Kelly Pisane

Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mohindar S Seehra

Committee Co-Chair

Edward Flagg

Committee Member

Mikel Holcomb

Committee Member

David Lederman

Committee Member

Aleksandr B Stefaniak

Abstract

Magnetic nanoparticles with large magnetic moments that can be manipulated with an external magnetic field, have potential uses in medicine because their sizes are comparable to biological scales. For such applications it is important to understand how their magnetic properties are affected by their size and size distribution inherently present in magnetic nanoparticles. For this purpose, maghemite (gamma-Fe2O3) nanoparticles with average diameters of 7.0+/-0.8 nm, 6.3+/-0.6 nm, 3.4+/-0.8 nm and 2.5+/-0.7 nm and Fe-Pt core-shell nanoparticles with an approximate core diameter of 2.2 nm were synthesized and investigated. To aid in the interpretation of the magnetic properties, the structural properties of these nanoparticles were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA).;For investigations of the magnetic properties, detailed ac and dc magnetic characterization is presented and discussed in terms of a distribution of particle sizes and magnetic moments. The dc magnetization measurements cover the temperature range from 2 K to 350 K and magnetic fields up to 90 kOe. The temperature dependence of the ac susceptibilities, &khgr;' and &khgr;'', was measured at various frequencies from 10 Hz to 5 kHz. From the zero field-cooled dc magnetization, the values of blocking temperature TB have been determined and the ac magnetic data was used to determine the contribution of interparticle interactions to the observed blocking temperature for different sized nanoparticles. The measured blocking temperatures of the maghemite nanoparticles are TB=35 K, 42 K, 21 K, and 29 K with contributions from interparticle interactions given in terms of To=0 K, 11 K, 2.5 K, and 12.5 K for the 7.0 nm, 6.3 nm, 3.4 nm, and 2.5 nm samples respectively. From the variation of TB with ac measurement frequency, the anisotropy constants Ka determined for the maghemite nanoparticles are: Ka=5.57, 7.51, 18.57, and 79.9 in units of 105 erg/cm3 for the 7.0 nm, 6.3 nm, 3.4 nm, and 2.5 nm samples with a Neel-Brown attempt frequency of fo=2.6x1010 Hz. The same approach applied to Fe-Pt nanoparticles yields TB=13 K, To=5 K, Ka=4.74x106 erg/cm3, and fo=5.3x1010 Hz. For maghemite nanoparticles, the size dependence of the anisotropy shows an increase with decreasing particle diameter consistent with data of other investigators. However this dependence is more rapid than the 1/D behavior typically used to discuss the size dependence of nanoparticle magnetic anisotropy.;The magnetic field dependence of the magnetization of the nanoparticles below their blocking temperature TB indicates negligible coercivity for the 7.0 nm, 3.4 nm, and 2.5 nm maghemite samples. However, for the 6.3 nm maghemite and the Fe-Pt samples, significant coercivity HC is observed with their magnitudes increasing with decreasing temperatures below TB and reaching 400 Oe and 750 Oe at 2 K, respectively. Above TB the field dependence of the magnetization of all the samples was analyzed in two different ways: in terms of a modified Langevin equation that ignores the distribution of particle diameters and in terms of a lognormal distribution of particle magnetic moments mimicking the size distribution. An important conclusion from this comparison is that the two approaches yield consistent and physically meaningful results as long as the width parameter, s, of the log-normal distribution is less than 0.83.;Another important result from these investigations is the derivation of the Eq. ф = фo [1 -- (To/TB )] relating the parameter ф used to describe the interparticle interaction strength to the Vogel-Fulcher temperature To in addition to providing the theoretical basis for the experimentally observed фo ≈ 0.11 to 0.15. Experimental verification of this relationship is presented using the published data on a variety of nanoparticle systems.

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