Saturation Magnetization and Coercive Force
Ferrite particles with their average particle size smaller than 20 nm were primarily applied to magnetic fluids, so that their magnetic properties, among others, saturation magnetization, have been studied in detail. Saturation magnetization of most of those particles decreases drastically when their particle size becomes below 20-30 nm. In the case of ultrafine magnetite, for instance, saturation magnetization amounts between 30 and 60 emu/g as compared with that of bulk material, 92 emu/g. Variation of the saturation magnetization is illustrated in Fig. 6 for some representative ferrites.
Crystalline magnetic anisotropy constant, K1, also plays an important role on the relative saturation magnetization. As shown in Fig. 7, the relative saturation magnetization is smaller for materials with a larger value of K1.
Fig. 6 Dependence of the relative saturation magnetization, ss/ sbulk, on average particle size D311. Solid line: particles by coprecipitation; broken line: grown particles by heat treatment.
Fig. 7 Relationship between ss/sbulk and K1 at constant particle size for several different ferrites. Points of a to f indicate materials corresponding to those of Fig. 6 and the symbols with parentheses indicate estimated values.
It is well known that saturation magnetization decreases with the decrease of the crystallinity. However, even when the particles are well crystallized as confirmed by high-resolution electron micrographs, decrease in the magnetization is unavoidable. One of the main reasons for the decrease is then the existence of the magnetically inactive surface layer. The relationship between the crystalline magnetic anisotropy constant and the thickness of the inactive layer is displayed in Fig. 8.
Crystalline magnetic anisotropy is closely related with the coercive force. Many attempts were therefore made to control large K1 by varying preparation methods without great success hitherto. It is to be noted, however, that coexistence of a-Fe2O3 in g-Fe2O3 brings about substantial increase in the coercivity. This kind of materials is prepared by careful annealing of conventionally precipitated hydrous oxide.
Characterization of detailed structure of nanoparticles is not particularly easy. In case of iron-containing compounds, however, Mossbauer spectroscopy serves as a versatile tool for characterization. A typical Mossbauer study was made on Mn-Zn ferrite nanoparticles embedded in silica matrix. The spectrum comprises two components superimposing with each other, i.e., one sextet as a result of ferromagnetic particles and a doublet as a result of superparamagnetic behavior of nanosized ferrite at the center. Quadrupole splitting of the nanoparticles is larger than that of bulk ferrite presumably because of the loss of cubic symmetry in the finer particles.
A more detailed study on the Mossbauer spectra is given on the pressed sample of 3 nm a-FeOOH.
When nanoparticles are compressed together, significant Mossbauer absorption is observed even at room temperature, with a quadrupole doublet typical for super-paramagnetic materials as shown in Fig. 9. This is not the case on the loose power. The difference is attributed to the suppression of the particle motion, which is significant for such nanoparticles because of the loss of recoilless fraction by the particle motion, overlapped with the lattice vibration. At temperatures as low as 10 K, we observe well-known hyperfine structure.
By detailed analysis of the spectra, the amount of the mean-square amplitude of the vibration in the direction of g-ray emission, (x2), was calculated and plotted as a function of temperature, as shown in Fig. 10.
FUNCTIONS AND APPLICATION
Functions and application of iron oxide nanoparticles, mostly with their assembly, are quite diverse in nature and large in quantity, if not of industrial significance for the time being. Some interesting case studies are given below.
Fig. 8 Relation between magnetically inactive layer thickness and K1.
Fig. 9 Mossbauer spectra of compressed 3 nm a-FeOOH at 10 K and room temperature.
Catalytic activity depends largely on the specific surface so that downsizing of the particles is always favorable for catalysis. Milling products are generally considered to have drawbacks for any characteristics attractive for modern industrial application just because of lower crystallinity. Milling zinc ferrites, for instance, were, nevertheless, revealed to be very effective for adsorption of gaseous species such as H2S. Mechanical activation leads to the substantial increase of its sulfur absorption capacity. Increased mobility of cations at sulfurization temperature, connected with their thermally stimulated return from the inversion into equilibrium sites, is the main reason for the increase in the absorption capacity.
A product of extended traditional technology of magnetic fluids is an intelligent ink, i.e., a colored water-based magnetic fluid. This is appropriate for an ink-jet-type printer and is extended to magnetofluidgraphy. By combining Mg-Zn and Mn-Zn ferrites with appropriate dyes, the concept for intelligent ink could be furnished by red and yellow azo dyes or mixed with copper phthalo-cyanine for blue magnetic inks. Viscosity of these magnetic inks ranges between 1.5 and 2.0 mPa • S, being fluid enough for practical rapid printing.
Fig. 10 Plot of (x2) as a function of temperature for the as-prepared sample. Theoretical line is also plotted with o = 2 x 1011.
Fig. 11 Proposed structure of the self-assembled films consisting of the BTHA dication, anionic polyelectrolytes, and Fe3O4 particles. Abbreviations: ATS (3-aminopropyltrimethoxysilane); PSS (sodium polystyrensulfonate); PAH (polyallylaminehydrochloride); and BTHA [4,4'-bis(trimethyl ammonium hexyloxy) azobenzene bromide].
When ferromagnetic iron oxide particles are well assembled in a thin film together with photoisomerizable molecules such as azobenzene-containing layers, the magnetic properties of the film can be controlled by photoillumination at room temperature. Self-assembled films were prepared by dipping a substrate, e.g., silanized quartz, into an aqueous solution of cationic and anionic polyelectrolytes alternatively. Cationic colloidal magnetite was subsequently adsorbed on the precoated substrate to obtain the final organized layer. A corresponding surface chemical structure is schematically illustrated in Fig. 11.
Photoisomerization of the film was examined by UV-vis spectra. The initial trans state was illuminated with 360-nm UV light to transform to the cis state. The latter, in turn, reversibly transformed to the trans state by illuminating visible light (400-700 nm). The photoisome-rization was accompanied by the reversible change in the magnetization of the film to exhibit photoswitching properties at room temperature.[ ]
Similar photoswitching properties were observed in conjunction with the light-induced excited spin state trapping (LIESST). Some ferric coordination compounds were found to be sensitive to LIESST so that their application toward molecular devices is expected.
Fine particulate ferrites prepared by self-combustion method are quite appropriate for multilayer chip ferrite inductor (MLCI).[10,11] Because of the low temperature necessary for sintering (<950°C), fineness of the grain is maintained without disturbing thermal stability.
Preparation, properties, and application of pure and complex iron oxides were presented with special emphasis on the particularity of downsizing as well as particles organization. Intrinsic properties of nanoparticles are symbolized by the increased significance of magnetically inert near-surface region and superparamagnetism. While unsupported, unorganized nanoparticles are not particularly attractive from application viewpoints; alignment and assemblage of nanoparticles look quite promising for various functional devices. Experimental characterization, particularly its topochemical aspect, is quite insufficient just because of the resolution limit of the existing analytical tools. Simulation studies with emphasis on the near-surface region might compliment these experimental difficulties.