MÖSSBAUER STUDY OF AMORPHOUS Fe75P15C10

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MÖSSBAUER STUDY OF AMORPHOUS Fe75P15C10

C. Chien, R. Hasegawa

To cite this version:

C. Chien, R. Hasegawa. MÖSSBAUER STUDY OF AMORPHOUS Fe75P15C10. Journal de

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JOURNAL DE PHYSIQUE _{Colloque C6, suppliment au}_no_12,_{Tome 37, Dtcembre 1976, page C6-759}

MOSSBAUER STUDY OF AMORPHOUS Fe7,Pl5Cl0

(**)

C. L. CHIEN

Physics Department, The Johns Hopkins University, Baltimore, Maryland 21218 U. S. A. R. HASEGAWA (**)

W. M. Keck Laboratory of Engineering Materials, California Institute of Technology Pasadena, California 91109 U. S. A.

Rbum6. - Le ferromagnetique amorphe Fe75PlsClo a et6 6tudik par spectroscopie Mossbauer (Fe57) de 4,2 K jusqu'8 1 100 K. Dans l'etat amorphe, le champ hyperfin Hefn(T) dkcroft par rapport

A

la temperature beaucoup plus vite que celui de Fe dans l'etat cristallin. De plus, Henn(T) decroit cornme BT312 pour les tempkatures basses, ce qui indique des excitations d'ondes de spin. La valeur de B est anormalement grande. La temperature Tc = 619 & 2 K est trouvke Etre bien defmie. La cristallisation a lieu a approxirnativement 710 K. Les phases cristallines subsequentes comprennent a-Fe (T, = 1 040 K), Fe3P (Tc = 715 K), et Fe3C (Tc = 490 K).

Abstract.

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Amorphous ferromagnet Fe75P15C10 has been studied by Fe57 Mossbauer spec- troscopy from 4.2 K to 1 100 K. In the amorphous state, the hyperfine field H e f f ( T ) decreases with temperature much more rapidly than that of crystalline Fe. Furthermore Heff(T) decreases as BT312 at low temperatures indicative of spin wave excitations. The value of B is anomalously large. Tc = 619 4 2 K is found to be sharply defined. Crystallization occurs at about 710 K. The subsequent crystalline phases consist of a-Fe (Tc = 1 040 K), Fe3P (Tc = 715 K ) and Fe3C (Tc = 490 K ) .

1. Introduction.

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The amorphous Fe-P-C system is one of the earliest reported amorphous ferromagnets. It has been subjected to a wide variety of structural [l], magnetic [2-41 and elastic [5] investigations. This work reports on a detailed study of amorphous Fe75Pl,Clo made by using 57Fe Mossbauer spectrosco- py from 4.2 K to 1 100 K. The properties of the amorphous state, the crystallization transition, and the crystalline phases after crystallization are discussed.

2. Experimental.

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The amorphous samples of Fe7,P15Clo were prepared by rapid quenching from the melt (the splat cooling method). The samples were thin disks of about 30 pm thick and 1.5 cm in diameter. These <( as-prepared samples were used as absorbers.

Typical Mossbauer spectra have 5 X 105 counts in

each channel.

3. 'Results and discussion.

-

For T

< T, amor-

phous Fe75P15Clo shows a well-defined but broadened six-line pattern (Figs. 1 and 2), due to the hyperfine field distribution commonly observed in amorphous ferromagnets [2, 6, 71. The areas of these six lines have a ratio of 3 : b : 1, where b varies from 0 to 4 depending upon the angle(s) between the y-ray direction and the

(*) Work supported by the National Science Foundation.

(**) Present address : Materials Research Center, Allied Chemical Corporation Morristown, New Jersey.

magnetization axis (axes) of the sample. For T 5 530 K, the area ratio is invariably close t o 3 : 4 : 1, indicating that the easy axis of magnetization lies in the sample plane. At about 530 K, the magnetization axis switches to become nearly perpendicular to the sample plane (Fig. 2) and remains so for higher temperatures. This type of unusual temperature dependence of the magnetization axis has also been observed in other amorphous ferromagnets [g]. The reason for this dependence is not well-understood, but strains frozen in the amorphous sample during the rapid quenching process have been suspected [g].

The hyperfine field Heff(T) of amorphous Fe7,Pl5C1, decreases with T much more rapidly than that of crystalline ferromagnets (e. g. Fe metal) as shown in figure 3. Similar behavior has been observed -

in amorphous Fe,oNi,,P1,B, [7, 91, Fe,oPl,C,Bl [g] and Fe,,B,, for Heff(T) and in other amorphous systems for the magnetization M(T). This is believed tb be a manifestation of structural disorder in amorphous ferromagnets, where the random atomic arran- gement dictates that the exchange constants between magnetic spins should have a range of values.. Theore- tical calculations [l l ] using the mean field approxima- tion and a varying exchange parameter indeed produce the lower magnetization curve as observed. However, quantitatively these approximate calculations, like most mean field approximations, do not predict the observed

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C6-760 C. L, CHIEN AND R. HASEGAWA

FIG. 1.

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Mossbauer spectra of amorphous Fe7sPlsC10 at low temperatures.

temperature dependences at low T and at T close to T,.

Heff(T) of amorphous Fe,,P,,C,, at low temperatures has a dependence of

as shown in figure 4. For amorphous Fe7,P1,C,, the values are B = (22 f 2) X 10-6 deg-3/2 and

B3/2

= 0.36 f 0.04. Recent bulk magnetization

( M ( T ) ) measurements [ 3 ] on Fe,,P1,Clo obtained the same T ~ / ~ dependence and B value, suggesting the proportionality between Heff(T) and M(T). It should be emphasized that the values of B and of

Fe,,P1,C1, are several times larger than those of crystalline ferromagnets ; for example, for Fe metal the values- are B = (3.4 f 0.2) X 1 0 - ~ deg-3/2 and

B,,, .=-0.114 f 0.007 -[12]. The T ~ / ~ relation and h g e .values-- of B3/2 have also-.been -observed- in other

Fe75P15C~~

I l I

-5 0 5 m w s

FIG. 2.

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Mossbauer spectra of amorphous Fe,sPlsClo at high temperatures.

FIG. 3.

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Reduced hyperfine field Hern(T)/Hent(O) versus reduced temperature T/Tc of amorphous Fe7sPlsClo and

crystalline Fe (dashed curve).

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MOSSBAUER STUDY OF AMORPHOUS FelsPl o C6-761

FIG. 4. - The T3/2 dependence of the change of hyperfine

magnetic field in amorphous Fe75Pl SCIO.

Of particular interest is the region of T close to T,,

where questions such as sharpness of T, and critical

phenomena may be explored. However, not every

amorphous magnetic system is suitable for this purpose, because the crystallization temperature TcR

may be below or too close to T,. For amorphous

Fe,,P,,C,,, the magnetic ordering temperature of

T, = 619

+

2 K is found to be sharply defined.

HefP(T)IHeff(0) behaves as D(l

-

TITc)* in the range

of 0.01

5

TITc

5

0.1 with D = 1.1

+

0.03 and

p

= 0.33 _f 0.05 [9].

At a heating rate of about 30 K/min the amorphous

sample rapidly crystallizes at 710 K. Measurements of the sample after crystallization indicate that there are three distinct crystalline phases : @-Fe (T, = 1 040 K),

Fe3P (T, = 715 K ) and Fe3C CT, = 490 K ) . The identification of these phases are made from the values of Tc and hyperfine parameters, and are in good

agreement with detailed crystallization studies [15].

We are indebted to Professor Duwez for kindly supplying the amorphous samples.

References

[l] LIN, S. C. H. and Duwez, P., Phys. Stat. Sol. 34 (1969) 469.

[2] TSUEI, C. C., LONGWORTH, G. and LIN, S. C. H., Phys.

Rev. 170 (1968) 603.

[3] TSUEI, C. C. and LILIENTHAL, H., to be published in Phys.

Rev.

[4] CHIEN, C. L., WALKER, J. C. and HASEGAWA, R., AIP

Con$ Proc. 24 (1975) 127.

[5] BERRY, B. S. and PRITCHET, W. C., J. Appl. Phys. 44

(1973) 3122.

[6] SHARON, T. E. and TSUEI, C. C., Phys. Rev. B 5 (1972)

1047.

[7] CHIEN, C. L. and HASEGAWA, R., AIP Con$ Proc. 29

(1976) 214.

[S] CHIEN, C. L. and HASEGAWA, R., J. Appl. Phys. 47 (1976) 2234.

[g] CHIEN, C. L. and HASEGAWA, R., Proceedings of the Structure and Excitations of Amorphous Solids Conf. (Williamsburg, 1976).

[l01 CARGILL, G. S., AIP Con$ Proc. 24 (1975) 138.

[Ill e. g., HANDRICH, K., Phys. State Sol. 32 (1969) K 55. [l21 ARGYLE, B. E., WRAP, S. H. and PUGH, E. W., Phys.

Rev. 132 (1963) 2051.

[l31 MOOK, H. A., WAKABAYASHI, N. and PAN, D., Phys.

Rev. Lett. 34 (1975) 104.

[l41 AXE, J. D., PASSEL, L. and TSUEI, C. C., AIP Con$ Proc. 29 (1976) 146.

[l51 RASTOGI, P. K. and D u w ~ z , P., J. Non-Cryst. Solid 5