MAGNETIC AND MÖSSBAUER INVESTIGATION OF AMORPHOUS Fe100-y-xCryBx ALLOYS

(1)

HAL Id: jpa-00220272

https://hal.archives-ouvertes.fr/jpa-00220272

Submitted on 1 Jan 1980

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MAGNETIC AND MÖSSBAUER INVESTIGATION OF AMORPHOUS Fe100-y-xCryBx ALLOYS

S. Dey, U. Gorres, H. Nielsen, M. Rosenberg, M. Sostarich

To cite this version:

S. Dey, U. Gorres, H. Nielsen, M. Rosenberg, M. Sostarich. MAGNETIC AND MÖSSBAUER IN- VESTIGATION OF AMORPHOUS Fe100-y-xCryBx ALLOYS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-678-C8-681. �10.1051/jphyscol:19808170�. �jpa-00220272�

(2)

JOURNAL DE PHYSIQUE CoZZoque C8, suppZ6ment au n08, Tome 41, aoct 1980, pageC8-678

MAGNETIC AND MOSSBAUER INVESTIGATION OF AMORPHOUS FelOO-y-xCryB, ALLOYS

S . Dey, U. ~orres*, H . J . V . ~ielsen**, M. Rosenberg and M. ~ostarich*

Institut fiir ExpenentaZphysik VI

*~nstitut fiir Werkstoffe der EZektrotechnik, Ruhr-Universitdt, 0-4630 B o c h m , R.F.A.

* * ~ e ~ a r t m e n t of EZectrophysics, TechnicaZ University, DK-2800 Lyngby, Denmark.

Abstract.- The saturation magnetization and the G s s b a u e r spectra of amorphous Fe Cr B have been measured, in order to determine the stifness constants of spin-waves andO!ii~-x *

hyperfine field distributions.

~ r o d u c t i o n rimental data to broad Gauss lines (disper Magnetic and Mossbauer studies of ferromg sion of about 14 % of the average hyper- gnetic amorphous alloys of 3d-metals have fine field) was used and the peak values contributed to a better understanding of were determined. Some degree of asymmetry the influence of electron transfer on the of the lines was observed in the Cr-rich magnetic moment and exchange interaction. compositions, but not analysed so far.

In the case of Cr substitution in FeX a-

morphous matrices (x=B,P) strong devia- Results and discussion

tions from the Slater-Pauling curve were Fig. 1 shows the reduced magnetization previously reported 1

.

The present study M(T)/M(o) versus the reduced temperature was undertaken in order to investigate T/Tc. As for most amorphous ferromagnets the influence of increasing Cr substitu- the curves are "flatterN than expected tion in FeB amorphous alloys on the stiff from the molecular field theory. This fas- ness of the spin-waves, on the average mg ter decrease of M(T)/M(o) with temperature ment and on the hyperfine interactions.

E m erimental

Amorphous ribbons of the compositions Feg5-xCr5Bx, with x = 15, 20, 25 and Fe85-yCr$1 5' with y = 5, 10, 15, 20, were prepared by melt-spinning as descri- bed elsewhere2. Both X-ray diffraction and MBssbauer spectroscopy confirmed the amorphous state of the ribbons. Magneti- zation measurements were made by a vibra-

ting sample magnetometer in fields of .I I

0 0.2 QL 0.6 a8 1.G Y

and 5.2 kOe. M6ssbauer spectra were taken ^f^/Tc

using a 5 7 ~ 0 in Rh source. The lines are Fig. 1. Reduced magnetization M(T)/M(o) much broader than in the case of FeB and versus reduced temperature T/Tc for an FeNiB amorphous alloys. A fit of the exPg applied field of 1 kOe.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808170

(3)

Fig. 2. Reduced magnetization M(T)/M(O) vs T ~ / ~ . Applied field - ^{5.2 kOe.}

is attributed3 change integral

to fluctuations of the ex-

, J. As seen in Fig. 1 ,

however, too large fluctuations, 6 k 0.5, where 6 = (<a J~>)'/~/<J>, would have to be assumed in order to fit the experimental curves. The flatness of the measured curves decreases with increasing boron conceg

B alloys 4 , but tration, like for FelOO,x

increases with the addition of Cr. At low temperatures the magnetization can be des- cribed by the simple spin-wave approxima- tion

M(T) = M(0)(1 - B T ~ / ~ )

as seen in Fig. 2. Like for other amorphm ferromagnets, this low-temperature beha*

persists up to T/Tc 0.3 - 0.7, that is up to much higher temperatures than for crystalline systems. The experimental values of B are listed in Table I together with the values of the spin-wave stiffness constant D, calculated using the standard relation

B = 0.0587 [g ,uB/M(0)] (kB/~)3/2 where g was taken 2.05. For Fe95,xCr5Bx alloys D and D/T, increase with x like for

B alloys in the same concentration FelOO-x x

range 5 . For the Fe85,yCryB15 alloys D is decreasing with increasing y, indicating a

"softening" of the exchange interaction by Cr substitution. However, as the Curie tern perature decreases even faster with y, the ratio D/Tc increases, which could be taken as an evidence for an increase of the

Table I. Spin-wave and magnetization parameters

- - -

B D D/Tc 'exp 'a u~

Compound Tc (K) ( ~o-~K-~'~) (mevA2) (rnevA2~-' ) (uB/TM at) (Y B/TM at) (uB/TM at)

a - ^{Ref.2, b}- present work, c - ^Ref.5.

(4)

C8-680 ^JOURNAL^DE^PHYSIQUE

range of the exchange interaction with in- x , a t % B

creasing chromium content. 0 5 10 15 20 25

The values o'f the average magnetic moment per transition metal atom ,ii, listed in

Table .I, show that ,iiis practically inde- pendent of x, while it drastically changes with y. The slope d,E/dy for 0 5 y = ¹⁰^is

-0.097pB/at% Cr, i.e. somewhat larger as the about -0.08 pg/at% Cr found for some (Fe, c ~ ) ~ ~ P ~ 3 ~ 8 alloys6. We tried to fit the experimental values of by two app- roaches. First we calculated the weighted average per formula unit, pa, attributing -

2 p B , -4 pB and 0 pB to the Fe, Cr and B atoms, respectively. A second set of values, p F , was obtained using Friedel's -

7 -

formula : p F= pmatrix - ⁽^A^Z⁺¹⁰⁾^cpB,

where pmatriX is the moment per Fe atom in the corresponding amorphous alloy with- out Cr, A Z = -2 and c is the Cr content in the transition metal. As seen in Table1

FF is systematically smaller than Fa, bo*

values, however, lie as a rule within 10 % from the experimental one.

The compositional dependence of the peak value of the Mossbauer hyperfine field, HF, at 6 and 77 K is shown in Fig. 3. As expected from the magnetic measurements, in Feg5,xCr5Bx alloys a relatively small decrease of HF with increasing x was observed. The values of HF are, due to the 5 % Cr, about 20 % smaller than for the correg ponding FelOO-xBx system8. The increase. of Cr content in Fe85-yCryB15 has a strong in fluence on HF. For y i 10, HF seems to decrease linearly with increasing y, whereas a sharper drop takes place for 10c y c 15 and the further increase to y = 20 does

Fig. 3. Concentration dependence of the hyperfine field (HF) at 6 and 77 K.

not change HF too severely. In the linear region a good fit to HF is: HF-(Fe) = (294 - 7y) kOe, as compared to HF in the

Cr crystalline alloys9: HFcrys(Fe) FelOO-y y

= (337 - 2.35~) kOe. The alternative plott of HF vs for Fe85-yCryBl alloys is quite similar to that vs y. In the linear region down to fizz1 pB, HF can be fitted to

HF(Fe) = (a pmatrix + b F ) kOe, where pmatrig 2 . 1 2 , ~ ~ and the coeffi-

cients are a = 69.9 and b = 68.8, as corn- pared to 118 f ⁸and 9

'

4, respectively, for the crystalline FelOO-y ~r alloys9.

One can see that the substitution with .Cr in the amorphous alloy has a much more dra_

matical influence on the decrease of HF at the Fe-sites than in the crystalline alloy. As compared, for instance, to the effect of Ni substitution in amorphous

(5)

FeB alloys 8 , the stronger decrease of HF Acknowledgements

with increasing Cr conkent suggests a sig- The authors are grateful to Professor nificant change of the band structure due E. Kneller for his support and helpful ^dis_

Fig. 4. Reduced hyperfine field vs T 3/2 for Fe80Cr5B15 and Fe70Cr15B15 alloys.

to the presence of chromium, with changes of the magnetic moments of both Fe and Cr, rather than a simple antiparallel orienta- tion of the 4,uB chromium moments against the 2 ,uB iron moments.

In Fig. 4 the reduced HF(Fe) is plotted vs T ~ for Fe80CrrB15 and Fe70Crl 5B, / ~ 5. The observed linear dependence as in the case of M(T)/M(O) leads to coefficients B of 6.5-10'~ K'~/~ and 24.10-~ K'~/~, respectively, which,compare reasonably with the ones given in Table I. This is an evidence for the fact, that the temperature dependence of the average moments of Cr and of Fe is.the same, i.e. determined by the

cussions, to P. Deppe and F. Kleimann for -

their assistance in the Mdssbauer experi- ments and to B. Liipkes and B. Miiller for technical assistance.

References

1

.

^T.^Mizoguchi^,^K.Yamauchi and H. Miyg jima, in "Amorphous Magnetismq1, ed.

Hooper and de Graaf, p. 325, Plenum Press, 1973.

2. H. 3. Vind Nielsen, J. Magn. & Magn.

Mater. 12, 187 (1979);; 2, 138 81980) 3. K. Handrich, Phys. Stat. Sol. x,

K55 (1969)

4. S. Hatta and T. Egami, J. Appl. Phys.

50, 1589 (1979)

5. R. Hasegawa and R. Ray, Phys. Rev. B

20, 211 (1979)

6. J. Durand, C. Thompson and A. Amamou, Rapidly Quenched Metals, Vol, 2, ,ed.

B. Cantor, The Metals Society, Ldndon, /

1978, p- 109

7. J. Friedel, Nuovo Cimento Suppl. 1,

287 (1958)

8. H. Fxanke, S. Dey, M. Rosenberg, F. $. Luborsky and J. L. Walter, 5 . Magn. & Magn. Mater., Proc. ICM '79, Miinchen (to be published)

9. H. Kuvano and K. Ono, J. Phys. Soc.

Jap. 42, ^{72 (1977)}

same type of long wavelength spin-waves with the stiffness constants given above,

in Table I.