磁性纳米材料的临界尺寸？ The criticalnanoscale for magnetism Kannan M. Krishnan's research group, University of Washington, WA, USA A basic introduction to the present-daymagnetism is given first. Then the influence of a nanometer length scale onmagnetic behavior is analyzed in detail including the origin of magnetism, the classification of magneticmaterials as well as the theoretical perspective. Twospecific applications about nanoscale magnetism of 0D nanoparticle inbiomedicine and GMR effect concerned 2D magnetic superlattices areintroduced in the end. The studyof magnetism has stepped into human science and technology eversince the invention of the compass needle thousands of years ago.Magnetic materials nowadays are very important for modern industrial society.However, the focus of magnetism had been on macroscopic magnetics for a long time, and the magnets supporting society are still largely produced viaprocesses perfected in the middle of the twentieth century and based ontraditional metallurgical concepts. Not until recently, with the emerging ofnanotechnology, haveresearchers discovered that manipulating the formationof magnetic materials at the nanometer (10-9) length scale(1~100nm) enables better magnetic performance than the best existing ones. For example, a nanometer-scale logic gate that works at room temperature and ismade entirely from metal has been developed. The ferromagnetic NOT gate is"a completely new class of device" that could be made even smaller. A13-bit shift registerhas been created by linking the devices together and afull set of logic gatesis made.1 In my opinion, weshould first traceback to the origin of magnetism to unravel why magnetic materialsstructured within this critical length scale expose such exciting properties. Physically speaking, magnetic materials are magnetic because theatoms of which they are made are magnetic. Atoms are magnetic because theelectrons comprising the atom occupy states have orbital or spin angularmomentum. Magnetic moment is always associated with angular momentum, which canbe the angular momentum of an electric dipole, or the angular momentum of anelectron orbiting a nucleus. In addition, angular momentum is also associatedwith the spin of an electron. Thus the magnetic moment is generated by the orbital and spin angularmomentum of electrons.2 An individual atomisolated from its neighbors may emanate a magnetic field arising from theplanetary-like electron orbit around the nucleus and the spin of the electrons. Through this magnetic field, each individual atom acts like a tinybar magnet. When millions of these “tiny bar magnets” are brought together toform a crystallite, their electrons interact, propagating the atomic-levelmagnetic information throughout the crystallite. Besides, a slight change inthe identity of the neighboring atom will alter the local magnetic forces. Forexample, substitution of a nonmagnetic zinc atom into an iron crystallite willchange the electronic interactions. Additionally, subtlechanges of the distance between atomic neighbors in a crystallite candrastically alter the local magnetic state as well. As a result, the nanometer appears to be a crucial length scale for the future design ofmagnetic materials. For one thing, the fundamental physical forces that createthe phenomenon of magnetism are operative only under this minute length scale3;for another, structuring at nanometer-scale would reduce the grain size incrystals and meanwhile largely increase the surface-to-volume ratio — on theorder of almost 1 surface atom for every 10 interior atoms. That is to say, theelectronic behavior of the surface atoms would make aconsiderable and unique contribution to the overall material’sbehavior. Hence, the slight atomic level changes of magnetic atoms, which mayhave a negligible influence on the conventional large-grained materials,may radically affect the magnetic behavior of nanostructured magnetic materialif a particular concentration is attained. In general, materials are classified accordingto their response to an external magnetic field, which designates all materialsto be one of five different classes, namely diamagnetic, paramagnetic,ferromagnetic, ferrimagnetic, and antiferromagnetic materials. Diamagnetic andparamagnetic materials exhibit no collective magnetic interactions betweenatoms and are not magnetically ordered. Ferromagnetic, ferrimagnetic andantiferromagnetic materials have long-range magnetic order below a certaincritical temperature. A ferromagnetic material exhibits a large susceptibilityin moderate magnetic fields and at higher temperatures (room-temperature),compared to the values of paramagnetic materials. Ferrimagnetism is one type ofmagnetic ordering with an antiparallel spin arrangement. However, thesub-lattice magnetic moments are of unequal magnitude and therefore produce anet magnetization in an applied field. Ferromagnetic or ferrimagnetic materialscan become uniformly magnetized (single domain) instead of breaking intomultiple domains in order to minimize their overall energy. For a single domainmagnetic nanoparticle, when the size is small enough, the spontaneous magneticreversal can occur due to the thermal fluctuation at room temperature, known assuperparamagnetism4. To sum up, the magnetism experiences severalmagnetic phase transition as the size decreases. Typically, two critical sizesare observed: one is a size limit below which magnetic materials can no longergain a favorable energy configuration by breaking up into domains, theso-called single domain nanoparticles; the other size limit is the occurrenceof superparamagnetism below a certain size, where thermal energy can drive themagnetic reversal freely. Both these sizes can be calculated based on thephysical parameters of magnetic materials and experimental constants. Theoretically, nanostructuralphenomena are often described by differential equations of the typefile:///C:/DOCUME%7E1/shenshen/LOCALS%7E1/Temp/msohtmlclip1/01/clip_image002.gif, where κ-1 is an interaction length.5,6R. Skomski et al. proposed that the fundamental magnetic interaction length scaleson the order of l0 = a0/α = 7.52 nm, where a0 isthe Bohr’s hydrogen radius and α is the Sommerfeld’s fine-structure constant.6,7It is also found that the critical single-domain and multi-domain sizeare strongly size and geometry dependent, and the critical single-domain radiusRSD ~ D1/2lex, where D is the demagnetizingfactor and lex is the exchange length (~10nm).6 Takemagnetite particles for example, when the diameter is over 30nm the particlestransform from superparamagnetic to single-domain state; when the particlediameter exceeds roughly about 80nm, the state transforms to multi-domain.According to the analysis above, it is proposed thatthe intrinsic properties of magnetics, such as the Curie temperature, arerealized on length scales of at most a few interatomic distances and tend toreach their bulk values on a length scale of less than 1 nm.5Therefore, they are unlikely to be improved by nanoscale structuring. Wherea**trinsic properties such as magnetic remanence and coercivity that are relatedto magnetic hysteresis, exhibit pronounced structure dimensional sizedependence. Thus nanometer-scale magnetics may exert a remarkable influence onthe extrinsic properties and lead to unusual phenomena. Since nanoscale is so criticalfor magnetism, many magnetic nanostructures are made by scientists, which opensup the potential of magnetism for broader and more advanced applications in thefuture. Magnetic nanoparticles (typically around 10nm), for example, exhibitappealing opportunities in biomedicine.8 Firstly, because of theircontrollable sizes, which are smaller than or comparable to those of a cell(10-100μm), a virus (20- 450 nm), a protein (5-50 nm) or a gene (2nm wide and10-100 nm long), they are able to interact with or bind to a biological entity,thereby providing a practical way of tagging it. Besides, manipulated by anexternal magnetic field gradient, they may act to deliver packages to targetedregions of the body. Moreover, they could be made to resonantly respond to atime-varying magnetic field, which enables advantageous results related to thetransfer of energy from the exciting field to the nanoparticles. The metallic magneticsuperlattice is another noticeable example, also known as magnetic 2Dmultilayers, which is a novel artificially made material system with uniquephysical effects. It is made by orderly deposition of alternating thin filmswith nanometer-scale thickness of two or more metals, at least one of which ismagnetic.9 One most promising and extensively investigated physical phenomenoninvolved in this system is the giant magnetoresistance (GMR) effect — theabrupt change of electrical resistance ofthe whole system when an applied field progressively overcomes the AF couplingand aligns the magnetizations of the magnetic nanostructures. A typical example is the 2D single-crystal(Fe/Cr)n superlattice oriented along [100] and grown by molecularbeam epitaxy, where the optimal thickness of each Fe and Cr layer are 2nm and5~20nm. When applying an external magnetic field over a specific value, asudden drop of electrical resistance as large as 50% would occur.10Due to the high sensitivity to the magnetic field, GMR effects have manyapplications. Especially in magnetic recording area, it is expected that thepossible use of GMR effect in the emerging magnetoresistance read heads, the socalled MRAM (magnetoresistive random-access memories), may substantiallysimplify the electrical circuitry and increase the read speed over one hundredtimes.3 In conclusion, the nanometer length scale (1~100nm) plays a significant role in the future application ofmagnetism. Compared with macroscopic magnetic materials, nanoscale magnetismmay have a profound influence on the extrinsic properties of magnetic materials,and structuring from this length scale with magnetic materials may give rise toa series of scientifically interesting and technologicallyimportant novel functional devices. However, to fully realize the range ofmagnetic nanostructures and their potential for exploring new applications still remainsto be a great challenge for future research. References: 1 D. A. Allwood, G. Xiong, M. D. Cooke, C. C. Faulkner, D. Atkinson, N.Vernier, and R. P. Cowburn Science 296 (2002) 2003-2006 2 Liu J P, Luo C P, Liu Y and Sellmyer D, High energyproducts in rapidly annealed nanoscale Fe/Pt multilayers , J 1998 Appl. Phys. Lett. 72 483 3 Laura Henderson Lewis, The New Future ofMagnetism , World and I, Sep. 2001, v16 p146. 4 C. P. Bean, J.Appl. Phys. 26, 1381 (1955) 5 R.Skomski, Nanomagnetics , J. Phys.Condens. Matter 15 (2003) R841–R896 6 R.Skomski, Nanomagnetic scaling , J. Magn. and Magn.Mater. 272–276 (2004) 1476–1481 7 R.Skomski, OepenH-P and Kirschner, Micromagnetics of ultrathin films with perpendicularmagnetic anisotropy , J 1998 Phys. Rev. B 58 3223 8 Pankhurst, Applications of magnetic nanoparticles inbiomedicine , J.Phys. D: Appl. Phys.36(2003)R167-R181 9 L.M. Falicov, Metallic Magnetic Superlatiice, PhysicsToday, Oct. 46-51(1992) 10 R. Lawrence Comstock, Introduction toMagnetism and Magnetic Recoding , John Wiley & Sons, Inc. (1999) Chapter 7, p393 查看更多0个回答 . 3人已关注
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