Das, Dipankar

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Indian J' P hvs.nx  (2). 185-188 (2004) Nanomaterials—2 0 0 3< U PMossbauer spectroscopy ; an excellent technique to characterize magneticmaterials of nanometer dimensionD ipankj|rDasIntel lliiiversily ('onsortium f o i  Dcpaftmcnt o f  Ati>mic Hneigy K i c i I h k s  LB 8, Sector III, Bidhannagtr, Kolkala-700 098, IndiaAbstract Effectiveness of MOsshauer spectroscopy in chaiacten/inj; nanonicinc magnetic maieiials aie highlighted with the spectia recordedat different temperatures for nanocrystalline cobalt femte, a~ Fc,0, and nanooornposilcs of Fe SiO, and Fc-Fe,()^ It is shown that in supcipaiamagnetic >,(.iie when the effective internal magnetic field at the probe nucleus is zero resulting in the collapse of the sextet palteni, the quadrupolai interaction can he used to identify the nanoparticles.Keywords Mdssbauer spectroscopy, superpaiamagnetisni, nanocrysiallitesPACS No. 76 801. IntroductionMossbauer sp ectroscop y  has b eco m e an essen tia l tool to characterize magnetic materials o f  nanom eter d im ension because of its ability to probe m icroscopically the im m ediate environment ot the nucleus through h y p erfin e  in teraction  nam ely  the interaction betw een the nuclear charge and the electrom agnetic  field generated by the extra-nuclear electrons. In the bulk or crysta lline s ta te , fe r r o m a g n e t ic ,  fe r r im a g n e tic  or an ti-  lerromagnetic c la sse s  o f  m aterials produce large internal magnetic field at the nucleus. This internal field  is m ainly created by the net .v-electron spin density g iv ing  rise to splitting of the nuclear energy  le v e ls  and a characteristic six -fin ger  M ossbauer sp ectrum . In the n a n o c r y s ta llin e  sta te , if  the  magnetic anisotropy energy o f  a crystallite w hich is proportional to the vo lum e o f  the cry sta llite  b eco m e s  sm aller  than the B()ltzman’s thermal energy at a particular temperature, the nci magnetic m om ent o f  the crystallite relaxes betw een the easy  axes o f m agnetization leading to superparam agnetism . If the frequency o f  such relaxation is faster than the Larmor precession  frequency o f  the nucleus ( - 10”  ^ sec), the average m agnetic  field at the nucleus b ecom es zero and the s ix -finger M ossbauer  pattern collapses to a doublet or sin g le t depending on whether the electric field  gradient at the nucleus is present or absent respectively. Thus, M ossbauer technique determ ines in a sim ple and stra igh tforw ard  w a y  th e  p r e s e n c e  o f  n a n o -p a r tic le s  undergoing superparam agnetic (S P M ) relaxation  in m agnetic rnaterials.In the present paper the e ffec tiv en ess o f  the M ossbauer  spectroscopy in characterizing the m agnetic nanoparticles are highlighted by taking specific exam ples i)f nanoparticles o f  cobalt ferrite and alpha ferric ox ide as w ell as nanocom posites o f  F'e- S iO t and F'e-Fe3 0 4 .2. Superparamagnetism and Mossbauer spectroscopyN anocrystalline m agnetic particles are often  sin g le  dom ain  particles. The m agnetic energy o f  such particles with uniaxial ani.sotropy is given  byft -  ^ '\/sin"l9where K =  ani.sotropy constant, V = volum e and 0 is the angle  b e tw e e n  th e  m a g n e tiz a t io n  d ir e c t io n  and e a s y  a x is  o f  magnetization.For extrem ely sm all particles, the energy barrier KV which  separates the tw o energy m inim a at (? =  0 ° and 0 =  1 8 0 ° , m ay be sm aller  than the therm al en ergy  (k^T ) ev en  at room  temperature. This leads to the spontaneous fluctuation o f  the m agnetization direction known as supeiparam agnetic relaxation  with frequency given  byT = To e x p ( K V / kffT)where Tq is the natural frequency o f  gyrom agnetic precession  and is o f  the order o f  constant.s and is Boltzm anns©20041ACS186 Dipankar DasSuperparam agnetic behavior m ay be ob served  using a technique with characteristic tim e ( T ^ , at temperatures above the so called  b locking temperature (T/,), defined  asT„=^KV/k, \n(rJr^)A b o v e  the b lock in g  tem perature, the relaxation  frequency  T » T ^  s for M bssbaucr spectroscopy) and theinternal m agnetic field  at the nucleus is averaged out g iving rise to usually a doublet M ossbauer pattern.B elow  the bliK'king temperature, r  «  and a characteristic M ossbauer sextet pattern is ovserved.W h en , a partially collapsed  sextet is observed.3. Nanocrystalline cobalt ferriteNanocrystalline cobalt ferrite .samples with average size ranging from 6  nm to 2 0  nm had been prepared at room temperature by a chem ical route and characterized by M ossbauer spectroscopy11]. M ossbauer spectra recorded at room temperature o f  the sam ples with different crystallite sizes are show n in Figure I As seen  from Figure 1, the 6  nm sized  sam ple g ives a strong  quadrupole doublet with a weak partially collapsed  sextet. The d o u b le t  is  a s s ig n e d  to u ltr a fin e  p a r t ic le s  u n d e r g o in g  superparamagnetic relaxation for which the relaxation frequency at room temperature r  is much larger than The partially collapsed  sextet is due to larger crystallites in the sam ple with  T -  , A nother point to be noted is the large quadrupolesplitting (Q S) 0 .76  m m /s o f  the doublet. In the bulk form, cobalt1 00-0 95-6nmFigure 1. M6.ssbaucr spectra at room temperature of C'oFejO^ samples with size 6 nm. 12 nm and 30 nm.ferrite being a cub ic sp inel [2] g iv es  n eg lig ib ly  sm all QS tort,, atom s in tetrahedral (A ) sites and Q S o f  the order o f  0 .40 mm/s for Fe atom s in octahedral (B ) sites. In nanocrystallm e state number o f  atom s in the grain boundaries b ecom es significant because o f  the large surface to vo lu m e ratio. These atoms experience large electr ic  field  gradient (EFG ) due to surface irregularities g iv in g  rise to large quadrupole splitting. Fnr 12 nm sized  particle the sextet pattern b ecom es prominent at the co .sto f the doublet b ecau se the num ber o f  large crystallites m the sam ple is exp ected  to be m ore with the increase o f  averau^ * crystallite size. Spectrum  o f  30  nm sized  particle show s a well defined sextet indicating that all the atom s experience static hyperfm e field  characteristic o f  a m agnetically  ordered stateFigure 2 sh ow s spectra o f  6  nm particles as a function oi temperature. At 150 K , the doublet seen  at room  temperature disappears sh ow ing  a sextet pattern w hich  b ecom es sharp and well define as the temperature is low ered to 4 0  K. With lowennc o f  temperature the superparam agnetic relaxation frequency i o f  nanocry.stallitcs b ecom es s low er and at temperatures when it becom es far less than a sextet pattern ev o lv es .1 000 95 -sR[Figure 2. M ossbauer spectra o f 6nm C oFc204 sample at different temperatures.4. Nanocrystalline a-Fc2 0 3N anocrystalline a -F e 2 0 3  w ith crystallite s ize  ranging from 17 nm to 55 nm have been  prepared u sing  a so l-g e l route (3| M ossbauer spectra recorded at room  tem perature o f  the sampleJ' with d ifferent s iz es  are sh ow n  in F igure 3, Spectrum  o f the sam ple with an average size  o f  17 nm show s a broad asymmetric sextet superposed on  a doublet. T he spectrum  w as fitted withMossbauer spectroscopy : an excellent technique to characterize maf^netic etc 187three symmetric sextets and a doublet. T he sharp sextet with fractional area 19% and isom er shift (IS) 0 .52  m m /s, quadrupole splitting (Q^>) 0-20  m m /s and internal fie ld  513 kOe is itiributed to crystalline com ponent present in the sam ple. The second sextet with 14% fractional area and larger QS (0.45 m m / s) and reduced //}„, (492  kO e ) in com p arison  w ith that o f  crystalline com ponent is attributed to finer particles w ithin the nanocrystallites experiencing enhanced EFG due to lattice strain 14 ]. The third sextet (fractional area 49% ) with very broad lines and larger QS (0 .62 m m /s) and very much reduced (447 kOe) compared with that o f  c ry sta llin e  part is  attributed to the interlacial atoms situated on the grain boundaries. The central Joublet with fractional area o f  18%> is assigned to single domain  nanocrystals undergoing SPM  relaxation. With the increa.se o f  particle size up to 2 2  nm the intensity o f  the crystalline part was iound to in crease at the c o s t  o f  the in terfacia l and SPM  tractions which clearly indicates the grain growth resulting in formation o f  crystallites o f  larger average size . B eyond  30 nm a magnetically ordered sextet is observed  with no signature o f  SPM fraction.‘12velocity (mm/sec)Hgure 3. Mttssbaucr spectra at room temperature of a-Fc20  ^ particles ot tliHerent sizx.Nanocomposites of Fe-SiOj^anocomposites o f  F e -S i0 2  average s iz e  varying from 1 .8  to 9.5 nm w ere characterized by M 5ssbauer spectroscopy  f‘^1 The sam ples w ere prepared by a so l-g e l technique. Figure 4  '^hows MOssbauer spectra o f  the sam ple w ith  average size  1 .8  A strong central doublet superposed  on a d istinct sextet isseen at room temperature. T he sextet with hyperfine field 337  kO e w as attributed to Fe particles for w hich  the b lock in g  tem perature is a b o v e  the room  tem p era tu re . T h e s lig h t  enhancem ent o f  the com pared with the bulk value (330  kOc)is signature o f  the ultrafine nature o f  the sam ple as has been reported by others [61. T he central d oub let is a ssign ed  to jparticles undergoing SPM  relaxation. The large QS (0 .69  m m /s) O f the doublet indicates loss o f  cub ic sym m etry at the Fe site because o f  surface effects [7 |. At 115 K , the intensity o f  the iex te t was found \o increase at the cost o f  the doublet w hich  iponfirms the assignm ent o f  the doublet to SPM  relaxation.100Ifo(/)XDTO0)>TO^  100\ i  I :  ’! ; I iil I;■ /’ ! I ; '•!RT97-A /I • i / \ I i i ! II I I II ' ! < 1115K-8 -2 0 2 4 6velocity (mm/sec)8 10F ig u re  4. MOsshducf speclra oi 1 8 urn fx -S i()2 nanocornposiicFor 3.7 nm and 9.5 nm sized  particles sim ilar M ossbauer  pattern as show n in F igure 4 were observed with gradual reduction o f  intensity o f  SPM  fraction as expected .12 6. Nanocomposites of Fe-FeN anocom posites o f  F c -F e i0 4  having d im ension  in the range 1 0 - 2 0  nm were prepared by a sim ple reduction m ethod starting with m icrom eter sized alpha ferric o x id e  [8 ]. Figure 5 sh ow s a typical spectrum recorded for particles with average size  o f  17 nm. As can be seen  from the figure the spectrum could be fitted  with three sextets and a doublet. The sharp sextet with a lm ost zero isom er shift and 330  kOe has been identified as due to alpha iron. The sextets with //,,^t495 kO e and 463  kOe have been  assigned to A site and B site o f  FC3O4 respectively. T hese are due to larger crystallites in the sam ple with blocking temperature above the room temperature. The doublet is due to SPM  particles188 Dipankar Dasof alpha Fe and/or F e ^ 4. At 70 K the doublet almost disappeared confirming its assignment as due to SPM particles.7. ConclusionFrom the examples as given above it is clear that Mossbaucr spectroscopy can be effectively  applied to characterize; nanometric magnetic particles. The MOssbauer spectra recorded at various temperatures can be used to confirm the exisienccM* nanocrystallites in a sample. In the superparamagnetic siatt the quadrupole splitting gives information about the aioi]i, located near the grain boundaries.Referencesi n  M Rajcnclran, R ( ' Pullar, A K.Bhaltacharya, D Das, s \ ('hintalupudi and C K Majumdar J .  M ag. M ag. Materials 2^ 1 i (2001)(2 1 M osshauer spertroseop\ A p p lied  to Inorganic Chem istry (cd (, i Long ar»d F Grandjean). Plenum Press, vol. 3 C'hap. 3 (l<)H9i[3] B K Nath, wS Bandyopadliyay. A Pandey and D Das P oh (>,yi So lu i State Pfiysivs (India) 44 95 (2001)|4] S J Campbell and H Olciter in M ossh auer Spectm scopsto M a g n e tism  a n d  M a te r ia ls  S c ie n c e  (ed G J Long i G randjean) Plenum Press vol. 1 (1993)[5] A Chatterjee, D Das and D Chakravorly A p p l Pliws Lvu 5" 1360 (1990)|6 | I Taniura and M Hayashi ./. M ag. M ag M ater. 3J-34 945 .[7 j T  Pannaparayil and L N Mulay J  A ppl. P h w  57 3S2(>ISl P Brahama, S Banerjee, D Das, P K Mokhopadhyay, S ( luttuiu A K Nigam and D Chaktavorty J  M ag M ag M a in  246 Id’ ( 2002)

Mossbauer Spectroscopy : An Excellent Technique to Characterize Magnetic Materials of Nanometer Dimension

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Mossbauer Spectroscopy : An Excellent Technique to Characterize Magnetic Materials of Nanometer Dimension

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