MÖSSBAUER SPECTRA OF THE AMORPHOUS ALLOYS DyM3 (M = Fe, Co, Ni)

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MÖSSBAUER SPECTRA OF THE AMORPHOUS

ALLOYS DyM3 (M = Fe, Co, Ni)

R. Arrese-Boggiano, J. Chappert, J. Coey, A. Liénard, J. Rebouillat

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dkcembre 1976, page C1-771

MOSSBAUER SPECTRA OF

THE AMORPHOUS

ALLOYS DyM3 (M

=

Fe,

Co,

Ni)

R. ARRESE-BOGGIANO and ' J. CHAPPERT

DRFIGroupe Interactions Hyperfines, Centre #Etudes NuclCaires, 85 X, 38041 Grenoble, Francz J. M. D. COEY

Groupe Transitions de Phases, Centre National de la Recherche Scientifique, 166 X, 38042 Grenoble, France

and

A. LIENARD and J. P. REBOUILLAT

Laboratoire de Magnktisme, Centre National de la Recherche Scientifique 166 X, 38042 Grenoble, France

Rbum6.

-

Les alliages amorphes DyM3, M = Fe, Co, Ni, ont kt6 ktudies par spectroscopie Mossbauer de 161Dy et s7Fe, avec dopage au fer si nkessaire. YNi3 dope 57Fe a aussi kt6 inclu dans cette etude. Les resultats sont compares avec ceux obtenus pour les alliages cristallins corres- pondants. Les champs hyperfins a saturation sur le noyau de fer sont considkrablement plus grands ( - 30 %) dans les alliages amorphes et on observe une distribution importante dont la largeur relative est

--

20 %. On discute Ies raisons qui pourraient expliquer la superioritk du moment du metal de transition dans les alliages amorphes. Les champs hyperfins B saturation sur le noyau de dysprosium sont infkrieurs de 1 % a ceux des alliages cristallins, avec une distribution dont la largeur relative est seulement 1 %. A partir des largeurs de raies dont les positions sont sensibles & l'interaction quadrupolaire, on dkduit que la direction du moment de Dy est fortement corrklke B la direction (alkatoire) de la contribution de rkseau au gradient de champ klectrique. On propose des structures magnktiques dans lesquelles les moments de Dy sont pour l'essentiel distribuks dans toutes les directions

a

I'intkrieur d'une hkmisphkre tandis que les moments de Fe ou Co sont ferromagnktiques. Le moment de Ni est trks faible.

'~bstract. - Amorphous alloys DyM3, M = Fey Co, Ni, have been studied by the 161Dy and 57Fe Mossbauer resonances, doping with iron where necessary. 57Fe-doped YNi3 was also included in the study. Results are compared with those for the corresponding crystalline alloys. Iron hyperfine fields at saturation are considerably greater ( - 30 %) in the amorphous alloys, and there is a broad distribution whose relative width is

-

20 %. Possible reasons for a greater transition metal moment in the amorphous alloys are discussed. The Dy hyperfine field at saturation is

1 % less than in the crystalline alloys, with a distribution whose relative width is only 1 %. From the widths of lines whose positions are sensitive to the quadrupole interaction, it is deduced that the Dy moment direction is strongly correlated with the (random) direction of the lattice contribution to the electric field gradient. Magnetic structures are proposed in which the Dy moments are essentially distributed over all directions within an hemisphere while the Fe or Co is ferromagnetic. The Ni moment is very small.

1. Introduction. - Research into the properties of amorphous alloys of the rare earths with the transition metals has been recently stimulated by two factors. First, their use in magnetic devices such as bubble domain memories make these materials potentially attractive for the computer industry. Many of the devices actually realized make use of Gd-Co based films which have both the necessary bulk anisotropy with the easy direction perpendicular to the plane of the film and a compensation point near room temperature [I, 21. Second, it is now becoming apparent t o the physicist that a rich variety of complex magnetic structures can

occur in amorphous solids. In particular some varie- ties of magnetic order seem to appear only in the absence of long range atomic order while others appear only in crystals [3,4]. For such fundamental studies it is highly desirable to be able to compare the magnetism of solids which can exist in both amorphous and crystalline states. Intermetallic rare earth-transition metal alloys are ideal in this respect. They also present the great advantag.: that by changing the rare earth, the transition metal or the composition, the exchange interactions and the single-ion anisotropy may be changed at will. Therefore a systematic study of the

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C6-772 R. ARRESE-BOGGIANO, J. CHAPPERT, J. M. D. COEY, A. LIENARD AND J. P. REBOUILLAT

influence of each of these parameters on the magnetic TABLE I

structure and properties is possible.

Here we report the beginnings of such a study. Sputtered DyM, alloys, with M = Fe, Co or Ni, have been examined by Mossbauer spectroscopy and magnetization measurements. Both the I6'Dy and 57Fe resonances have been used, iron being introduced as an impurity in DyCo, and DyNi,. Fe-doped YNi, has also been examined. For this paper we focus our attention on the Mossbauer data, and compare it with previously published work on the crystalline alloys. Some discussion of the magnetic structures of these alloys is also included. Applications of the Mossbauer spectroscopy in amorphous solids have been discussed in two recent reviews [5,6].

2. Results.

-

Thin film samples approximately 5 p thick were deposited at room temperature on kapton substrates by DC argon ion sputtering from a target DyM, or YNi,. The Mossbauer absorbers were made of several sheets in order to obtain the adequate thickness. For iron-doped samples, a small piece of 57Fe metal was fixed at the centre of the target with iron wire. The iron concentration was estimated as 2-4

%.

The films were all X-ray amorphous except for DyNi, which showed some signs of partial crystallization. Their composition was Dy2,M7, or Dy,,M,, but we will usually refer to them as DyM, for conve- nience. The l6'Dy Mossbauer source kept at room temperature was 16'Tb obtained by neutron irradiation of 90

%

enriched l6'Gd~,. 5 7 C ~ into Rh or Cr matrices was used for the 5 7 ~ e resonance.

The temperature dependence of the macroscopic magnetization induced by a magnetic field of 15 kOe is shown in figure 1. Only DyCo, presents the characte-

FIG. 1.

-

Variation of the spontaneous magnetization of the Fe, Co and Ni amorphous alloys as a function of the reduced

temperature TITc.

ristic minimum at a compensation temperature of 230 K. At saturation the global moments are

-

1.8, 2.4 and 5.1 p, for the Fe, Co and Ni alloys respectively. Their Curie temperatures are listed in table I.

161Dy hyperJne interactions and ordering tem- peratures o f amorphous (a) and crystalline (c) DyFe,,

DyCo, and DyNi, at 4.2 K. Distribution widths are given in parentheses. go fin Hhf e2 qQ (MHz) - (cmls)

-

161DyFe3 (a) 890 (12) 10.5 (1.0) (c) 904 12.8 161DyCo3 (a) 870 (10) 10.0 (1.9) ( 4 883(*) 12.0 (*) 161DyNig (a) 840 (12) 10.4 (0.7) (c) 850 (**) 12.5 (**)

(*) YAKINTHOS, J. K. and CHAPPERT, J., Solid State Commun. 17 (1975) 979.

(**) YAKINTHOS, J., ROSSAT-MIGNOD, J. and BELAKHOVSKY,

M., Phys. Stat. Sol. (b) 47 (1971) 247.

(***) This temperature is difficult to evaluate because of crystallization.

Typical Mossbauer spectra of the 16'Dy and S7Fe resonances taken with the y-rays perpendicular to the plane of the film are shown in figures 2 and 3 respectively. In figure 2 are represented the spectra of amorphous and crystalline DyFe, at 4.2 K. It is obvious that the over-all splitting is about the same but that some lines are rather broadened for the amorphous alloy. This arises from a distribution in magnetic and quadrupole interactions. The same behaviour is found for DyCo, [4] and DyNi,. The spectra were fitted to eight independent lorentzian doublets corresponding to pairs of lines with the same intensity which are dis- placed equally and in the same sense by the quadrupole interaction. The distribution in magnetic hyperfine field was derived from the widths of those lines whose positions are independent of the quadrupole interaction, and it was then used in the derivation of the

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MOSSBAUER SPECTRA OF THE AMORPHOUS ALLOYS DyM3 C6-773

-10 -8 -6 -4 -2 0 2 4 6 8 10 VELOCITY (mmls)

FIG. 3. - 57Fe Mossbauer spectra at various temperatures of amorphous DyFe3 and Y(Ni1-.Fes)3. The y-source was 57C0

into Rh and Cr for Dy and Y alloys respectively.

distribution in quadrupole interaction from the widths of the remaining lines. The results are listed in table I.

In figure 3 are shown typical spzctra for DyFe,

and Y(Ni,-,Fed, at various temperatures. Very broad lines are observed even at T E 0. The spectra were

fitted with three doublets from whose positions and widths, the average values and distribution widths of

the magnetic hyperfine field and quadrupole interaction were deduced, assuming a common isomer shift for all the iron. These parameters are listed in table 11, together with the relative intensities of the Am = 0 lines. The Fe moments are estimated by assuming a proportionality of 150 kOe/y,. Data on crystalline alloys are shown in tables I and I1 for comparison.

3. Discussion.

-

Magnetization measurements provide the global moment of a domain which is the sum of the net moments of the rare earth and transition metal sublattices. The net moment of a sublattice depends in turn on the magnitude and distribution in direction of the individual atomic moments. It is just such information which is provided by Mossbauer spectra. Ni and Co alloys were tagged with 5 7 ~ e whose moment is expected to couple strongly to that of Ni and Co. In that case information is obtained about the direction but not the absolute magnitude of the Ni and Co moments.

We consider the magnitudes of the moments at T

--

0 first of all. From the hyperfine fields, it appears that the Dy moments in the three alloys at 4.2 K are close to the free-ion value, 10 y,. The same result was found in a previous study of DyFe2 [7]. The Dy hyperfine field is only reduced by about 1

%

compared with the crystalline alloys, and its distribution width is also only 1

%.

These data confirm that the Dy 4f shell is well shielded from the influence of neighbouring atoms in so far as its moment is concerned.

The situation is quite different for Fe, where the distribution in Fe hyperfine field may reach 20

%

of its absolute value. The 3d shell is very sensitive to the variety of nearest-neighbour environments in the amorphous alloys, whose structure may be described by a Bernal model, a random close-packing of spheres [8,

51. Furthermore, it may be seen from table I1 that the Fe hyperfine fields are invariably greater in the amorphous alloys than in their crystalline counterparts. Possible contributing factors to this difference are :

i) lower density, hence greater average interatomic distance in the amorphous alloys,

5 7 ~ e hyperjine interactions in Dy(Col-,Fed,, DyFe,, Dy(Nil-,Fez), and Y(Ni, -,Fe>, ; 0.02

<

E

<

0.04.

Distribution widths are given in parentheses. The iron moment p(Fe) is estimated from Hhf assuming 1 pB/150 kOe.

I,.,/I,

-,

is the relative intensity of the Am = 0 transitions.

Comments

-

- 0.00 (0.20) 0.23 f 0.02 4.2 K Average at 77 K, ref. [12] 0.00 (0.16) 0.49 4.2 K Average at 77 K, ref. [13]

0.00 (0.50) 0.45 4.2 K, sample partially crystalline Average at 4.2 K, ref. [14] 0.00 (0.30) 0.20 1.6 K

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C6-774 R. ARRESE-BOGGIANO, J. CHAPPERT, J. M . D. COEY, A. LIBNARD AND J. P. REBOUILLAT

ii) greater probability of an iron atom to have another transition-metal atom among its nearest neighbours in the amorphous alloy,

iii) tendency towards electron localization due to disorder (Anderson localization).

It is impossible to assess the relative importance of these contributions at present, though experiments under pressure should show the influence of i). The Mossbauer experiment gives information about the Fe moment, but there is independent evidence that the Co moment is greater in the amorphous alloys (1.6 pB) [2] than in the crystalline ones (0.8 y,) 191. In crystauine RNi, alloys the Ni moment is very small (0.05 pB), or possibly zero [lo]. The same seems to be true of the amorphous alloys. Magnetic measurements indicate that amorphous YNi, is a weak ferromagnet with an ordering temperature of 20 K and a moment of 0.04 pB/Ni at 4.2 K.

The differences in ordering temperature between the amorphous and crystalline alloys (Table I) reflect both the differences in moment on the transition metal, and the differences in exchange interactions J(r) in the amorphous and crystalline structures. Obviously no general statement is possible about ordering temperature being lower (or higher) in an amorphous solid than in its crystalline counterpart.

We now consider the question of the magnetic structures of these amorphous alloys. None of them can possibly be collinear, because the moments at T = 0 (Fig. 3) are incompatible with the sum or the difference of the Dy moment (10 pB) and three times the transition metal moments. In contrast, GdCo, is believed to be a collinear ferrimagnet (Fig. 4a) [I, 21.

in the plane with a small external field and varying the angle between the film and the y-direction. The Dy moments however have almost no preferred orientation relative to the film plane ;

3 <

sin2 8

>

= 0.37 f 0.04. The magnetic structure compatible with these results is shown on figure 4b.

A sperimagnetic structure, as we term it, is defined as a two sublattice magnetic structure where the moments of one (or both) sublattice are scattered at random in directions in space according to a probability P ( Y ) . The name is by analogy with ferrimagnetic and spero- magnetic structures, the former being a two sublattice, collinear, antiparallel structure, whereas the latter is a one sublattice structure where the moments are scattered at random uniformly in all directions in space, Naturally the term sublattice in-these amorphous solids has a chemical, not a structural significance. Further- more, the magnetic structure is defined in the absence of domains, just as in a crystalline solid.

The physical basis for the dispersion of the Dy moment directions throughout a wide cone is the local anisotropy field which varies randomly in direction in an amorphous solid [I 11. The second order term in the crystal field is the dominant one 181 and it is greater than the Dy-Co exchange. The ferromagnetic Co-Co exchange is the dominant exchange interaction, so a Dy moment simply chooses the direction in its easy plane defined by the local crystal field most nearly antiparallel to the Co sublattice moment [4]. The fact that the lattice contribution to the electric field gradient does not average to zero is direct evidence that the Dy moment direction is strongly correlated with the local crystal field axis in all three amorphous alloys [4]. In DyFe,.,, the observed moment of 1.8 p, may be explained by a sperimagnetic structure similar to that in DyCo,,. If the Fe were ferromagnetic and the Dy

moments uniformly distributed over a hemisphere whose axis is antiparallel to the Fe moment direction, the resulting moment would be

In fact the Dy-Fe exchange interaction will tend to reduce Yo from 900 changing the hemisphere into a FIG. 4. - Magnetic structures proposed for a) GdCos ;

b) DyCos and DyFej, with ' Y o 2 70" and c) DyNi3 with

yo 2: 90°. a) is ferrimagnetic, b) is sperimagnetic and

c) is asperomagnetic.

The magnetic structure of DyCo, was discussed in a previous publication

141,

but we recapitulate the results here. The 57Fe moments, hence the Co moments, are seen to lie essentially in the plane of the film from the relative intensities of the Am = 0 transitions. 1,,,/11-, gives

3 <

sin2 8

>

= 0.49 & 0.02 where 8

is the angle between the moment and the y-direction, and

< >

means the average value. Moreover the transition metal moments were shown to be ferro- magnetically coupled by saturating the magnetization

broad cone, and increasing the angular factor

<

cos Y

>

beyond 0.5. A secondary effect will be to open up a narrow cone on the Fe sublattice.

The magnetic moment of our sample of DyNi,, although it is not completely amorphous, may be

explained by the magnetic structure shown in figure 4c.

We suppose that there is no moment on Ni, and that the Dy-Dy exchange is much weaker than the single- ion anisotropy so that the Dy moments are uniformly distributed over a hemisphere giving a moment of 5 pB. Such a one sublattice structure where the moments are scattered at random in directions according to a non- uniform probability P(!P) may be termed asperomagnetic.

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M ~ ~ S B A U E R SPECTRA OF THE AMORPHOUS ALLOYS DyM3 C6-775

for DyCo,. There the Dy sublattice moment is larger in three alloys has just recently been explained by Bhatta- magnitude than that of Co at T = 0, but it falls off charjee et al. [15] on the basis of the Harris, Plische more rapidly with temperature because J,,-,<Jc,-,. and Zuchermann model [ll].

In DyFe, the iron sublattice moment dominates at all Finally, we note that there is a clear tendency for the temperatures whereas in DyNi, the dysprosium is the Fe moments to lie out the plane in DyFe, and only sublattice with a moment. The temperature Y(Ni,-,Fee),, which may result from a texture in the dependence of the spontaneous magnetization of these amorphous structure.

References

[I] CHAUDHARI, P., CUOMO, J. J. and GAMBTNO, R. J., I.B.M.

J. Res. Dev. 11 (1973) 66.

[2] MEYER, R., JOWE, H. and REBOUILLAT, J. P., B E E Trans. Magn. 11 (1975) 1335.

[31 COEY, J. M. D., Phys. Bull. 27 (1976) 294.

[41 COEY, J. M. D., CHAPPERT, J., REBOUILLAT, J. P. and WANG, T. S., Phys. Rev. Lett. 36 (1976) 1061.

[5] COEY, J. M. D., J. Physique Colloq. 35 (1974) C 6-82. [6] LITTERST, F. J. and KALVIUS, G. M., Proc. Znt. Con$

Mossbauer Spectroscopy, Cracow 1975, vol. 2, p. 189. [73 FORRESTER, D. W., ABBUNDI, R., SEGNAN, R. and

S ~ E G E R , D., AZP Con5 Proc. 24 (1975) 115.

183 COCHRANE, R. W., HARRIS, R. and PLISCHKE, M., J. Non- Cryst. Solids 15 (1974) 239.

191 YAKINTHOS, J. K. and MENTZAFOS, D. E., Phys. Rev. B 12 (1975) 1928.

[lo] ARIF, S. K. and BUNBURY, D. S. P., Phys. Stat. Sol. (a)

33 (1976) 91.

[Ill HARRIS, R., PLISCHKE, M. and ZUCKERMANN, M. J.,

Phys. Rev. Lett. 31 (1973) 160.

[12] ARIF, S. K., BUNBURY, D. St. P., BOWDEN, G. J. and DAY, R. K., J. Phys. F 5 (1975) 1048.

[13] ARIF, S. K., BUNBURY, D. St. P. and BOWDEN, G. J., J. Phys. F 5 (1975) 1792.

[14] TSAI, S. C., NARASIMHAN, K. S. V. L., KWH, C. 3. and BUTERA, R. A., J. Appl. Phys. 45 (1974) 3582.