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磁性的同步辐射研究新趋势
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磁性的同步辐射研究新趋势 Current research in magnetism is driven by the interesting physics of rather complex materials and by technology relevance, as is indicated, for example, by the rapidly increasing demands of the information-storage and -processing industry. The discovery of the Giant Magneto-resistance 20 years ago, honoured by the Nobel Prize in 2007, laid the foundation to the entirely new research field of Spintronics, which attempts to exploit the electron spin as the basic carrier for the functionality and information transfer in electronic devices. The fifth School on Magnetism and Synchrotron Radiation held at Mittelwihr in October 2008 focussed on current and likely future research trends in the area of magnetism and magnetic materials and posed the question about the special tools needed. Advances in the synthesis of new materials and complex structures, often with nanometerscale dimensions, require increasingly sophisticated experimental techniques that can probe the electronic states, the atomic magnetic moments and the magnetic microstructures responsible for the properties of these materials. Tools are needed to explore the microscopic interactions between a spin-polarized current and the magnetization. Processes like spin-transfer torque and spin transfer at interfaces are in the focus of interest. In the last two decades, experimental techniques based on synchrotron radiation have provided unique capabilities for the study of magnetic phenomena. One reason is that X-ray techniques have the unique advantage of coupling directly to the spinresolved electronic states of interest. X-ray Magnetic Circular or Linear Dichroism (XMCD or XMLD) spectroscopy is a unique tool for measuring element-specific 3d, 4f and 5d magnetic moments, frequently separated into spin and orbital components. Inelastic X-ray scattering, in resonant and non-resonant mode, is a powerful emerging spectroscopic probe that, due to the advent of new instrumentation, provides a wealth of information on electronic states in strongly correlated materials or in materials under high pressure and in strong magnetic fields. Such experiments need the high brightness of a third-generation synchrotron source, like the ESRF or SOLEIL and others. An important aspect in magnetism research is dimensionality. Many modern magnetic materials like thin films, multi-layers and clusters, self-organized or laterally patterned structures show spatial extensions with at least one dimension on the nanometer scale. These novel materials, often heterogeneous or multi-component, exhibit structural, electronic and magnetic properties different from those of bulk materials. The ability to control spatial dimensions of magnetic features at the nanometer level opens the possibility to study the fundamental magnetic interactions on this scale. Synchrotron radiation sources of the third generation have made it possible to perform magnetic imaging using X-ray techniques on a sub-micrometer level. This technique combines X-ray microscopy or X-ray photoelectron microscopy with spectroscopy and permits imaging of magnetic domain structures with a lateral spatial resolution of a few 10 nm. A new development is lens-less imaging by Fourier transform X-ray holography, where the diffraction pattern of a coherently illuminated sample is recorded in Fourier space. These methods provide a key technique for research on small structures important in microelectronics, which are often heterogeneous and composed of several elements. Together with the temporal structure of the synchrotron radiation, they permit element-sensitive time-domain studies, which are of prime importance for magnetic recording. Examples are the dynamics of domain-wall displacements and transformation or dynamics of the magnetization of mesoscopic magnetic structures. The underlying processes occur at times in the nano- to femto-second range. Understanding is limited due to the lack of a microscopic theory. A class of materials of particular scientific interest are the actinide metals and their compounds. Their physical properties, deriving from the 5f electron states, show many similarities with the lanthanides, such as electron correlations, superconductivity, or ordered magnetism. But compared to the 4f metals, their properties and their magnetic structure, in particular, remain poorly understood. This is due to experimental complications and the exotic behavior of the 5f states that appear to be delocalized for the light actinide metals, but become localized in the latter part of the series. Considerable insight into the electronic ground state can be obtained from core-level X-ray absorption spectroscopy and electron energy loss spectroscopy, together with recent theoretical results. Current interest in magnetic materials includes molecular magnets. They bridge the gap between the atomic and the mesoscopic length scale. A special case is the Single Molecule Magnets, which are coordination compounds of paramagnetic metal ions held together by suitable ligands. Interest in this material is focussed on the understanding of their magnetic hysteresis that occurs at low temperature and presumably is of pure molecular origin. This Mittelwihr School on the interrelation of magnetism and synchrotron radiation was meant, like the preceding ones, to introducing into the basics of the topic. Hence the first lectures were devoted to the major fundamental phenomena and aspects in magnetism, to the modern theoretical concepts for the description of the interaction of an electromagnetic wave with matter, focussing on core-level X-ray spectroscopies, and to the fundamentals of synchrotron sources and devices. A new spectroscopic tool was presented, X-ray detected magnetic resonance, which uses XMCD to probe the resonant precession of local magnetization spin and orbital components in a microwave pump field. A lecture important for future developments was devoted to report on the progress in the realization of free-electron laser sources in the UV and X-ray range. These sources will produce spatially coherent, ultra-short (100 fs) pulses with very high brilliance and mark the transition from third- to fourth-generation light sources. In the above lines, I have only addressed what appeared to me as the strong points of the school. The reader interested in the fascinating actual aspects of magnetism as studied by synchrotron radiation will find an excellent presentation in these Lecture Notes. |
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2014-10-31 01:36:32, 9.12 M
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