Synthesis, microstructure, and magnetic properties of monosized Mn x Zn y Fe3 − x − yO4 ferrite nanocrystals
© Yoon et al.; licensee Springer. 2013
Received: 28 September 2013
Accepted: 9 December 2013
Published: 17 December 2013
We report the synthesis and characterization of ferrite nanocrystals which exhibit high crystallinity and narrow size distributions. The three types of samples including Zn ferrite, Mn ferrite, and Mn-Zn ferrite were prepared via a non-aqueous nanoemulsion method. The structural, chemical, and magnetic properties of the nanocrystals are analyzed by transmission electron microscopy, X-ray diffraction, X-ray fluorescence, and physical property measurement system. The characterization indicates that the three types of ferrite nanocrystals were successfully produced, which show well-behaved magnetic properties, ferrimagnetism at 5 K and superparamagnetism at 300 K, respectively. In addition, the magnetization value of the ferrites increases with the increasing concentration of Mn.
Ferrite nanocrystals have been interestingly studied due to their tunable and remarkable magnetic properties such as superparamagnetism [1–3], as well as catalytic properties not existing in the corresponding bulk materials [4, 5]. There have been extensive investigations on ferrite nanocrystals for potential applications in magnetic storage, ferrofluid technology, and biomedical fields from drug delivery, hyperthermia treatments, to magnetic resonance imaging [6–10].
A ferrite has the spinel structure basically constructed from face-centered cubic lattices formed by oxygen ions and assumes a general formula described as (M2+1 − δFe3+δ)tet[M2+δFe3+2 − δ]octO4. The element M in the formula can be a transition metal, like Mn, Co, and Zn. Moreover, the round and square brackets indicate the tetrahedral site (A site) and octahedral site (B site) created by oxygen ions, respectively. The subscription, δ, in the range from 0 to 1, represents the inversion parameter of the spinel structure. The parameter could be adjusted in terms of various factors, for example, synthesis methods, particle size, and heat treatments [12–18]. The ferrimagnetism of the ferrite is originated from the exchange energy between the A and B sites (A-B interaction) which is larger than other interactions (A-A, B-B). Since the A-B interaction has a negative value, the ions located in both sites have antiparallel orientations; consequently the net moments between both sites result in ferrimagnetism [19–23]. Therefore, possible variation of ion arrangements in the lattices may affect the magnetic properties of the ferrite.
In this study, we report the synthesis and characterization of Mn x Zn y Fe3 − x − yO4 ferrite nanocrystals, i.e., x = 0, y = 0.9 for Zn ferrite, x = 0.6, y = 0 for Mn ferrite, and x = 0.315, y = 0.45 for Mn-Zn ferrite via a nanoemulsion method. The structure, chemical, and magnetic properties of the nanocrystals were comparatively analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) spectroscopy, and physical property measurement system (PPMS).
The ferrite nanocrystals of the three types were synthesized by the nanoemulsion method with a biocompatible polymer [24, 25]. The synthesis was performed by thermal decomposition of precursors including iron(III) acetylacetonate, manganese(II) acetylacetonate, and zinc(II) acetylacetonate hydrate. In the case of the Zn ferrite, the iron and zinc precursors were added at a molar ratio of 2:1. In the same manner, the iron and manganese precursors were added at a ratio of 2:1 for the Mn ferrite, while for the Mn-Zn ferrite, the iron, manganese, and zinc precursors were added at a ratio of 4:1:1. 1,2-Hexadecanediol and octyl ether were used as the reductant and the solvent, respectively. The completion of the reactions was achieved in the nanoreactors formed by poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO-PPO-PEO) polymer surfactant. All chemicals were purchased from Sigma-Aldrich Corporation (St. Louis, Missouri, USA), except for octyl ether (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan). The mixture was first heated to 120°C for 1 ~ 2 h, and then the temperature was raised rapidly to 280°C for refluxing. After 1 h of refluxing, the solution was air-cooled and washed with ethanol several times. The washed solution was subsequently centrifuged to precipitate the nanocrystals.
The crystal structures, particle sizes, and shapes of the nanocrystals were investigated by XRD (D/MAX-2500 V/PC; Rigaku Corporation, Tokyo, Japan) and TEM (JEM-2100 F; JEOL Ltd., Tokyo, Japan) including high-resolution transmission electron microscopy (HRTEM), while the chemical compositions of the nanocrystals were determined by an energy-dispersive spectroscopy (EDS) system in TEM and XRF (S2 PICOFOX; Bruker Corporation, Billerica, MA, USA). In addition, the magnetic behaviors of the nanocrystals were analyzed by a PPMS (Quantum Design Inc., San Diego, CA, USA).
Results and discussion
The reactions were completed through the thermal decomposition of the appropriate precursors in the nanoreactors formed by the polymer molecules, resulting in high-quality nanoparticles as desired . The use of the polymer, PEO-PPO-PEO, is distinctive, which has many merits and broad applications. In particular, the polymer is bio-friendly  and has an amphiphilic property , so the synthesized nanoparticles can be well dispersed in an aqueous solution without any additional surface modifications, which is especially benign for biomedical purposes .
Chemical compositions of the ferrite nanocrystals
Precursor molar ratio
We have synthesized the ferrite nanocrystals which exhibit high crystallinity and narrow size distributions via the non-aqueous nanoemulsion method and compared three types of samples from Zn ferrite, Mn ferrite, to Mn-Zn ferrites. The structural and chemical measurements performed by XRD and XRF indicated that the ferrite nanocrystals were successfully produced. All samples behave ferrimagnetically at 5 K and superparamagnetically at 300 K, individually. As the concentration of Mn increases, the magnetization value of the ferrites increases. Furthermore, the M-T curves obtained by the ZFC-FC modes clearly substantiate the superparamagnetism of the ferrite nanocrystals.
This work was supported through the National Research Foundation of Korea which is funded by the Ministry of Science, ICT and Future Planning (NRF-2010-0017950, NRF-2011-0002128).
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