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Nanomagnetism and Spin Electronics: Materials, Microstructure and Novel Properties 40TH ANNIVERSARY J MATER SCI 41 (2006)793–815 Nanomagnetism and spin electronics: materials, microstructure and novel properties K. M. KRISHNAN∗, A. B. PAKHOMOV, Y. BAO, P. BLOMQVIST, Y. CHUN, M. GONZALES, K. GRIFFIN, X. JI, B. K. ROBERTS Department of Materials Science and Engineering, University of Washington, Box 352120, Seattle, WA, 98195-2120 E-mail: [email protected] We present an overview of the critical issues of current interest in magnetic and magnetoelectronic materials. A broad demonstration of the successful blend of materials synthesis, microstructural evolution and control, new physics and novel applications that is central to research in this field is presented. The critical role of size, especially at the nanometer length scale, dimensionality, as it pertains to thin films and interfaces, and the overall microstructure, in determining a range of fundamental properties and potential applications, is emphasized. Even though the article is broad in scope, examples are drawn mainly from our recent work in magnetic nanoparticles and core-shell structures, exchange, proximity and interface effects in thin film heterostructures, and dilute magnetic dielectrics, a new class of materials for spin electronics applications. C 2006 Springer Science + Business Media, Inc. 1. Introduction sions quantum-mechanical tunneling effects are expected Magnetism, subtle in its manifestations, is electronically to predominate [2]. driven but weak compared with electrostatic interactions. In the ideal case, considering only dipolar interactions, It has its origin in quantum mechanics, i.e. the Pauli ex- the spin-flip barrier for a small magnetic object [3]is clusion principle and the existence of electron spin. How- a product of the square of the saturation magnetization, 3 ever, it is also known for a variety of classical effects, Ms and its volume (V ∼ a ). As a first approximation, 2 3 arising from both short- and long-range forces and is one can set this equal to the thermal energy, i.e. Ms a widely associated with “microstructure” or the morpho- ∼ kBT ∼ 25 meV at room temperature, and for typical logical arrangements of phases, grains or individual atoms ferromagnets obtain a characteristic length a ∼ 4 nm, be- themselves. These are, in part, the reasons for the rich- low which they do not behave as ferromagnets any more. ness of structures and properties encountered in magnetic In practice, changes in magnetization of a material oc- systems from which various useful and technological ap- cur via activation over an energy barrier and associated plications arise [1]. These are also, in part, the reasons with each type of energy barrier is a different physical why the collective magnetic behavior of materials is com- mechanism and a characteristic length. These fundamen- ∼ plex, poorly understood and many fundamental questions tal lengths are the crystalline anisotropy√ length (lK (J/K), remain unanswered. 2 the magnetostatic length (ls∼ J/(2π·Ms )) and the ap- There are clearly two limits to magnetic behavior as plied field length (lH∼(2J/H·Ms). Here J is the inter- a function of size and dimensionality. At one end of the atomic exchange, K is the anisotropy constant of the bulk spectrum (bulk) the microstructure determines the mag- materials and H the applied field. In principle, if multi- netic (hard and soft) behavior. It is a function of the pro- ple barriers are present, for a given time, the one with cessing method and our understanding of it is qualitative the shortest characteristic length determines the materials and empirical at best. At the other end, as the length scales properties [4]. For a general anisotropy, K, a characteristic approach the size of domain wall-widths (nanostructures), time, τ, for reversal is determined as τ∼τ 0 exp (K·V/kBT). lateral confinement (shape and size) and inter-particle If the measurement time (typically 100 s) is included, exchange effects dominate, rendering classical descrip- one can then determine a characteristic size (Vsp)at tions grossly inadequate, until finally, at atomic dimen- room temperature or, for a given volume, a characteristic ∗Author to whom all correspondence should be addressed. 0022-2461 C 2006 Springer Science + Business Media, Inc. DOI: 10.1007/s10853-006-6564-1 793 40TH ANNIVERSARY Figure 1 Single domain size, Dcrit and magnetic stability size or the super- paramagnetic limit at room temperature, Dsp for some common ferromag- netic materials. temperature called the blocking temperature, Tb that de- fines a transition from ferromagnetic to a thermally unsta- ble or superparamagnetic behavior. Further, if the particle sizes become larger, instead of being uniformly magne- tized (single domain), the particles can break into multiple domains to minimize their overall energy. Using theories for domain stability in fine particles [5] and bulk prop- erties available in the literature, one can determine the characteristic size up to which single domains are stable. This series of magnetic “phases” as a function of size is shown (Fig. 1) for different ferromagnets and includes a “single domain” size (Dcrit) below which the material will not support a multi-domain particle [6] and a size (Dsp) defined by the super-paramagnetic effect [7] below which a spontaneous flip in magnetization occurs due to thermal effects at room temperature. Such nanostructures in the size range identified in Fig. 1, which can be considered to be zero-dimensional objects, can be made by three distinct approaches – met- allurgy, chemical synthesis and lithography (Fig. 2). The Figure 2 Alternative routes for the fabrication of zero-dimensional struc- metallurgical approach involves the synthesis of alloys by tures. (a) Metallurgical—Hard Nd2Fe14B precipitates in a non-magnetic rapid solidification that on subsequent annealing phase Nd matrix [10]; (b) Chemical—10 nm dia cobalt nanoparticles synthe- segregates with a microstructure on the nanometer length sized by chemical routes and self-assembled on a TEM carbon grid and (c) scale. Such an approach is used for preparing very soft Lithography—arrays formed by ion-implantation through stencil masks—a top down approach [12]. [8] commercial magnets or very hard [9] magnets. The former is achieved by creating nanoscale crystallites that are randomly oriented but interact strongly to give a very small effective anisotropy by directional averaging. On mask [12]. The third approach is to chemically synthesize the other hand, large anisotropies and coercivities very nanoparticles by the rapid decomposition of a metalor- close to the theoretical maximum can be obtained by cre- ganic precursor by injecting it in a coordinating solvent ating isolated magnetic particles in a non-magnetic matrix containing surfactants at elevated temperature [13, 14]. [10, 11]. The alternative, well-known method to create Details of our work using the last method, including the nanoscale objects is by lithography. This top-down ap- synthesis of core-shell particles, their magnetic behavior proach, carried out with either electrons or photons, is and self-assembly, as well as their potential in biomedical both time consuming and expensive but recent variations applications are discussed later in this article. also include locally patterning a specific property, such as The effect of dimensionality on magnetic behav- the magnetic anisotropy, by implantation through a stencil ior is best studied in thin film heterostructures. Their 794 40TH ANNIVERSARY magnetic behavior is rich in new phenomena and includes contribution is predominant and the magnetization lies recent highlights such as surface/interface anisotropy in the plane of the film. For ultrathin films and mul- that leads to perpendicular anisotropy in multilayer tilayers (tM<∼1 nm), it has been suggested that the structures [15] as well as frustration, proximity and surface/interface anisotropy contribution, arising from a interface effects that lead to exchange bias (ferro- break in symmetry at the interface [28] and proportional −1 magnetic/antiferromagnetic bilayers) [16] and exchange to tM , can overcome the shape anisotropy and result in spring (ferromagnetic/ferromagnetic bilayers) [17] be- a spontaneous magnetization perpendicular to the film. havior. These magnetic properties are dominated by the Such perpendicular anisotropy in multilayers is now well physical, chemical and magnetic structure of their inter- known [15] but, in practice, an understanding of the ori- faces and it is important that appropriate tools be devel- gin of this phenomenon requires a careful evaluation of oped for studying them most effectively at relevant length the evolution and control of their complex microstruc- scales [18, 19]. ture at the atomic scale [29]. Recently, we have combined The basic energies involved in the manipulation and these two phenomena, i.e. perpendicular anisotropy and control of the magnetic properties of thin film het- exchange bias and critically evaluated the role of inter- erostructures are exchange (controls magnetic order) and faces in such perpendicularly exchange-biased systems. anisotropy (controls magnetic orientation). These are phe- Details of our work in such thin film structures is also nomenological descriptions of fundamental correlations discussed later. and energies arising from the electronic and crystalline Part of the interest in magnetic anisotropy and ex- structure of the material [20]. A soft ferromagnet (FM) change bias in thin films and multilayers is related to such as iron has a large exchange
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