MÖSSBAUER EFFECT ON Fe-Pd ALLOYS

Mossbauer effect has been observed for disordered and ordered Fel-,Pdx alloys. The internal field and the quadrupole and isomer shifts were obtained. The asymmetrical spectrum for 0.45 < x < 0.75 and the temperature dependence of isomer shifts for x = 0.32 were interpreted as a distribution of the quadrupole field and a second order Doppler effect respectively. With respect to the invar effect and the martensite X10.65 as well as order-disorder transformations for Fel-,Pd, disordered alloys, a number of studies has been reported mainly by us on the structural [I], magnetic [2-41 and thermal [5] properties. Particularly, we have found the martensite transformation concerning on a lattice change from fcc to fct at low temperature in x < 0.34. In this paper, from a microscopic standpoint, the Mossbauer effect on the whole ferromagnetic fcc phase will be presented to make clear roles of Fe atom in relation to the lattice 1 0 8 6 4 -2 0 2 ' i ' 6 ' B q l b and ordering transformations. V (mmlsec) Disordered alloys, Fel-,Pd, (0.30 5 x 5 0.80) were Fig. of the 'pectrum for = perepared by quenching into ice water from 1 100 OC, 0.65. while ordered ones (0.50 5 x 5 0.75) by slow cooling with a rate of 50 OC per day from 1 000 OC. The fcc structure and the ordered CuAu and CusAu types were confirmed by X-ray diffraction for above samples. Mossbauer measurements were carried out in room temperature for all the samples and temperature variations were observed on x = 0.30 and x = 0.32 for the fcc-fct transformation. In the observed spectra of disordered Fel-,Pd,, the quadrupole shift is given to be zero and the isomer shift is not so large (0.01 to 0.03 mm/s) at toom temperature. While a remarkable quadrupole shift, 0.2 mm/s, was observed in the ordered FePd. It is noted that, as shown in figure 1, the spectra for 0.45 5 x 5 0.75 show a remarkable asymmetric behavior for its width and height of the six absorption lines. One the other hand, an appreciable temperature variation of the central shift between 15 and 300 K was observed on x = 0.30 and 0.32, which will be explained below. The spectra were analysed by the following usual expression. s (v) = f: /"= Ai P (H) dH i=l 0 1 + ((v (Y;H 6c9 f eqQ) / 7j2 ,. . ure 2 is obtained from the over all splitting of the spectrum, which coincides with H = Ho value at the peak of P (H) . A linear relation of Hi, (2) on x > 0.45 can be expressed by the Wertheim formula, but that for x < 0.45 is interpreted as a change of coefficients of the formula with the concentration. The half width, AH (x) , for the P (H) peak showns a parabolic behavior in figure 2, suggesting a binomial distribution of Fe (1) where the notations are standard and T = 0.14 mm/s is used to fit the observed spectra. The distribution Fig. 2. Concentration dependence of the internal field function of internal field, P (HI , is calculated from the half with AH of (H) and the correlation term a histogram method. The internal fie1d (x) in figQm in the quadrupole effect for Fel-,Pd, alloys. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988854 C8 136 JOURNAL DE PHYSIQUE atoms in the disordered state. For the ordered alloys, AH takes 113 and 112 value for FePd and FePd3 respectively as compared with the disordered ones. In order to explain the asymmetric behavior shown above, the following expression is offered as a correlation between the internal field and the quadrupole shift, with Ho A H 5 H 5 Ho + AH, where HO is the internal field at the maximum of P (H) and Qo is an ordinal quadrupole term and Qo = 0 for the perfect disordered alloys. The term Qm is attributable to a &polar magnetic field due to a local ordering for the neighboring Fe atoms [TI. Thus, as shown in figure 2, Qm shows a maximum at x 0.65 corresponding to the maximum of the order-disorder transition point in the equilibrium phase diagram, where the local ordering remains appreciably. As for the martensite transformation, fcc to fct, though some experiments revealed as an anomaly of the second order transition [l, 3, 41, the thermal expansion and the electrical resistivity did not show any anomaly at the transition. Then let us reconfirm this by means of the Mossbauer effect on the sample x = 0.32 with the transition at Tt = 188 K. The temperature dependence of the internal field, Hi, (T) , and the half width, AH (T) , are shown in figure 3, where a small depression for Hi, and an inflection for AH can be seen at Tt. On the other hand, the central shift of the spectra for x = 0.30 and 0.32 shows a remarkable temperature dependence, which usually consists of the isomer shift 61s and the second order Doppler (SOD) shift, viz. For all Fel-,Pd, alloys, the 61s observed at room temperature ( 6 ~ 0 ~ = 0) is very small, so its temperature variation can be neglected. While the SOD shift is a difference of SOD shift between the source kept at room temperature and the absorber: &OD (T) = (R.T.) S$OD (T) (4) where each 6goD and 6 h D is expressed as follows by the Debye temperature OD, the averaged atomic mass m and the light velocity c. Therefore, we can determine the SOD shift 6iOD of the absober from the observed SCs (T) , where the Debye temperature at 0 K is quoted from our specific heat measurement [5]. The remarkable anomaly at Tt in beoD (T) , as shown in figure 3, means an effect of lattice softening due to the temperature variation of OD for this marensite transition. Fig. 3. -Temperature dependence of the internal field Hi,, the half width AH and the second order doppler shift btoD for Feo.6sPdo.32. [l] Matsui, M., Yamada, H. and Adachi, K., J. Phys. Soc. Jpn 48 (1980) 2161. [2] Kussmann, A. and Jessen, K., 2. Metallkd. 54 (1963) 504. [3] Matsui, M., Shimizu, T. and Adachi, K., Physica 119B+C (1983) 84. [4] Matsui, M., Kuang, J. P. and Adachi, K., J. Magn. Magn. Mater. 54-57 (1986) 912. [5] Kuang, J. P., Kontani, M., Matsui, M. and Adachi, K., Physica 149B+C (1988) 209. [6] For example, Kolk, S., Studies of Dynamical properties of Solid with the Mossbauer Effect (NorthHolland) 1984, p. 26. [7] Billard, L. and Chamberod, A., SoEd State Con mun. 17 (1975) 113.