INTRODUCTION
Magnetic particles are well known for their use in recording devices. For such an application that involves information storage, the nature, size, and shape of the particles are important parameters. These particles are usually made of magnetite (Fe3O4), maghemite (g-Fe2O3), cobalt ferrite (Fe2CoO4), or chromium dioxide (CrO2). They are spherical or acicular, and are usually micron-sized.[1] These particles are deposited on a substrate, and only magnetic properties of the systems are considered.
In this entry, we shall not focus on magnetic particles as magnetic pigments, but as special colloids. Indeed, we shall describe the synthesis and properties of particles whose diameters are smaller than 15 nm, and thus are subject to Brownian motion.
OVERVIEW
Nanometric magnetic particles can be prepared by grinding a magnetic material, typically a ferrite, for several weeks using chemical synthesis.[2] Many synthesis procedures are used, according to the nature of the magnetic particles: iron oxide particles are usually obtained using a precipitation method; synthesis of metallic particles needs more subtle processes. The main difficulty is in getting monodispersed samples in a nanometric range. Nevertheless, it is an important point because the magnetic properties of particles vary based on their size. The materials constituting the particles are ferromagnetic or ferrimagnetic materials.[3] Because of their small diameters, the particles under consideration are magnetic monodomains, which means that each particle has a permanent magnetic moment whose intensity is proportional to the particle’s volume and to the specific magnetization of the material. Depending on this volume, this moment is either blocked in the easy axes of magnetization, or is free to rotate inside the grain. In this last case, the grain is said to be ”superparamagnetic.”1-4-1 For even smaller particles, the magnetic order will disappear.
Because of their small size, magnetic nanoparticles can be dispersed in a liquid carrier.1-5-1 A superpara-magnetic colloidal dispersion (usually called ferrofluid) is then obtained.1-6-1 Such a dispersion is stable for years,contrary to the so-called ”bitter fluid” or to the mag-netorheological fluids that are dispersions of micron-sized particles. It does not settle with time—neither when a magnetic force is applied and is not magnetic in zero field but becomes magnetized when a magnetic field is applied.[7] It is able to move under the action of a magnetic force and indeed finds numerous applications in the field of mechanics. Because the entry on ”Magnetic Nanoparticles in Fluid Suspension: Ferrofluid Applications” covers this aspect in depth, this entry will only touch on this area of research. A key condition for the development of ferrofluid-based applications is that the colloidal dispersion has to be stable. To prevent particle aggregation because of van der Waals forces or magnetic dipolar interactions, interparticle repulsions have to be ”chemically” created. Oxide particles can be directly dispersed in water, provided the pH value is such that the surface hydroxyl groups are ionized.[8] In this case, electrostatic repulsions prevent particles from aggregation.1-9-1 Particles can also be coated by surfactants or polymers, especially when they have to be dispersed in oily media, and steric repulsions are, in this case, responsible for the stability of the system.1-10-1 Obtaining stable dispersions of magnetic particles is an important point for applications, but on the theoretical point of view, such dispersions are also original examples of colloids, with additional interparticle interactions, that are tunable using an external parameter (i.e., the magnetic field). These dispersions have, in addition, many original properties because of their global response to a magnetic field. This response can be a ”magnetic” one (the medium becomes magnetized), but also an optical one (the medium becomes birefringent) or a rheological one (the medium becomes structured).1-6,7,9-1
These properties can be transmitted to more complex media: glasses,1-11-1 polymeric gels,’12- lyotropic systems (nematics[13- and smectics'14-1), and microscopic colloids (emulsions,[15- latex,[16- and liposomes[17-), which then obtain a response to a magnetic field of small intensity. For example, orientational transition of self-organized phases of surfactants1-18-1 and deformation of liposomes'-19-1 have been observed in low-intensity magnetic fields. Magnetic nanoparticles have also been introduced in inorganic or polymeric gels as viscosity probes, analyzing their magneto-optical response.'20-
These magnetic nanoparticles should find numerous applications. Nevertheless, until now, except for the ''ferrofluid applications'' in the field of commercial mechanics,1-21-1 few applications have been developed. The challenge seems to be in the field of biomedical applications in the next few years.[22,23]
PREPARATION PROCEDURES
According to the chemical nature of the particles (metallic or oxide particles), synthesis procedures will be very different.
Chemical Nature of Particles
Magnetic nanoparticles are, of course, constituted of a magnetic material: a ferromagnetic material or a ferri-magnetic one.[3] In a ferromagnetic or ferrimagnetic material, specific interactions exist between the spins of atoms and lead to an ordering of the spins in parallel or antiparallel directions (Fig. 1). This spontaneous ordering of individual magnetic moments results in a magnetic moment per unit volume that is called its specific magnetization Ms, but it disappears above a given temperature, which is called Curie temperature. Please note that although ferromagnetism and ferrimagnetism are formally different, they lead to the same macroscopic magnetic properties, and the word ”ferromagnetism” is often used in place of ”ferrimagnetism,” even in this article.
Table 1 lists several ferromagnetic or ferrimagnetic materials, in order of increasing magnetization.
Unfortunately, the more magnetic materials (metals) are, the more easily they are oxidized; this tendency to oxidation is enhanced when materials become powder. Consequently, the most commonly used magnetic materials as colloidal particles are iron oxides because they are cheap and easy to obtain.
Fig. 1 The three types of cooperative magnetism. Antiferro-magnetism does not lead to macroscopic magnetism.
Table 1 Some commonly used ferromagnetic or ferrimagnetic materials (Tc is the Curie temperature, and Ms is the saturation magnetization of the material)
|
Material |
Type of magnetism |
Tc (°C) |
Ms (kA/m) |
|
Chromium |
Ferrimagnetic |
117 |
410 |
|
dioxide (CrO2) |
|||
|
Maghemite |
Ferrimagnetic |
590 |
414 |
|
(y-Fe2O3) |
|||
|
Cobalt ferrite |
Ferrimagnetic |
520 |
422 |
|
(CoFe2O4) |
|||
|
Magnetite |
Ferrimagnetic |
585 |
470 |
|
(Fe3O4) |
|||
|
Nickel |
Ferromagnetic |
358 |
485 |
|
Ni(a) |
|||
|
Cobalt Co(a) |
Ferromagnetic |
1131 |
1400 |
|
Iron Fe(a) |
Ferromagnetic |
770 |
1707 |
Magnetite and Maghemite Particles
The most widely known magnetic iron oxide is magnetite (Fe3O4). Indeed, the first magnetic nanoparticles were obtained by grinding magnetite grains in ball mills, in the presence of a surfactant and a liquid carrier. The surfactant, at the same time, helps in grinding and prevents particle aggregation. This process costs time (about 1000 hr) and leads only to surfactant-coated particles.1-2,24-1 Chemical synthesis of colloidal magnetite is now widely used. It has been known for a long time: aqueous mixtures of ferric and ferrous salts are mixed with an alkali to induce the precipitation of a black hydrophilic magnetic product, which is constituted of magnetite par-ticles—negatively charged and associated with the counterions of the alkali.1-25-1 According to the experimental conditions of the synthesis, the particle size can be roughly controlled.1-26,27-1 The particles are rocklike (Fig. 2a) and the system is always polydispersed in size, but the average diameter can be monitored between 5 and 14 nm through the nature of the alkali, its concentration, the concentration of the ferric and ferrous salt, the Fe(II)/ Fe(III) ratio, the ionic strength, the temperature, the way of admixing the reactants, and the efficiency of stir-ring.[26] Once all these parameters are fixed, the result of the synthesis is reproducible. Concerning magnetite, the situation can be summarized this way. Magnetite nano-particles are obtained at room temperature and, for a given concentration of ferric and ferrous species, an increase of pH induces a decrease of particle size, sodium hydroxide leads to particles smaller than ammonia (for the same pH), an increase in ionic strength induces a decrease in particle size, and an increase in the Fe(II)/ Fe(III) ratio increases the particle size. Another way to reduce particle size is to add to the synthesis mixture a molecule-chelating iron species. For example, it is known that carboxylate species interfere with the formation and growth of ferric oxides. Performing the synthesis of colloidal magnetite in citrate or tartrate species allows the reduction of the average particle diameter to 3 nm.’28-However, particle size distribution remains rather wide. Synthesis using micelles has been proposed to control the size distribution.’29- Processes using iron(III) acetyl-acetonate in organic media have been recently proposed to produce monodisperse magnetite particles.’30- Please note that nature is also able to synthesize colloidal magnetite (e.g., magnetotacticum bacteria produce nanometric magnetite particles).’31-
Fig. 2 TEM pictures of ferrite nanoparticles (a) and iron particles (b).
Colloidal magnetite gets oxidized easily. If it is kept under usual atmosphere, it quickly oxidizes, and the product often called magnetite is, in fact, bertholide ‘i.e., a ferric oxide whose composition is between that of magnetite (Fe3O4) and maghemite (g-Fe2O3)-. That is the reason why magnetite is sometimes deliberately oxidized to maghemite, by dispersing the magnetic particles in acidic medium and stirring them with ferric nitrate.’10-The maghemite particles obtained are then chemically stable in alkaline or acidic medium. The saturation magnetization of maghemite is slightly lower than the one of magnetite, but the difference of magnetization is largely counterbalanced by the gain of chemical stability.
Other Ferric Oxides
Many ferric oxide materials with a spinel-like structure (MFe2O4, M=Co, Mn, etc.) can be obtained as nano-particles. The synthesis of cobalt ferrite particles and manganese ferrite particles has been widely described.’32-35-The process is again a precipitation of the oxide using an alkali, but the synthesis conditions to obtain magnetic nanoparticles depend on the nature of the oxide synthesized. For example, cobalt ferrite particles are more rapidly obtained at 90°C, using sodium hydroxide or methylamine.’33- Such particles are highly unstable in acidic medium: they are completely dissolved, leading back to the initial mixture. The same problem exists with manganese ferrite particles. Coating particles with a thin amorphous ferric oxide shell prevent this dissolution and stabilize the samples in acidic media.’32-
M can stay for a mixture of two metallic species (e.g., Zn and Mn, Ni or Co). Compounds with such a composition M1 _ xZnxFe2O4 have magnetic properties that are continuously dependent on the value of x (when x = 1, the compound ZnFe2O4 is no more ferrimagnetic at room temperature). The problem is to control the particle size at the same time, which is dependent on x and the magnetic properties.’34-
Again synthesis using micelles has been described to produce calibrated cobalt ferrite,’33,36- or zinc-cobalt ferrite nanoparticles.’37- Surfactant of the type metal dodecyl sulfate ‘Fe(DS)2, Co(DS)2, or Zn(DS)2- are synthesized and mixed in micelles; addition of an alkali again induces the formation of particles.
Metallic Particles
Metallic particles seem to be more interesting than oxides because their specific magnetization is higher (Table 1). Nevertheless, metallic particles are highly oxidizable and often lose their high magnetization because of oxidation. Different processes have been developed.
Decomposition of metal carbonyl compounds is a convenient method that allows one to obtain large amounts of monodisperse metal particles. Cobalt particles are prepared from dicobalt octacarbonyl (Co2(CO)8).’38-40-Iron particles are obtained from iron pentacarbonyl (Fe(CO)5).’41,42- In the case of iron particles, the solvent is usually toluene or decalin, and the stabilizing agent is either oleic acid, or a polymer as polyisobutene. Usually, the metallic particles obtained this way have narrower size distributions than the ferrite particles (Fig. 2b). Nevertheless, their magnetization is always lower than the one of the bulk material because of surface oxidation and carbon incorporation. The surfactant or the polymer dispersed in the solvent is supposed to catalyze the reaction, to prevent particles from aggregating and to protect them against oxidation. Unfortunately, for this last point, the efficiency of the surfactants or polymers tested is not as promising.
Decomposition of iron octacarbonyl in the presence of ammonia has been described as leading to iron-nitride particles (Fe3N), which has a high saturation magnetization and a good chemical stability.’43- The particle diameter is monitored between 5 and 10 nm, through the Fe(CO)5 content.
Reduction of metal salts by strong reducing agents such as borohydrides is another method to obtain metallic particles, but it is difficult to obtain colloidal objects with reasonable polydispersity without using reverse micelles. For example, nanosized particles, with diameters on the order of 6 nm, are obtained from the reduction of cobalt bis(2-ethylhexyl)-sulfosuccinate (Co(AOT)2) involved in Na(AOT) micelles.’44- Nearly monodispersed cobalt nanocrystals were obtained by reduction of cobalt chloride at high temperature in the presence of stabilizing agents.’45- A special case is the one of iron particles dispersed in mercury that have been obtained by electrolytic reduction of metal salt solutions, or of colloidal solutions of iron oxide in a supporting electrolyte.’46- Nanometric particles are obtained, but the prevention of particle agglomeration is the key problem and is still under study.
A third method uses the evaporation of metal in a vacuum in the presence of a solvent containing a surfac-tant.’47- This technique produces metal particles, the size of which depends on the time during which samples are heated and the amount of surfactant.
