Electron Microscopy Study of Submicron Size Antiferromagnetic KMnF3 Nanoparticles and Their Self-assembly
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Electron Microscopy Study of Submicron Size Antiferromagnetic KMnF3 Nanoparticles and Their Self-assembly Jeffrey Anderson1, Rubi Garcia, and Weilie L. Zhou Advanced Materials Research Institute, University of New Orleans, New Orleans LA 70148 1 The Academy of the Sacred Heart, New Orleans LA, 70115 ABSTRACT Submicron KMnF3 cubic and spherical nanoparticles were synthesized using the reverse micelle method. The nanostructures of the nanocrystals were studied by field emission electron microscopy and transmission electron microscopy. KMnF3 nanocrystals synthesized at room temperature started with cubic submicron particles (~100 nm) and consisted of KMnF3 nanocrystallites (10-15 nm). As the reaction continued, the nanocrystals fused together and transformed into perfect cubic nanocrystals. Spherical beads composed of KMnF3 nanocrystallites were observed at low temperature synthesis. As the reaction continued, the spherical particles grew larger, however, no characteristic cubic shape of KMnF3 nanoparticles were observed. Even as they grew larger, there was no evidence of homogeneous crystal morphology as seen in the room temperature samples. Cubic shape KMnF3 nanocrystals were self-assembled into large area self-assembling patterns.
INTRODUCTION Most of the research into magnetic phase has been focused on ferro- and ferromagnetic materials, little research has been done in the field of antiferromagnetic compounds [1-2]. The magnetic behavior of small-sized grains of antiferromagnetic materials was first proposed by Neel in 1961 [3]. Due to the high area/volume ratio, small antiferromagnetic particles should possess a net magnetization below the ordering temperature. KMnF3 is a typical antiferromagnetic compound, which is a paramagnet with a cubic perovskite structure at room temperature. At 88.3 K it undergoes a transition to an antiferromagnetic ordered state. It was found that KMnF3 nanophase is easily synthesized using the reverse micelle method [4-7]. CTAB reverse micelle synthesis produces easily obtainable monodispearsed particles. Reverse micelles function as microreactors created by dissolving a surfactant (CTAB: cetyltrimethylammoniumbromide) into a nonpolar (octane) solvent. When an aqueous salt solution is added, the surfactant surrounds the polar water solution with its hydrophilic regions oriented inward and the hydrophobic regions, attracted by the nonpolar solvent, oriented outward. When two microemulsions containing reactive salt solutions are mixed the reverse micelles, through a variety of different models [8], function as reactors for the formation of the particles. Final particle size can be controlled by manipulating surfactant and water concentrations in order to form different sized micelles [8,9]. Unlike other spherical particles, KMnF3 nanocrystals are cubic in shape [9,10]. No discussion regarding the specific manipulation of KMnF3 nanocrystals has been addressed yet. In this
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