Size-Dependent Crystalline and Magnetic Properties of 5–100 nm Fe ₃ O ₄ Nanoparticles: Superparamagnetism, Verwey Transition, and FeO–Fe ₃ O ₄ Core–Shell Formation

Abstract

Due to their non-toxicity and their ability to be functionalized, magnetite (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) nanoparticles (NPs) are good candidates for a variety of biomedical applications. To better implement their applications, it is crucial to well understand the basic structural and magnetic properties of the NPs in correlation with their synthesis method. Here, we show interesting properties of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> NPs of various sizes ranging from 5 to 100 nm and the dependence of these properties on particle size and preparation method. One synthetic method based on heating Fe(acac) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with oleic acid consistently gives 5 ± 1 nm NPs. A second method using the thermal decomposition of Fe(oleate) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> in oleic acid led to larger NPs, greater than 8 nm in size. Increasing the amount of oleic acid caused the average NP size to slightly increase from 8 to 10 nm. Increasing both the reaction temperature and the reaction time caused the NP size to drastically increase from 10 to 100 nm. Powder X-ray diffraction and electron-microscopy imaging show a pure single crystalline Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> phase for all NPs smaller than 50 nm and spherical in shape. When the NPs get larger than 50 nm, they notably tend to form faceted, FeO core-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> shell structures. Magnetometry data collected in various field-cooling conditions show a pure superparamagnetic (SPM) behavior for all NPs smaller than 20 nm. The observed blocking temperature, TB, gradually increases with NP size from about 25-150 K. In addition, the Verwey transition is observed with the emergence of a strong narrow peak at 125 K in the magnetization curves when larger NPs are present. Our data confirm the vanishing of the Verwey transition in smaller NPs. Magnetization loops indicate that the saturating field drastically decreases with NP size. While larger NPs show some coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) up to 30 mT at 400 K, NPs smaller than 20 nm show no coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 0), confirming their pure SPM behavior at high temperature. Upon cooling below TB, some of the SPM NPs gradually show some coercivity, with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> reaching 45 mT at 5 K for the 10 nm NPs, indicating emergent interparticle couplings in the blocked state.