Magnetism amorphous materials

Solid state materials have been studied by nuclear magnetic resonance methods over 30 years. In 1953 Wilson and Pake ) carried out a line shape analysis of a partially crystalline polymer. They noted a spectrum consisting of superimposed broad and narrow lines which they ascribed to rigid crystalline and amorphous material respectively. More recently several books and large articles have reviewed the tremendous developments in this field, particularly including those of McBrierty and Douglas 2) and the Faraday Symposium (1978)3) —on which this introduction is largely based. 

Virtually all catalysts studied by the ESR technique are composed of small crystallites or an amorphous material. For such samples, the spectrum is the envelope of the spectra from all possible orientations of the radical with respect to the external magnetic field. In order to obtain meaningful data it must be possible to extract the principal g and hyperfine values from these polycrystalline spectra. A relatively straightforward analysis of the spectra can be made provided the resolution is adequate. 

However, commonly due to a spread of quadrupole parameters caused by structural variation, with the extreme case being glassy or amorphous materials, the distinct features of fhe lineshape can be lost. This is particularly true if Cq is small. Then an approach which records spectra at several magnetic fields will allow bofh fhe second-order quadrupole effect parameter Pq, defined as... 

The topics of surfaces and sintering will be new to most students. The short chapter on bonding and the chapters on amorphous materials and liquid crystals introduce new concepts. These are followed by treatment of molecular morphology. The final chapters are on magnetic materials, porous and novel materials, and the shape memory. 

The primary function of magnetic core materials, that is magnetic flux multiplication, requires both high saturation magnetization and high permeability. Both of these are important for miniaturization. The desire for miniaturization also results in steadily increasing the operation frequencies which, of course, require low eddy-current losses. Similar to amorphous metals, their production-inherent low thickness of about 20 pm and their high residual resistivity of about 100 pQcm minimize eddy currents in nanocrystalline ribbons and make them attractive up to frequencies of 100 kHz or even more. 

To fully understand the performance of amorphous materials, it is necessary to be able to measure the molecular mobility of the samples on interest. This is because at temperatures as far as 50 K below the glass transition temperature, pharmaceutical glasses exhibit significant molecular mobility that can contribute to both chemical and physical instability.The main techniques that have been developed for monitoring molecular motions in amorphous materials are nuclear magnetic resonance (NMR) and calorimetric techniques (e.g., DSC and isothermal microcalorimetry). Average molecular relaxation times and relaxation time distribution functions obtained from these... 

Mossbauer spectroscopy is also able to give local moment orientations, with respect to the crystalline lattice, or the correlations between moment orientations and local distortion axis orientations in a chemically disordered or amorphous material. This arises from the interplay between the structural (electric field gradient) hyperfine parameters and the magnetic hyperfine parameters. In this way, the spin flop Morin transition of hematite, for example, is easily detected and characterized (e.g., Dang et al. 1998). The noncollinear magnetic structures of nanoparticles can also be characterized. 

The characterization of an amorphous material is difficult because it lacks characteristic XRD diffractions, so that Auger or Mossbauer spectroscopies are preferred, along with other conventional analytical assays such as spot test and iodo-metric titration [71]. The Fe203 nanoparticles are converted to crystalline Fe304 nanoparticles when heated to 420 °C under vacuum or when heated to the same temperature under a nitrogen atmosphere. The magnetization of pure amorphous Fe203 at room temperature is very low (< 1.5 emu g ) and it crystallizes at 268 °C. 

Fig. 3.11. Normalized fraction of amorphous material Xa derived from the saturation magnetization vs. milling time. The dashed line corresponds to the amorphous-phase production rate dX,/dt...

The chemical shift anisotropy arises from the nonspherical electron density around the nuclei, and is particularly prominent for aromatic and carbonyl (C=0) carbon types. These carbon types experience different shieldings of the magnetic field depending on whether the bond axes are parallel or perpendicular to the external magnetic field. For polycrystalline or amorphous materials all orientations are possible, including these two extremes. 

Boll, R. Hilzinger, H. R. (1983). Comparison of amorphous materials, ferrites and permalloys. IEEE Transactions on Magnetics, Mag-19, 1946-51. Buschow, K. H. J. (1990). New Developments in hard magnetic materials. Reports of Progress in Physics, 54, 1123-213. 

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