Molecular crystals, magnetism principles

THEORETICAL STUDY OF THE MAGNETISM IN MOLECULAR CRYSTALS USING A FIRST-PRINCIPLES BOTTOM-UP METHODOLOGY... 

All of these quantities can (in principle) be obtained in the gas phase or in molecular beams with high-resolution microwave spectroscopy and molecular beam magnetic or electric resonance. Such data provide isolated molecule values (usually selected as to vibrational and rotational state) with which to compare liquid crystal and solid state values. These techniques reveal the desired tensor components by their relationship to the principal inertial axes of the molecule. 

The non-volatile bubble memories used for data storage are based on the principle of formation of magnetic domains in thin films grown on a gadolinium gallium garnet crystal substrate. Molecular engineering creates the precise film compositions for optimum performance. 

We have written this book with two main purposes in mind. One aim is to acquaint chemists and biochemists with the general principles of crystal structure analysis so that they can critically appraise articles in the crystallographic literature, and extract, with a reasonable comprehension of the precision of the results of the experiment, any structural information they are interested in. The second aim is to make the reader aware of the vast amount of structural information that has resulted from this method of analysis, to inform him or her of how to access the results in the most useful way, and to indicate the manner by which these types of data have enhanced our understanding of chemistry and biochemistry. It is our aim to place the method in context with other methods of structure analysis, such as solution studies (nuclear magnetic resonance and infrared analyses) and molecular modelling. 

During the Stone Age, the material research was limited to the mechanical treatment of natural products. When Dalton discovered atomicity and Mendeleev revealed the periodic table, the research trends drastically changed in the intervening period and research was focussed on fundamental principles of basic molecular structure and simple chemical reactions. During the late twentieth century and the early twenty-first century, an exciting revolution in chemistry has taken place, with multidisciplinary approaches in nanoscience and nanotechnology to the creation of molecules with pre-specified complex structures to perform novel functions, hi the present century, research is focussed on control of crystal structures, nanostractures and microstructures with distinct mechanical, electrical, optical and magnetic properties [1-5]. 

Curie understood that under stress, or in the presence of external electric or magnetic fields, the symmetry of a system is changed. The Neumann principle still applies but should no longer be based on the symmetry of the isolated crystal, but on that of the combined system of crystal and external field, as we have considered in Sect. 3.9. In the case of ammonia, application of an electric field has the Coov symmetry of a polar vector. The symmetry that results from the superposition of the field with the molecular point group 3 depends on the orientation (see Appendix B). In the coordinate frame of Fig. 3.1 one has ... 

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