Magnetic Nanoparticles: a Subject for Both Fundamental Research and Applications

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 952540, 22 pages http://dx.doi.org/10.1155/2013/952540 Review Article Magnetic Nanoparticles: A Subject for Both Fundamental Research and Applications S. Bedanta,1 A. Barman,2 W. Kleemann,3 O. Petracic,4 and T. Seki5 1 School of Physical Sciences, National Institute of Science Education and Research (NISER), IOP Campus, Bhubaneswar 751005, India 2 Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake City, Kolkata 700098, India 3 Department of Physics, University Duisburg-Essen, 47057 Duisburg, Germany 4 Julich¨ Centre for Neutron Science JCNS and Peter Grunberg¨ Institute PGI, JARA-FIT, Forschungszentrum Julich¨ GmbH, 52425 Julich,¨ Germany 5 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Correspondence should be addressed to S. Bedanta; [email protected] Received 16 July 2013; Accepted 19 October 2013 Academic Editor: Gaurav Mago Copyright © 2013 S. Bedanta et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Single domain magnetic nanoparticles (MNPs) have been a vivid subject of intense research for the last fifty years. Preparation of magnetic nanoparticles and nanostructures has been achieved by both bottom-up and top-down approaches. Single domain MNPs show Neel-Brown-like´ relaxation. The Stoner-Wohlfarth model describes the angular dependence of the switching ofthe magnetization of a single domain particle in applied magnetic fields. By varying the spacing between the particles, the inter- particle interactions can be tuned. This leads to various supermagnetic states such as superparamagnetism, superspin glass, and superferromagnetism. Recently, the study of the magnetization dynamics of such single domain MNPs has attracted particular attention, and observations of various collective spin wave modes in patterned nanomagnet arrays have opened new avenues for on-chip microwave communications. MNPs have the potential for various other applications such as future recording media and in medicine. We will discuss the various aspects involved in the research on MNPs. 1. Introduction timescales. For various applications, exploration of a vari- ety of new phenomena is required, and this ranges from Modern technologies aided the invention of various new slower processes such as domain wall and magnetic vortex magnetic materials, synthetic structures, micro- and nanos- dynamics to faster processes such as spin wave propagation tructures, and metamaterials. Magnetism has come a long and localization, ultrafast demagnetization, and relaxation. way and found applications in a range of multidisciplinary Thisintroducesmagneticstructuresatvariouslengthscales fields in present and future nanotechnologies like nonvolatile such as nanodots, microdisks, magnetic nanowires, and magnetic memory [1], magnetic storage media [2], magnetic nanostripes. Here, we shall review the experimental and recording heads [3], magnetic resonance imaging [4], and in numerical studies of properties of magnetic nanoparticles biomedicine and health science [5]. Emerging technologies (MNPs) together with their leading preparation techniques. such as spin logic [6, 7], spin torque nano-oscillators (STNOs) Research on MNPs has been a vivid research subject [8], and magnonic crystals [9]havealsobecomeveryactive. over the last few decades not only for technological reasons, The new technologies demand the invention ofnew but also from the fundamental research point of view. material properties, which requires structuring of known The field of MNPs has been an interdisciplinary subject, materialsinallthreedimensionsatvariouslengthscales where researchers from physics, chemistry, and biology put and exploiting dynamical magnetic properties over various equal interest in synthesizing, understanding, and moving 2 Journal of Nanomaterials forward for various applications. In the last decade, thorough 2. Synthesis of Magnetic Nanoparticles investigations have been made in the field of nanosized magnetic particles because of their potential for applications There are basically two types of approaches: (i) bottom- such as data storage, memory, magnonic crystals, permanent up and (ii) top-down approaches to synthesize magnetic magnets, in biology, for example, improving the quality of nanoparticles and nanostructures. magnetic resonance imaging (MRI), hyperthermic treatment There are several important issues of nanoparticle syn- for malignant cells, site-specific drug delivery, and manip- thesis [19] such as (i) obtaining a monodisperse particle size ulating cell membranes [10, 11]. Frenkel and Dorfman [12] distribution; (ii) control of the particle size in a reproducible first predicted that a particle of a FM material is expected manner; (iii) obtaining materials with satisfactory high crys- to consist of a single magnetic domain below a critical size. tallinity and the desired crystal structure; (iv) control over the Kittel [13] made rough estimates of critical particle sizes. An shape of nanoparticles; (v) stability of the nanoparticles over approximate radius of 10–1000nm is found for a spherical long time. MNPofaFMmaterial.Themagnitudeofthemagnetic Synthesis of MNPs by chemical methods has been widely moment of a particle is proportional to its volume. Such used in the last few decades because it is one of the cheapest monodomain FM particles can be viewed as a large magnetic ways of producing large quantities of the desired MNPs. In unit, each having a magnetic moment of thousands of . the chemical method, the particle size can be well controlled Therefore, these single domain magnetic nanoparticles are in the range from a few nanometers to micrometers. In called “superspins” or “macrospins.” Usually, an ellipsoidal chemical synthesis, a short burst of nucleation followed by shapeoftheparticlesisassumed,wherethemagnetic slow controlled growth is critical to produce monodisperse moments have the tendency to align along the longest axis, particles [19]. which defines the direction of lowest “shape” anisotropy Figure 1 shows the representative procedure for the energy [14]. preparation of MNPs by chemical synthesis. In the first The critical radius belowwhichaparticleactsasasingle part, the rapid injection of the reagents, often organometallic domain particle is given by [15] compounds, into hot surfactant solution induces the simul- taneous formation of nuclei [20]. In the second part, reagents are mixed at low temperature and the resulting reaction ( )1/2 ≈9 , mixtures are slowly heated in a controlled manner to generate 2 (1) 0 nuclei. The particle growth occurs by subsequent addition of reactive species. The particle size can also be increased where is the exchange, is the uniaxial anisotropy by aging at high temperature by Ostwald ripening, during constant, 0 is the vacuum permeability, and is the which smaller nanoparticles dissolve and deposit at the bigger saturation magnetization. Typical values for are about ones [19]. In chemical synthesis, the particle size can be 15 nm for Fe, 35 nm for Co, and 30 nm for -Fe2O3, while for controlled by systematically adjusting the various reaction SmCo5 it is as large as 750 nm [16]. Depending on size and parameters such as time, temperature, and concentration of material, the magnetic moments of single domain particles reagents and stabilizing surfactants. Also during this process, 3 5 organicreagentscanbeadded,whichwillformashellaround canbe10 –10 B [17]. There are various models for the magnetization reversal the magnetic core, an option to avoid any agglomeration of of single domain particles. A model for the coherent rotation MNPs. of the magnetization was developed by Stoner and Wohlfarth Another important technique to synthesize MNPs is the [18]. They assumed noninteracting particles with uniaxial microemulsion approach. In this technique, two immiscible anisotropy in which the spins are parallel and rotate at unison. liquids form a thermodynamic stable isotropic dispersion In this paper, we highlight the state-of-the-art prepara- denominated microemulsion, where the microdomain of one tion techniques for MNPs, the magnetic states observed in or both liquids is stabilized by an interfacial film of surfactant ensembles of MNPs, and also some future applications of molecules [21]. Various magnetic nanoparticles have been MNPs. In the beginning, we go through both bottom-up and prepared by the microemulsion technique, some bodies of top-down approaches for preparing magnetic nanoparticles literature about which are given by references of Lu et al. [21]. and nanostructures. We also briefly discuss nanoparticle Details of various approaches of chemical synthesis of MNPs superlattices or supracrystals and templated self-organization are described in various recent articles [19, 21–26]. of nanoparticles. Then, we review some of the magnetic Sun et al. [27] synthesized FePt nanoparticles using a ground states such as superparamagnetism, superspin glass, chemical method. FePt is a highly interesting alloy, since surface spin glass, and superferromagnetism in ensembles L10-ordering of FePt leads to the large uniaxial magnetic 6 3 of MNPs. Further, we will discuss the recent progress in anisotropy ( ∼7×10J/m ), which is comparable

Load more