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Low-temperature magnetic properties were investigated on the gold nano particles (GNPs) with an average size of 11.5 nm, assembled with molecules of ruthenium complex (Ru0), and phenylboronic acid (B0) by the proton nuclear magnetic resonance (1H-NMR) and susceptibility measurements. The temperature dependence of the NMR shift and the uniform susceptibility was described as the sum of the Curie–Weiss term and a positive constant term. From the former, the average number of Ru0 on each GNP was estimated to be 118, which is 23% of the calculation. The finite positive constant term shows a clear contrast with the well-known fact that the bulk gold is diamagnetic. Finally, a disappearance of the motional narrowing effect in the proton NMR spectra below 60 K indicates that the wavering motion of Ru0 complexes on GNP at room temperature is frozen at low temperatures. [ABSTRACT FROM AUTHOR]

The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios (rods, foils, wires, powders) it was found that the impurity contribution is correlated with surface area. Tunneling measurements which use the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field >4T, consistent with the Appelbaum-Anderson model refid="ref1"/ for spin flip tunneling. Viewed together the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments caused by oxygen vacancies in Nb2O5. The computation of the surface impedance (Rs) in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of Rs, suggesting that magnetic impurities are responsible for the so-called residual resistance. These properties may have an impact on Nb based superconducting devices and shine a new light on the origin of the paramagnetic Meissner effect (PME). [ABSTRACT FROM AUTHOR]

Using a Cu stabilized Nb3Al strand with Nb matrix, a 30 meter long Nb3Al Rutherford cable was made by a collaboration of Fermilab and NIMS. Recently the strand and cable were tested. In both cases instability was observed at around 1.5 Tesla. The magnetization of this Nb3Al strand was measured first using a balanced coil magnetometer at 4.2 K. Strands showed an anomalously large magnetization behavior around at 1.6 T, which is much higher than the usual Bc2 ∼ 0.5 Tesla (4.2 K) of Nb matrix. This result is compared with the magnetization data of short strand samples using a SQUID magnetometer, in which a flux-jump signal was observed at 0.5 Tesla, but not at higher field. To explain this magnetization anomaly, the interfilament coupling through the thin Nb films in the strands due to the proximity effect, which is preventing the full penetration of the external field into the inside part of strand, is suggested. The instability problems observed in low field regions of the Nb3Al Rutherford cables and the small racetrack magnet wound with this cable are attributed to this effect. [ABSTRACT FROM AUTHOR]

Heterostructured Co-Au nanoparticles are direct synthesized from gas phase. Magnetic properties of Co-Au nanoparticles are characterized. Low-temperature and room-temperature hysteresis loops show that the magnetic properties of Co are kept in Co-Au nanoparticles. A blocking temperature of 290 K is measured for Co-Au particles with an average size of 12 nm. A field-cooled low-temperature hysteresis loop of Co-Au nanoparticles indicates the existence of oxide, even the particles have a core-shell-like structure. [ABSTRACT FROM AUTHOR]

SQUID magnetometry and electron magnetic resonance (EMR) spectroscopy at 9.28 GHz were employed to probe magnetism and surface spin disorder in Fe3 O4 nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed magnetite particles with size ≃8 nm. Magnetization (M) versus temperature studies yielded blocking temperature TB ≃ 170 K in H = 50 Oe. For T > TB, M scales approximately as H/T with μP ≃ 5400 μB as the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity Hc ≃ 37 Oe and saturation magnetization M8 = 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe3 O4 particles. In EMR studies, a single symmetric line at g ≃ 2.11 with linewidth ΔH = 850 Oe is observed at 300 K with the g-value and ΔH increasing with decreasing T. These results are interpreted in terms of a disordered surface spin layer of 0.5 nm thickness. [ABSTRACT FROM AUTHOR]

We report the calculation of free energy with constrained magnetization for L10 FePt nanoparticles. We employ an effective spin Hamiltonian model constructed on the basis of constrained density functional theory calculations for L10 FePt. In this model, the Fe spins (treated as classical spins in this paper) are coupled directly and via induced Pt moments with both isotropic and anisotropic interactions. Interactions mediated by the Stoner-enhanced Pt moment stabilize the ferromagnetic order and lead to a pronounced coordination dependence and long-range interactions. The free energy of these nanoparticles, as a function of the temperature and the constrained magnetization F(T, Mz), is calculated from the joint density of states g(E, Mz), using the extended Wang-Landau algorithm, The free energy barrier for magnetization reorientation is found to depend fairly linearly on the temperature in the ferromagnetic phase and vanishes in the paramagnetic phase. [ABSTRACT FROM AUTHOR]

We recently introduced a Preisach vector model for isotropic media. In this aper, we extend the model to anisotropic media by using the magnetization variation of the outside critical surface of the particle similar to the Stoner-Wohlfarth model. The resulting model can calculate efficiently the vector magnetization variations due to any sequence of vector applied fields. [ABSTRACT FROM AUTHOR]

Well-defined CoFe2 O4 nanoparticles embedded in an amorphous SiO2 matrix have been synthesized by a sol-gel method, characterized and investigated by Mössbauer spectroscopy and magnetic properties measurements. The mean particle size increases from 3 to 15 nm consistently with the temperature of the subsequent annealing, which varied from 800°C to 1100°C, respectively. The composites exhibit superparamagnetic behavior with the blocking temperature TB increasing with the mean particle size from 80 to 350 K, respectively. Sufficiently high value of Hc ∼ 2.5 T (at 2 K) has been observed for the sample annealed at 1100°C. [ABSTRACT FROM AUTHOR]

We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at TB = 135 K and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field H = 100 Oe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below TB it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated Cu2+ ions at the surface of the particles. [ABSTRACT FROM AUTHOR]

Superparamagnetic regions and magnetic anisotropic properties in randomly orientated Co nanodots (NDs) are investigated as a function of dot diameter. The Co NDs are fabricated by intentionally exposing a laser source on ultrathin Co film. Various dot sizes are ultimately realized by changing laser power, scan condition, and initial film thickness. Magnetic hysteresis loops and temperature dependence magnetization of the Co NDs are measured with a superconducting quantum interference device. The analysis of magnetization and hysteresis loops is effectively used to determine superparamagnetic regions of the Co NDs. The experimentally observed results repealed that room temperature superparamagnetic limit of our Co NDs was about 30 nm in diameter, with the confirmation of high resolution transmission electron microscope. [ABSTRACT FROM AUTHOR]

The properties of Pr0.7Ba0.3MnO3 composition have been studied by means of magnetic and resistance measurements. According to X-ray CuK α diffraction analysis, the sample was single-orthorhombic phase. The temperature dependence of resistance in the paramagnetic region followed the nearest neighbor hopping of the small polaron model. The value of activation energy obtained with the resistivity measurement was higher than that deduced from the temperature dependence of electron paramagnetic resonance line intensity. [ABSTRACT FROM AUTHOR]

The dependence of the media dynamic coercivity dependence on field sweeping rate was obtained for a general case with the media energy barrier ΔE = KuV(1 - H/Ho)n. The effective coercivity measurement time teff for a linearly sweeping field is derived. The general solution was applied to perpendicular media (n = 2). The difference in perpendicular media coercivity with a step field and linearly sweeping field is calculated for various Ho and KuV/(kBT). The dynamic coercivity of perpendicular media samples was measured using a Kerr-effect magnetometer with both field sweep rate and finite time duration schemes. The calculated coercivity is compared to measurement results. [ABSTRACT FROM AUTHOR]

The leading magnetic material for use in data recording tapes is the metal particulate (MP) composition. The trend to smaller particle size necessarily entails an increase in thermally induced time effects. An Arrhenius-Néel switching model is used, together with the dependence of the experimental coercivity on field duration, to determine an effective switching volume. The mode! takes account of the highly imperfect magnetic orientation present in actual recording media. Results for coated media made with current advanced particles having various degrees of orientation (squareness) show that the calculated switching volume is not sensitive to orientation above a squareness value of about 0.6 (essentially random in plane). This switching volume correlates well with particle volume, as determined by transmission electron microscopy (TEM). These results confirm the practical relevance of the model, both for realistic tapes and for disks that have random grain orientation, and are evidence that the model adequately takes account of preferred-axis misalignment. [ABSTRACT FROM AUTHOR]

The giant magnetoresistance (GMR) effect and magnetization curves were measured for Cu80 Co20 granular thin films above the blocking temperature. Their correlation was analyzed in a unified scheme. When the GMR and the magnetization curves are fitted to the Langevin functions using an averaged single particle size, there is a lack of agreement for both. The discrepancies were removed by assuming that there are two size distributions in the Co nano-particle population. Under such assumptions, good fittings to the experimental curves were achieved for both the GMR and the magnetization, using the same set of parameters. The obtained average particle sizes for the smaller and the larger particles, and their populations, are 1.0 and 2.8 nm, and 0.35 and 0.65, respectively. The study revealed that a certain degree of magnetic bulk scattering is present in this system. [ABSTRACT FROM AUTHOR]