Effect of atomic environment on 57Fe hyperfine structure in FePt alloys

Journal of Magnetism and Magnetic Materials 104-107 (! 992) 2051-2052 North-Holland II

Effect of atomic environment on 57Fe hyperfine structure in F e - P t alloys T. Goto, H. Utsugi and A. Kashiwakura Faculb, of Engineering, Tohoku Gakuin Unieersit).', Tagajo 985, Japan

57Fe M6ssbauer spectroscopy has been made on the Fe-Pt alloys with a CuAul-type structure. The concentratien dependence of the magnetic hyperfine field, the quadrupole shift and ~he profile of absorption lines is explained from a model based on the effect of atomic environment on the hyperfine structure.

In F e - P t binary system, a tetragonal CuAuI-type structure exists in the concentration range from 38 to 65 at% Fe. This system exhibits an excellent hard-magnetic property that depends strongly on the concentration and the degree of crystalline order of the alloy

[t,21.

In the present study, the 57Fe M6ssbauer effect study is made on the well ordered F e - P t alloys containing 50-64 at% Fe in order to investigate the effect of atomic environment on the 57Fe hyperfine structure which may be concerned with the excess iron atoms substituted in Fe sites. The samples were prepared by arc melting of 99.9% Fe and Pt, and then filing and finaly annealing at 750°C for 50 h. The M6ssbauer spectra at 295 K are shown in fig. 1. The spectra are split into t~o sextets except tor the 50% Fe alloy. The sextets with strong and weak intensities would correspond to the spectra of 57Fc o~ Fe. and Pt sites, respectively. The magnetic hypcrfinc field and quadrupole shift for each site estimated by separating two sextets are shown in fig. 2. The lines are asymmetric in width and height with respect to the center of a spectrum. This behavior may be caused by the distribution of magnetic hyperfine field and quadrupole shift. The model proposed by Billard et al., which is based on the effect of atomic environment on hyperfine structure, is considered to be useful to understand the present spectra [3]. According to their model, the quadrupole shift A is defined using a structure factor S by

giver central iron atom. The effective magnetic hyperfine field is defined by

H = H o + hl(n

-

where H o, h~ and h 2 are constants. The second arid third terms of H express the field which depend on tl-c number n of nearest neighbor iron atoms, and on their spatial disposition, respectively. ( n ) is the mean number of nearest neighbor iron atoms and the value in th.~ disordered state of 64 at% Fe alloy ( = 7.6) is chosen as (n). The simulation spectrum f(c), which is the surl ot the Fe-site (A-site) and Pt-site (B-site) spectra, i+

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Fig. 1.57Fe M6ssbauer spectra of Fe-Pt alloys in the highly ordered state of CuAul-t3,pe structure at 295 K.

0312-8853/92/$05.00 © 1992 - Elsevier Scienze Publish'~'s B.V. All rights rese~'ed

T. Goto et al. / STFehyperfine structure of Fe-Pt alloys

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I I 55 60 ot *l, Fe Fig. 2. Co~centration dependence of magnetic hypert'ine field ( H * ) and quadrupole shift ( A * ) for 57Fe on Fe and Pt sites.

Dotted lines indicate the values estimated from the sirtmlation spectra.

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Fig. 3. Simulated M6ssbauer spectra of ordered Fe-Pt alloys. calculated as a function of Doppler velocity v for given concentration x, and order parameter r a, where the molecular formula of the alloy is defined by Fel+ x P t l _ , and r n is the probability that a Fe site is occupied by an iron atom. Now, the spectrum is given by

f ( v ) = r a Y A ( V ) + (l + X - r a ) f B ( v fK(")= ,,, =

6 EI, ,=,

),

(3)

PKA(nKA) PK.(nKB) E

E

l+[(,,-,,,)/r]:

n , , + h I ( , , - <,,>) +

h S] +_wS,

,

(4) (5)

where nl
quadrupole shift (0.16 m m / s ) of the 50 at% Fe alloy, and S = - 4 . The simulation spectra calculated for r A = I are shown in fig. 3, where h l = - I kOe is adopted for a suitable value, and hence h 2 is determined to be 12.1 kOe from eq. (2) using 280 kOe of the hyperfine field in 50 at% Fe alloy. The simulation spectra express well the characteristics of the observed spectra such as the feature of splitting, the asymmetry in intensity and their concentration dependence. The magnetic hyperfine fields and quadrupole shifts estimated from the approximate peak positions of the simulation spectra are also shown in fig. 2 by dotted iincs The results express well thc concentration dependence of the hyperfine field, the quadrupole shift and the sign of quadrupole shift. A large value of h 2 would mean that the spatial disposition of atoms has a strong effect on the transfered hyperfine field in F e - P t alloys. References

PAA(r, AA) = 4C,,AAr~x'XA(1 -- rA) 4-nAA,

(6)

where mC,, is the binomial coefficient. In calculating S, the magnetic field is assumed to be along the c axis according to the result of neutron diffraction [4]. The field of 325 kOe, which is the value in the disordered alloy of 64 at% Fe obtained by filing, is taken as Ho. The value of w is determined to be - 0 . 0 4 from the

[1] K. Watanabe, J. Jpn. Inst. Met. 29 (1988) 80. [2] K. Watanabe and H. Masumoto, J. Jpn. Inst. Met. 47 (1983) 699. [3] L. Billard and A. Chamberod, Solid State Comunn. 17 (lo.~

1•1 , 3 .

[4] A.Z. Menshikov, Yu.A. Dorofeyev, V.A. Kazantsev and S.K Sidrov, Fiz. Met. & Meta!loved. 38 (1974) 505.