Magnetic and structural properties of nickel zinc ferrite nanoparticlessynthesized at room temperatureShannon A. MorrisonThe George Washington University, Washington, D.C. 20052 and U.S. Naval Research Laboratory,Washington, D.C. 20375Christopher L. CahillThe George Washington University, Washington, D.C. 20052Everett E. Carpentera)U.S. Naval Research Laboratory, Washington, D.C. 20375Scott CalvinSarah Lawrence College, Bronxville, New York 10708Raja Swaminathan and Michael E. McHenryCarnegie Mellon University, Pittsburgh, Pennsylvania 15213Vincent G. HarrisNortheastern University, Boston, Massachusetts 02115共Received 5 December 2003; accepted 4 March 2004兲Nickel zinc ferrite nanoparticles (Ni0.20Zn0.44Fe2.36O4) have been produced at room temperature,without calcination, using a reverse micelle process. Particle size is approximately 7 nm asdetermined by x-ray powder diffraction and transmission electron microscopy. Saturationmagnetization values are lower than anticipated, but are explained by elemental analysis, particlesize, and cation occupancy within the spinel lattice. Extended x-ray absorption fine structureanalysis suggests that a significant amount of Zn2⫹, which normally occupies tetrahedral sites,actually resides in octahedral coordination in a zinc-enriched outer layer of the particles. This‘‘excess’’ of diamagnetic Zn can thus contribute to the overall decrease in magnetism. Further, thismodel can also be used to suggest a formation mechanism in which Zn2⫹is incorporated at a laterstage in the particle growth process. © 2004 American Institute of Physics.关DOI: 10.1063/1.1715132兴I. INTRODUCTIONNickel zinc ferrite 共NZFO兲 has found use in electromag-netic applications that require a high permeability, such asinductors and electromagnetic wave absorbers.1Current in-terest has been to make nanosized NZFO particles in order toreduce energy losses associated with bulk powders. Further,most electronic applications require these materials bepressed into larger shapes with near theoretical density,which is difficult to obtain if the particles have a wide sizedistribution. Though several methods of nanoscale ferritesynthesis have been successful in making NZFO, such ashydrothermal,2–5coprecipitation,6–9and ball milling,10con-trol of the particle size and distribution has remained elusive.Reverse micelle synthesis is a technique that has demon-strated considerable control over nanoparticle size and distri-bution in other oxide systems.11–13Briefly, reverse micellesare water-in-oil emulsions in which the water to surfactantratio controls the size of water pools within which aqueouschemical syntheses take place, and consequently control thesize of resultant particles.14This technique is particularly at-tractive for room temperature reactions such as the precipi-tation of oxide nanoparticles.15Synthesis of various nanopar-ticles within reverse micelles, specifically ferrites, hasdemonstrated the ability to control the particle size, sizedistribution,13,16chemical stoichiometry, and cationoccupancy.17However, previous work has often produced‘‘precursor’’ particles that require subsequent firing.8,18Our recent efforts have focused on the room-temperaturesynthesis of nanoscale NZFO ferrites that do not require fur-ther processing. In the study presented herein, we have em-ployed a surfactant system for the room-temperature reversemicelle synthesis of NZFO nanoparticles. Sodium dioctylsul-fosuccinate 共AOT兲,19,20was combined with a 2,2,4-trimethylpentane 共isooctane兲 oil phase to make the reversemicelle solution. This allowed comparison to similarly pro-duced materials from other research groups in which NZFOwas synthesized in reverse micelles and subsequently fired toproduce the desired product; our material did not need asubsequent firing step. We have optimized the reaction con-ditions of this system to produce, at room temperature, purephase nanoscale NZFO particles over a narrow size distribu-tion. This paper outlines the synthetic route in the AOT sys-tem and examines the magnetic behavior and cation distribu-tion within this ferrite structure.II. EXPERIMENTFor the AOT/isooctane reverse micelle system, a stocksolution of 0.5M AOT was prepared in isooctane. An aque-a兲Email address: [email protected] OF APPLIED PHYSICS VOLUME 95, NUMBER 11 1 JUNE 200463920021-8979/2004/95(11)/6392/4/$22.00 © 2004 American Institute of PhysicsDownloaded 02 Jul 2004 to 128.2.77.202. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspous metal solution was then prepared using 0.045M FeCl2,0.011M NiCl2, and 0.011M ZnCl2. The AOT solution wasadded volumetrically to the aqueous metal solution to form areverse micelle solution. An analogous solution was madeusing water and concentrated ammonia in a 1:1 ratio. Theammonia solution was used to adjust the pH of the aqueousmetal solution to induce precipitation and oxidation withinthe water pools. The mole ratio of ammonia to metal cationswas greater than 12:1 and the relative volume of AOT solu-tion to water was 6:1, corresponding to an␻of 16.5.Both the metal containing reverse micelle solution andthe corresponding ammonia solution were agitated until eachwas visibly clear, sonicated for 5 min and then purged withflowing nitrogen for 5 min. The metal solution was placed inan addition funnel and added to the ammonia solution whilestirring. Once the addition was complete, the reaction wasstirred for 2 h. Excess methanol was added and mixed withthe reverse micelle solution to disrupt the micelles and re-move surfactant from the particles. This mixture was centri-fuged and the supernatant removed. The material waswashed and centrifuged repeatedly 共⬃10⫻兲 with methanoland then methanol/water until the AOT was removed. Thematerial was then dried over-night in a vacuum chamber.X-ray powder diffraction was performed on the NZFOparticles using a Phillips x-ray diffractometer employing CuK␣radiation from a sealed tube 共50 kV, 30 mA兲 source.Analysis of the material revealed a match with PDF #08-0234,21indicating the nickel zinc ferrite structure 共Fig. 1兲.Using the Scherrer equation, a crystallite diameter of 6.8 nmwas determined.A TECNAI F20 model high-resolution transmissionelectron microscope 共TEM兲 was used to characterize themorphologies and the particle size distribution of the nano-particles 共Fig. 2兲. The NZFO nanoparticles were