Magnetic Response of Iron Oxide Nanoparticles as Measured by AC Faraday Rotation
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Magnetic Response of Iron Oxide Nanoparticles as Measured by AC Faraday Rotation Maarij Syed and John Moore Department of Physics & Optical Engineering, Rose-Hulman Institute of Technology, Terre Haute, IN 47803, U.S.A. ABSTRACT Metal ferrite nanoparticles are of considerable technological and theoretical interest. Magnetic response of these systems is a function of various system properties like saturation magnetization, growth orientation, average particle size and size distribution, volume concentration, etc. [1]. This preliminary study investigates the magnetization dynamics (and thereby the Verdet constant) of aqueous Fe3O4 nanoparticle solutions through precision AC measurements of the Faraday Rotation (FR) at 633 nm for three different Fe3O4 nanoparticle solutions that are all prepared to have the same average particle size (~10 nm) but differing saturation magnetization values. For each of these nanoparticle solutions precision measurements for FR are carried out over a given range of volume concentrations. The study shows simple linear dependence of FR on volume concentration and a more involved dependence on saturation magnetization. Some preliminary results relating to the frequency dependence of Verdet constants are also presented. Future directions for this work are also discussed. It is hoped that that these results will help in the development of better models to characterize response of these technologically and fundamentally useful systems. INTRODUCTION Transition metal ferrite nanoparticles are attracting increasing attention because of their chemical stability, easy fabrication, and low cost. These attractive qualities make them ideal candidates for many current and potential applications in technologies including ultrahighdensity magnetic recording, optoelectronics, MEMS magnetic actuators, and biomedicine. FR has been an area of technological and fundamental significance for a very long time. It is observed as the rotation of the electric field vector of a linearly polarized light beam as the beam propagates through a medium in a magnetic field applied along the direction of propagation. It arises from magnetically-induced circular birefringence in which right-hand and left-hand circularly polarized optical field components propagate through the medium at different rates due to differing indices of refraction. The result is a net rotation of the light’s plane of polarization. The amount of rotation is given by:
T
VHl ,
(1)
where l is the length of the medium, H is the magnetic field intensity, and V is a material-specific constant called the Verdet constant [2]. This constant is strongly dependent on the optical wavelength, but at any given wavelength it can vary widely depending on the magnetic properties of the medium. Materials with a large Verdet constant have applications as optical
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isolators, modulators, and magnetic field sensors, to name a few. Often, thin film samples or weakly magnetic samples (both cases lead to the case of very small net rotation) are also investigated with
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