Pergamon NanoStructurcd Materials, Vol. 12, pp. 763-768, 1999 Elsevier Science Ltd PI1 SO%59773(99)00232-9 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 096%9773/99/$-secfront matter zyxwvutsrq NANOSCALE CHARACTERIZATION OF MAGNETIC NANOPARTICLES Y. Jin, C. L. Dennis, and S. A. Majetich Department of Physics, Carnegie Mellon University, Pittsburgh, PA, 15213, USA Abstract -- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 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. SINGLE PARTICLE MAGNETIZATION DIRECTION 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 763