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Novel ZnO/MgO/Fe2O3 composite optomagnetic nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 J. Phys.: Condens. Matter 25 194105 (http://iopscience.iop.org/0953-8984/25/19/194105) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
JOURNAL OF PHYSICS: CONDENSED MATTER
J. Phys.: Condens. Matter 25 (2013) 194105 (7pp)
doi:10.1088/0953-8984/25/19/194105
Novel ZnO/MgO/Fe2O3 composite optomagnetic nanoparticles ´ I Kaminska, B Sikora, K Fronc, P Dziawa, K Sobczak, R Minikayev, W Paszkowicz and D Elbaum Institute of Physics, Polish Academy of Sciences, 32/46 Aleja Lotnik´ow 02-668 Warsaw, Poland E-mail:
[email protected]
Received 17 October 2012, in final form 10 February 2013 Published 24 April 2013 Online at stacks.iop.org/JPhysCM/25/194105 Abstract A facile sol–gel synthesis of novel ZnO/MgO/Fe2 O3 nanoparticles (NPs) is reported and their performance is compared to that of ZnO/MgO. Powder x-ray diffraction (XRD) patterns reveal the crystal structure of the prepared samples. The average particle size of the sample was found to be 4.8 nm. The optical properties were determined by UV–vis absorption and fluorescence measurements. The NPs are stable in biologically relevant solutions (phosphate buffered saline (PBS), 20 mM, pH = 7.0) contrary to ZnO/MgO NPs which degrade in the presence of inorganic phosphate. Superparamagnetic properties were determined with a superconducting quantum interference device (SQUID). Biocompatible and stable in PBS ZnO/MgO/Fe2 O3 core/shell composite nanocrystals show luminescent and magnetic properties confined to a single NP at room temperature (19–24 ◦ C), which may render the material to be potentially useful for biomedical applications. (Some figures may appear in colour only in the online journal)
1. Introduction
They are relatively unstable in a physiologically relevant environment. However, those nanoparticles (NPs) coated with a magnetic Fe2 O3 shell and specific antibodies can be useful in a rational design of diagnostic probes for the detection and the quantitative determination of specific biomarkers. In addition, a bio-reactive Fe2 O3 shell is capable of generating reactive oxygen species (ROS), which can be potentially useful for site-specific cellular toxicity [4]. We have recently reported on the mechanism of ZnO NP sol–gel synthesis and confirmed that while the initial rapid nucleation and growth is kinetically controlled, subsequent nanocrystal growth is thermodynamically controlled through diffusion limited Ostwald coarsening [5]. Thus by applying solution phase colloidal methodology, nanostructure sizes can be tailored to match the dimensions of biological cells (10–100 µm), viruses (20–450 nm) or proteins (5–50 nm). The application of sol–gel technology to Fe doped ZnO nanofibers obtained by an electrospinning method has been recently reported by us [6]. The magnetic nanomaterials can be manipulated with an external magnetic field gradient, and therefore can be concentrated in the proximity of target delivery. Suitable magnetic NPs can be synthesized to resonantly respond to an alternating magnetic field. Both
Multimodal nanostructures can be a source of valuable information, leading to an understanding of the processes responsible for medical pathology, early diagnostics and for the ultimate treatment of diseases. Theranostics, a fusion of therapeutic and diagnostic strategies, is a relatively new, dynamically growing field of medicine. The future success of this field directly depends on the development of a new generation of biocompatible, luminescent, magnetic and photo-triggered ‘smart’ theranostic agents. An example of such an agent is a subject of this study. ZnO/MgO heterostructures have been useful in the study of chemical sensors, optical devices, scanning probes and heterojunction materials with quantum confinement effects [1]. Bera et al [2] have reported MgO as an ideal candidate for a shell material in ZnO passivation. ZnO/MgO core/shell structures have optical properties which can be potentially useful for bio-imaging applications. While functionalized by beta-cyclodextrin and Texas Red, the ZnO/MgO nanostructures are effective donors in fluorescence resonance energy transfer to the aromatic acceptor [3]. 0953-8984/13/194105+07$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
J. Phys.: Condens. Matter 25 (2013) 194105
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Fe3 O4 and Fe2 O3 exhibit superparamagnetic properties which enable NPs to be used as agents for magnetic resonance imaging [7, 8]. Currently, magnetic NPs have four major, clearly defined biomedical applications: cell separation, drug delivery, hyperthermia (which involves the introduction of either ferromagnetic or superparamagnetic particles into the tissue [9, 10]), and magnetic resonance imaging [11–18]. Recent advances in solution phase colloidal physical chemistry have enhanced the production of nanomaterials with multifunctional properties for various biomedical applications. In spite of intensive research activity, only a few examples of optomagnetic NPs, which are both chemically stable under biologically relevant aqueous conditions and biocompatible, have been reported. Recently, a study on the seeded growth of Fe3 O4 /ZnO bifunctional nanocrystals possessing optomagnetic properties was reported [19]. However, neither emission spectra nor biologically relevant solution stability results were provided. Biocompatible core/shell optomagnetic NPs are of considerable importance for site-specific therapeutically motivated drug delivery. Optical (especially near-infrared) and magnetic imaging have made a significant impact on medical diagnostics. Future advances in these fields are in the confinement of specific optical and magnetic properties on single, multimodal NPs. Recently, several groups reported a successful synthesis of particles that possess both fluorescent and magnetic properties. Qian et al [20] have reported on the synthesis of bifunctional magnetic–fluorescent hollow ZnO/ZnFe2 O4 nanostructures by a co-precipitation method. Guskos et al [21] have synthesized fine particles of Fe2 O3 /ZnO using a wet chemical method, based on ferromagnetic resonance (FMR) measurements. Analysis of the FMR spectra indicates the presence of strongly anisotropic interactions. Yi et al [22] have synthesized a water soluble nanocomposite consisting of both: magnetic nanoparticles γ -Fe2 O3 and CdSe quantum dots (QDs) encapsulated within a silica shell and showed that the nanocomposite preserved both the magnetic properties of γ -Fe2 O3 and the optical properties of CdSe QDs. The aim of this work is to synthesize NPs which are useful for a new generation of biosensors exhibiting biocompatible optomagnetic properties. To the best of our knowledge, this paper is the first communication on the properties of core/shell ZnO/MgO/Fe2 O3 NPs under physiologically relevant environments.
Synthesis (1) was performed using 0.5 mM Zn(CH3 COO)2 ×2H2 (Chempur pure p.a.) in 65 ml ethanol (Chempur, min 99.8% pure p.a. (per analysis)) and 1.25 mM NaOH (Chempur, min 98.8% pure p.a.) in 70 ml ethanol. All components were pre-heated to 30 ◦ C, stirred to dissolve for 1 h, followed by cooling at room temperature. The reaction was initiated by adding the NaOH solution dropwise into the Zn2+ solution at 0 ◦ C with vigorous stirring [23]. The resulting solution was incubated for 30 min and then warmed up to 35 ◦ C. After 2 h an aliquot of 3 ml was placed in the spectrophotometric cell to record absorption and emission spectra. In the next step (2) a solution of 0.5 mM Mg(CH3 COO)2 × 4H2 O (Sigma-Aldrich, ≥99%) (mass ratio Mg/Zn = 17%) in 15 ml ethanol was added dropwise and incubated overnight for 15 h. The unreacted material was discarded after centrifugation (6000 rpm, 15 min, 20 ◦ C). The spun material was washed in heptane–ethanol (3:1) and the product allowed to dry at room temperature. Spectrophotometric measurements were then performed on the newly synthesized ZnO/MgO core/shell structure at 25 ◦ C [24]. 2.2. Synthesis of ZnO/MgO/Fe2 O3 nanoparticles In order to synthesize the optomagnetic NPs the previously prepared ZnO/MgO NPs were covered with an Fe2 O3 shell : Fe3+ + 6OH− → Fe2 O3 + 3H2 O.
Synthesis (3) was performed using 37 mM Fe(NO3 )3 × 9H2 O (Chempur, min 99.8% pure p.a.) in 150 ml of methanol (Chempur, min 99.8% pure p.a.). Subsequently 0.26 g of ZnO/MgO was dissolved in 100 ml of methanol (prepared as described above). Then 0.11 mol NaOH in 1:1 distilled water to methanol was added dropwise to a stirred mixture of preformed ZnO/MgO and the Fe(NO3 )3 × 9H2 O solution at 5–10 ◦ C. The resulting mixture was incubated for 30 min. It was then warmed up to 35 ◦ C and incubated for 2 h on a magnetic stirrer followed by centrifugation. The ZnO/MgO/Fe2 O3 NPs were alternately washed with water and ethanol, and dried for several days at room temperature. The stability of ZnO/MgO/Fe2 O3 NPs dissolved in water and several phosphate buffered saline (PBS) solutions (5, 20, 50 and 100 mM phosphates at various pH = 5.0, 6.0, 7.0, and 8.75) was determined by absorption and emission spectroscopy. The concentration of the magnetically stirred NP suspension was 0.2 mg ml−1 . This paper is confined only to 20 mM PBS at pH = 7.0 and water. Absorption and emission spectra for the NPs were monitored using a Cary 50 Scan UV–vis spectrophotometer and a Fluorolog III Phosphorimeter with a 550 W xenon lamp, respectively. Both instruments were equipped with a thermostated cuvette holder. The measurements were carried out on samples kept in a quartz cuvette with dimensions of 1 × 1 × 4 cm3 [5]. A structural characterization of ZnO/MgO/Fe2 O3 was performed using XRD, at a synchrotron-radiation source. Diffraction studies were carried out using a powder diffractometer with 2θ from 2◦ to 70◦ and a step size of 0.004 at the B2 (HASYLAB/DESY)
2. Materials and methods 2.1. Synthesis of ZnO/MgO core/shell nanoparticles In order to prevent spontaneous aggregation and to improve the efficiency of ZnO luminescence in aqueous solutions, their surface was passivated with a shell of MgO. The following reactions took place: Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2 O
(1)
Mg2+ + 2OH− → Mg(OH)2 → MgO + H2 O.
(2)
(3)
2
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To calculate the percentage ratio emission intensity (exciton peak) for ZnO/MgO and for ZnO/MgO/Fe2 O3 NPs we used the following formula: 1IL (t)/IL (t = 0) and 1IL (t) = IL (t = 0) − IL (t), where IL (t) is the intensity of a peak at a wavelength of approximately 370 nm at time t. The results are summarized in table 1. For the case of ZnO/MgO NPs dissolved in water, we observed a decrease in the exciton peak intensity to about 12.2% (after 168 h), which indicates relatively good stability for the NPs in water (figure 1(b)). However, these NPs are not stable in PBS (20 mM, pH = 7.0) (figure 1(f)). Note only a slight decrease in the intensity of the exciton peak for ZnO/MgO/Fe2 O3 in PBS from 6.3%, to 8.7%, to 14% in 1.5 h, 120 h and 168 h, respectively (figures 1(h) and (j)). We can conclude that the passivation of the ZnO/MgO surface by Fe2 O3 stabilizes the NPs by making them inert to aqueous solutions of inorganic phosphates present in physiological fluids such as blood, plasma, saliva, urine, and other body fluids. A typical physiologically relevant plasma phosphate level is (2 ± 0.09) mM [26] which implies that the core/shell NPs are relatively stable when exposed to these fluids. TEM studies showed the presence of ZnO (wurtzite type, space group: P63 mc) (figure 2(e)) and the diameter of the whole heterostructure to be approximately 5 nm. High-resolution (HR) TEM micrographs showed that the interplanar distance for the fringes is 0.247 nm, which is in good agreement with the (001) plane for wurtzite ZnO (figure 2(a)). Chemical mappings from ZnO/MgO/Fe2 O3 NPs extracted from electron energy loss spectroscopy are shown in figures 2(c) and (d). Iron and zinc (line M) have been identified. Several TEM images were used to plot the histogram for the size distribution of the ZnO/MgO/Fe2 O3 NPs. The histogram (figure 2(b)) revealed a median diameter of d = 4.8 nm. The distribution derived from Mg is not observed, probably due to the relatively small amount of MgO in the shell. Figure 3 shows the diffraction pattern obtained from ZnO/MgO/Fe2 O3 NPs. We observed the main phase resulting from the ZnO (zincite) wurtzite structure (black line); the ¯ marked by a blue MgO (periclase) phase (space group: Fm3m) line; and γ -Fe2 O3 (maghemite) (space group: P41 32), marked with a red line. The magnetic field dependence of the magnetization was measured in the temperatures range T = 5–250 K and an applied magnetic field up to 2 kOe (figure 4(a)). Both zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were taken at H = 10 Oe (figure 4(b)). One should notice that a NP contains a nonmagnetic core. This fact implies an effect on the magnetic properties at least in two simple ways. Firstly, the magnetization of such NPs should be reduced due to the smaller amount of magnetic material, and secondly, there is a possibility of stress caused by lattice mismatch responsible for a structural rearrangement in the Fe2 O3 coating. Both factors depend on the radius ratio rshell /rcore . Furthermore, as compared to typical Fe2 O3 NPs, the anisotropy constant should be influenced by the additional inner surface of the magnetic shell.
bending–magnet beamline, using Debye–Scherrer geometry [25]. An imaging-plate detector was applied for data collection. The sample was mounted within a thin-wall glass capillary (diameter 0.3 mm). The signal from the capillary was subtracted from the diffraction pattern. A JEOL JEM2000EX transmission electron microscope operating at 200 kV was used for the determination of the size and morphology of ZnO/MgO/Fe2 O3 NPs. Superconducting quantum interference device magnetometry (home built with Cryogenics SQUID sensor) was utilized to measure both the magnetic and the temperature dependences of magnetization, which allow us to estimate the blocking temperature as well as the saturation magnetization for ZnO/MgO/Fe2 O3 NPs.
3. Experimental results and discussion Optomagnetic properties for various nanostructures, such as hollow nanospheres, Fe2 O3 /ZnO and SiO2 /MP-QDs (magnetic nanoparticles-quantum dots) nanocomposite particles, have been presented previously [20–22], but with no information reported concerning their stability, which is new to this study. A comparison of the UV–vis absorption and emission spectra for ZnO/MgO and ZnO/MgO/Fe2 O3 NPs in the presence of 20 mM PBS, pH = 7.0 or water, are summarized in figure 1. In order to examine the influence of two typical environments we measured the absorbance in the range 275–550 nm and the emission in the range 365–400 nm in the presence of 20 mM PBS (pH = 7.0) (figures 1(e)–(j)) or water (figures 1(a)–(d)). For the case of ZnO/MgO NPs dissolved in water, the absorption onset is unchanged (355 nm), even after 168 h (figure 1(a)). A constant (to within experimental error) absorption wavelength onset indicates a constant band gap energy, thus insignificant ZnO/MgO particle growth [2]. ZnO/MgO/Fe2 O3 NPs dissolved in water are stable for at least 168 h, as judged from the absorption spectra (figure 1(c)) and emission spectra (figure 1(d)). We speculated that this is related to Fe2 O3 surface passivation. On the other hand, the disappearance of absorption for ZnO/MgO NPs dissolved in PBS (20 mM, pH = 7.0) was already observed after 90 min (figure 1(e)), which can be attributed to the NPs’ partial dissolution. The absorption onset for ZnO/MgO/Fe2 O3 NPs dissolved in PBS ∼270 nm (4.6 eV) is constant. The peak wavelength shifted towards shorter wavelengths of 323–320 nm (figure 1(g)) and 309–305 nm (±2 nm) (figure 1(i)). The position of the sample absorption band below 350 nm indicates the presence of small particles. This is consistent with the transmission electron microscopy (TEM) measurements. Additional Fe2 O3 coating of ZnO/MgO significantly stabilized the nanostructure. A possible explanation is the relatively low reactivity of Fe2 O3 with phosphate anions − 2+ and Zn2+ cations. (HPO2− 4 , H2 PO4 ) compared to the Mg Interestingly, ZnO/MgO/Fe2 O3 NPs are also stable in the presence of higher phosphate anions: 50 and 100 mM at various pH = 5.0, 6.0 and 8.75 (data not shown). A dominant exciton peak at 370 ± 2 nm was observed for all the emission spectra summarized in figure 1, which is typical for ZnO based nanostructures. 3
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Figure 1. Absorption ((a), (c), (e), (g), (i)) and emission ((b), (d), (f), (h), (j)) spectra for ZnO/MgO and ZnO/MgO/Fe2 O3 NPs as a function of time: (a)–(b) ZnO/MgO and (c)–(d) ZnO/MgO/Fe2 O3 NPs dissolved in distilled water; (e)–(f) ZnO/MgO and (g)–(j) ZnO/MgO/Fe2 O3 NPs dissolved in PBS (pH = 7.0). Inset: intensity change versus time. Excitation wavelength was 325 nm.
For NPs with such a complicated structure one can expect one of the two possible stable phases for Fe2 O3 , α and γ . Despite their different magnetic properties in bulk (antiferromagnetic and ferromagnetic, respectively), both of the phases are superparamagnetic in nanometric sizes (less
than 10 nm) due to their single domain structure. Aside from the differences in the crystal structure, phases can be distinguished in a M(T) plot by the Morin transition around 290 K, which occurs only for α-Fe2 O3 [27]. We did not observe this type of transition in our sample, hence we infer 4
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Figure 2. (a) HRTEM images for ZnO/MgO/Fe2 O3 NPs. (b) Size distributions for ZnO/MgO/Fe2 O3 NPs. Electron energy loss spectroscopy chemical mapping for ZnO/MgO/Fe2 O3 NPs. (c) Zinc mapping. (d) Iron mapping. (e) Electron diffraction pattern corresponding to a wurtzite crystal structure. Table 1. Change of the emission intensity for ZnO/MgO and for ZnO/MgO/Fe2 O3 NPs (details in the text). 1IL (t) (arb. u.) Environment
Sample
t = 90 min
t = 120 h
t = 168 h
Water
ZnO/MgO ZnO/MgO/Fe2 O3 ZnO/MgO ZnO/MgO/Fe2 O3
Not tested 0% 31.9% 6.3%
5.6% 0% Not stable 8.7%
12.2% 0% Not stable 14.0%
PBS
Interestingly, the slight difference of 1 K between both the average and the maximal blocking temperatures, TB and TB,max respectively, suggests a small dispersion of NP sizes. The magnetic anisotropy constant can be calculated from the relation K = 25kB VB /TB , where kB is the Boltzmann constant, TB is the average blocking temperature and VB is the corresponding volume of NPs. For an average diameter estimated from TEM studies (d = 4.8 nm), the value of K is 2.6 × 106 erg cm−3 , which is relatively high. One should keep in mind that this estimation is not dedicated to heterostructures with magnetic shells. Hysteretic behavior is well observed below the blocking temperature TB = 43 K, while above this point the NPs reveal a superparamagnetic response in applied magnetic fields. Saturation of magnetization (MS ) in the Fe2 O3 NPs can be attained at magnetic fields much higher than 2 kOe [28]. However, the experimental points M(H) at temperatures above TB are well fitted by a Langevin function, giving the highest saturation magnetization at TB equal to 55 emu g−1 . This value is around 0.6 × MS , as compared to bulk γ -Fe2 O3 [29]. In order to demonstrate the magnetic properties of ZnO/MgO/Fe2 O3 NPs at room temperature they were
˚ for the Figure 3. XRD patterns (Cu Kα1 , λ = 1.5406 A) ZnO/MgO/Fe2 O3 nanocrystalline powder.
that the shell is built from the γ -phase. It can be stabilized by cubic MgO—the middle part of the structure. Magnetization dependences on temperature (figure 4(b)) show a behavior known for typical superparamagnetic NPs. 5
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Figure 4. (a) Mass magnetization as a function of applied external magnetic field measured for various temperatures in the range T = 5–250 K. (b) ZFC and FC magnetization curves as well as their difference MFC − MZFC for ZnO/MgO/Fe2 O3 NPs.
biological labeling imaging and separation in an external magnetic field of biomolecules attached to the NPs. In addition, the material could be used for diagnostically relevant biosensing when passivated with a biologically active molecule capable of binding specifically with a pathological target. Our ZnO based heterostructure material is shown to be stable for seven days in physiologically relevant environments and is potentially biologically compatible.
Acknowledgments We wish to thank Dr Tony Bell for assistance with the operation of the B2 powder diffraction beamline at HASYLAB/DESY. This work has been supported by grant NN 518 424036 from the Ministry of Science and Higher Education and Innovative Economy grant POIG.01.01.02-00-008/0.
Figure 5. ZnO/MgO/Fe2 O3 NPs dissolved in distilled water. (a) In a magnetic field the particles are attracted to the side of the vessel (indicated by dashed ovals). (b) Image of ZnO/MgO/Fe2 O3 NPs after the removal of or without the magnetic field.
suspended in distilled water and inserted between neodymium magnets (in a magnetic field of approximately 600 mT). The magnetic field attracts NPs, as shown in figure 5(a). The removal of the external magnetic field dispersed the NPs homogeneously in the suspension and no visible aggregates were observed [30]. As one can see, superparamagnetism enables the NPs to respond to an applied magnetic field without any permanent magnetization and thus to be re-dispersed rapidly when the magnetic field is removed (figure 5(b)) [31].
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4. Conclusions In this work, using an efficient and low cost sol–gel method, we synthesized sizable quantities of ZnO/MgO/Fe2 O3 NPs of a defined geometry, crystalline structure, and optical and magnetic properties. The fabricated material has potential, after proper biological passivation, in biomedical applications. The ZnO/MgO/Fe2 O3 NPs of mean diameter 4.8 nm, while in aqueous solutions, maintain their optical and superparamagnetic properties at room temperature. The optomagnetic dual properties are potentially useful for 6
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