W. A. Hines

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.

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.

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.

Open‐ended coaxial‐line technique for the measurement of the microwave dielectric constant for low‐loss solids and liquids

A system which enables fast and reliable measurements of the dielectric constant over continuous microwave frequency ranges for both solid and liquid low‐loss materials is described. The main thrust of this work is the application of the open‐ended coaxial‐line probe technique, which has been used previously for soft biological materials, to low‐loss solid samples. Using the instrumentation and procedure presented here, the dielectric constant for low‐loss solids can be measured absolutely to ±2%–3% with routine care. The uncertainty can be reduced by about a factor of 2 by averaging several measurements. It is also smaller for liquid samples. This application features the use of relatively simple and readily available microwave components. Also, it is shown that a simple empirical relationship can be used to obtain the bulk dielectric constant from samples of a material in the form of thin slabs. The experimental results which are presented here for kapton, Teflon, Corning glass No. 0211, soda lime glass...

Magnetic properties of SiO2-coated Fe nanoparticles

SiO 2 -coated Fe nanoparticles were synthesized using a wet chemical method, and their structural and magnetic properties were studied. The SiO 2 material was in an amorphous state. The Fe nanoparticles were in a bcc state and contained an inner ferrihydrite core whose size decreased with increasing calcination temperature. The nanoparticles were basically in the ferromagnetic state. Their saturation magnetization increased with increasing calcination temperature, whereas their coercivity decreased with increasing calcination temperature. Different from bulk Fe, the nanoparticles exhibited strong temperature-dependent magnetic behaviors. The Bloch exponent fell from 1.5 to smaller values and decreased with increasing ferrihydrite content, while the Bloch constants were much bigger than that for bulk and increased significantly with ferrihydrite content. The value of coercivity decreased notably with increasing temperature. The exchange anisotropy arising from the exchange coupling across the Fe/ferrihydrite interfaces was examined and was used to interpret the observed temperature behaviors.

Some applications of NMR to the study of magnetically-ordered materials with emphasis on the short-range order in (Fe-B)-based crystalline and amorphous alloys

Abstract In this review, we summatize recent developments in nuclear magnetic resonance (NMR) studies on (Fe-B)-based crystalline and amorphous alloys, focusing on the application of NMR in identifying the existence of short-range order (SRO), determining the types of SRO, characterizing the behavior of the SRO and exploring the effect of the SRO on the magnetic properties for the Fe-B system. NMR experiments reveal that certain local environments surrounding the B atoms exist in both crystalline and amorphous Fe-B alloys. The type of SRO existing in this rapidly quenched system can be either o-Fe 3 B or bct-Fe 3 B, or a mixture, depending on the composition and processing factors, especially the carbon content and quenching speed. The SRO originates from a strong covalent bonding between the B and Fe atoms. As this interaction plays the same role in both crystalline and amorphous Fe-B alloys, the SRO which occurs in the amorphous Fe-B alloys is similar to the SRO which exists in their crystalline counterparts. NMR, in combination with magnetization measurements, provides evidence indicating that the SRO existing in the amorphous Fe-B alloys has a significant effect on their soft magnetic properties and that different types of SRO may act differently, thus providing an opportunity to improve the magnetic properties by changing the SRO. In connection with reviewing the achievements of NMR studies in recent years, brief comments concerning the advantages and potential of NMR experiments in the investigation of other magnetically-ordered materials will also be presented.

Crystallization of Fe-B amorphous alloys: A NMR and x-ray study

Based on a knowledge of the NMR spectra for FeB, Fe2B, orthorhombic Fe3B (o‐Fe3B), and body‐centered‐tetragonal Fe3B (bct‐Fe3B), the phases produced during the annealing of Fe‐B amorphous alloys can be identified with greater sensitivity by NMR techniques than by x‐ray diffraction. In the present work, a combination of x‐ray diffraction experiments and spin‐echo NMR measurements of the hyperfine field distributions for the 10B, 11B, and 57 Fe nuclei has been performed on both as‐quenched and annealed Fe100−xBx (14≤x≤25) amorphous alloys. In general terms, it was found that annealing at temperatures near 400 °C resulted in crystallization products which included bct‐Fe3B, while annealing at approximately 800 °C leads to the formation of o‐Fe3B. The behavior of the B hyperfine field indicates that, for the Fe100−xBx amorphous alloys, an o‐Fe3B‐like local order is favored in the low B concentration regime, and a bct‐Fe3B‐like local order is favored in the high B concentration regime.

Strain-Induced Change in Local Structure and its Effect on the Ferromagnetic Properties of La0.5Sr0.5CoO3 Thin Films

We have used high-resolution extended x-ray absorption fine-structure and diffraction techniques to measure the local structure of strained La0.5Sr0.5CoO3 films under compression and tension. The lattice mismatch strain in these compounds affects both the bond lengths and the bond angles, though the larger effect on the bandwidth is due to the bond-length changes. The popular double exchange model for ferromagnetism in these compounds provides a correct qualitative description of the changes in Curie temperature TC, but quantitatively underestimates the changes. A microscopic model for ferromagnetism that provides a much stronger dependence on the structural distortions is needed.

Microstructure and magnetic behavior of carbon-coated Co nanoparticles studied by nuclear magnetic resonance

Spin-echo nuclear magnetic resonance (NMR) experiments have been carried out at 4.2 K and room temperature on carbon-coated nanoscale (average diameter ≈20 nm) face-centered-cubic (fcc) Co particles prepared by the Kratschmer carbon arc process for 0⩽H⩽9.0 kOe. Information concerning the magnetic structure and paramagnetic relaxation behavior of the nanoscale particle system has been obtained.