V N IJ Joimial ofvScicncc, M athem a tics - Physics 25 (2009) 199-205
Signficantly improving magnetic properties o f Sr-La-Co hexagonal ferrite
N g u y e n K h a n h D u n g ' * N g u y e n T h i L e H u y en "
' V/oC/i/A/Z/i/; O n ' U niversity o/ỉtHỈitsỉry
Ỉ2 Ni^uycn Van Bao, Ward 4. Go Vap District, ỉĩochim inh city, Vietnam
Institute o f Physics in ỉỉoC hiM inh City. Ỉ M ac Dinh Chi, I District, H ochim inh city, Vietnam Rcceivcd 5 S eptem ber 2009
A b s t r a c t : 'Ỉ he liard inamielic fcn ite system o f S rj.xL a xl'ei2-vCoyOi9 with X ^ 0.04).3 and y = 0.0- 0.3 has been pre p are d bv tratlilional ccramic tcchnolouy and s h o w ed hexagonal crysĩalline siruclure o f M - Ỉ \ p c .1'he anisotropic sample Sro.gLao^Fci 17C00.3O19 0.5% weight SÌO2 sintered al
ì2 8 0 '’C/2h has llic best hard magnetic piopcrlies, nam ely Bf = 4 .66 kG; bHc = 3.46 k O e and
~ 5.70 M G .O c. I'h e role o f La and Co as well as the reasons leading to these perfect propei tics o f the studied ferrites have been discusscd based on llie overall studies o f structure and chdiaclcristics o f c x a n i i n c d samples.
Ỉ. In tro d u ctio n
In m ore than a h a lf century from the discovery o f hexagonal fcư ite, ferrite m agnets alw ays used very popular and evcrvvvhcrc in scicncc. tcchnoloịzv. life and especially in civil electronics industry, in c o m m unica tions and other diffcrcnl areas due to chcap price as well as sim ple technology. But still now Ihc problem to i m p r o v e hard m agnetic properties ÒÍ’ this kind o f materials alw ays attracted the rcscaichcrs and tcchnolouists. VVc arc talking about two main rollow m g trends:
- Using diỉTcrcní doping elem ents including 3d and R arc-carth metal oxides with hoping that s a t u r a ti o n n ia g n cti/atio n and m agnctocrystallinc anisotropy will be im proved [1-4J.
- A pplying advancc technologies such as coprccipitation, sol-gcl, high pressure com pressing, hot p r e s s i n g , isostalic prcssiim ...to m anufacture l e m tc pow der and product Ị5-8].
In this report \vc study the struclurc and magnetic properties o f icrritc system Sr,.xI^axI'ei2_yCOyO|9
(x = 0.0-0.3 and y = 0.0-0.3) to im prove the quality o f hard m agnetic ferrite and explain the reasons leading to these results.
2. E xperim ent
Substitution in our study w as the sim ultaneous doping o f La and C o in system Sri.xLaxFci^.y CoyO|ọ + 0.5% w eight o f SiOi. R aw materials used here arc SrCO j, FC2O3, La2Ơ3, C o O and SiO. with
Corresponding author. F-mail: nkdung 2 0 0 9 @ y alio o .c o m 199
200 N.K. D unẹ, N J '.L ỉ ỉ u y e n / \'N U J o u rn a l o fS c w n c c . M athem atics - P h ysics 25 (2009j Ỉ 99-205
high purity (3 - 4 N). Wet planetar) ball mill lias been used lo proxidc line ỊK)udci with main si/C around IỊ.II1Ì. riic ratio betw een powder: ball; w ater was 1 :4: I. We kept the moisture o f p o u d c r about 35% for anisotropic pressing. T o orientate the particles along the m agnetic field, the field strength o f
!T and llic pressure OÍ' 500 kG/c'iii have been applied. Tlic sam ples w ere sintered at temperatures 1260‘’C. !27()"C, 1280''c and keeping lime was 2h. A pproxiniatelv, the clicmical coniỊX)silion o f samples was checked bv lEDS iacilily. The thermal transitions w ere pcrfoniiL'd by D ['A - D'l'G machine, [ lie crystallographic struclure o f samples was exam ined by X -ray dirfractoniclci (Siemens D5005 X-ray, G erm any). M icroslriicture o f isotropic and anisotropic sam ples has been studied usinu SHM (J li0 l.- J S M 5 4 1 0 L V , Japan). M agnetic properties are invcslii^atcd by V SM (200C Nini, USA), hystcrcsisgraph (A M H 5 0 -2 0 , U S A ) and m agnctom clcr using high pulse nuii^nctic n d d to measure SPD curve.
3. R esults and d iscu ssion
Fig. 1 shows d em agnetizing curves o f Sr-La-Co anisotropic sam ples s i n t e r e d at 1280‘'C/2h . T h e main characteristics derived from this figure are presented in Table 1.
Table 1. T he main hard m agnetic param eters o f anisotropic sam ples sintcrd at 1280"C72h
Sample C o m p o u n d B r lk G ] BHclkOc] j H c l k O e ] (B1 [MG.Oe]
Ml Sr!-'ei:0 } 9 4.60 2.15 2.45 4.78
M2 Srygi-a(i2Ỉ'Ci20Ị9 4.58 2.73 2.73 5.34
M3 Sro7Lao3Í‘C|Ị yCoo )Oi9 4.50 2.90 3.25 5.60
M4 Sro gLa<) 2Ỉ*ei 1 7CO0 3^19 4.66 3.46 3.60 5.70
It is d e a r l y seen from this table that with appropriate substitution o f La and Co, the magnetic characteristics o f sam ple M4 evidently improved in com parison wilh pure hexagonal ferrile M l , nam ely nllc increased up to 60% and (BU),„ax achieved the rccord value o f 5.70 M G.Oc.
T o understand the reasons leading to so very high quality o f this sample, \vc rem e m b er that for materials consisting o f single dom ain particles, the magnetizing process is ihc rotation proccss o f magnetization vcctors o f d om ains and coercivily should be determ ined by expression:
K Xt
H(. = a - ^ + b ( N , - N , ) l s + c ^ (1) [9]
I s
Here a, b and c are contants, magnctocrystalline anisotropic constant,
Ki is N| and N2 are dem agnetization factors determ ined parallel and perpendicular to the axis o f particles,
strain and Is is saturation m agnetization. The
Coercivity (kOe)
Fig. 1. D e m a g n etizin g curves o f anisolropit sam ples M l - M4.
first term in expression (1) relating vith
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niauneiocr> stall me anisotropy and norm ally plays a dccidcd role for creating high cocrcivity o f m a t e r i a l s . I'or this reason alm ost authors focusing to find the w ay for increasing K | o f hexagonal ferrite mainly by substitution effects. T he sccond term in (1) dealing with shape anisotropy o f particles Íoriiiiiìíi material and takes part o f several tens percent o f He and finally the last term in (1) determ ined by Iiiagnctic elastic energv and oflcn is small and could be ignored. Apart from above iiidicalion. the general orientation to im prove hard m agnetic properties o f hexagonal ferrite is to make raise niaunclocr>'stallinc anisotropy, shape anisotropy o f particles as well as saturation m agnetization
Constant Ki w as m easured in m a gnetom eter with hiuh pulse m agnetic field using special point dcicction (SIM)) tcchnicque. I'ig.2 show s the m easurem ents o f M (H ) curvc and d M / d li (H ) curvc for s a m p l e M 4 .
so 70 60 5(»
4U M)
2U iO
/ \
puralie
\ »
pcrpcndicular ' ^1.
II (T) I 2 3 4 5 6 7 I I ( T )
Fig. 2. M (II) curve (left) and ^ curve (right) o f sam ple M 4 sintered at 1260‘’c/2h.
J / / '
able 2. Cotìstaiiỉ Ki o f sam ples M i and M4 derivci T ab ic 3 l.altice p aram eters o f studied Sri aT o-M
Irom SIM) m easu rem en t ferrites
■Sample KịXỈÍ? [J 'm' SampL- l A i
Ml M4
3.60 3.93
M l M3 M4
5.8868 5.8775 5.8740
l A i 23.0370 23.1034 23.1091
N'casurcd results from T a b .2 show ed the value Ki = 3.6.10^ J/m^ for pure SrM ferrite which is com p etcly agreem ent with the results given by other authors, for ex am ple [5, 10, 11 ]. N ote that ferrite Lal-C has K ,= ( Ỉ 0 - Ỉ 3 ) .10' J/m^ at OK [12], whereas ferrite C o F c2 0 4 has Ki = 3.9.10^ J/m^ at room tem perature [13]. n . Zijistra [14] pointed out that in La-Co ferrite anisotropic field lla could reachtlic value o f lO^A/m and Ki = 5.10^J/m \
Th i s with llie sim ultan e o u s substitution o f La for Sr and C o for Fe in the SrM-ferrite hard
magnetic properties o f hexagonal ferrite significantly improved.
T i c crystallographic structure o f sam ples has been ex am ined by d iffractom eter and the X RD p a ltc n s o f samples M l, M3 and M4 are presented in Fig.3. T he results indicated that the structure o f studied ferrite co rresp o n d in g lo m agnetoplum bile structure with c-axis is a little longer but a-axis is shrinlcd a little in co m parison with pure SrM -fcrrite as show n in T a b .3
202 .V-A', Dunị^, N. T L ỉĩu y e n ■ \'N V J o u rn a l o f Science. M a them atics - P ỉiysics 25 (2009) ỈỌ 9-205
M l S iO .6 F e a 0 a
ci u
c M4
M3 M l
20
Fig. 3. X -ray diffraction patterns o f the samples M I , M3 and M4 sintered at 12 6 0 ‘’C/2h.
From T ab .3 w e can con clu d e that by substitution in our study the unit lattice has m ore shape anisotropy wilh longer needle form.
To understand about particle size o f fine milled S rLaCo-fcrrite, the p o w d er vsas m e asu re d by Microtrac m achine S 3000 and ihe results are indicated in 7'ab.4. It is clearly seen that in sa m p le s M |- M3 alm ost the particles have small size and especially by optim al substitution in sam ple M4, 100% o f particles has single dom ain structure with high shape anisotropy (Tabs. 3 and 4).
T able 4. Crystal grain size o f SrLaC o- ferrite, w hich was presintered and fine milled
Sample M l M2 M3 M4
Crystal grain size 33.69 (5% ) 30.93 (3% ) 26.33 (7% )
[urn] (ratio) 1.55 (5 4 % ) 1.62 (54% ) 1.69 (5 8 ” o) 1,59 (46"i,)
0 . 3 9 ( 4 1 % ) 0.39 (43% ) 0.41 (3 5 % ) 0.37 (5 4 % )
Figs. 4 and 5 show the S EM pictures o f the samples. From both figures we can sec the iiiiliicncc OÍ substitution to m ake finer and more hom ogeneity o f particlcs which arc correspond to siniile domain structure. It is also obviously seen from Fig. 5 that the appropriate substitution (and small dopinu o f SÌO2) nol only restricting the grain grovvth but m aking the m ore shape anisotropy o f them and both these factors leading to enhan c e the hard magnetic properties o f studied samples.
O ur purpose is not only to achieve high He but also lo reach high value o f magncti/.ation. I'o llnd the solution for this problem , alm ost the authors focusing on the w ay to m ake increase Iip-spin o f positions 2a and 2b. A bout the role o f La to improve hard m agnetic properties o f hexagonal ferrite, the V ietnam ese researchers have published a large num ber o f publications am o n g them several has been occured on the International Jo u rnals [15 -18].
In order to understand and discuss on the advance o f La and La-Co as well as applied leclinology o f authors inside and outside V ietnam w e derive here som e main related results:
- In previous our w ork [14], the structure and magnetic properties o f ferrite (SrO )i.v(La:0 3)^n.
5,3(F c2 0 3) + 0.5% w eight SÌO2 (x = 0.00 to 0.12) have been detailly investigated, riic new icchnology w as applied there, nam ely the wet isostatic pressing w as perform ed in a rubber die pressing. Í3y that tool we have succeded to prepare anisotropic sam ples with very high density (d > 9 9 % d x ) and ver\
high orientation degree o f particles along magnetizing fiek' applied in pressing process and \vc have
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rcccivcci icrritc sam ple with rccord quality for quite long time: Br = 4.3 kG; nHc = 3.2 kOe, (BH), 5.5 M Q X )c).
l-iii. 4. sr.M Imaiics o f isotropic bulk samples Ml siiitcrcd at Ỉ260‘’c/2h (bright line is 1 0
Fig. 5. SEM Images o f anisotropic bulk samples MĨ, M4 perpendicular (left) and parallen (rigth) to the preferred
magnetizing direction (bright line is 1 0 |im).
li is well know n lhai the ions which are origine o f m agnetic m o m en t o f hexagonal ferrite noniiall} localcd at 2a, 2b and 12k positions (up-spin) and 4fi and 4Ỉ2 positions (dow n-spin) in the crystallo^rapllic lattice. T herefore m agnetic m om ent o f a m olecule Sr0 .6F e i0 3 is equal lo 20|ÌB.
The substilution o f l.a for Sr leads to the following change:
s r " + hV* La
fV * (2)
Si
Hie peribrm cd ions possibly are located at 4fi and 4Ỉ2 positions (dow n-spin) and because ion
! c has siiiallcr magnetic m o m en t than o f ion this valency conversion led to the increase o f total n i a u iic ti/a lio ii o f f e rrite .
- Sc-Dong Yang ct al. [19] studied two compositions S rFcii 7C003O19 and Sro7l-ao3Fcn 7 C003O19 iuid showed that C o alone enhanced Br but both La and C o im proved He-
- R Cirossinger ct al. [20] indicated that the sim ultaneous replacem ent o f La and Co in SrM - I c rrilc Iioiably i n c r e a s e d l i e d u e to i n c r e a s i n g o f a n i s o t r o p i c f ie ld Ha a n d e s p e c i a l l y t h e y a l s o h e l p e d stabiỉi/c i !(, on tem perature but one ncccssar>' thing is the replacem ent fractions arc not exceeded 0.25 mol.
-
K.
Masuzawa ct al. [2 1] have studied ferrites Sri.xLaxFeii 7C003O19 and showed that with X = 0.3-0.4 ferrites have phase M. W hen X <0.3 He becom es low er due to spinel phase (Co-ferrite) existed and while \ > 0.4 magnetic properties o f ferrite strongly decreased, particularly for the rem anentinduction Bj. bccausc o f occurring hem atite phase.
- Y. Kubota et al. [2] ex am ined ferrite system (Sr^^. x )0 .n{(Fe^''i. yCo^'‘y)2 0 3} w here X = 2ny and n = 5.4-6.0 and show ed that by substitutions o f La for Sr and C o for Fe, respectively, jHc o f ferrite is significantly increased w hile Br only slightly enlianced (jHc - 4.5kO e; Bj. = 4.4kG). The
authors explained that ihc residual orbital m agnetic m om ent o r i o n plays the main lolc to m ake high value o f He-.
- F. Kools ct al. [22] studied f c m te system Sr].J.aJ*'C|2.^CoxC)i9 <0.25) and indicated that Co plays the im portant role for notably im provem ent o f Ha (i.e H ( ) and ions C o “' possible located 12k and 4Í2 positions. T hey also sho w e d that rem anent induction could be determ ined by expression:
Br = s ( d / d " ) ũ \ (3)
w here s IS I'errite fraction in the solid body, f IS alignm ent factor, d and d'’ arc e x p erim en t and theoretical density, rcspcctivcly, is saturation m agnetization. Beside J°s is intrinsic p aram eter, the others are extrinsic ones and strongly d epena on the technological processes.
A ccording to our know ledge, recently m axim um energy product obtained in this report b e lo n g s lo the highest value and very closed to the theoretical prediction.
204 N.K. Díỉiĩg. N .T.L. Ịỉu y c n / \'N U J o u rn a l o f Science. M iiihem atics - P hysics 25 Ỉ2009) Ị 99-205
4. C onclu sion
By optimal substitution o f La for Sr and Co for Fc in SrM hexagonal ferrite and used a p p ro p n a ic technology, the studied anisotropic sample S r j.x Lax I‘Ci2-y COy O i9 (x = 0,2; y = 0.3) + 0 .5‘!o w eight o f SiO . gives Br = 4.66 kG; »Hc = 3.46 kO e and (BIỈ) m ax = 5.70 M G .O e.
In our study, La and C o (together with small am ount o f S iO l) played the role to reducc the sintering temperature, restricted the grain growth, crcated the single dom ain particles with needle shape, changed the valcncy o f iron ions from to Fe"' w hich located at dow n-spin positions and as the results leading to desirably increase o f anisotropy (both m agnciocrystalline and shape anisotropy o f particlcs) as well as magnetization.
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