纳米磁性材料的测量和表征？ Ways to measure andcharacterize magnetic nanostructures Kannan M. Krishnan's research group, University of Washington, WA, USA The basic information about conventionalmeasurement and characterization techniques such as XRD, SEM and TEM is given first. Then characterization techniques for magnetic nanomaterials are discussed in detail, according to thegeneral classification of the magnetic fieldmeasurements and magnetization measurements. These include methods measuring themagnetic field strength, magnetization and techniques for imaging theconfiguration of magnetization. People’s desire to observe the natural world beyond the limitations ofour five senses has driven the development of many new tools. For example, wehave developed telescopes, microscopes, and cameras to help us to see thoseobjects which are too small, distant, or faint, or are moving too quickly orslowly to observe with the unaided eye. To measure and characterize magneticsystems is an interesting case in point, for here we are dealing with aphysical phenomenon that is detectable only by our sense of touch, with themagnetic force on strongly magnetized objects. Tosatisfy the fundamental curiosity and practical interest, people have beingstriving to develop a series of techniques and facilities to unravel the innermystery of magnetic structures, especially the new-born magnetic nanomaterials. Conventional techniquesto measure andcharacterize materials such as X-ray diffraction (XRD),scanning electron microscopy (SEM) and transmission electron microscopy (TEM)can be applied to obtain the general structure information of the magneticnanostructures. Firstly, XRD is a very important experimental technique that has long been usedto address all issues related to the crystal structure of solids, includinglattice constants and geometry, identification of unknown materials,orientation of single crystals, preferred orientation of polycrystals, defects,stresses, etc. In XRD, a collimated (the line of sight of an optical device isadjusted) beam of X-rays, with a wavelength typically ranging from 0.7 to 2 Å,is focused on a specimen and is diffracted by the crystalline phases in thespecimen according to Bragg’s law, which shows the relationship between thespacing d of atomic planes in the crystalline phase and the X-ray wavelengthλ. In addition, one can also estimate the size ofcrystallite in a powder sample applying its XRD pattern along with the ScherrerEquation, which describes the relationship between the size and othermeasurable parameters such as the half height widths of individual peaks andrelated diffraction angles.1 SEM, on the other hand,through emitting electrons to strike the sample surface and detecting thesecondary low-energy and backscattered electrons, is capable of providing highresolution images of a sample surface within nanometer-scale (1~100nm).Furthermore, even higher resolution down to atomic-scale image of samplestructure information can be generated by TEM, which employs a beam ofelectrons to focus on a specimen and records the passing through of electronintensity variation to produce a high contrast image on a fluorescent screen orlayer of photographic film. Specially for the magnetic properties, peoplehave developed many characterization methods, most of which can be roughly divided into twogroups: the magnetic field measurements and magnetization measurements. Tomeasure the magnetic field strength of certain nanostructures, one can make useof the changes in various properties of materials caused by the presence of amagnetic field. For example,magnetoresistance, the change in electrical resistance of a material, whensubjected to a magnetic field, can be applied to determine the field strengthaccording to the known variation pattern of resistance with field. The main advantage of this methodis that very small probes can be fabricated to measure the field to a point.Magnetoresistive probes are particularly useful for field measurements at lowtemperature. On the other hand, if the magnetostriction(another unique phenomenon which is the change in the shape ofspecimen when subjected to a magetic field) of a material as afunction of field is known, it can also be utilized to measure the field strength.Additionally, magnetic anisotropy and magnetic resonance are other usefulproperties available for field strength measuring, especially for magneticthin-films and particles. Moreover, one recently developed advancedtechnique for magnetic field measurement is SQUID, which means the superconducting quantum interference device. It basically utilizesthe relation between the magnetic field and the quantized flux in asuperconducting circuit containing a poor conducting weak link. SQUID turns outto be highly sensitive and capable to provide an ultimate resolution on theorder of 10-14 T for the reason that the weak link enables theflux trapped in the ring to change by discrete amounts and thereby very smallchanges in flux can be measured.2 To image the magneticstray field, theiron filling method, as refined by Bitter, offered the greatest spatialresolution for a long period. In that method, thesurface of a magnetic material is dusted with magnetic nanoparticles, and thesubsequent particle agglomeration in the stray magnetic fields at domain wallsreveal the magnetic features down to 100 nm. Subsequent to Bitter,the instrument most widely used now is the magnetic force microscope (MFM),which is a variant of the noncontact atomic force microscope (AFM). Byrecording the frequency and amplitude of the vibration change of aferromagnetic tip corresponding to the stray magnetic field gradient at thesample surface, MFM imaging can achieve a very high spatial resolution of lessthan 10 nm. However, due to the rather complicated interaction between themagnetic tip and sample surface, it is difficult to extract quantitativeinformation directly from MFM images at present. In fact, the highestresolution among current methods available to map the field distribution isprovided by techniques applying electron microscopy.3Lorentz microscopy, for example, which derives the contrast image from thedeflection of electrons due to the Lorentz force upon passing through themagnetic induction in the sample, can obtain a lateral resolution of betterthan 10 nm. Therepresentative applications include the study of the detailed magnetic elementsand patterned spin tunnel junction material.4 The primary method to measure a sample’s magnetization is to use the magneto-optic effects, whichinvolve rotation of the polarization angle of linearly polarized light. The two principal magneto-opticeffects are the Faraday effect, which occurs when light is transmitted througha transparent medium in the presence of a magneticfield along the direction of propagation of the light, and the Kerr effect,which occurs when light is reflected from a ferromagnetic medium. They providea good way to measure the magnetization of ferromagnetic or ferrimagneticmaterials through other variables like the thickness of the specimen and therotation angle. In physics, one most often prefers to know the actual configuration ofmagnetization within a sample. Numerous high-resolution magnetic imaging techniquesmeasure quantities proportional to the local sample magnetization. Theseconsist of interactions of electron, photon, or neutron beamswith the sample have been developed to imaging the magnetization. 3For instance, imaging techniques based on interaction of electron beams, electron microscopy series, areparticularly useful for investigation on ferromagnetic materials of surface andthin-film magnetism. One powerful method is the scanning electron microscopy with polarization analysis, or SEMPA, which can be regarded as a variant of the SEMdevice that the emitted secondary electrons were detected using aspin-polarization analyzer. It has several special merits over most othermagnetic imaging techniques: it is able to measure the magnitude and directionof the magnetization directly and it is sensitive only to the few outermostatomic layers of the sample due to the small inelastic mean free path of thesecondary electrons,5 which thus allows SEMPA a high resolution ofaround 40 nm. So far, SEMPA is proved to be particularly powerful ininvestigating fundamental problems like interlayer exchange coupling ofmagnetic multilayers. To conclude, conventional techniques to measure and characterizematerials such as XRD, SEM and TEM can be applied to obtain the generalstructural information of the magnetic nanostructures, while a number ofmethods can be used to investigate the magnetic properties concerned withmagnetic field, magnetization and the their possible configuration. Althoughexcellent resolutions within nanometer length scale or even higher have alreadybecome reality, analytic methods, along with many fundamental mechanisms ofmagnetic nanostructures, are still in strong demand for future research. References: 1 B. D. Cullity and S. R. Stock, Elements ofX-Ray Diffraction, 3rd edition, Prentice Hall, Upper Saddle River,NJ, 2001 2 D. Jiles, Introduction to Magnetism andMagnetic Materials , Chapman and Hall. 3 M.R. Freeman and B.C. Chol, Advances in Magnetic Microscopy, Science16 Nov. 2001, vol294 4 K. J. Kirk, J. N. Chapman, S. McVite, P. R.Aitchison, C. D. W. Wilkinson, Appl. Phys. Lett. 75, 3683 (1999) 5 D.L. Abraham, H.Hopster, Phys.Rev. Lett. 58, 1352(1987) 查看更多0个回答 . 2人已关注
化工原理精馏单板效率？ 在连续 精馏塔 中分离苯- 甲苯 混合物 。在全回流条件下,测的相邻两层塔板上的液相组成分别为0.41和0.28（摩尔分数），操作条件下苯-甲苯的平均相对挥发度为2.5，则相邻两层塔板中较下层者塔板的单板效率EMV为 A 0.610 B 0.550 C 0.650 D 0.450 答案为A但是我的计算结果和答案对不上，请大家把你们的计算过程写出来，感激不尽。查看更多2个回答 . 1人已关注
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