Rai, Davendra Kumar

Upadhya, Chandan

Verma, H C

IACS Institutional Repository

Imium J. rhys. 7 5 A  (4 ) . 4 0 5 -4 0 7  (2 0 0 1 )U P  Ai\n international )ournalSite preferences for cobalt and cobalt-titanium in substituted barium ferritesC handan  U p a d h y a y D a v c n d r a  K um ar H C  Vcnna* and Jilcndra K u m a r' l^epartmcnt of Physics. Ituiiail Institute of Technology.Kanpur 20« 0 |6 . India-Materials ScicMice Prograiiunc, Indian Institute of Technolog>.Kanpur-208 ()|6, India7B-mail chaiulaf|<<f»'iitk ac inVbstract : Cobalt and cobalt-titaniiim substituted barium hexaferrite samples having composiiion BaFe,, ^Co/),., (0*rx<l 6) and HaFc,, ,^ Co^ Ti^ O,.^ii)< \< ]  0) were prepared using an aiitoignition method and characien/cd by Mbssbauer spectroscopy and magncii/ation incasuiemcnts It is shown that ihc sobalt ions prefer to occupy 12k and 2b crystallographic sties of BaFe,^  ^ Mossbauer spectra of BaFc,, .^ Co^ Ti^ Oj., icvcal that the magnetn.vdhalt and nonmagnetic titanium ions assume 12k, 2b and -4f siysiallogiapiiic sites prefeicntially with no significant involvement of othci sites I he .'vuivity {H  ) decreases considerably with increasing substitution amount In CoJ),.^^ ,^, H  decreases from 422.  ^ Oe to 2072 Oe as x vanes fioin') I 6, while in BaF'Ci, it decreases much faster and becomes 407 Oe lor x -  1 It is suggested that the site prefeiences of suhsiitulcdJunents w *aken the uniaxial anisotropy of the material which, in turn, leads to significant decrease in the coercivity valueKeywords Ferrites. Mossbauer spectra, magnetic recordingl'V<\S Nos. 7S .SO Gg. 70 80 -fy. 7.S 00 Hj1. IntrcKluctionM a ^ n c ltc  r e c o r d in g  t e c h n o l o g y  in  c o n j u n c t io n  w ith  sem iconductor e le c tr o n ic s  has con tr ib u ted  s ig n ifica n tly  to the gtowih o f  in ex p en s iv e  com p u ter p ow er and a u d io -v id eo  storage. Barium ferrite, a m aterial for perm anent m agn et and m icro w a v e  devices 11, 2], is a lso  a su itab le  cand idate for m agn etic  recording  aficT a su b stitu tion  o f  by d iv a len t and tctravalent e lem en ts  such as Co '^*’ and |3 j .  A  n um ber o f  stu d ies  have been  m ade  on C o-T i su b stitu tio n  in barium  ferrite prepared by d ifferen t rncihods su ch  as cera m ic  m eth o d , co -p rec ip ita tio n  etc. It has  been found that s ite  o c c u p a n c y  o f  the su b stitu ted  ca tion  at the c r y s ta llo g r a p h ic  s i t e s  o f  b a r iu m  fe r r ite  d e p e n d s  o n  th e  preparation m eth o d s [4-8J. F or the first tim e w e  h ave prepared  Co and C o -T i su b s t itu te d  b a r iu m  ferr ite  by a u to ig n il io n  com bustion  m eth o d  [9 ]. T h e  uniquenc.ss o f  the procc.ss lie s  in ibc in itiation  o f  c o m b u stio n  at lo w  tem perature and u se  o f  the beat liberated by exoth erm ic an ion ic  oxidation -reduction  reaction  betw een th e  c itra te  and  n itra te  io n s . In v e s t ig a t in g  th e  s ite  preferences o f  c o b a lt  and  c o b a ll- l ila n iu m  in barium  ferrite, prepared by a u to ig n itio n  m eth o d  and e f fe c t  on  the c o erc iv ity  to substitution, are the objectives o f  this w ork.Corresponding AuthorT he hariurn ferrite has m a g n cto p lu m b itc  siructure. T he unit ce ll c o n s is ts  o f  tw o  sp inal b lo ck s  (S  and S * ) and tw o  h exagon a l b lo c k s  c o n s is t in g  o f  barium  (R  and R *). 7 'hcse  b lo ck s  are arranged in the seq u en ce  RSK^S**", w here R * and S* can he obta ined  by a rotation o f  R and S b lo ck s  about the c ax is  though  1 S O '. T here ate fiv e  d istinct crysta llograph ic  sites 4fy^ , 2/?, 12A, 4fjy and 2a for m e ta llic  c a t io n s  f lO ). T h e cry sta llo g r a p h ic  properties and sp in s o f  th ese  s ite s  arc g iv en  in T able 1.Table 1. The crystallographic piopcrlics and spin characteristics of barium hexafemteSites Coordination No of ions per forimilu unitBlock Spin4/‘w octahedral 2 R Down3/> bipyramidal 1 R UpI2it octahedral 6 R-S U Ptetrahedral 2 S Down2 u octahedral 1 S UpT h e three s ite s  2 a , 4/^,, and 12^ h a v e  octahedral sym m etry  w h ereas 4fjy h as tttrahedral sy m m etry  and the sy m m etry  o f  2b is  in  b e tw e e n  te tr a h e d r a l an d  o c ta h e d r a l . T h e  m a g n e t ic  in teraction  is  d o m in a ted  by su p erex ch a n g e  in teraction  b etw een©2001 lACS406 Chandan Upadhyay, Davendra K um ar Rat, H  C  Venna a n d  J itendra K um arIrc^  ^ io n s  through the 180^ b on d s o f  T hesu b stitu tio n  o f  Co^’*' and in the ca tio n  s ite s  a lter th ese  s u p e r e x c h a n g c  in t e r a c t i o n s  a n d  h e n c e  th e  m a g n e t ic  ch aracter istics.T h e  ^^Fe M d ss b a u c r  s p e c t r o s c o p y  is  an id e a l to o l to  in v e s t ig a te  the s ite  o c c u p a n c y  in c o m p o u n d s  h a v in g  m etal ca tio n s at d ifferen t cry sta llo g ra p h ic  s ites . T h is is b eca u se  the  Fc n u cle i h a v in g  d ifferen t a to m ic  en v iro n m en ts g iv e  rise to  M o ssb a u e r  c o m p o n e n ts  w ith  ch a ra c ter istic  param eters and  th ese  co m p o n en ts  can  be separated  out by su itab le com p u ter  program s. In th is paper w c  present the resu lts o f  M ossb au er  and m a g n etic  m ea su rem en ts on sa m p les  o f  B a F c j2^ C o^O ,Q ,^2 and B a F e ^  varying from  0 to 1.6 and 0 to  1.0 rc.spcctivcly , in step s  o f  0.2.2. ExpeiimentalPure barium  ferrite (B a F e ,20,g) has b een  prepared by m ix in g  sto ich io m etr ic  prop ortion s o f  barium  and Icrric nitrates w ith  a q u e o u s  s o lu t io n  o f  c itr ic  a c id  (th e  pH  is  m a in ta in e d  by  cth yien ed iarn in e at 4 .0 )  and heating  at tem perature around 9 0 ”C  to  form  a g e l. D u e  to heat liberated  in the c x o th c m iic  reaction  that fo llo w s  the form ation  o f  ash , w h ich  is ca lc in a ted  at 90(TC ~  10(X)X' for 8 to 9  hours to p rod u ce the final product.For co b a lt su b stitu tio n , cob a lt nitrate w as added to the citric  acid  .solution and for cob a lF titan iu tn  su b stitu tion , co b a lt nitrate and titan ium  o x id e  h ave  b een  used .M a g n etic  m ea su rem en ts w ere carried  out u sin g  a vibrating  sa m p le  m agn etom eter . M o ssb a u er  spectra  w ere recorded  u sin g  a con stan t a cce lera tio n  M o ssb a u er  sp ectrom eter  w ith  a 5 -m C i ^^Co so u rce  in rhod ium  m atrix. A ll spectra  w ere  recorded  at room  tem p eratu re  and a n a ly z e d  u s in g  lea st sq u a res fittin g  program m s.3. Result and discussion3,1 Interpretation of Mossbauer spectra :M o ssb a u e r  sp ec tra  fo r  BaFcj^.^x d ifferen tam ounts o f  substitu tion  recorded  at room  tem perature w ere fitted  w ith  d isc r e te  s e x te t  su b p a ttern s  c o r r e sp o n d in g  to  the f iv e  cry sta llo g ra p h ic  ca tio n  s ite s  4^ ;.^ ., 4/;. ,^ 2a, 2b and \2k o f  pure  barium  ferrite. A d d in g  a sm all contribution  from  a sixth  sex tet in the spectra  o f  sub stitu ted  sa m p les  im p ro v es the fittin g . S o m e  o f  the typ ica l M d ssb a u cr  sp ectra  arc g iv e n  in F igure 1. T h e so lid  c ir c le s  d e n o te  th e  e x p e r im e n ta l  d a ta  p o in t s  a n d  th e  l in e  co rresp o n d s to  the com p u ter-g en era ted  data acco rd in g  to  the  M ossb au er  param eters ob ta in ed . T h e re la tiv e  spectral areas o f  sex te ts  corresp o n d in g  to  th e  cry sta llo g ra p h ic  s ite s  12k, and  2b are found  to  d e c r e a se , w h erea s th o se  for 4 / and 2a s ite s ^ vtremain almost constant with increase in Co-Ti substitution. The area of a sextet corresponds to the number of iron ions present at the respective crystallographic site. The decrease in the area of the sextets denotes the substitution o f Co and Ti ions to \2k4 /^  sites with a slight involvement of 2b site for small x. Theh y p erfin c  m a g n etic  f ie ld  (H M F ) at a g iv e n  s ite  can changi becau.se o f  n o n m a g n etic  su b stitu tio n  at n e ig h b o r in g  sites as w e ll as b eca u se  o f  any ch a n g e  in the bond  a n g le  due to crystalFigure 1. Mossbauer speetta o f BaFc,, samples at nuuntemperature, (a) \ =: 0.0, (h) \ = 0.6, (c) .r ~ 1 0.strains. T h e  H M F  at d ifferen t c r y sta llo g r a p h ic  s ite s  varies s lig h tly  as a fu n ctio n  o f  am ou n t o f  C o -T i substitu tion  The variation  is  sh o w n  in F ig u re  2. T h e s ig n ific a n t d ecrease m h y p e r fin e  f ie ld s  at 4/^^, 2a a n d  4/^^  s i te s  in d ic a te  that the su rro u n d in g s o f  Fe '^*' io n s  in th e se  s ite s  h a v e  ch a n g ed  ami p o ss ib ly  con ta in  n o n -m a g n ctic  ion  T h is  p rov id es furthei e v id e n c e  for su b stitu tion  o f  Fc^”*^ io n s at \2k and 2b sites. As stated  earlier, ad d ition  o f  a six th  sex te t w ith  m uch  reduced and»2a'4 F0}O.£r0.0 0.2 0.4 0.6Doping (x)0.8' 12k * 2b1.0Figure 2. Variation of hyperfine field H with doping x for difK*reni crystallographic sites for BaFc,j,^  •Site preferences fo r  cobalt and  cobalt-titanium  in substitu ted  barium  ferr ite s 407broad lin es im p ro v es  the fittin g  s ig n ifica n tly . S u ch  n ew  H M F  may com e from  the statistical probability  o f  several nonm agnetic  ions o cc u p y in g  the n e ig h b o rin g  s ite s  o f  particular iron sites. This r e su lts , in  r e d u c t io n  o f  h y p e r fm e  f ie ld  o f  that s ite  significantly. Z h o u  et al [4 ] h a v e  a lso  reported  n ew  H M F s on  C'o-Ti su b stitu tion  and in terpreted  as c o m in g  o f  due to ch a n g es  ,n the en v iro n m en t o f  iron io n s  at \2k s ite  w h en  su b stitu tion  takcj> p lace in R b lo ck s .The trend o f  re la tiv e  sp ectra l area and H M F  are not so  w e ll J c lin c d  w h e n  o n ly  C o  is  s u b s t i t u t e d .  W e h a v e  s tu d ie d  BaFcp.^Co^Ti^Oj<^ for 0 to 1.6 in step  o f  0.2. 1'or large am ountol substitution  (.v >  1.0) the re la tiv e  sp ectra l area o f  12A' and 2b siies d ecrea ses , sh o w in g  that C o  a lo n e  a lso  p refer  th ese  s ites .j  2 Effect on coercivity :The variation  o f  c o e r c iv ity  as a fu n ctio n  o f  co b a lt-tita n iu m  cuntent is sh o w in  in F igure 3 w e  se c  that there is a large decrease  in the co erc iv ity  v a lu e  from  4 2 2 3  O c for jr =  0  0  to 4 0 7  O c for .v =1.0. The variation  se e m s  to  fo l lo w  the ex p o n en tia l equation  =  H  ^exp  (“ OU) w h ere  is  the c o e r c iv ity  va lu e  for.v =  0 and a  is lound to be 2 .2 7  ±  0 ,0 6 .A c c o r d in g  to  A lb a n e s c  [ 5 ]  th e  o r ig in  o f  u n ia x ia l  m agnctcxrysta lline an iso tro p y  in BaFCj^Ojj^ lie s  in the iron ion s  of h ipyram idal ( 2b ) and o cta h ed ra l (4/^;, 2a and 12^ ) s ite s  l>clonging to R  b lo ck . In the present ca se , w e find cobalt-titan ium  prefers 12k, 2b and 4fjy s ite s . T h e rep lacem en t o f  iron in 12k and 2h sites w ill th erefore  g iv e  r ise  to  d ecrea se  in a n iso trop y  and hence co erc iv ity . In C o  su b stitu ted  sa m p le s  a lso  the c o e r c iv ity  has decreased  from  4 2 2 3  fo r  x  =  0  to  2 0 8 2  for  x =  1.6 but not so  >*iipidly as in the C o -T i su b stitu ted  ferrite. T h is  m ay be due to  tlic fact that the C o  i t s e lf  is  m a g n e tic  ion  and rep la cem en t o f  Fe  hy Co m ay n ot be as e f f e c t iv e  in red u cin g  the c o e r c iv ity  as the  nonm agnetic T i su b stitu tio n . ^ 3 Comparison with samples prepared through other routes :The sites o cc u p a n c y  and varia tion  o f  c o e r c iv ity  o f  B aFC j2.2x Co^Ti^Ojf^ stu d ied  in  the p resen t w ork  a g rees  by and large w ithth ose  prepared by other m eth o d s T u rilli e t [6| ex p la in ed  the  ch a n g es  in m a g n etic  order in M n -T i sub stitu ted  barium  ferrite by ceram ic  m eth od , a ssu m in g  preferentia l o ccu p a tio n  o f  the  cry sta llo g ra p h ic  site  12A: hy both  the Mn^'  ^ and n o n -m a g n etic  Ti"^"*^  ion s.A g r e s li et al \1] in tro d u ced  a lo w -tem p era tu re , am bient pressure cop recip ila tion  m ethod  to prepare BaFCj^ ^^Mo^I’l^Oj^. "Hie saturation  m a g n etiza tio n  w a s found  to  be ind ep en en t o f  loca l cry sta llin e  order and c o e r c iv ity  in the range o f  5 0 0 -1 4(K) O c. T h e se  w ork ers a lso  fou n d  a m o n o to n ic  d ecrea se  in the re la tive  in ten sity  o f  the \2k s ite s  in M o ssb a u er  spectra o f  C o- M o  substitu ted  sa m p les  su g g e s t in g  preferentia l o ccu p a n cy  o f  th is very lo ca tio n  w ith  little  d istortion  in the crystal .structure.? R ane ct al |S ) in their recent study on N i-Z r d op ed  sy stem  lilive a lso  slu)W'n that su b stitu tion  o f  Zr^^ o ccu rs at 4/^ ,^  and 2b c r y sta llo g r a p h ic  s ite s  and N r^  su b stitu te  Afy  s ite  for sm all v t lu c s  o f  d o p in g  but for larger d o p in g  it substitu te  the \2k site.Z h o u  et al 14J stu d ied  the M o ssb a u er  spectra  o f  C o , Ti substitu ted  sy s te m s sy n th es ized  by ceram ic  m eth od . T h ey  have  su g g e s te d  that and Ti^^ ca tio n s  p referab ly  o ccu p y  the 4f^ and 2b crysta llograph ic  s ites , in variance w ith  our interpretation  that su b sta n tia l su b s titu tio n  ta k es  p la ce  in \2k s ite . T h e se  authors a lso  o b serv ed  new^ H M F s w ith  m uch  reduced  va lu e, w h ich  they a ss ig n  to  I2A: s ite s  w ith  m o d ifie d  en v iro n m en t. A d d itio n a l ex p er im en ts  arc n eed ed  to  support th is as.scrtion.4. ConclusionB arium  fe n itc  sa m p les  d o p ed  w ith  C o , T i u sin g  au to ign ition  m eth od  have the sa m e crysta l structure as the und op ed  sy stem . T he sa m p les  prepared by a u to ig n itio n  m eth od  sh o w s that the substitu ted  e le m e n ts  prefer s ite  o ccu p a n cy  in \2k, 4 ^  and 2b. rh e  su b stitu tion  g iv e s  co n tro ls  to  vary co e r c iv ity  o v er  a lon g  range m aking th ese  sy s te m s attractive for m agn etic  app lication .R cfcrcncc.s[IJ D J Oc Hitetto J  Apt>! P h \ s  3.^  3482 (1‘^ M)[2] K Hancila, C Miyakawa am) K Goto lE ,h E  T ra n s  M af^n  MA<i-23 3134 (1987)[3] O Kuho, T Ido, H Yokoyaina and Y Koikc J  A p p l  Phws 57 4280 (1985)[41 X Z Zhou, A H Monsh, Z W Li and Y K Hong IE E E  ’T r a m  M a ffn  27 4654 (1991)[51 G AIbnesc J  D e  P h y s iq u e  38 CI-85 (1977)(61 G Tunlli, F Licci, S Rinaldi and A Denu J  M a fin . M a ^ n  M a te r  59 127 (1986)(71 D J Agresti, T D Shclicr, Y K Hong and Y J Paig I E E E  T u rn s  M a tin , 25 4069 (1989)[8] M V  Ranc, D Bahadur, A K Nigain, C M Sriva.stava J  M af*n  M a g n  M a te r . 192 288 (1999)[9] P Sujaia Devi and H S Maiti /  S o l id  S ta te  Chem 109 35 (1994) [lOJ J Smil and II P J Wijn, E e r r i t e s  (New York : John Wiley) p i80(1959)

Site Preferences for Cobalt and Cobalt-Titanium in Substituted Barium

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Site Preferences for Cobalt and Cobalt-Titanium in Substituted Barium

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