Size and surface effects on the magnetism of magnetite and maghemite nanoparticles
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Size and Surface Effects on the Magnetism of Magnetite and Maghemite Nanoparticles V. N. Nikiforova*, A. N. Ignatenkob, and V. Yu. Irkhinb aMoscow
bMikheev
State University, Moscow, 119991 Russia Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, ul. S. Kovalevskoi 18, Yekaterinburg, 620990 Russia *e-mail: [email protected] Received July 20, 2016
Abstract—The size effects of magnetite and maghemite nanoparticles on their magnetic properties (magnetic moment, Curie temperature, blocking temperature, etc.) have been investigated. Magnetic separation and centrifugation of an aqueous solution of nanoparticles were used for their separation into fractions; their sizes were measured by atomic force microscopy, dynamic light scattering, and electron microscopy. A change in the size leads to a change in the Curie temperature and magnetic moment per formula unit. Both native nanoparticles and those covered with a bioresorbable layer have been considered. The magnetic properties have been calculated by the Monte Carlo method for the classical Heisenberg model with various bulk and surface magnetic moments. DOI: 10.1134/S1063776117010046
1. INTRODUCTION The first magnetic material known to humanity, magnetite, still remains in many respects mysterious owing to the complex interactions of spin, orbital, and charge degrees of freedom [1]. Apart from magnetite, maghemite (γ-Fe2O3), which also has the crystal structure of spinel, belongs to the magnetic iron oxides. The existence of a planar nanophase ε-Fe2O3 has also been discussed in recent years [2]. In magnetite one Fe3+ ion occupies a tetrahedral site, while each of the two other ions, Fe3+ and Fe2+, occupies octahedral sites. Owing to the cation and vacancy ordering, maghemite can have a tetragonal superstructure. The formulas for magnetite and + ]O4, maghemite are Fe3+[Fe3+Fe2+]O4 and Fe3+[Fe 30.33 respectively. Usually, there is a continuous series between (suboxidized) magnetite and the state completely oxidized to maghemite. Maghemite is a very common mineral on the Earth’s surface. It is also found in corrosion products and proteins, is used in medicine as an agent for drug delivery [3] and in nuclear magnetic resonance tomography [4], and is widely applied as a magnetic recording medium [5]. Maghemite is unstable: it loses its magnetic properties and transforms into hematite (α-Fe2O3) as the temperature rises (when heated, they form a continuous metastable magnetic solid solution). The transfor-
mation temperature depends on prehistory: in slightly oxidized samples its value is near 300°C, while in more oxidized ones it is above 450°C [6]. Its value is also affected by the particle size, water content, and stoichiometry [7]. Magnetite and maghemite nanoparticles are widely used in information recording and storage systems, biological studies, and medical tests (hyperthermia). Nanomagnetite is used in magnetic hyperthermia as an efficient material at low frequencies (below 1 MHz [8]), because the
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