Journal of Magnetism and Magnetic Materials 239 (2002) 160–163
Effect of Zn doping on the magnetoresistance of sintered
Fe
3
O
4
ferrites
C.T. Lie
a,*, P.C. Kuo
a, Wei-Chih Hsu
a, I.J. Chang
a, J.W. Chen
ba
Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan b
Department of Physics, National Taiwan University, Taipei, Taiwan
Abstract
The Zn doped Fe3O4 ferrites were prepared by mixing ZnO with Fe3O4 powder and then sintering in argon
atmosphere at 11001C for 3 h. The effects of Zn content and sintering temperature on the magnetoresistance (MR) and microstructure of sintered samples were investigated. From the analysis of Fe2+ and Fe3+ ions contents, X-ray diffraction, and scanning electron microscopy, we found that the nonstoichiometric phases of Fe3O4+a
and ZnFe2O4d coexisted in a sintered sample and Zn ions were dispersed uniformly in the sample. From the
measurement of electrical resistivity r at temperatures between 80 K and room temperature, the relationship between log r and T1=2 is linear, which means that the dominant MR effect is spin-dependent tunneling. The sample with Zn=0.86 at% has the highest MR which is about 7% at room temperature under a magnetic field of 8 kOe. r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Sintered Fe3O4ferrites; Magnetoresistance; Zn doped Fe3O4ferrites
1. Introduction
Recently, research was focused on the transport properties of the ferrimagnets Fe3O4and CrO2[1–3] in
view of potential applications. These magnetic materials are half metallic and therefore ideal candidates for the emergence of oxide spin electrons. However, the intrinsic magnetoresistance (MR) of these componds is small. The MR of pure Fe3O4 thin films has been
intensively investigated [3–6]. The epitaxial Fe3O4films
show no MR in low fields, whereas the polycrystalline film exhibits an MR of 1.7% at room temperature, indicative of spin-polarized tunneling between the adjacent grains. The MR behavior of Fe3O4 in
polycrystalline thin film, powder compact, and single-crystal has been compared by Coey et al. [4], but not in the sintered sample. The MRs of Fe3O4are still too low
in these studies to be used at room temperature. Therefore, developing a suitable fabrication process
and modifying the composition of Fe3O4,in order to
increase its MR at room temperature is necessary. In this study, the mechanism of the magneto-transport properties of sintered Fe3O4 ferrite and the
effects of Zn doping on its MR were investigated.
2. Experiment
The samples were prepared by the conventional ceramic method. The starting materials are high-purity ZnO and Fe3O4powder. According to the formula of
(ZnO)X(Fe3O4)100X (where X ¼ 0225), each starting
material was weighted, added into acetone and ball mill to complete mixing. The mixed powder was compressed into a pellet shape (10 mm diameter, 1 mm thick) under a pressure of 53 393 lb/in2and then sintered in Ar atmo-sphere at 11001C for 3 h.
The crystalline structure of sintered samples was examined by X-ray diffractometer (XRD) with a Cu-Ka radiation and their microstructures were
ob-served with a scanning electron microscopy (SEM). The chemical composition was analyzed by energy disperse
*Corresponding author. Tel.: 2-23648881; fax: +886-2-23634562.
E-mail address:[email protected] (C.T. Lie).
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 3 8 - 8
spectrometer (EDX). The Fe2+ and Fe3+ion contents of the sintered sample were examined by the method of chemical titration [7]. The magnetic properties were measured by vibrating sample magnetometer (VSM) at room temperature with a maximum applied field of 12 kOe. The MR of the sintered sample was measured at room temperature with the four-probe method and the applied field was parallel to the direction of current, the maximum applied field was 9 kOe. The electric resistivity r was measured by the four-probe method at tempera-tures between 80 K and room temperature.
3. Results and discussion
Fig. 1 shows the X-ray diffraction patterns of the sintered samples with various amounts of ZnO powder (2–25 mol%) in mixed powder. The sintering tempera-ture Tsis 11001C. We can observe that the two phases of
Fe3O4 and ZnFe2O4 coexisted in all sintered samples.
This indicates that the ZnO oxide will react with a part of Fe3O4to form ZnFe2O4ferrite during sintering. The
X-ray diffraction peaks of ZnFe2O4 ferrite are quite
close to that of Fe3O4and almost overlapped. The peaks
of ZnFe2O4 ferrite phase are not easy to identify
separately as the ZnO powder content in mixed powder
iso5 mol%. Table 1 shows the ZnO powder contents of various mixed powder samples and the measured Zn contents in these samples after sintering.
From the examination of Fe2+and Fe3+ion contents
in the sample by the chemical titration method, the (Fe2+/total Fe) value is about 27 mol% which is lower
than that of pure Fe3O4 (33 mol%). This means that
some Fe3O4 are oxidized to Fe2O3 during sintering.
After ZnO is added, we inferred that ZnO would combine with Fe2O3or a part of Fe3O4ferrite to form
ZnFe2O4ferrite or nonstoichiometric ZnFe2O4dferrite
during sintering, because Fe2O3peaks did not appear in
the X-ray diffraction patterns. We speculate that the content of Fe2O3 is small and cannot be detected by
X-ray diffraction. Fig. 2 shows the SEM micrographs of the sintered samples with various Zn contents and their Zn mapping. The average grain sizes of all s amples are almost the same (it is about 3 mm) and we can observe some pores dispersed in the grain boundary. The sintering density of all these samples is about 4.93 g/cm3, that is, about 94% of the theoretical density. By comparing between Zn mapping and the corresponding SEM micrograph, it is revealed that Zn ions disperse uniformly in the grain. We believe that the Zinc-rich component may be ZnFe2O4 ferrite or
nonstoichiometric ZnFe2O4d ferrite, but it is difficult
to determine the d value.
We investigated the relationship between log r and T1=2of various samples with different Zn contents, as shown in Fig. 3. It shows good linear relationship between log r and T1=2 in all samples. This implies
that the transport of electrons is in tunneling mode, where the electrons flow through barriers (e.g. Fe2O3, Zn
ferrite, etc.) between the two magnetic Fe3O4phases [8].
On the other hand, the Zn ions are uniformly distributed within the sample and grains (see Fig. 2), which also confirms this tunneling mechanism.
The MR was defined as MRð%Þ ¼ ðRH R0Þ=R0;
where RH is the resistance in applied magnetic field
Hand R0is the resistance in zero-field. Fig. 4(a) shows
20 25 30 35 40 45 50 55 60 ▼ ● Fe3O4 ZnFe2O4 (5 1 1 ) (4 2 2 ) (4 0 0 ) (2 2 2 ) (3 1 1 ) (2 2 0 ) ▼ ● ▼ ● ▼ 5 mol.% ZnO 25 mol.% 15 mol.% ZnO 10 mol.% ZnO 3 mol.% ZnO 2 mol.% ZnO Intensi ty (arb. unit) 2 (deg.) ● ▼ ● ▼ ● ▼ ●
Fig. 1. X-ray diffraction patterns of the sintered samples with various amounts of ZnO powder in mixed powder, sintered at 11001C.
Table 1
The ZnO powder content in the mixed powder of various samples and the measured Zn content in these samples after sintering
ZnO content in mixed powder (mol%)
Zn content in sintered sample (at%) 0 0 2 0.42 3 0.47 5 0.86 10 1.72 15 2.55 25 4.16
the MR curve of the sintered Fe3O4 ferrite. When the
applied field increases, the value of negative MR increases rapidly with H; the solid arrows indicate the increase of H and the dotted arrows indicate the
decrease of H: The MR value of this sintered Fe3O4
ferrite at room temperature is about 5.4% as H ¼ 8 kOe. The Hc value of this sample is small and about 30 Oe as shown in the M–H loop of Fig. 4(b).
Fig. 5 shows the relationship between MR value and Zn content in sintered samples. The maximum MR value is about 7% as Zn content is 0.86 at%. The amount of ZnFe2O4ferrite or ZnFe2O4dferrite affects
the MR value obviously, that was controlled by the doping amount of Zn. The ZnFe2O4 ferrite or
ZnFe2O4d ferrite provides the tunneling barrier for
enhancing the MR value. As the doping amount of Zn is more than 0.86 at%, the length of spin-dependent tunneling barrier bocomes very large and the MR value is decreased with increasing Zn content, as shown in Fig. 5.
4. Conclusions
The effect of Zn doping on the MR of sintered Fe3O4
ferrite was studied. It was domonstrated that a little amount of Zn doping improves the MR value of Fe3O4
0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 1 10 100 1000 10000 Zn content 0 at.% 0.42 at.% 0.47 at.% 0.86 at.% 1.72 at.% 2.55 at.% lo g (
Ω
-c m ) T -1/2 (oK -1/2 )Fig. 3. Log r vs. T1=2of various samples which were sintered
at 11001C. -12000 -6000 0 6000 12000 -6 -4 -2 0 (a) MR ( %) H (Oe) -12000 -6000 0 6000 12000 -1.0 -0.5 0.0 0.5 1.0 M/MS (b) H (Oe)
Fig. 4. (a) The magnetoresistance curve and (b) M2H loop at room temperature of the Fe3O4 ferrite which is sintered at 11001C.
Fig. 2. SEM micrographs of the sintered samples with various Zn contents and their Zn mapping. The Zn content of (a) is 0.42 at%, (b) is 0.86 at%, and (c) is 4.16 at%.
C.T. Lie et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 160–163 162
ferrite. The maximum MR value at room temperature is about 7% when the Zn content is 0.86 at%. The dominant MR effect is spin-dependent tunneling. From
the analysis of Fe2+ and Fe3+ ion contents, X-ray diffraction, and scanning electron microscopy, it was found that the nonstoichiometric phases of Fe3O4+aand
ZnFe2O4dcoexisted in the sintered sample and Zn ions
were dispersed uniformly in the sample.
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0 1 2 3 4 2 3 4 5 6 7 8 MR (% ) Zn Content (at.%)
Fig. 5. Relationship between MR value and Zn content in sintered sample. The sintering temperature is 11001C.
