MÖSSBAUER STUDY OF THE TERNARY SYSTEM Ho(Fe, Co)2

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MÖSSBAUER STUDY OF THE TERNARY SYSTEM Ho(Fe, Co)2

A.M. van der Kraan, P. Gubbens

To cite this version:

A.M. van der Kraan, P. Gubbens. MÖSSBAUER STUDY OF THE TERNARY SYSTEM Ho(Fe, Co)2. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-469-C6-472. �10.1051/jphyscol:1974697�.

�jpa-00215854�

JOURNAL DE PHYSIQUE Colloque C6, suppl&ment au no 12, Tome 35, DCcembre 1974, page C6-469

MOSSBAUER STUDY OF THE TERNARY SYSTEM Ho(Fe, Co),

A. M. VAN DER KRAAN and P. C. M. GUBBENS Interuniversitair Reactor Instituut, Delft, the Netherlands

RCsum6. - On dkcrit des mesures de champ magnetique hyperfin et d'kclatement quadrupolaire dans les composBs intermktalliques H0(Fel-~Co~)2. Le champ magnktique hyperfin augmente d'abord avec la concentration en cobalt et atteint un maximum pour x = 0,35 environ. Pour des concentrations de cobalt plus elevees le champ hyperfin diminue. Cependant si l'atome defer n'a que des atomes de cobalt comme proches et seconds voisins, un champ effectif de 188 kOe est determine a T = 20 K, ce qui reprksente 84 % du champ mesure dans HoFez pur. L'kclatement quadrupolaire observk augmente avec x dans tout le domaine de concentration. Cet effet peut &re expliquk par une diminution du champ d'anisotropie dans ces composks.

Abstract. - Magnetic hyperfine field and electric quadrupole splitting measurements in the inter- metallic compounds H o ( F ~ I - ~ C O ~ ) ~ are reported. The magnetic hyperfine field first increases with Co-concentration and reaches a maximum at approximately x = 0.35. For higher Co-concentra- tions the hyperfine field decreases. However for an Fe-atom with only Co-atoms as nearest neigh- bours and next nearest neighbours an effective field of 188 kOe is determined at T = 20 K, which is 84 % of the field in pure HoFe2. The observed electric quadrupole splitting is increasing with x over the whole concentration region. This effect can be explained by a decrease of the anisotropy field in these compounds.

1. Introduction. - Much research has been done in the past concerning the nature and magnitude of the moment associated with the transition metal ions in the cubic Laves phase compounds of stoichiometry RB, formed between the rare-earth (R) and transition metals (B). As the magnetic behaviour of the cobalt based compounds seems to be very different from that of the iron based compounds, it is of interest to inves- tigate a ternary system R(Fe, Co),. The rare-earth element holmium is chosen, because in HoFe, the magnetization is parallel to a [OOl] direction [I] and consequently the Mossbauer spectrum consists of one six line hyperfine pattern only.

Furthermore HoCo, shows a pronounced first order magnetic phase transition at T = 78 K, which has not been found in HoFe, [2]. This first order phase transi- tion is probably due to the fact that the magnetic moment of the cobalt atoms in RCo, compounds is induced by the moment of the rare-earth atoms.

Apparently the magnetic properties of the RCo, compounds are mainly determined by the rare-earth elements, while in the RFe, compounds the Fe atom dominates.

Recently GuimarBes and Bunbury [3] reported their results of a Mossbauer study of the same ternary sys- tem Ho(Fe, Co),. However their experiments are less accurate and less complete.

2. Experimental. - The intermetallic compounds were prepared by K. H. J. Buschow of the Philips

Research Laboratories in Eindhoven by arc melting in an argon atmosphere. In order to avoid formation of Ho(Fe, CO), the initial composition was Ho(Fe, Co), ,,. The samples are vacuum annealed for 18 days at 900 OC in a sintered Al,O, crucible. After the annealing procedure the crystalline structure is exami- ned by X-ray diffraction. The samples consist of a mixture of the Laves phase and a small amount of metallic holmium.

The Mossbauer spectra at temperatures between 20 and 295 K were obtained using a 57Co source in a Rh foil mounted on a constant acceleration drive system [4]. The isomer shift is measured relative to the N. B. S. standard reference material no. 725 (Na,Fe(CN), . N O . 2 H,O). The hyperfine fields were determined by a calibration relative to a-Fe,O, at T ⁼ 295 K (He = 515 kOe). The measured spectra are fitted by a least-squares analysis program of Lorentzian line shapes.

Intermetallic Ho(Fe, -,Cox), compounds throu- ghout the whole range of composition were studied using the Mossbauer effect. Since the probability of atomic absorption of the gamma rays increases with the cobalt concentration the samples with 4.5 and 9.0 atomic percent Fe are enriched in 57Fe up to 85 % and 40 % respectively.

3. Results and discussion. - The Mossbauer experi- ments have been carried out on ten samples of HO(F~,-~CO,), with x = 0, 0.022, 0.044, 0.111,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974697

C6-470 A. M. VAN DER KRAAN AND P. C. M. GUBBENS 0.222, 0.333, 0.444, 0.555, 0.91 1 and 0.955 respectively

from T ⁼ 20 K up to T = 295 K. Only for pure HoFe, we have extended the measurements from 295 K up to T,. However it is very difficult to deter- mine T, from the Mossbauer measurements due to the fact that the recoilless fraction strongly decreases with increasing temperature. Furthermore a large increase of the relative intensity of the fourth absorption line in the six-line hyperfine pattern has been observed and moreover hyperfine lines due to metallic iron appear in the spectrum at temperatures of 400 K and above. These effects are shown in figure 1.

0 9 9 5 -

DOPPLER VELOCITY (mm/s)

DOPPLER VELOCITY (rnmh) FIG. 2. - Mossbauer spectra of some Ho(Fe~-~Coz)n compounds measured at T = 77 K.

FIG. 1. - Mossbauer spectra of HoFez.

In figure 2 the Mossbauer spectra of some Ho(Fe, -,Cox), samples measured at T = 77 K are

r -

^--

given. The spectrum of HoFe, consists of one six-line

.

\

T=20K

1

hyperfine pattern which is consistent with an easy , , direction of magnetization along one of the [OOl] direc-

.., ^{- -.} ,.,

tions. In this case the electric quadrupole splitting

-

*-+..

vanishes [I] and consequently all the iron atoms are ^{x / * - \}

_. 1

magnetically equivalent. Essentially the same result is 5

.

200L'

^.

^,

obtained for all the other samples. The effective

= ^{. .}

hyperfine fields (He) are deduced from the spectra by

'.

fitting these spectra with one six-line hyperfine pattern.

In figure 3 He at T = 20 K is given as a function

1

of the Co-concentration. The same compositional

"'1

dependence of He is found at T = 77 K, except for the _IS0

sample with the highest Co-concentration. For this _{HO Fez} 40 ⁸⁰ _{HO Cop} sample the effective field is lower than the value

deduced from the straight line through the other FI,. 3. - Variation of the iron hyperfine field in H o p e , C O ) ~ measured points. This is due to the relatively low with Co-concentration at T = 20 K.

MOSSBAUER STUDY OF THE TERNARY SYSTEM Ho(Fe, C O ) ~ C6-471

Curie temperature of this sample at T = 130 K. For the sample with x = 0.91 1 the Curie temperature is at 210 K, while for the other samples T, is above room temperature. The increase of T, with lowering the Co-concentration is qualitatively in agreement with the results of the magnetization measurements of Slanicka et al. [5].

From the computer fitted Mossbauer spectra it follows that the line width remains essentially the same in the Ho(Fe, Co), compounds with 0 up to 35 % Co, while in the compounds with a higher Co-concentration the line width suddenly increases. Since a cobalt atom has one 3d electron more than an iron atom, the hyperfine interaction at an iron site in Ho(Fe, -,Cox), will change with the cobalt concentration x. The linear increase of the hyperfine field with cobalt concentra- tion below 35 % is most likely due to the filling of the spin up and spin down d sub bands through the extra electron of the cobalt atom. This itinerant character of the d-band electrons is consistent with the observed constant line width in this cobalt concentration region.

The highest effective magnetic field is reached at x = 0.333 and the increase in He from HoFe, up to Ho(Feo.,,Co0,,,), is approximately 24 kOe at T = 20 K.

Above x ⁼ 0.444 the effective hyperfine field seems to decrease linearly with increasing cobalt concentra- tion, while the line width suddenly increases. These effects can be understood when in this Co-concentra- tion region the d-electrons are mainly localized. In that case broadening of the hyperfine lines takes place as a result of the presence of different nearest and next-nearest neighbour configuration of the iron atoms. By extrapolating He as a function of Co- concentration to 100 % Co in figure 3, it follows that for an iron atom in pure HoCo,, He = 188 kOe which is equivalent to a magnetic moment of 84 % of the iron moment in HoFe, at T = 20 K. The same result is obtained by analysing the Mossbauer spectrum of the sample Ho(57Fe,~,,Co,~,,)2 using a binomial distribution. From this analysing procedure it follows that He(O.O) = 188 kOe, He(O.l) = 198 kOe and

He(l.l) ⁼ 208 kOe, where He(i. j.) means the effective field at an iron nucleus with i iron atoms as nearest neighbours and j iron atoms as next nearest neighbours.

It thus appears that the extrapolated field at Fe in pure HoCo, is equivalent to He(O.O). This is in agreement with the suggestion that in the cobalt rich region of the ternary system Ho(Fe,-,Cox),, the d-electrons are mainly localized.

An other interesting phenomenon observed in the Mossbauer spectra of Ho(Fe, -,Cox), is the variation in the electric quadrupole splitting with composition.

In table I the values of 4 E' are given for the different samples, and it follows that the observed electric quadrupole splitting is increasing with Co-concentra- tion. The electric quadrupole splitting for an uniaxial symmetric electric field gradient is given by

where 2 ^E is the electric quadrupole splitting above T, and 0 the angle between the symmetry axis and the easy direction of magnetization. In the Laves phase compounds the [ I l l ] direction is the symmetry axis.

In pure HoFe, the easy direction of magnetization is the [001] direction, so that

0 = 54044' and (3 cos2 8 - 1) = 0 and consequently no first order quadrupole splitting is to be expected. In spite of this a significant quadru- pole splitting has been observed for HoFe,. Bowden et al. [6] calculated the dipole fields at the iron sites in HoFe,. When the magnetization is in the [001] direc- tion the dipole field is in a [loll direction as indicated in figure 4. According to these authors the easy direc- tion of magnetization in HoFe, is then determined by the vector summation of the dipole field and the effective hyperfine field. Although the results of this model seem to be in agreement with the experimental data of HoFe,, the easy direction of magnetization is not determined by this vector summation but by the vector summation of the dipole field and the anisotropy field.

HyperJiYle parameters of Ho(Fe, -,Cox), intermetallic compounds

C6-472 A. M. VAN DER KRAAN AND P. C. M. GUBBENS

- x

Z = 5 / 8 A Z * 7 / 8 9 Z = 3 / 8 0 Z = 1 / 8

FIG. 4. - The directions of the dipolar fields at the different iron atoms in the crystal HoFez. The total magnetic moment is taken

parallel to the [OOl] direction.

the easy direction of magnetization. Since the magni- tude of the dipole fields in HoFe, and HoCo, are mainly determined by the large moment of the hol- mium atom, these dipole fields are nearly the same

( E 8 kOe) in both compounds. However in table I it is shown that for the cobalt rich compounds 2 ^E is much smaller than for HoFe,, while 4 ^E' becomes more negative by going from HoFe, to HoCo,. This means that 0 is increasing and the direction of magne- tization deviates more and more from the [OOl] direc- tion. For instance in the case of Ho(57Feo,08Coo~92)2 13 is observed to be 640 while for HoFe, 0 = 580.

From the deviations of the direction of magnetization from the [OOl] direction it follows that the anisotropy fields are approximately 45 and 175 kOe for Ho(57Fe,,o,Coo~92)2 and HoFe, respectively. Follow- ing the model of Bowden et al. [6] one needs a dipole field of approximately 32 kOe in order to explain the observed angle 0 of 640 in Ho(57Feo~08Coo,92)2 and such a dipolar field is much too large for this cubic Laves phase compound.

The experimental isomer shifts are also given in table I. Although the number of 3d-electrons is increas- ing by going from HoFe, to HoCo,, the effect on the isomer shift is very small.

Now it is possible to estimate the order of magnitude Acknowledgment. - The authors are indebted to of the anisotropy field from the calculated dipole field Dr. K. H. J. Buschow of the Philips Research Labora- and the observed angle 0 between the [ I l l ] axis and tories for the preparation of the samples.

References

[I] BOWDEN, G. J., BUNBURY, D. ST. P., GUIMAR~ES, A. P. and [4] VAN DER KRAAN, A. M., Thesis University of Technology SNYDER, R. E., J. Phys. C. 1 (1968) 1376. Delft (1972).

[2] LEMAIRE, R., Cobalt 33 (1966) 201. [5] SLANICKA, M. I., TAYLOR, K. $4. R. and PRIMAVESI, G. J ,

[3] GUIMAR~ES, A. P. and BUNBURY, D. ST. P., J. Phys. F : MetaE J. Phys. P : Metal Phys. 1 (1971) 679.

Phys. 3 (1973) 885. [6] BOWDEN, G. J., J. Phys. F : Metal Phys. 3 (1973) 2206.