Nanoscale characterization of magnetic nanoparticles

Nanoscale characterization of magnetic nanoparticles

NanoStructurcd Materials, Vol. 12, pp. 763-768, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 096%97...

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NanoStructurcd Materials, Vol. 12, pp. 763-768, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 096%9773/99/$-secfront matter


PI1 SO%59773(99)00232-9



Y. Jin, C. L. Dennis, and S. A. Majetich Department of Physics, Carnegie Mellon University, Pittsburgh, PA, 15213, USA Abstract -- A new technique for determining the magnetization direction of individual nanoparticles using the Foucault method of Lorentz microscopy is described Experimental images for isolated SmCo, nanopartkles as afunction of the objective aperture shift direction are shown. In preparation for studies of interparticle coupling using this approach, preliminary results describing the preparation of or&red arrays of magnetic nanopartkles are presented Here nonmagnetic nanorodr of uniform size are shown to self-assemble into arrays, which can be fixed in a silica matrix and heated to transform the rob into a fem’magnetic phase. 01999 Acta Metallurgica Inc. INTRODUCTION While scientists have studied the physics and chemistry of magnetic nanoparticles for many years, theories have focused on isolated individual particles (l-4) while experiments have concentrated on disordered assemblies of particles which often have substantial size distributions and significant interparticle interactions (5-7). Here we report two results which will enable future experiments to better bridge the gap between theory and experiment: 1) the ability to determine the magnetization direction of individual nanoparticles using Lorentz microscopy, ind 2) the preparation of self-assembled magnetic nanoparticle arrays, in which interactions occur but are more readily modeled because of the ordering.





We have developed a method using Lore& microscopy to uniquely determine the magnetization direction in SmCog nanoparticles as small as 5 nm. With the tremendous strides being made in micromagnetics calculations (3,4), there is renewed interest in magnetic measurements on single particles. Several groups have performed experiments on individual nanoparticles using magnetic microscopies (g-lo), or small numbers of magnetic nanoparticles using microSQUID methods (11,12). The advantage of the microscopy methods is the ability to readily determine the direction of the magnetic moment for a large number of particles. An electron passing through a magnetic particle is deflected by a Lorentz force due to its magnetic field. Because TEM specimens are very thin and the deflection is small, conventional (bright field) electron micrographs are still possible. However, with slight modifications evidence of the magnetic deflection, and therefore information about the magnetization structure of the sampEe, can be obtained. In the Foucault method of Lorentz microscopy the objective




aperture is slightly shifted to block electrons with similar deflections. The aperture is at the back focal plane of the objective lens, and without magnetic deflection all electrons ideally pass through the same point in this plane. Compared with a conventional image, the Lorentz image shows dark regions in areas with similar in-plane magnetization components. The in-plane magnetization structure is revealed from analysis of images for different aperture shift directions. Lorentz microscopy is standard for thin films but has received considerably less attention in the fine particle community. The Foucault method has previously been applied to larger, needle shaped “I-Fe,O, particles (8), and the Fresnel method has been used to investigate 50-100 nm Co precipitates in Co-Au thin films (10). Here we build on this work, extending the Foucault method to the smallest particles to date, and noting some differences in the images in extremely small spherical particles, due to the small magnitude of the electron deflection. Details of the procedure for generating ball milled SmCog nanoparticles and dispersing them in a thermoplastic matrix have been reported elsewhere (13). TEM samples were prepamd using standard ultramicrotomy sectioning techniques. Foucault Lore& microscopy was carried out in a JEM-120CX transmission electron microscope equipped with a Lorentz pole piece. Particles were examined under bright field conditions, and then the objective aperture was shifted to cut off part of the electron beam. Images and diffraction patterns for the same fields of view were recorded for a series of different aperture shift directions. The aperture shift angles 8, were found from the diffraction patterns and corrected for rotation, relative to the bright field image. A single SmCo, particle is shown in bright field and Lorentz images for different aperture shift angles in Figure 1. The most notable feature of the Lorentz images is the appearance of a pair of dark lobes on either side of the particle. These lobes rotate with the aperture shift angle, 6l, (14). In addition, for a range of shift angles the center of the particle darkens, reaching minimum intensity at a shift angle 8, * (14). The dark lobes arise when the aperture shift blocks electrons which have passed through the fringe field of the particle. The orientation of the dark lobes rotates with the aperture shift angle and not twice as fast because the back focal plane contains the Fourier transform of the object wavefunction. Just as in single slit diffraction, electrons passing through the particle along a line at an angle 8 are spread out in a line at an angle &c/2 in the back focal plane. When the aperture is shifted at an angle 8,, dark lobes appear in the image at B&/2. These lobes are arcs because a range of fringe fields have a substantial component in this direction. The appearance of the dark center with intensity maximized for a unique value of 8, can be explained solely from the Lorentz deflection. All electrons are deflected in the same direction while passing through the particle, but those going through the center are deflected most. This creates a unique aperture shift angle 8, * to reverse contrast in the particle interior, and causes the center of the particle to darken first. After accounting for image inversion by the lens, the true orientation of the particle magnetic moment, 8,, is given by 8, = e,* + r~t2.


The particle in Figure 1 therefore has the in-plane component of its magnetic moment directed at approximately 55’ relative to the vertical. Lorentz microscopy images of SmCog nanoparticles 15-50 nm in diameter behave like tiny dipoles, which is not surprising considering they are well below the theoretical limit for monodomain particles (2 pm). It is possible to uniquely determine their magnetic moment directions from images under different aperture shift directions, focusing on the angle which leads to a dark interior in each particle, rather than on the orientation of the prominent dark lobes



Fig. 1. Brig;ht field image of a 20 nm diameter SmCog nanoparticle (Upper Left), and Lorentz images of the same particle for different aperture shift angles. which appear on either side. The different procedure for interpreting the images arises because in nanostructured materials the magnitude of the Lorentz deflection is small relative to the intensity distribution of the electrons in the back focal plane of the electron microscope.





Successful self-assembly of nanoparticle arrays (15-17) depends on the ability to prepare monodisperse particles and to balance the interparticle forces so that ordemd structures form spontaneously. Compared to other self-assembling systems, magnetic nanoparticles have an additional magnetostatic force, which favors the formation of magnetically aligned chains of magnetic dipoles, rather than two- or three-dimensional structures. The previously reported tine particle magnet arrays have been made by lithographic methods (l&21), by a scanning tunneling microscope (STM) (12), or by electrodeposition of metals in the cylindrical pores of anodized aluminum (22). The lithographic approach has been used to make nearly perfect arrays, but extending this technology to the nanometer scale has been extremely difficult. The STM approach can form very small structures, but is slow and not readily adapted to fabricating large quantities. Self-assembly can produce nanostructures and is scaleable for the production of large quantities at low cost. If successful, the most obvious application would be in magnetic data storage. However, the production of magnetic nanoparticle arrays is in :its infancy, and much must be learned about the process of self-assembly. To make magnetic arrays we first prepared nonmagnetic arrays and then transformed them into a magnetic phase once the particles were solidified in a matrix. Randomly oriented needle like T-Fe203 particles are used in magnetic tapes, but they are poor candidates for forming



nanorod arrays because they, and their “nonmagnetic” precursors, YFeOOH, are not uniform in size, and form irregular lath-like structures (23) rather than rods. However, slow hydrolysis of FeC13 at pH 2 and below (24,25) generates uniform rods of j3-FeOOH, which self-assemble to form smectic liquid crystals called Schiller layers. A schematic of the Schiller layer structure deduced from light scattering studies and optical microscopy (24) is shown in Fig. 2a. Characteristic iridescence with colors ranging from pink to blue occurs because the separation between layers of rods is on the order of a wavelength of light. Schiller layers ate readily distinguishable from the brownish orange disordered slurries of P-FeOOH nanorods. Schiller layers were prepared by centrifuging the solution until iridescence was seen in the deposited sediment, and then most of the supematant was removed. The acid catalyzed reaction of tetraethylorthosilicate (TEOS) in a mixture of ethanol and water at 50 ‘C was used to form a Si02 matrix around the particles. Iridescence remained, but the color changed upon addition of the TEOS, suggesting a contraction of the interlayer spacing. Solid samples were sectioned with a diamond knife in a microtomy device to prepare specimens for TEM. To make ferrimagnetic y-Fe203, the antiferromagnetic P-FeOOH arrays were slowly heated (l”C/min., up to 240-260 “C) in an inert atmosphere and then exposed to H2 overnight. The particle phase was monitored through x-ray and electron diffraction. Without the silica coating this reaction does not preserve the rod-like shape of the original particles, but generates spherical a-Fe203, and then reduction to Fe,O,, due to dissolution of the more soluble p FeOOH followed by nucleation of or-Fe203 (26). Figure 2b illustrates the range of structures present in the Schiller layers embedded in silica. In the upper left hand comer is a section where the order has been preserved and the particles are oriented with their long axis normal to the page. The unusual square superlattice arises from the tetragonal crystal structure of the P-FeOOH, which leads to faceted nanorods that pack most closely with this structure. Spherical nanoparticles tend to form hexagonal close

Fig. 2. Left: Schematic of Schiller layers; Right: TEM of Schiller layers embedded in silica; the inset shows a magnification of the ordered region in the upper left.



packed or face centered cubic crystal structures, which are closest packing arrangements for spheres. The central region of Figure 2b shows what appear to be layers of small particles oriented on their sides. The effect of the silica and heating in the current protocol is to decouple the layers and to introduce additional disorder, factors which should be minimized. While bulk magnetic measurements include contributions from both ordered and disordered particles, preliminary dhta from SQUID magnetometry shows that the heat treated samples are no longer antiferromagnetic and have a small ferro- or ferrimagnetic component, as well as a significant paramagnetic component from residual Fe3+trapped in the silica matrix.

CONCLUSIONS The magnetization direction of individual SmCo5 nanoparticles 15-50 nm in diameter is determined using the Foucault method of Lorentz microscopy. While the observation method is similar to that used previously, for these very small particles the interpretation of the images is different. The: experimental data can be understood in terms of a model which accounts for both the Fourier transform of the image in the back focal plane on the electron microscope and the Lorentz deflection of the electrons by the sample. The degree of ordering is currently lower in the heat treated silica coated samples, relative to that for samples embedded in polymer resins at room temperature. However, the silica is necessary to withstand the temperature requited for the phase transformation, which was determined by differential thermal analysis (DTA). Future work will concentrate on improving the degree of order by minimizing the stress applied to pores within the silica matrix during the drying process.

ACKNOWLEDGMENTS Support from the National Science Foundation under grants DMR-9258308 and DMR 95003 13, and assistance from M. Grant are gratefully acknowledged.

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