Preparation and magnetic properties of (Co,Zn)-doped γ-Fe2O3 particles

4 104

IEEE TRANSACI'IONS ON

MAGNETICS, VOL. 30, NO. 6, NOVEMBER 1994

PREPARATION

AND

MAGNETIC PROPERTIES OF (Co,Zn)-DOPED

r-Fez03

PARTICLES

C.H.

Lin

P.C.Ku0

8 J.A.

Chen,

Y.D.

Yao,

Dept.

of Materials Science, Tsing Hua Univ., Hsinchu, Taiwan

Institute

of Materials Science, Taiwan Univ., Taipei, Taiwan

Institute

of

Physics, Academic Sinica, Taipei, Taiwan.

Abstract-idation tenrperatures and ti- of Fe304 to produce (Co,Zn)-doped r-Fez03 particles were examined in this study to relate these two parameters to void, particle size, saturation magnetization, coercivity, and squareness. The effect of Zn/Co ratio on magnetization and coercivity of the particles was also studied.

The partial substitution of Zn for Co in 8-r-Fez03 in- creased the U value and environmental stability, but de- creased the iH, and the squareness of T-Fez03.

INTRowcrIoN

Co-r-Fez03 particles, which find extensive application in high desity reoxding media, are produced by the dehydra- tion, reduction, and oxidation of wet-chemically prepared u-

FeoOH. The addition of Zn, which was produced by absorbing

Co and Zn ions on the surface of r-Fez03' ' - ' I , has been found to be capable of increasing the stability of Co-r-FezO3 par- ticles. A preliminary studyc3', in which Co and Zn ions were initially absorbed on the surface of U-FeOOH instead of T-

Fez03 and then through the dehydration, reduction, and oxi- dation processes, has shown that (CO,Zn)-r-Fez03 partides have some interesting magnetic properties. In this article a further study is undertaken on the preparation of (Co,Zn)- doped r-FezO3 particles by the oxidation of (Co,Zn)-FesO4 and their magnetic properties.

EXPERIHENTAL

Acicular (CO,Zn)-doped Fe304 particles containing 5 and 8 wt% of of Co and Zn ions in total (Co+Zn+Fe) and having various Zn/Co ratios were produced by first wet-chemically synthesizing U-FeOOH particles. This was next followed by adding CO and Zn ions onto the surface of the u-FeoOH par- ticles, dehydrating u-FeOOH to form cy-FezO3, and reducing u-Fez03 to form Fe304 by hydrogen.

parative prooedures are reported elsewherec 33.

Various ternperatures and times were used to oxidize (CO, Zn)-FesO4 particles.

the complete oxidation temperature of r-Fe304 is 252"C, oxi- dation tmperatures ranging from 320 to 420°C were selected.

VSn were used to examine the particles.

The details of the pre-

Because DTA analysis indicated that

After the preparations of the particles, XRD, TM, and

RESVLTS AM) DISCUSSION

The U value (saturation magnetization) of T-Fez03 indi- cated in Figure 1 and 2 was found to decrease with increases

of both the oxidation temperature ranging from 310°C to 420°C for 1.5 hrs and oxidation time ranging from 1 to 53 hrs at 350°C. XRD analysis indicates that this phenomena occurred because s ~ n e Of the T-Fez03 was transformed into a-Fez03. The U value of T-Fez03 illustrated in Figure 3 was found to

increase with the Zn/Co ratio ranging from 0 to 0.7 since cobalt zinc ferrite has higher magnetic moments than pure

aobalt ferrite. The Q value of lowest curve in Figure 1 and

Figure 2, which contained no zinc ion, decreased more rapiely than other (Co,Zn)-r-Fez03 at high temperature region. This would imply the effect of zinc is to increase the high tern- perature stability of Co-r-Fez03, i.e., the resistance of r- Fez03 to @-Fez03 transformation. The U values of (CotZn)-r- FezO3(8wt%) given in Figure 3 were always slightly smaller than those of (Co,Zn)-doped T-Fez03(5wt%).

difficult to explain since the U values of pure coO.Fe~03(80 emu/g) is larger than that of pure r-FezOz(76 em~/g)'~'. It may be due to scnne nonmagnetic phase, which can not be de- tected by XRD analysis, existing in the (Co,Zn)-doped r-FezO3.

The iH,(coercivity) of (CO,Zn)-doped r-Fez03 (Figure 4 and Figure 5) increased with a higher oxidation temperature or longer oxidation time of Fe304 because more Do diffuses inside ?--Fez03 and fonns more cobalt ferrite which has higher coercivity than pure r-FezO3.

u-FezO3, which was produced either at a higher oxidation ternperature or a longer oxidation period, mignt introduce a stress field in the sample and subsequently increase the

i H , value of r-Fez03. The removal of pores (Figuire 6) at a higher oxidation temperature removes the demagnetizing field and, subsequently, also increases the coercivity of r-FezOs. The iH, of the sample was found to increase with an in- creasing ( W Z n ) content. However, it would decrease with an increasing Zn/@ ratio because of a large anisotropy wnstant of ccbalt ferrite and the addition of zinc ion de- creases ita anisotropic constant(K1).

was found to have the same properties as iH,, whioh increases with either a higher oxidation temperature or longer oxida- tion time of Fe304.

MA analysis in Figure 10 shows that the partial substi- tution of Zn for CO in co-doped T-Fez03 particles stabilized r-Fez03 because the transformation temperature from 7-Fez03 to u-FezO3 increased with Zn/Co ratio. The iH, of the (Cot

Zn)-doped r-Fez03 (Figure 11) increased with the aging time at 60°C in air within 90 days because of r-Fez03 oxidation. doped Fez03 in this study, the magnetic properties were found

to be similar to those of previous works'

'-".

This phenomena is

A trace amount Of

The squareness of (Co,Zn)-doped r-Fez03 (Figure 8 and 9 )

Although a different method was used to produOe (Co,Zn)-

CONCLUSIONS

The U value of (Co,Zn)-r-Fez03 decreased with increased of both the oxidation temperature and oxidation time of FesO4. The U value of r-FenOo increased with the Zn/Co ratio but decreased with the (CBtZn) content. The partial substi- tution of Zn for CO in CO-r-FezO3 increased the resistance of r-Fez03 to u-FezO3 transformation.

The iH, and squareness of (Co,Zn)-doped r-FezO3 increased

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Figure 1 s value of (Co,Zn)-~-Fez03 related to various oxidation temperatures Of r-Fez03 (oxidation time = 1 hr, (CotZn)/(FeMDtZn) = 8 ut%)

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Figure 2 Q value of (Co,Zn)-r-FezOs related to various oxidation t i m a of f-Fez03 (oxidation tenper- ature

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value of (Co,Zn)-r-Fez03 related to various Zn/Co ratios (l'=35OoC, b1.5 h m )

with a higher oxidation tenperature or longer oxidation time

of ~ e 3 0 4 beaause of the fonmtion of more cobelt ferrite, the stress field of a-FezOs, and the removal of pores. The iH,

of

f-Fez03 was increase with an increasing (CotZn) content,

decreased with an increasing Zn/@ ratio because of snmller

K I value of zinc ferrite.

The partial subetitution of Zn for ~o in ~o-r-Fez03 in- creased the Q value and environmental stability, but de-

creased the ,He and squareness of r-Fez03.

[l] 0 . Kaganowicz, E.F. Hooking and J.W. Robinson, IEEE

[21 A. Roueset, C. Bonino, M. Gougeon and P. Mollard, ibid. 131 P.C. K w and J.A. &en, J. Hatl. &i., pp.817, 1993. [41 B.D. Cullity, Introduiction to Magnetic Materials, 1972,

Trans. Magn. MAO-11, pp.1194, 1975.

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Figure 4 i€Ie value of (Co,Zn)-r-Fez03 related to various oxidation hnperatures of r-Fez03 (oxidation time = 1.5 hrs, (Co+Zn)/(FeMXwZn) = 8 ut%)

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Figure 5 iH, value of (CO,Zn)-r-FezOs relatcd to varicils oxidation times of f-Fez03 (oxidatiori tenrper. ature = 350"C, (Co+Zn)/(FeMDtZn) = 8 vt%)

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Figure 8 Squareness of (Co,Zn)--FezO3 related t4 various oxidation tenperatures of ?-Fez03 (oxidation time = 1.5 hrs, (Cc+Zn)/(Feeco+Zn) = 8 wt%)

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Figure 9 Squareness of (Co,Zn)-~-FezOa related to various oxidation times of T-Fez03 (oxidation temper- ature = 350"C, (cotZn)/(Feeco+Zn) = 8 Wt%)

Figure 10 Transfornvltion temperature of (Co,Zn)?-Fez03

t4 u-FezO3 (Cc+Zn)/(FetcoeZn) = 5 Wt%)

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