J. I. Budnick

Microwave magnetic properties of Co50/(SiO2)50 nanoparticles

Co50/(SiO2)50 nanoparticles were synthesized by a wet chemical method, and their microwave permeability was measured in the 0.1–18 GHz range. The synthesized nanoparticles exhibit two loss peaks at microwave frequencies: one appears around 7.0 GHz and is believed to result from the eddy current effect, the other appears around 250 MHz and is probably caused by natural ferromagnetic resonance. Compared with micrometer-size Co particles, the synthesized nanoparticles exhibit high permeability μ′ and low magnetic loss, especially over 10–18 GHz.

A local environment description of hyperfine fields and atomic moments in Fe3−xTxSi alloys

This paper presents a comprehensive description of a local interaction model of the systematic variations with concentration of the atomic moments and hyperfine fields in Fe3Si and its 3d transition metal ternary alloys, which had previously been presented only in parts. The model which accounts for the internal fields requires two principle contributions, one linearly dependent on the site moment and the other on the average moment of the first-near neighbors. The good results obtained by applying these ideas to Fe3-xMnxSi alloys at concentrations where they have complex spin structures, is a further confirmation of the validity of the model. The different behavior of both the Fe and solute moments for substitutions of atoms from the left and right of Fe in the periodic table is discussed. The effect of electron concentration in Fe3Si systems on the bulk magnetization (very like the left side of the Slater-Pauling curve), on the limits of solubility of transition metals in Fe3Si and on the stability of the DO3 structure is presented. Much of this material had not been presented previously. Finally, some preliminary comments are made on conduction electron changes with concentration and charge and spin transfer in these alloys.

Hyperfine fields in metallic glasses

We review recent experimental data obtained on hyperfine fields (hf) in amorphous metallic alloys. We restrict ourselves to magnetically ordered materials. Both the average value and the distribution of the hf are discussed for three different cases. First, we summarize the hf data on majority constituents such as transition metals (Fe, Co) and rare earth elements (Gd, Eu, Dy). In a second section, we first analyze the hf data on magnetic impurities (Ni, Co, Mn) diluted in Fe based amorphous ferromagnets (FePB, FePC). The case of 59Co diluted or concentrated in amorphous GdCo alloys is analyzed with some detail. Finally, we review the data on transferred hf on simple metals (Au) and s-p elements (P, B) is various amorphous ferromagnets. For those three cases, the mean hf value is discussed in the light of hf data on both pure elements and compositionally related crystalline compounds. The hf distribution is analyzed in relation with the structure of these amorphous alloys (electronic structure, magnetic structures, atomic-scale or medium-range atomic order).

Dependence of the Nèel-temperatures of La2CuO4 on Sr-doping studied by muon spin rotation

The magnetic behaviour of La2-xSrxCuO4 was studied using the method of muon spin rotation (μSR). We found that the system orders magnetically for x ≤ 0.05 and the Neel-temperatures vary from 250 K for x = 0 to 6 K for x = 0.05. In spite of these drastic changes of the Neel-temperatures, the extrapolated internal fields at T = 0 K are similar for all samples indicating that the moments at the magnetic (Cu) ions are not changed appreciably. The depolarization rates measured in this experiment are analyzed in terms of an inhomogeneous field distribution in the sample.

Structural, morphological, and magnetic study of nanocrystalline cobalt-copper powders synthesized by the polyol process

Nanocrystalline Co{sub {ital x}}Cu{sub 100{minus}{ital x}}(4{le}{ital x}{le}49 at.%) powders were prepared by the reduction of metal acetates in a polyol. The structure of powders was characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS) spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy, and vibrating sample magnetometry (VSM). As-synthesized powders were composites consisting of nanoscale crystallites of face-centered-cubic (fcc) Cu and metastable face-centered cubic (fcc) Co. Complementary results of XRD, HRTEM, EXAFS, NMR, and VSM confirmed that there was no metastable alloying between Co and Cu. The NMR data also revealed that there was some hexagonal-closed-packed (hcp) Co in the samples. The powders were agglomerated, and consisted of aggregates of nanoscale crystallites of Co and Cu. Upon annealing, the powders with low Co contents showed an increase in both saturation magnetization and coercivity with increasing temperature. The results suggested that during preparation the nucleation of Cu occurred first, and the Cu crystallites served as nuclei for the formation of Co.

Suppression of superconductivity in FeSe films under tensile strain

We have studied the effect of tensile strain on the superconductivity in FeSe films. 50, 100, and 200 nm FeSe films were grown on MgO, SrTiO3, and LaAlO3 substrates by using a pulsed laser deposition technique. X-ray diffraction analysis showed that the tetragonal phase is dominant in all of our FeSe films. The 50 nm FeSe films on MgO and SrTiO3 are under tensile strain, while the 50 nm FeSe film on LaAlO3 and the other thick FeSe films are unstrained. Superconducting transitions have been observed in unstrained FeSe films with Tonset≈8 K, which is close to the bulk value. However, no sign of superconductivity has been observed in FeSe films under tensile strain down to 5 K. This is evidence to show that tensile strain suppresses superconductivity in FeSe films.

Y-Ba-Cu-O/silver composites: An experimental study of microstructure and superconductivity

Abstract An experimental study is presented of Y-Ba-Cu-O/Ag composites employing a variety of different experimental techniques: electrical transport, magnetometry, light and electron microscopy, electron microprobe analysis, X-ray diffractometry, and X-ray absorption. Three different distinct chemical environments for Ag in such composites are identified. The nature of the chemical interactions of Ag and YBa 2 Cu 3 O 7− x and the benefits of the inclusion of silver are discussed in terms of these results.

Magnetic Susceptibility of Au–Fe Alloys

Over the past years there has been considerable interest in the magnetic properties of the Au(Fe) system. Mossbauer measurements indicate that a complicated type of long‐range ordering occurs below concentrations c of ∼12 at.% Fe and extends down to c<1%. Yet no definitive susceptibility study has been carried out over a complete range of temperatures and concentrations in order to characterize the magnetic ordering. We have measured the low‐field magnetic susceptibility χ(T) for a series of eight Au–Fe alloys (1–22 at.% Fe), and we observe a definite peak in χ at those ordering temperatures determined from the Mossbauer data. The magnitude of χ greatly increases with c, while the shape of χ(T) in the region of the peak changes from a cusp‐like behavior at low c to a behavior for c≳17% in which χ(T) gently rises, then sharply falls. These behaviors confirm the existence of a long‐range order suggtesive of antiferromagnetism at low c (≤8%) with a more typical ferromagnetic ordering at high c. At intermedia...

Structure and magnetic properties of SiO2-coated Co nanoparticles

SiO2-coated Co nanoparticles in a size range of 10 to 50 nm were synthesized by a wet chemical approach, and their structure and magnetic properties were investigated using x-ray diffraction, high-resolution transmission electron microscopy, and a superconducting quantum interference device magnetometer. The structure of the synthesized nanoparticles varied with calcination temperature. When the calcination temperature was as high as 900 °C, the nanoparticles had a core/shell structure: the core was fcc Co and the shell was amorphous SiO2 . When the calcination temperature was 800 °C or below, the nanoparticles had a nano-onion structure: the shells from the exterior to the interior were amorphous SiO2 , fcc Co, and CoO, and the innermost core was Co3O4 . The SiO2 shell had the ability of hindering Co from particle growth during the synthesis procedure and protecting Co against oxidation after the synthesis procedure. The nanoparticles were ferromagnetic. At both low and room temperatures, the saturation magnetization increased with increasing calcination temperature, while the coercivity decreased with increasing calcination temperature. For the nanoparticles calcined at 800 °C or below, the low temperature coercivity was found to be notably higher than the room temperature one due to Co/CoO exchange coupling. For the nanoparticles calcined at 900 °C, the coercivity was relatively low and the saturation magnetization reached the expected values.

Effect of magnetic field on the superparamagnetic relaxation in granular Co-Ag samples

In order to study the effect of applied magnetic field on the superparamagnetic relaxation behavior of small Co particles, magnetization measurements were carried out on as-prepared and annealed granular samples of Co20Ag80 and Co25Ag75. Values of the superparamagnetic blocking temperature TB− were obtained from the characteristic peak in the zero-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease of TB− with applied magnetic field is quadratic. An estimate of the magnetic anisotropy “energy density” Ku yielded a value which is two orders of magnitude greater than the value for bulk cobalt. The results reported here underscore the importance of considering the effect of superparamagnetic relaxation on the performance of nanostructured magnetic materials.