K and superconductivity occurs at ?40 K. The exact nature of Ru spin magnetic ordering is still being debated and no conclusion has been reached yet. Here, we found the superconducting transition temperature T _c=15 K from the dc magnetization measurements for undoped sample. It is observed that the superconducting transition temperature decreases with the Mn doping at Ru site. DC magnetic susceptibility measurements exhibited ferromagnetic like transition for all the synthesized samples. It was also observed that the net magnetic moment decreases gradually with Mn doping, though not monotonically. It seems that doping of Mn in EuRu-1222 at Ru site enhance the AFM ordering of Ru spins and suppress the FM component. Our results point out possible coupling between superconductivity and magnetism.     

NMR Study of 3He Adsorbed in Zeolite Pores

The NMR properties of ³He adsorbed in the pores of MCM-41 zeolite have been studied in the temperature range 1.4 to 15 K, at pulsed NMR frequencies of 1.66 and 3.26 MHz. At a coverage x=0.84 monolayer, the linewidth 1/T ^* ₂ scales approximately linearly with the magnitude of the static magnetic field, and T ^* ₂ increases linearly with increasing temperature with an extrapolated low temperature limit of order 80 µs. However T ₂ is significantly longer, increasing from 0.7 µs at the lowest temperatures investigated in a way suggesting thermally activated motion. We attribute T ^* ₂ to static field variations between pores arising from paramagnetism of the zeolite and the random orientation of the pores. On the other hand T ₁ increases monotonically with decreasing temperature to around 80 ms at 1.6 K, indicating that the correlation time of the local magnetic field fluctuations τ _c>ω ^?1 ₀. A detailed temperature dependence of T ₁ and T ₂ has been carried out at a coverage of 0.32 monolayer. Here a minimum in T ₁ is observed at 11 K for a Larmor frequency of 3.26 MHz corresponding to a correlation time τ _c of 5×10^?8 s. T ₁ increases by around three orders of magnitude on cooling to 1.8 K. At this temperature T ₁ decreases significantly with increasing coverage while T ₂ shows a very much weaker coverage dependence.     

Synthesis Pressure–Temperature Effect on Pinning in MgB2-Based Superconductors

The volume pinning force, F _p(max), increases with increasing synthesis or sintering pressure (0.1 MPa–2 GPa) in materials prepared at high temperature (1050 °C) while it stays practically unchanged in those prepared at low temperature (800 °C). The position of F _p(max) can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC, or C, for example), and (3) in-situ preparation. Grain boundary pinning (GBP) dominates in materials prepared at low temperatures (600–800 °C), while high-temperature preparation induces strong point pinning (PP) or mixed pinning (MP) leading to outstanding properties. In materials produced by spark plasma sintering (SPS), the position of F _p(max) is higher than expected for both grain boundary and point pinning. The distribution of boron and oxygen in MgB₂ based material, which can changed by additions or the preparation conditions, significantly affects the type and strength of pining. Materials prepared under a pressure of 2 GPa with a nominal composition of Mg:7B or Mg:12B consist of 88.5 wt % MgB₁₂, 2.5 wt % MgB₂, 9 wt % MgO or 53 wt % MgB₁₂, 31 wt % MgB₂₀ 16 wt % MgO, respectively. Their magnetic shielding fractions at low temperatures are 10 % and 1.5 %, with a transition temperature, T _c of 37.4–37.6 K. Although their magnetic critical current density at zero field and 20 K was 2–5×10² A/cm², they were found to be insulating on the macroscopic level.     

RKKY Interactions in a Bilayer Olympicene Structure: A Monte Carlo Study

Using Monte Carlo simulations, we study the effect of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions in a bilayer olympicene structure, with mixed spins (σ = 3/2 and S = 2) in the Blume–Capel model. The studied system is formed by two magnetic layers (N ₁ and N ₂), separated by a number of non-magnetic layers (NM). We first elaborate the ground-state phase diagrams and then we found that from 4 × 5 = 20 phases possible, only 12 configurations are found to be stable. For low reduced temperature values, the partial magnetizations are found to be in good agreement with the corresponding ground-state phase diagrams. The critical temperature is displaced towards lower temperatures. This is due to the fact that the increasing of the number of non-magnetic layers increases the effect of the RKKY interaction. We have interpreted the behaviors of the magnetic hysteresis for this model.     

Magnetic Measurements on Single Crystal of Double Perovskite Ca2FeMoO6

We report the magnetism properties on single crystal of Ca₂FeMoO₆ double perovskite prepared by a floating zone technique. This high quality material has been studied by X-ray powder diffraction (XRD), and magnetic measurements. The field dependence of the magnetization at 5 K, 100 K, and 300 K suggest a ferrimagnetic behavior with a saturation magnetic moment of approximately 2.1 μ_B, 2.0 μ_B, and 1.4 μ_B per formula unit, respectively. From dc susceptibility, we observed two magnetic transitions at T _C1=380 K (paramagnetic–ferrimagnetic) and T _C1=336 K (orthorhombic–monoclinic phase).     

Diamagnetic and Electrical Properties of Si x Mo2S3−x (x=0.1, 0.2, 0.33, 0.5) and A0.1Mo2S3−x (A=C, B, and Ru)

Mo₂S₃ doped with Si, C, B, and Ru, is identified to bear the same crystalline structure P2₁/m as that of Mo₂S₃ through XRD analysis. Diamagnetic transitions with χ _m ～10^?4 emu/g?Oe at temperature ranging from 2 K to 6 K were observed in the doped samples of Si _x Mo₂S_3?x (x=0.1, 0.2, 0.33, 0.5). And both of the x=0.2 and 0.5 samples were found to have double diamagnetic transitions with higher T _c at the same temperature of 6.01 K, while Si _x Mo₂S_3?x of x=0.33 displayed an extra ferromagnetic-like response at 63 K. The corresponding transition in resistivity of Si _x Mo₂S_3?x with x=0.1 was noticed to show a mild drop with less than 10 % of its original transition values as measured down to 2 K. But a superconducting-like magnetic field dependence on the phase transition of resistivity was also noted. Its diamagnetic signals were greatly reduced when the applied magnetic fields were raised to 10³ Oes. In the doped samples of A_0.1Mo₂S_2.9 (A=C, B, and Ru), the phase transition in resistivity at 4.08 K, 4.62 K, and 4.35 K, respectively, exhibited similar fashion as that in the case of Si_0.1Mo₂S_2.9.