Aqueous Solution Preparation, Structure, and Magnetic Properties of Nano-Granular Zn x Fe3−x O4 Ferrite Films
© The Author(s) 2010
Received: 11 March 2010
Accepted: 7 June 2010
Published: 22 June 2010
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© The Author(s) 2010
Received: 11 March 2010
Accepted: 7 June 2010
Published: 22 June 2010
This paper reports a simple and novel process for preparing nano-granular ZnxFe3−xO4 ferrite films (0 ≤ x ≤ 0.99) on Ag-coated glass substrates in DMAB-Fe(NO3)3-Zn(NO3)2 solutions. The deposition process may be applied in preparing other cations-doped spinel ferrite films. The Zn content x in the Zn x Fe3−x O4 films depends linearly on the Zn2+ ion concentration ranging from 0.0 to 1.0 mM in the aqueous solutions. With x increasing from 0 to 0.99, the lattice constant increases from 0.8399 to 0.8464 nm; and the microstructure of the films changes from the non-uniform nano-granules to the fine and uniform nano-granules of 50–60 nm in size. The saturation magnetization of the films first increases from 75 emu/g to the maximum 108 emu/g with x increasing from 0 to 0.33 and then decreases monotonously to 5 emu/g with x increasing from 0.33 to 0.99. Meanwhile, the coercive force decreases monotonously from 116 to 13 Oe.
Spinel ferrites, Me x Fe3−x O4 (Me = Mn, Co, Ni, Zn, Mg, Cu, etc.), are a technologically important group of materials, having numerous applications in magnetic devices, recording materials, photocatalysis, ferrofluid technology, magnetic refrigeration, and humidity sensors [1, 2]. The typical spinel ferrite, Fe3O4, has 64 tetrahedral sites (A sites) and 32 octahedral sites (B sites) of which only 8 A sites and 16 B sites are occupied by the Fe2+ and Fe3+ ions, respectively. The inverse nature of this spinel implies that all the A sites are occupied by Fe3+ ions, while an equal number of Fe2+ ions and Fe3+ ions share the B sites . Structural and magnetic properties of spinel ferrites MeFe2O4 strongly depend upon the nature, concentration, and distribution of the substituted Me cations on A and B sites as well as the method of preparation. If the dependence of physical properties on substituted cations is known for a given complex ferrite, then it is possible to design a ferrite possessing the desired physical properties by choosing the appropriate compositions.
In recent years, the Zn- or NiZn- incorporated ferrite films with high resistivity and high permeability are widely used in micro-inductors, micro-transformers, magnetic recording, and high-frequency field . For example, if the electronic circuits are covered with the NiZn ferrite film, the magnetic films render additional resistance to the circuits, thus attenuating the conducted-electromagnetic noises . Fujiwara et al. studied the Zn2+ ions-doped ferrite films having high real part (μ′) and imaginary part (μ″) of permeability in 10 to 100 MHz region. The films may be used as high-frequency magnetic film devices that are required at present for the current information technology infrastructure development . Zn x Fe3−x O4 ferrite films were applied to field effect spintronic devices prepared by a pulsed-laser deposition technique .
Various preparation techniques such as liquid phase epitaxy, sputtering, plasma splay, and molecular beam epitaxy have been employed for preparing ferrite films. In all these processes, post-heat treatments or high deposition temperatures (>600°C) are required to induce the desired crystalline phases . The high temperature would deteriorate the non-heat-resistant substrates, e.g., GaAs integrated circuits, plastics, and biomaterials . The low-temperature wet chemical preparation methods such as spin spray plating , electrodeposition , and chemical deposition  offer a good alternative as it can overcome the drawbacks of the conventional methods. Abe et al.  developed a widely used spin spray method. In the process, long deposition time and complex equipment are needed; and a great deal of reactants is wasted. Recently, Izaki developed a very simple method to prepare magnetite and Zn-incorporated magnetite film by immersing a Pd/Ag-catalyzed substrate in the stable aqueous nitrate and dimethylamine borane complex (DMAB) solution [14, 15]. In Izaki’s method, the incorporation of Zn into magnetite was finished by the deposition of magnetite film on ZnO thin film, so the Zn content of ferrite film and the magnetic properties cannot be well adjusted.
In order to overcome the drawbacks of the above-mentioned methods, a modified Izaki’s method was used to prepare nanostructured Zn x Fe3−x O4 ferrite films (0 ≤ x ≤ 0.99) on Ag-coated glass substrates in the DMAB-Fe(NO3)3-Zn(NO3)2 solutions at 80°C by adjusting the Zn(NO3)2 concentration. The dependence of structure and magnetic properties of the Zn x Fe3−x O4 ferrite films on the Zn content was studied.
Prior to deposition, glass substrates were cleaned ultrasonically in ethanol and acetone for removing organic impurities and rinsed in distilled water. Then Ag layer was deposited on the glass substrates (24 × 12 × 0.5 mm) using a two-step Sn/Ag activation process at room temperature. This two-step Sn/Ag activation process includes sensitizing the glass substrates by dipping in solution containing 10 g/l SnCl2 and 0.08 M HCl, rinsing the substrates by distilled water, and activating the glass substrates by 1–2 g/l AgNO3. The above steps were repeated three to six times to form Ag nano-granules with small sizes and a high density over the entire substrate surface. The Zn x Fe3−x O4 ferrite films (0 ≤ x ≤ 0.99) were prepared by immersing the Ag-catalyzed substrates in a 50-ml tube containing 15 mM iron nitrate 9-hydrate, 0–1.4 mM zinc nitrate hexahydrate, and 30 mM DMAB at 80°C for 1 h.
The pH values of the mixed solutions were measured by using a PHS-29A meter (Shanghai Precision Scientific Instrument Co.). X-ray diffraction (XRD) patterns of the films were measured using a Rigaku D/Max-2400 X-ray diffractometer with Cu K α radiation (40 kV, 60 mA). The average grain sizes of the films were estimated from the diffraction peak widths through Scherrer equation. For the Rietveld refinements, the XRD data were recorded in a range from 15 to 90° (2θ) with a step width of 0.02° and a counting time of 3 s per step. The chemical compositions of the films were analyzed by a Thermo IRIS Advantage inductively coupled plasma atomic emission spectrometer (ICP-AES). The morphology and thickness of the films were analyzed by a Hitachi S-4800 field emission scanning electron microscope (SEM). The Raman spectra were recorded on the Horiba Jobin–Yvon LABRAM-HR800 laser micro-Raman spectrometer with 532-nm radiation that provides a typical spatial resolution less than 1 μm and spectral resolution better than 1 cm−1 in peak position. The magnetic properties of the films were measured at room temperature by a Lake Shore 7304 vibrating sample magnetometer (VSM).
Figure 7 shows that the coercive force H c decreases monotonously from 116 to 13 Oe with x increasing from 0 to 0.99. The variation of the H c can be explained by the inverse relation between H c and M s , H c ∝K 1/M s , where K 1 is the anisotropy constant , which decreases with increasing Zn content . Decrease in K 1 and increase in M s with x increasing from 0 to 0.33 lead to a decrease of H c . For x > 0.33, both K 1 and M s decrease with increasing x. It implies that the decrease of K 1 (K 1 = −1.1 × 104 J/m3 for Fe3O4 and K 1 = 0 for ZnFe2O4) is the dominant factor compared to the decrease of M s . Although H c decreases with increasing Zn content, the H c values are a little larger than those of the Zn x Fe3−x O4 films with the same compositions prepared by the spin spray method . The H c of the Zn x Fe3−x O4 films prepared by the spin spray method decreases from 100 to 5 Oe with x increasing from 0 to 0.7 . First, it may be due to the small grain size in the films that can induce large K 1. Second, it may be attributed to the rough surface, which provides pinning sites for domain walls .
The Zn x Fe3−x O4 (0 ≤ x ≤ 0.99) ferrite films were deposited in the DMAB-Fe(NO3)3-Zn(NO3)2 solutions at low temperature (80°C) on the Ag-coated glass substrates. The Zn content x in the Zn x Fe3−x O4 films increases linearly with the Zn2+ ion concentration in the solution. As x increases from 0 to 0.99, the lattice constant of the films increases from 0.8399 to 0.8464 nm; and the microstructure of the films change from the non-uniform nano-granules to the fine and uniform nano-granules of 50–60 nm in size. The saturation magnetization of the Zn x Fe3−x O4 films first increases from 75 emu/g to the maximum 108 emu/g with x increasing from 0 to 0.33 and then decreases to 5 emu/g with x increasing from 0.33 to 0.99. Meanwhile, the coercive force decreases monotonously from 116 to 13 Oe with x increasing from 0 to 0.99.
Qiang Tian and Qian Wang contributed equally to this work.
The work was supported by the National Natural Science Foundation of China under grant No. 50872046, the International S&T Cooperation Program (ISCP) under grant No. 2008DFA50340, MOST, and the Specialized Research Foundation for the Doctoral Programs under grant No. 20070730022, MOE, China.
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