*Corresponding author. Fax: #82-32-866-0131.E-mail address: jaeheeoh@inha.ac.kr (J.-H. Oh).

Journal of Magnetism and Magnetic Materials 226}230 (2001) 1415}1417

Magnetic properties of NiCuZn ferrites with addition oftungsten trioxide

Kwang-Soo Park, Joong-Hee Nam, Jae-Hee Oh*

Department of Ceramic Engineering, Inha University, 253 Yonghyun-Dong Nam-Ku, Inchon 402-751, South Korea

Abstract

The e!ect of WO�addition on the properties of NiCuZn ferrite, (Ni

���Cu

���Zn

���O)

����(Fe

�O

�)����, has been

investigated in this study. The enhanced electrical and magnetic properties obtained from WO�addition into NiCuZn

ferrites were also shown in this study. Q-factor at 10MHz of NiCuZn ferrites increased at higher electrical resistivity, butno signi"cant change in initial permeability. The resonance frequency of those NiCuZn ferrites was shifted to a higherrange, which means relaxation time was also decreased over 0.6wt% of WO

�content. � 2001 Elsevier Science B.V. All

rights reserved.

Keywords: Ferrites*soft; Complex permeability; Magnetic relaxation; Microstructure

NiCuZn ferrites such as (Ni�Cu

���Zn

����O)

����(Fe

�O

�)����

have been generally used as materials formultilayer chip inductor. Microstructural and magneticproperties of those ferrites are highly sensitive to theprocessing of preparation such as sintering conditionsand the amount of constituent metal oxides or additivesin their composition [1,2]. NiZn ferrites are generallysuitable for using at a relatively high frequency rangebecause of its high electrical resistivity with su$cient lowlosses for applications.The present work is aimed at the AC properties and

microstructures of NiCuZn ferrites [2] prepared by ce-ramic processing for multilayer chip inductor in theaspect of initial permeability, Q-factor, complexpermeability. All samples were sintered at 9003C for 5 hin air. The composition of NiCuZn ferrite acts a crucialrole to control their parameters of properties. In applica-tion as a chip inductor, the reduction of magnetic loss athigh frequency ranges is essential to get a good perfor-mance for electronic components. In order to improvethe electrical and magnetic properties of NiCuZn ferrites,WO

�was added up to 1.0wt% and its bulk density

(measured by pycnometer), initial permeability ��(cal-

culated from those inductance values measured by LCRmeter) and saturation magnetization (measured by VSM)were characterized. The samples with 0.4wt%WO

�pre-

sented the lowest value of porosity for the sinteredsample of high performance of (Ni

���Cu

���Zn

���O)

����(Fe

�O

�)����

as presented in this study. An increase ofinitial permeability in Fig. 1 is explained on the basis ofthe changes in saturation magnetization due to graingrowth in NiCuZn ferrite. Furthermore, it appears a de-crease of initial permeability over 0.6wt% WO

�which

have contributed to make the magnetization by domainwall from the larger pore volume in the sintered NiCuZnferrites [3]. Fig. 2 shows the variation of �

�Q in terms of

magnetic loss with WO�amount in NiCuZn ferrite of

x"0.2. In this case, an increase of ��Q values can be

supposed to form the demagnetization "eld due topolarization e!ect by the e!ective W�� ion in NiCuZnferrites [3].The permeability of ferrimagnetic materials, such as

ferrites that exhibit Snoek's limit, can be a!ected bycomposition and processing. The real part of permeabil-ity decreases with the frequency and the imaginary partrepresents a broad peak, which is related to the relax-ation phenomena. As the frequency of RF excitationincreases, the material reaches a point beyond whichthe spins cannot fully respond to the excitation. In the

0304-8853/01/$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 0 2 8 - 2

Fig. 1. Initial permeability and porosity as a function of WO�

content of NiCuZn ferrite sintered at 9003C for 5 h in air.

Fig. 2. ��Q measured at 1MHz as a function of WO

�additive

content of NiCuZn ferrite sintered at 9003C for 5 h in air.

Fig. 3. XPS spectrum and curve-"tted W��spectra of NiCuZn

ferrite with addition of 1.0wt%WO�sintered at 9003C for 5 h in

air.

Fig. 4. Relaxation time and anisotropy "eld of NiCuZn ferritewith WO

�content sintered at 9003C for 5 h in air.

characterization of frequency dependence of spinel ferritematerials, the dispersion due to relaxation can be ob-served at some frequency range and is also related withthe ferrite composition and initial permeability [3].Fig. 3 shows the result of XPS analysis for WO

�-

containedNiCuZn ferrites. From the result of XPS, thosetungsten ions (W�,W��) with di!erent electronic val-ences as seem to be coexisted and provided the oxygenvacancies as materials transportation path by electrontransition of chemical state in a small amount of WO

�.

With increasing WO�content, the demagnetizing "eld

can be formed due to polarization by addition of WO�

and resulted in the increasing of magnetic loss. Thesaturation magnetization can also be reduced by substi-tution of tungsten ions with strong preference of te-trahedral site by ferric ion [3].The relaxation time (�) and anisotropy "eld (H

�) were

calculated from the complex permeability to ensure theloss phenomena of NiCuZn ferrites with WO

�addition

as shown in Fig. 4. As much of WO�content over

0.6wt%, the magnetic loss of WO�}NiCuZn ferrite was

1416 K.-S. Park et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1415}1417

reduced at a higher frequency. In general, the coercivityof spinel ferrite increases with decreasing initial permeab-ility, which leads to larger anisotropic "eld and reducesthe magnetic loss with frequency [4].

References

[1] J.Y. Hsu, W.S. Ko, H.D. Shen, C.J. Chen, IEEE Trans.Magn. 30 (1994) 4875.

[2] J.H. Nam, H.H. Jung, J.Y. Shin, J.H. Oh, IEEE Trans.Magn. 31 (1995) 3985.

[3] J. Smit, H.P.J. Wijn, Ferrites, Philips' Technical Library,Eindhoven, Netherlands, 1959, pp.136}326.

[4] A. Globus, M. Guyot, IEEE Trans. Magn. 6 (1970) 614.

K.-S. Park et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1415}1417 1417