IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
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Magnetic Properties of High Frequency Ni–Zn Ferrites Doped with CuO Ovidiu F. Caltun, Leonard Spinu, Member, IEEE, and Alexandru Stancu, Member, IEEE
Abstract—Cu substituted Ni–Zn ferrites, with different copper content, were prepared by a conventional ceramic technique. Structural and magnetic properties of the ferrites were characterized by X-ray diffraction, scanning electron microscopy, frequency dependent power loss, and high frequency (106 –109 Hz) permeability measurements. These studies revealed that the magnetic performances increase considerably with the optimum concentration of the additions. The contribution of two magnetization mechanisms, domain wall motion and magnetization rotation, on the high frequency complex permeability, is also discussed. Index Terms—Copper substituted Ni–Zn ferrites, high frequency complex permeability.
I. INTRODUCTION
N
i–Zn FERRITES with their ease of preparation and versatility for use in wide ranging applications are very attractive materials from the commercial point of view. Microstructure and magnetic properties of these ferrites are highly sensitive to preparation method, sintering conditions and amount of constituent metal oxides, including impurities or dopants. Many efforts are focused in obtaining low-power loss material operating in the MHz region in accordance with the miniaturization of cores. The studies on microstructure and composition related magnetic properties have been reported for Ni–Zn ferrites by several researchers [1]–[4]. The low-temperature synthesis of Ni–Zn ferrite produces small ferrite particles ( 50 nm), which gives sintered compacts of bulk density ( 4.5 g/cm ) at a sintering temperature of 1100 C/8 h. The maximum grain size obtained was 700 nm, which was insufficient to give permeability values higher than 50 in the 20–70 MHz frequency range. The absence of the contribution of domain wall motion to the permeability explains this fact [3]. Nakamura [5] has reported on the low-temperature sintering of Ni–Zn ferrite at 900 C resulting in magnetic permeability over 200 at 1 MHz and density greater than 4.5 g/cm . The high permeability has been attributed to the domain wall contribution, which is controlled not only by the higher post sintering density but also by increased grain size. Doping increases the density, decreases the sintering temperature and increases the resistivity of the material. Manuscript received October 12, 2000. The research at Advanced Materials Research Institute (AMRI), University of New Orleans (UNO), is supported by DARPA through Grant MDA 972-97-10003. O. F. Caltun and A. Stacu are with the Faculty of Physics, “Al. I. Cuza” University, Iasi, 6600, Romania (e-mail: {caltun; alstancu}@uaic.ro). L. Spinu is with AMRI, UNO, New Orleans, LA 70148 USA (e-mail:
[email protected]). Publisher Item Identifier S 0018-9464(01)06784-X.
Fig. 1. X-ray diffraction patterns for the sintered composition of (NiZn) Cu Zn Fe O , (R1 x = 0:1, R2 x = 0:2, R3 x = 0:3).
This paper focuses on the dependence of hysteresis loops, of the initial permeability and of the losses on frequency for three different samples of Ni–Zn ferrites with different copper oxide content added in the sintering process. The obtained experimental results were related to the sample microstructure and morphology. The contribution of the domain wall motion and of the spin rotation to the complex permeability is also discussed. II. EXPERIMENTAL DETAILS A. Sintering Condition and Microstructure Ni–Zn ferrite ceramics doped with CuO were synthesized using the usual ceramic technique [5]. Starting powders of Fe O , NiO, ZnO and CuO were mixed in suitable proportions for 20 hours in steel ball-mill using water as the mixing medium. The mixture was dried and calcined at 800 C for 2 hours in air. The presintered powders were milled in water to obtain BET surface 2–3 m /g. The green density was 2.9 0.2 g/cm . The ring cores, with outer radius of 12 mm, inner radius of 8 mm and height of 6 mm, were pressed at 150 MPa and sintered at 1100 C/4h in air. Three types of Cu Zn Fe O samples with general formula (NiZn) with ranging in the from 0.1 to 0.3 in steps of 0.1 denoted by , and were obtained. All the samples were structurally characterized using a anode, high-resolution X-ray diffraction Philips X’Pert, Cu system. The – scans of the three samples are shown in Fig. 1, and are typical for Ni–Zn ferrites as shown by the corresponding identification bars displayed on bottom of Fig. 1.
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Fig. 2. Scanning electron micrograph of the microstructure of samples (a) R x 0:2 and (b) R x = 0:3.
=
Fig. 4. Hysteresis loops for all the samples at 20 kHz and high applied drive field amplitude.
Fig. 3. Hysteresis loops for the three samples at 2 kHz for low applied drive field amplitude.
Also, it must be noted the sharp peaks are indicating for a high crystallinity of the samples. Analyzing the FWHM of all the spectra it was concluded that crystallite size decreases from sample 1 to sample 3. This was confirmed by the direct analysis of the samples performed by scanning electron microscopy (SEM), using a JEOL 5410 system. Fig. 2 shows the SEM images of samples 2 and 3. We note that upon increasing the copper substitution, the average grain size decreases and the microstructure becomes more uniform with fewer pores. The characterization of the samples was completed by the EDAX analysis (DXPRIME) that confirmed the different concentration of the Cu ions in our samples. B. Magnetic Properties 1) Hysteresis Loops and Losses: In order to obtain the hysteresis loops for all the samples, a storage scope interfaced to the computer and a sinusoidal function generator ranging in 2 Hz–20 MHz were used [7], [8]. The hysteresis loops were obtained, for low (10 A/m) and high (40 A/m) applied drive field amplitudes, at some selected frequencies in the relaxation region [9], by exciting the ring cores with a sinusoidal waveform. Fig. 3 shows the hysteresis
loops at low magnetic field strength for the three samples at 2 kHz, while Fig. 4 features the hysteresis loops at high magnetic field strength for the three samples at 20 KHz. For low applied drive field amplitude we observe, for all three samples, a change in the shape of the hysteresis loop as the frequency increases (in Fig. 3 the hysteresis loops at 2 KHz). The characterized change in shape is significant for the sample by smallest grain size. For the same sample as the frequency increases, we notice a slowly decrease of the saturation magnetic flux density. For high applied drive field (Fig. 4) we note that the addition of copper oxide contributes to the decrease of the coercive magnetic field and of the hysteresis losses without important changes in saturation magnetic flux density, as we would expect by increasing of the nonmagnetic phase. Also, in our study we observed an important decreasing of remanence magnetic flux density vs. frequency in the case of . Similar behavior was obtained for the frequency sample dependence of hysteresis losses, which is presented in Fig. 5. 2) Complex Permeability vs. Frequency: Spectra of the complex initial permeability at room temperature was measured by using the conventional technique based on the determination of the complex impedance of a circuit loaded with a toroid shaped sample (Hewlett Packard 4191B Impedance Analyzer with Option 002 and HP 16454A magnetic material test fixture). The results presented in Fig. 6 confirm a relation between the relaxation of the permeability and the changing in shape of hysteresis loop. The real part of the permeability is about 750, and respectively for 1 MHz 350 and 250 for samples , and begins to decrease near 10 MHz in the same manner like in [5]. The imaginary part has maximum values of 350, 150 and and at 5, 7 and 12.5 MHz respectively. 125 for , The study of the complex permeability and of the hysteresis loop dependence on frequency confirms that the wall relaxation occurs in the 1 MHz frequency range. The high permeability ) shows the values at low frequencies (greater for sample
CALTUN et al.: MAGNETIC PROPERTIES OF HIGH FREQUENCY Ni–Zn FERRITES DOPED WITH CuO
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The wall motion is damped and becomes out of phase with the excitation field. For the sample wall relaxation occurs in the and in the 10 MHz range. 1 MHz range, while for III. CONCLUSIONS
Fig. 5. The hysteresis loss vs. frequency in 2 to 160 kHz region for all the samples for low applied drive field amplitude.
The influence of the additions on the magnetic properties of Ni–Zn ferrite cores for high frequency applications were investigated. The samples sintered in the same conditions present different behaviors depending on doping level, which provide ( ) charmicrostructural differences. The sample acterized by smallest grain, smallest porosity on the boundary and on the surface of the grains displays interesting magnetic characteristic. The domain wall motion plays an important role in magnetization processes and in loss mechanism in sample . Despite high values of permeability the relaxation of wall at a low frequency. Sample motion occurs for sample ( ) presents high magnetic performance and suggests that the limiting amount of copper substitution is favorable for the grain growth of Ni–Zn ferrite operating in MHz domain with low power loss. ACKNOWLEDGMENT The authors would like to thank Dr. W. Zhou and Dr. J. A. Weimann from AMRI, for SEM characterization and for XRD analysis of the samples. REFERENCES
Fig. 6. (a) Real part, , and (b) imaginary part, , of the complex permeability at room temperature. B < 0:5 mT for all the samples.
dominant role played by wall motion. Increasing the frequency the shape of the hysteresis loop at low field amplitude changes from straight line to elliptical and the mean slope decreases.
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