ES-07
EFFECTS OF Mg DOPING AND SINTERING TEMPERATURE ON THE
MAGNETORESISTANCE
OF
SINTERED
FelOI
FERRITES
C . T. Lie'. P. C . Kuo', C . Y. Chou', 1. I. Changl. and 1. W. Chen'
'
Institute o f Millcrials Science and Engineering, National Taiwan University, Taipei. Taiwan 'Depwimentof Physics. National Taiwan University, Taipei. Taiwanlmroductinn
Cumently, the msgneloresirtance (MR) o f pure Fe?Ol has been investigated by few groups 11-21. Thc M R YBIUC of FmO, in thesc studies w m iuund vcry m a l l al room lcmpcramrc ( MR = 0-1.7 Yo). For the sintered FelOr ferrite. i t s M R wlue weis dependent "11 the micro~truc~urc and sintering conditions. I n thia work. we iinpruve MR vidue of the sintered Fc,04 ferrite ;iI mom tcmpcwturc to a h u t 7.3 R with small amount of Mg addition. Thc effects o f Mg content, sintering temperature (Tsj. and miCrOStrUCNre on i f s magnetoresistance at rmm temperitlure were investigalcd.
Exoerimental mocedure
The high-purity FelOr powder was mixed with various amount o f M g O powder (0-25 mol.%). After fully mixed, the mixed powder was compressed into B pellet ( 10 mm diameter. I mm thick) under a pressure o f 53393 Iblin' then sinlering in argon atmosphere at temperatures hetween 1050 @C and I200 "C for 3 hours. Composition o f the sinlered samples was analy,,ed hy energy disperse sppectmmelrr (EDS). Crystalline slructure o f the sintered sample was delermined by using X-ray diffractometer (XRD) with CuKa radiolion. Microslnwlure was observed with a scanning electron microscope (SEMj. The Fe'' and Fe'* ion contents o f the sintered rample were examined h y chcmical titriltim mcthod. The magnetoresistance of the sintered sample was measured at mom temperature with Ihe four-pruhc mclhud and the milximuin applicd ficld was 9 kOc.
Results and discussion
I t is found that the Fe304 and MgFelOr phaqes are co-existed iii all sampler which sintered at temperatures between 1050 T and 1200
T
from their X-ray diffraction pettems. This ilxlicalcs that thc MgO oxidc rcscts with wmc Fc104 tu form MgFczOd fcrritc during sintering. From the analysis of SEM element mapping, we find that the Mp ions we dispersed uniformly in 1111 sintered samples. Figure I shows the relationship between MR value andT,
of various sintcrcd samplcs with diffcrcnt M g contcnts at room Icmperafurc. Thc optimum T.. which meam that maximum MR value c m he ohtnined i s ohout I100 il for a11 samples. as shown in Fig. I. Figure 2 shows Ihe relationship between M g conten1 and M R value at room temperature of the sintered sample which was sintered a1 I100n.
The maximum M R value i s ahout 7.3 % which occurs at Mg content o f 0.93 at.%. The g n i n Size o f t h k sample is about2.5
wm and its density
i s 4.95dcm'
which i s about 94 % theoretical density o f the pure F c i Q (Thal is 5.24 dcm').Chemical titration analysis o f Fe'* and Fe" ions contents indicates that the
nonrtoichiomelric phases of Fe304+y and M@I:e>O,.x are also existed in the sintered sample. T h c elcclric rcsislivity p of Ihc sintcrctl sample was i n c i i ~ w ~ by Ihc four-prube mclhud ill
temperillurer between XI1 K nnd rooill lempernture. The relatiomhip hetween lag 1) and ' T I i i o f sintered snmples with differen1 Mg contents was shown in Fig.3. T and log p arc linearly
rehtcd lirr all samples. implying that lhe magnelore.iirtnnce effcci n i t h e w r i m e r d vilmples i* *pin-dependem tunneling 131. This means than the insulators Fe203. MglFe104. and M ~ F C ~ O A . X pruvidc lhc tiinncling bnrricrs fur cnhsncing thc MR wluc. Thc lcnglh o f spin-dependent tunneling barrier becomes too large and the M R value decreases with incre;iaing M g content as the M g doping amoiint is higher than 0.93 st.%, as shown in F i g 2
8 I +o.s3 ;,M - - o ~ 3 : ~ t . ~
I / ,
, , ,,
-I.n8,.=l ,,
1
---Z.ffiid.", U 1Mo I l W 11% I" s i " r c n , ~ ' l ' ~ u l m l *Cl Fig. I. Relalionship between M R value and T, o f various sintered samples which have diffcrcnl Mg cmtcnts.T.'"I X " n I
Fig. 3. log 1) YS. TIc o f various samples which were sintered at IIU0"c.
0 1 2 3 4 5
*cmn, U,'%] Fig. 2. Rehlionship helween MR value and M g coiitcnt o f the sintered wmple. KCfCrCIlCW
I I I G Q. Gong. A. Gupta. G Xiao, W. Q i m . and V. I? Drwid, Phyr. Rev. B 56. 50Y6, (19971.
12lJ. M. D. Coey, A. E. Berkowilr. L. I. B.ilvells. F. F. Putris, and F.
T.
Parker,.
Appl. Phys. Lctt. 12.734. (19910. 13 I Ping Sheng. E. Abclea and Y. Arie, Phyr.Rve. Letl. 31.44. (1973). 0 L
