Magnetic properties of nanocrystalline Fe86.7Zr3.3B4Ag6 thin film

The changes of magnetic properties with annealing temperature were studied in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film. The thin films were deposited by a DC magnetron sputtering method, annealed at 300–700°C for 1 h in vacuum under a field of 1.5 kOe parallel to the film plane, and then furnace-cooled. As a result, it has been found that the Ag addition to Fe–Zr–B amorphous thin films resulted in the decrease of crystallization temperature to 400°C due to promoted crystallization ability. Also, it gave rise to formation of fine BCC α-Fe crystalline precipitates with a grain size smaller than 10 nm in the amorphous matrix near 400°C, and led to prominent enhancement in the magnetic properties of the Fe_86.7Zr_3.3B₄Ag₆ thin films. Significantly, excellent magnetic properties such as a saturation magnetization of 1.7 T, a coercive force of 1 Oe and a permeability of 7800 at 50 MHz were obtained in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film containing 7.2 nm-size BCC α-Fe, which was annealed at 400°C. Also, core loss of 1.4 W cm⁻³ (B_m=0.1 T) at 1 MHz in the thin film was obtained, and it is a much lower value than had been obtained in any existing soft magnetic materials. Such excellent properties are inferred to originate from the uniform dispersion of nano-size BCC α-Fe in the amorphous matrix.     

Synthesis and characterization of nanocrystalline HAp powders prepared by using aloe vera plant extracted solution

Nanocrystalline hydroxyapatite (HAp) powders were synthesized by a simple method using aloe vera plant extracted solution. To obtain nanocrystalline HAp, the prepared precursor was calcined in air at 400–800 °C for 2 h. The phase composition of the calcined samples was studied by X-ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase. With increasing calcination temperature, the crystallite of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter, a, in a range of 0.9520–0.9536 nm and c of 0.6739–0.6928 nm. The particle sizes of the powder were obtained to be 43–171 nm. The optical properties of the calcined powders were characterized by Raman and FTIR spectroscopies. The Raman spectra showed a main peak of the phosphate vibration mode (ν₁(PO₄)) at ∼963 cm⁻¹ for all the calcined samples. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcined powders. The morphology tends to change from a spherical shape to a rod-like shape with increasing calcination temperature as revealed by TEM.     

Time dependence of isothermal magnetization in single-phase B1 MoCy encapsulated in multiwall carbon nanocages

Isothermal magnetization M(t) in nanocrystalline single-phase B1 MoC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 5000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with increasing temperature T, and decreases linearly with increasing J. The behaviors of J(t), U_eff(T), and U_eff(J) can be described by the Anderson–Kim flux-creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. The slower relaxation of current density above the broad peak field in the isothermal magnetization curves suggests that the peak is a result of vortex dynamics.     

Analysis of Tb3+- and melittin-binding with the C-terminal domain of centrin in Euplotes octocarinatus

Centrin is a low molecular mass (20 KDa) protein that belongs to the EF-hand superfamily. In this work, the interaction between the Tb³⁺-saturated C-terminal domain of Euplotes octocarinatus centrin (Tb₂-C-EoCen) and 2-p-toluidinylnaphthalene-6-sulfonate (TNS) was investigated using difference UV–vis spectra and the fluorescence spectra methods. In 100 mM N-2-hydroxy-ethylpiperazine-N-2-ethanesulfonic acid (Hepes) at pH 7.4, with the addition of Tb₂-C-EoCen, four new peaks were observed at 265 nm, 278 nm, 317 nm and 360 nm by absorptivity compared with blank solution of TNS. At the same time, the reaction could be measured by fluorescence spectra. The fluorescence emission of TNS was shifted from 480 nm to 445 nm in the presence of Tb₂-C-EoCen. Meanwhile, its fluorescence intensity was increased markedly. The 1:1 stoichiometric ratio of C-EoCen to TNS was confirmed by fluorescence titration curves. The conditional binding constants of TNS with C-EoCen and Tb₂-C-EoCen were calculated to be log K_{(C-EoCen-TNS)}=5.32±0.04  M^?1 and log K_{(Tb2-C-EoCen-TNS)}=5.58±0.12 M^?1, respectively. In addition, the protein of Tb₂-C-EoCen binding with melittin was also studied. Based on the fluorescence titration curves, the 1:1 stoichiometric ratio of Tb₂-C-EoCen to melittin was confirmed. And the conditional binding constant of C-EoCen with melittin was calculated to be log Ka′=6.79±0.17 M^?1.     

Electrical,magnetic and magnetocaloric properties of polycrystalline Pr0.63A0.07Sr0.3MnO3 (A=Pr,Sm and Bi)

We studied the effects of the partial substitution (10%) of praseodymium by samarium and bismuth, on the structural, magnetic, magnetocaloric and electrical properties of the Pr_0.63A_0.07Sr_0.3MnO₃ (A=Pr, Sm and Bi) manganites prepared using the solid state reaction. Refinement of the X-ray diffraction patterns shows that all our samples are single phase and crystallize in the orthorhombic structure with Pnma space group. Magnetic studies indicate that all the samples exhibit a ferromagnetic–paramagnetic transition with increasing temperature. Curie temperature T_C decreases by substitution. M(H) curves indicate the presence of some antiferromagnetic domains in the substituted samples testifying the phase-separated nature of these samples. The magnetic entropy curves –ΔS(T) show a maximum in vicinity of T_C. Important values of maximum of −ΔS are recorded for our compounds. For the parent compound, we found 4.59 J/kg K for an applied magnetic field of 2 T at T_C=266 K which raises the possibility of using this compound as a magnetic refrigerant. The temperature dependence of the electrical resistivity ρ(T) indicates that all compounds exhibit a metal-insulator transition with increasing temperature. Electrical study suggests the presence of a correlation between electrical and magnetic properties.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Microstructure refinement and significant improvements of magnetic properties in Pr2Fe14B/α-Fe nanocomposites

Exchange coupled (Pr,Tb)₂(Fe,Nb,Zr)₁₄B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c-axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10 m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17 m/s and annealed at 700°C for 20 min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6 kOe and 14.7 MGOe for the (Nb,Zr)-free sample to 8.2 kOe and 20.3 MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20 nm, leading to a high remanence of 0.73 M_s. Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.     

Improved critical current density in Zn doped YBCO single crystals

Magnetic measurements were made using pure YBCO and Zn doped YBa₂(Cu_1?xZn_x)₃O_7?σ. Single crystals with Zn concentration of 0.5%, 1.5%, 3.0% and 4.3%. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 ? T/T_c ? 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density J_c increased for low Zn content samples up to 3% Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of J_c in the temperature range of 0.7–0.9T_c, close to critical temperature compared to pure YBCO sample. The irreversibility field H_irr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field H_p above H_c1 and irreversibility field H_irr, both show power law dependence of the form H = m₁(1 ? T/T_c)^m2 in the temperature range of 0.75–0.96T_c. The values of parameter m₂ increased from 1.44 to 1.95 for the samples up to 3% Zn content and decreased to 1.37 for higher Zn contents. The ratio H_irr/H_p was found to be 3–4 for the lower Zn content samples and was 7–8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field H_p and irreversibility field H_irr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution.     

Crystal-field effect on the hyperfine splittings,Mössbauer spectra and specific heat of Er3+ in pyrogermanate and diglycollate hosts

Hyperfine splittings and the Mössbauer spectra (MS) of the two isomers ¹⁶⁶Er and ¹⁶⁷Er in the pyrogermanate (or ErPG) and diglycollate (or ErDG) hosts were calculated using the previously determined values of the crystal field (CF) parameters and the CF levels. MS for the 166 isomer in both the hosts show only 5 lines, whereas 27 lines are expected for the 167 isomer. In case of ErDG, the quadrupole interaction P increased considerably between 300 and 4 K, i.e., from 1.5 to 21.3 MHz in case of ¹⁶⁶Er and from −1.0 to −14.0 MHz in ¹⁶⁷Er. However for ErPG, the corresponding changes in P for both the isomers are insignificant ∼1 MHz. The hyperfine magnetic fields H_hf in ErDG and ErPG were found to be nearly the same being equal to 4.4±0.01 MG because the g_∥-values are close, i.e., 10.31 and 10.58, respectively. Specific heat C_p of ErPG was measured between 30 and 0.65 K and a sharp λ peak was detected with a transition temperature T_N=0.95±0.01 K. The calculated hyperfine and Schottky specific heat components, C_hf, and C_sch, respectively showed peaks at 32 mK and 110 K. From the experimental results of C_p, the lattice specific heat C_L/R in ErPG was found to be 3.34×10⁻⁵ T³ and the thermal characteristics of the magnetic specific heat C_M was determined. The internal energy U_M(T) was obtained from C_M by numerical integration and its change ΔE/R=[U_M(∞)/R–U_M(0)/R] was found to be 1.02 K and the corresponding value of the Weiss constant θ in the ordered state was 2.01 K. The value of the exchange interaction constant |J_ex/k| and the dipole–dipole interaction constant for nearest neighbours were found to be ∼0.5 and ∼1.03 K, respectively. The molecular field (H_mol) for ErPG was equal to 4.1 kG. From future epr and MS measurements, interesting results are expected regarding the magnetic ordering in ErPG and the phase change in ErDG.     

Structural changes in ab-plane of Zn doped Bi-2212 HTSC single crystals

Single crystals of undoped and 1% Zn-doped Bi-2212 are grown by self-flux slow cooling method under similar growth conditions. T_c(ΔT_c) are found to be 84 K (2 K) and 82 K (8 K) for undoped and doped samples respectively. Effect of Zn doping on structure and modulation is studied. Different change in the value of a- and b-axes are observed resulting in a transformation of structure from tetragonal to orthorhombic one. However, no effective change in modulation is there. Distortions are compared in c- and b-axes for undoped and doped samples. The asymmetry in position and intensity of satellite peaks has been discussed. The value of a in excess of b has been reported for the first time. These results are discussed in terms of valancy, coordinates and position of occupancy of Zn ions vis-à-vis those of Cu⁻ ions in Cu–O plane of the structure.     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.     

Hydrothermal synthesis and magnetic studies of transition metal nocerites M3(BO3)F3 (M=Fe,Co, Ni)

Polycrystalline samples of M₃(BO₃)F₃ (M=Fe, Co, Ni), isostructural with nocerite Mg₃(BO₃)(OH,F)₃, have been prepared in supercritical hydrothermal conditions. These compounds represent with boracites, M₃B₇O₁₃F (M=Mg, Cr, Mn, Fe, Co, Zn), the only transition metal fluoride borates known to date. Co₃(BO₃)F₃ and Ni₃(BO₃)F₃ are antiferromagnetic with T_N=17(2) and 40(2) K, respectively. Spin-flop transitions at B_C1=4.0 T and B_C2=7.5 T occur at 1.6 K in Co₃(BO₃)F₃, while a parasitic ferromagnetism (0.02 μ_B/Ni²⁺ at 1.6 K) appears below T_N in Ni₃(BO₃)F₃. The magnetic structures consist of three spin sub-lattices of double rutile-type ferromagnetic chains.     

Structure and magnetic properties of co-sputtered Co–C thin films

We studied the structure and magnetic properties of co-sputtered Co_1−xC_x thin films using a transmission electron microscope (TEM) and a SQUID magnetometer. These properties were found to depend critically on deposition temperature, T_S, and composition, x. Generally, phase separation into metallic Co and graphite-like carbon phases proceeds with increasing T_S and decreasing x. Plan view and cross-sectional TEM images of the films prepared showed that Co grains about 10–20 nm in diameter and 30–50 nm in height are three-dimensionally separated by graphite-like carbon layers 1–2 nm thick. Optimum magnetic properties with saturation magnetization of 380 emu/cc and coercivity of 400 Oe were obtained for a film with x=0.5 and T_S=350°C.     

Thermoluminescence,photoluminescence and EPR studies on Mn2+ activated yttrium aluminate (YAlO3) perovskite

Thermoluminescence (TL) measurements were carried out on undoped and Mn²⁺ doped (0.1 mol%) yttrium aluminate (YAlO₃) nanopowders using gamma irradiation in the dose range 1–5 kGy. These phosphors have been prepared at furnace temperatures as low as 400 °C by using the combustion route. Powder X-ray diffraction confirms the orthorhombic phase. SEM micrographs show that the powders are spherical in shape, porous with fused state and the size of the particles appeared to be in the range 50–150 nm. Electron Paramagnetic Resonance (EPR) studies reveal that Mn ions occupy the yttrium site and the valency of manganese remains as Mn²⁺. The photoluminescence spectrum shows a typical orange-to-red emission at 595 nm and suggests that Mn²⁺ ions are in strong crystalline environment. It is observed that TL intensity increases with gamma dose in both undoped and Mn doped samples. Four shouldered TL peaks at 126, 240, 288 and 350 °C along with relatively resolved glow peak at 180 °C were observed in undoped sample. However, the Mn doped samples show a shouldered peak at 115 °C along with two well defined peaks at ～215 and 275 °C. It is observed that TL glow peaks were shifted in Mn doped samples. The kinetic parameters namely activation energy (E), order of kinetics (b), frequency factor (s) of undoped, and Mn doped samples were determined at different gamma doses using the Chens glow peak shape method and the results are discussed in detail.     

Transport and magnetic properties of bulk polycrystalline (YBa2Cu3O7)1−x(Nd0.7Sr0.3MnO3)x nanocomposites

We have prepared a series of bulk polycrystalline samples with the nominal compositions (YBa₂Cu₃O₇)_1?x(Nd_0.7Sr_0.3MnO₃)_x (x = 0–1) by a conventional solid-state reaction method using the larger difference in sintering temperature of the two constituent oxides and a well conceived sintering sequence. XRD patterns show that the samples are composites consisting of YBa₂Cu₃O₇ and Nd_0.7Sr_0.3MnO₃ particles with average grain size of ～65 nm. For x ? 0.55, with increasing x, the zero-resistance superconducting transition temperature, T_C0, measured at zero magnetic field decreases and the normal state resistivity increases rapidly. The T_C0 for the sample with x ～ 0.48 is estimated to be 0 K. The M–H hysteresis loops indicate the coexistence of ferromagnetism and superconductivity in the samples. The depression of T_C0 can be attributed to the proximity effect between ferromagnetism and superconductivity.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

Magnetotransport properties of ferromagnetic LaMnO3+δ nano-sized crystals

Transport and magnetic properties of LaMnO_3+δ nanoparticles with average size of 18 nm have been investigated. The ensemble of nanoparticles exhibits a paramagnetic to ferromagnetic (FM) transition at T_C～246 K, while the spontaneous magnetization disappears at T≈270 K. It was found that the blocking temperature lies slightly below T_C. The temperature dependence of the resistivity shows a metal–insulator transition at T≈192 K and low-temperature upturn at T<50 K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. The low temperature I–V characteristics are well described by indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates.     

Giant magnetoresistance in the Ge-rich magnetocaloric compound,Gd5(Si0.1Ge0.9)4

We have measured the zero-field electrical resistivity in the temperature range 5–295 K and magnetoresistance in magnetic fields of up to 12 T of Gd₅(Si_0.1Ge_0.9)₄. The resistivity changes drastically at the magnetostructural first-order transition (T_C≅80 K on heating). This transition can be induced reversibly by the application of an external magnetic field above T_C, producing a concomitant giant magnetoresistance (GMR) effect, Δρ/ρ≅−50%. This study demonstrates that (in addition to giant magnetocaloric and magnetoelastic effects) GMR can be tuned between ∼20 and ∼290 K in Gd₅(Si_xGe_1−x)₄ with x⩽0.5 by simply adjusting the Si : Ge ratio.     

Dielectric and magnetic properties of Bi1−xYxFeO3 ceramics

Y doped BiFeO₃ polycrystalline ceramics were prepared by sol–gel method. Crystal structure examined by X-ray diffraction indicates that the samples were single-phase and crystallize in rhombohedral structure. An anomaly in the dielectric constant and dielectric loss in the vicinity of the antiferromagnetic Neel temperature (T_N) was observed. Saturated magnetization loops were observed for all sample with saturated magnetization M_s=0.678 emu/g and remnant magnetization M_r=0.084 emu/g for x=0.3.     

Giant magneto-caloric effect around room temperature at moderate low field variation in La0.7(Ca1−xSrx)0.3MnO3 perovskites

Among the perovskite manganites, a series of La_1?xCa_xMnO₃ has the largest magneto-caloric effect (MCE) (|ΔS_m|_max=3.2–6.7 J/kg K at ΔH=13.5 kOe), but the Curie temperatures, T_C, are quite low (165–270 K). The system of LaSrMnO₃ has quite high T_C but its MCE is not so large. The manganites La_0.7(Ca_1?xSr_x)_0.3MnO₃ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) have been prepared by solid state reaction technique with an expectation of large MCE at room temperature region. The samples are of single phase with orthorhombic structure. The lattice parameters as well as the volume of unit cell are continuously increased with the increase of x due to large Sr²⁺ ions substituted for smaller Ca²⁺ ions. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic measurements at low field and low temperatures indicate that there is a spin-glass like (or cluster glass) state occurred. The Curie temperature T_C increases continuously from 258 K (for x=0) to 293 K (for x=0.25). A large MCE of 5 J/kg K has been observed around 293 K at the magnetic field change ΔH=13.5 kOe for the sample x=0.25. The studied samples can be considered as giant magneto-caloric materials, which is an excellent candidate for magnetic refrigeration at room temperature region.