Conversion electron Mossbauer spectra of composition modulated FeSi thin films have been analysed within the framework of a quasi shape independent model in which the distribution function for the hyperfine fields is assumed to be given by a binomial distribution. Both the hyperfine field and the hyperfine field distribution depend on the modulation characteristic length

Briones Fernández-Pola, Fernando

Labarta, Amílcar

Martínez Benjamin, Joan Josep

Obradors, Xavier

Ruiz, A.

Tejada Palacios, Javier

IEEE Transactions on Magnetics

English

Diposit Digital de la Universitat de Barcelona

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 5, SEPTEMBER 1987 3581 MOSSBAUER  STUDIES  OF  AMORPHOUS FeSi COMPOSITIONALLY MODULATED THIN  FILMS B. Martinez, A. Ruizt, A. Labartat t ,  X. Obradors**, F. Brionest and J .  Tejada** ICMAB, C.S.I.C., c /  Marti i Franquhs, s /n .  Apartado  de  Correos 30102.08028 BARCELONA. * CNM - C.S.I.C., c/  Serrano, 144  28006 MADRID tt Facultad de Fisica, U. de Barcelona, Diagonal, 647, 08028 BARCELOBA i2An Bbstract Conversion  electron  Rossbauer  spectra of composition modulated FeSi thin  f i lms have been analysed  within  the framework of a quasi  shape  independent model in  which the  distribution  function  for  the  hyperfine  fields is assumed t o  , b e  given by a binomial distribution. Both the hyperfine  field  and  the  yperfine  field  distribution depend on the modulation characterist ic length.  Introduction Thin solid fi lms are of act ive scient i f ic  interest  as well as of great  technological  importance  and  considerable theoretical  nd  experimental works t o  understand  the electronic  and  magnetic structures of surfaces  and interfaces  have been undertaken in  recent  years'"'z). Several Mossbauer works"-".' have been carried  out  with the ' a im to  study  the  iron  "in  contact with"  diverse material by using the hyperfine magnetic fields as a test for   the  interfacial   effects .  The main conclusions af the  Xossbauer s tud ies   r fe r   tha t   the moment of the "interfacial" Fe is different  from the  bulk  value  and depends on the coating material":'. - Our FeSi  composition modulated thin  f i lms were prepared in  a special  triode-sputtering  system  onto  glass substrates held at  room temperature.  Films were grown by codeposition from two independently  polarized Fe and S i  cathodes The resulting  films were magnetically s o f t  with  an induced uniaxial  anisotropy  axis  in  their  plane, which was deduced from transverse  biased  init ial  suscept ibi l i ty  (TBIS) measurements In t h i s  work w e  have studied  single  layers of an amorphous alloy of composition Fe76Sizs with different thicknesses separated by amorphous s ingle   layers  of Si. The total   thicknesses of the  samples, of about l O O O A ,  were determined by using a Tolonsky interferometer. The amorphous, character of the  films was determined by  X-ray diffraction. By means of low angle X-ray scat ter ing we have tested the modulating structure of the  multilayers. X-ray results agree well with the A values predicted by the preparation method. In Table I we sumarize the thickness and composition of the samples. Table  I:  Thickness  and composition of the  different  samples I F e 7 5 S i 2 5  s i n g l e  Amorphous Si s i n g l e  Charac te r i s t ic  layer  thicknesses  layer  thicknesses  modulation (a)  (a) length X ( A )  4.4 8.8 17.6 35.2 2.2  6.6 4.4 13.2 8.8  26.4 17.6  52.8 The electron  conversion Mossbauer s tudies  have been carried  out by using a home  made acetone  gas  detector The single  l ine  source was 20 mCi of  s'CoRh. A l l  the  spectra were recorded a t  room temperature  and  the experimental  rrangement is tha t   the  gamma rays are perpendicular to  the substrate  plane.  Because of in amorphous systems a wide dis t r ibut ion i n  local neigbourhood of the  iron  atoms  exists  a  a consequence of both chemical and structural  disorder,  the I l -10 I 0 VELOCITY ("IS) 10 Figure 1: Spectra of Fe.76 Si.%= modulated s t ructures   for   dif ferent  A values. Mossbauer spectra  of our  sample show 6 broad  partially overlapped  lines (see Figure 1). That means tha t  in  order  t o   g e t  a correct  information  regarding  thelocal surroundings of the  iron  atoms we have t o  do further assumptions. 0018-9464/87/0900-3581$01.0001987 IEEE Authorized licensed use limited to: Universitat de Barcelona. Downloaded on February 17, 2009 at 08:12 from IEEE Xplore.  Restrictions apply.3582 For the  analysis  of the Xossbauer spectra we have assumed that :  1) the distribution functions for the isomer s h i f t  and  quadrupole  interaction  are s o  narrow compared with the distribution function for the magnetic hyperfine f ie ld  (P(Il) 1 that  they  can be approximated  with 6- functions, 2) P(H) is given by a binomial distribution": P(X,n) = xn (1 - X)'+' (1) with H(n)= HDtnAH. We have fixed the number of f i e lds  to  21  and  consequently 2=20 and n varies between 0 and  z. Furthermore we have assumed tha t  the  in tens i ty  ra t io  for  a l l  the  samples is 3:4:1, as indicate  magnetic m e a s ~ r e m e n t s ~ - ~  > .  The least-squares fitting procedure run to  f ind  the  optimun values of the  shape-parameter  x,  the internal  width AH, the  lower limit H e  and  the  line  width of the  basic  sextuplet4'.  We have also  introduced a pure quadrupole  doublet in  order  to  take account for   the  existence of non magnetic Fe atoms. In Table I1 we summarize the  yperfine  parameters obtained from the  above  exposed f i t  procedure  and  in Figure 2 w e  show the  volution of the  result ing P(H) distributions  with thencrease of the  modulation characterist ic length A . 0.2 P ( H )  0 . 1  H (KOe 1 Fig. 2. Binomial dis t r ibut ion of the hyperfine fields. Discussion The fac t  tha t  the  P(H) functions corresponding to samples having A < 6 t  have practically  the same values  for  the mean < H >  and  width  allow u s  to   in te rpre t  them as due mainly to   interfacial   i rons.  Hence we assume tha t   the  interface has an extension of about 6 A  in  agreement  with the  results  obtained by Van Poort et  al.'.' in   the Cu-Fe compositionally modulated thin fi lms. The  P(H) distributions  for  the  samples  having A > 6 t  have been deconvoluted  in two P(H) d is t r ibut ions.  One  of them has the H ~ T  dist r ibut ion at t r ibuted to  interfacial  effects  and the second one corresponds t o  t h e  iron located a t  the bulk of the amorphous  Fe76SiZ6 single layers,  The area of t h i s  second  istribution  increases when increases  the thickness of the amourphous Fe 75 Si25 single layers,  as it is shown in Figure 4. The magnitudes of the hyperfine fields of the interfacial  irons are smaller than those of the irons located at  the bulk which agrees  well   with  the  fact   that   the  iron magnetic moment decreases when increases  the number  of S i   f i r s t  and second neighbours. 29 23C 210 / /*/ /. / /' , / / Increasing  the  characteristic  modulation  length, A, the  P(H) distribution of the "bulk" iron moves to  higher fields  (see  Figures 2  and 4 ) .  This may not be at t r ibuted to   var ia t ion  in   the magnetic moments but t o  an  increase of the exchange interactions  and  consequently  in  the Curie ternperatureslo'. Authorized licensed use limited to: Universitat de Barcelona. Downloaded on February 17, 2009 at 08:12 from IEEE Xplore.  Restrictions apply.3583 . 2  t I SAMPLE 3 . 2  I SAMPLE 4 7.4 Figure 4: deconvolution of hyperfine  field  istribution for  sample 3 and 4.  hsehd  l ines   correspond to dis t r ibut ions of Figure 2. Table 111. Hyperfine parameters of the quadrupole doublet Sample Isomer s h i f t  ( m m / s )  Q S .  ( m m / s )  Intensity 1 0.28(2) -1.58  (2 ) 7990 2 0.21 (1) -1.28(1) 8250 3 0.16(1) -1.52 (1) 9920 4 0.18(1) -0.73(1) 24270 (arbi t rary uni ts)  Ve thanks  the CAICYT for  f inancial   support ,   contract  number 916184 RGFEREACBS 1) B,K. Flevaris, J.B. Ketterson and J.E. Hilliard. J. Appl. Phys. 53  (1983)  80-46 2) G. Marchal, P.H. Mangin and Chr. Janat Solid  State Commun.18 (19761739-742 3) B.K. Jaggi, L.H. Schwartz, H.K. Wong and J.B. Ketterson. J, Magn.  Magn. Hat. 49 (1985)  1-4. 4) H.W. Van Boort, F.J.A. den Broeder and H.J.D. Draaisma J .  Wagn.  Wagn. Hat. 51(1985)  273-279. 5) C.B. Afonso, A.R. Lagunas, F. Briones and S. Gir6n. J .  Magn.  Magn. Mat. 15-18  (1980)  833-835. 6) J.M. Alameda, M.C. Contreras, W .  Torres and A.  Gonzalez Arche. J. Magn.  Wagn. Mat. 62 (1986)  215-220 7) I. Vincze, Solid State Commun. 25(1978) 689. 8) J.M. Alameda, M.C. Contreras and H. Rubio Phys. S ta t .  Sol.(a),85(1984)511 9) G. Marchal, Ph. . h u g i n ,  X. Piecuch, Chr .  k n o t  and J. Hubsch. J. Phys. F. Vol. 7, B. 6(1977) 10) 3. Kwo, E.W. Gyorgy, D.B. McVhan, 1. Hong, F.J. Disalvo C. Vettier and J.E. Bower, Phys. Rev. Lett. 55, i985)402. 11) B.S. Kazama and H. Fujimori Proceedings of the Thin films conference. Asilomar, California. U.S.A. (1985) The intensi ty  of the quadrupole  doublet  introduced t o  f i t  the  spectra  increases  with  t e  thickness of the  Si amorphous sublayers  (see  Table 111). This  effect  can  be explained  taking  into  account  he  increase of both,  the interface  region  and  the non magnetic Fe atoms present in the bulk as increasing the modulation length. Authorized licensed use limited to: Universitat de Barcelona. Downloaded on February 17, 2009 at 08:12 from IEEE Xplore.  Restrictions apply.

Mossbauer studies of amorphous FeSi compositionally modulated thin films

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Mossbauer studies of amorphous FeSi compositionally modulated thin films

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