Magnetic resonance of the NiFe2O4 nanoparticles in the gigahertz range
© Shi et al.; licensee Springer. 2013
Received: 18 June 2013
Accepted: 5 August 2013
Published: 1 October 2013
We report an adjustable magnetic resonance frequency from 1.45 to 2.54 GHz for NiFe2O4 nanoparticles which were prepared by a sol–gel process. X-ray diffraction and scanning electron microscopy results indicate that the samples are polycrystalline nanoparticles, and the size of the particles increases obviously with the thermal treatment temperature. The consequence of the surface composition suggests that the oxygen defects are present in the nanoparticle surface, and this surface magnetic state can show a strong surface anisotropy. With decreasing size of the particle, the surface magnetic effect is predominant, resulting in an increase of resonance frequency for NiFe2O4 nanoparticles. This finding provides a new route for NiFe2O4 materials that can be used in the gigahertz range.
Soft magnetic ferrites have attracted much attention in recent years because they have large saturation magnetization (Ms), low electrical conductivity, and excellent chemical stabilities [1, 2] and can be used as ferrofluids , in magnetic resonance imaging , and in microwave devices [5, 6]. Furthermore, nanoscale soft magnetic ferrites exhibit special magnetic-like, magneto-resistive, and magneto-optical properties compared with bulk magnetic materials . Because the surface-to-volume ratio becomes very large with the reduction of the particle size at nanoscale, they are potentially useful for a broad range of applications. Soft magnetic ferrites have a potential application in electronic devices when used in the gigahertz (GHz) range. This is because in this frequency region, magnetic metals exhibit strong eddy current loss  compared to soft magnetic ferrites [9, 10]. For soft magnetic ferrites, there is magnetic resonance, resulting in magnetic losses. This provides some limitations (like threshold frequency) of the application. Nakamura  and Tsutaoka et al.  reported that the resonance frequency of bulk soft magnetic ferrites is much lower than 1 GHz. It seems to be an urgent issue to improve the resonance frequency of soft magnetic ferrites. Guo et al. reported that Ni-Zn ferrite thin films exhibit much higher natural resonance frequency, thanks to bianisotropy . There is strong surface anisotropy in ferrite nanoparticles (NPs), which has been reported before [14–16]. Owing to this surface anisotropy, ferrite NPs will likely show high resonance frequency. NiFe2O4 is a typical soft magnetic ferrite with high electrical resistivity , and it is an inverse spinel with metal ions occupying the octahedral and tetrahedral sites. The magnetic moments placed in the tetrahedral site and octahedral site couple in an antiparallel manner by a superexchange interaction which is mediated through adjacent oxygen atoms and forms a collinear ferrimagnetic ordering. Additionally, the magnetic behaviors of nanoscale NiFe2O4 are extremely sensitive to their size . There is already a significant interest in synthesizing NiFe2O4 NPs for achieving optimal magnetic properties [19–21]. In this work, NiFe2O4 NPs were prepared using the sol–gel method. The morphology, structure, and magnetic characterization of the NiFe2O4 NPs have been systemically investigated. Importantly, an adjustable magnetic resonance has been observed in the GHz range, implying that NiFe2O4 is a candidate for microwave devices in the GHz range.
NiFe2O4 NPs were synthesized by the sol–gel method with a postannealing process . All chemical reagents used as starting materials are of analytical grade and purchased without any further treatment. In a typical synthesis process, 0.01 M Ni(NO3)4·5H2O, 0.02 M Fe(NO3)3·9H2O, and 0.03 M citric acid were firstly dissolved in 100 ml of deionized water. The molar ratio of metal ions to citric acid was 1. A small amount of ammonia was added to the solution to adjust the pH value at about 7 with continuous stirring. Then, the dissolved solution was stirred for 5 h at 80°C and dried in the oven to form the precursor at 140°C. The precursor was preannealed at 400°C for 2 h and then calcined at different temperatures (700°C, 800°C, 900°C, and 1,000°C) for 2 h in the air, which were denoted as S700, S800, S900, and S1000, respectively.
X-ray diffraction (XRD; X'Pert PRO PHILIPS with Cu Kα radiation, Amsterdam, The Netherlands) was employed to study the structure of the samples. The morphologies of the samples were characterized using a scanning electron microscope (SEM; Hitachi S-4800, Tokyo, Japan). The measurements of magnetic properties were made using a vibrating sample magnetometer (VSM; LakeShore 7304, Columbus, OH, USA). The chemical bonding state and the compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS; VG Scientific ESCALAB-210 spectrometer, East Grinstead, UK) with monochromatic Mg Kα X-rays (1,253.6 eV). The complex permeability μ of the particles/wax composites were measured on a vector network analyzer (PNA, E8363B, Agilent Technologies, Inc., Santa Clara, CA, USA). Then, 63 vol.% of particles and 37 vol.% of wax were mixed together and pressed into a coaxial cylindrical specimen, in which the magnetic particles were randomly dispersed. Electron spin resonance (ESR) measurements were performed with a Bruker ER200D spectrometer (JEOL, Tokyo, Japan).
Results and discussion
The evidence for the composition of products in the surface was obtained by XPS. Figure 3b shows the XPS survey scan spectrum of a representative sample, S700, indicating that no impurities were detected in the sample within the detection limit. An asymmetric high-resolution XPS spectrum for the O 1s peak is observed (shown in Figure 3c), which has a shoulder at the higher binding energy side. By fitting, we obtained three peaks at 529.8, 531.2, and 532.4 eV. The dominant peak located at 529.8 ± 0.2 eV (Oa), which corresponds to O2− ions of the pure composites [27, 28], and the highest binding energy peak at 532.4 ± 0.2 eV (Oc) can be attributed to the chemisorbed oxygen of surface hydroxylation, oxygen atoms in carbonate ions, and adsorbed H2O or O2. Furthermore, the medium binding energy component (Ob) located at 531.2 ± 0.2 eV (Oc) is associated with the O2− ions in the oxygen-deficient regions (O vacancies) . The result obviously demonstrates the presence of oxygen defects in the surface, and the oxygen defects can destroy the superexchange interaction. This indicates that surface and internal magnetic states are different, and the surface magnetic state can show a strong surface anisotropy .
In summary, NiFe2O4 NPs were obtained using the sol–gel method, and the magnetic properties of NiFe2O4 NPs regularly change with the sintering temperature. Notably, NiFe2O4 NPs exhibit magnetic resonance in the GHz range. Through the study of the surface composition, the presence of oxygen defects, which can destroy the superexchange interaction, in the surface can be deduced. The strong surface anisotropy field for NiFe2O4 NPs has an effect on the core of particles, and this leads to a strong effective anisotropy field, thereby generating high-frequency magnetic resonance for NiFe2O4. ESR spectra measured at room temperature further confirm that surface magnetism plays a great role.
This work is supported by the National Basic Research Program of China (grant no. 2012CB933101), the NSFC (grant no. 11034004 and no. 51202101), the National Science Fund for Distinguished Young Scholars (grant no. 50925103), and the Fundamental Research Funds for the Central Universities (no. lzujbky-2012-28).
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