MÖSSBAUER EFFECT STUDIES ON Tm(Fe, Co)2 COMPOUNDS

(1)

HAL Id: jpa-00216634

https://hal.archives-ouvertes.fr/jpa-00216634

Submitted on 1 Jan 1976

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished 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.

MÖSSBAUER EFFECT STUDIES ON Tm(Fe, Co)2

COMPOUNDS

G. Wiesinger

To cite this version:

(2)

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, De'cembre 1976, page C6-585

MOSSBAUER EFFECT STUDIES

ON Tm(Fe, Co), COMPOUNDS

G. WIESINGER

Institut fiir Experimentalphysik, Techn. Universitat, Vienna, Austria

Rhumb.

-

Les paramktres hyperfins dans les composes intermetalliques Tm(Fel-ZCoZ)2 ont etk analysks. Le champ magnetique hyperfin augmente avec la concentration de cobalt. x &ant approximativement 0,35 on observe un maximum suivi d'une diminution linkaire. L'interaction quadrupolaire et le deplacement isomerique montent jusqu'a x = 0,s. La largeur de raie des spectres augmente lentement avec la concentration de cobalt ce qui indique une longueur de liaison mktallique. L'influence de l'environnement local est estime. Les resultats des analyses Mossbauer sont compares avec des mesures magnetiques et avec des calculations de champ cristallin utilisant un seul modele d'ion.

Abstract.

-

Hyperfine parameters in the intermetallic compounds Tm(Fe1-~Co~)2 have been investigated. The magnetic hyperfine field increases with Co concentration. At x 1:0.35 a maximum

is observed followed by a linear decrease. The quadrupole interaction and the isomer shift increase until x = 0.5. The line width of the spectra increases slowly with Co content indicating a long range metallic bond. The influence of the local enviroment is estimated. The results of the Moss- bauer investigations are compared with magnetic measurements and crystalline field calculations using a single ion model.

1. Introduction. - The crystallographic and bulk magnetic properties of the ferrimagnetic compounds TmFe, and TmCo, have been investigated extensively [I-31. They both cristallize in the same cubic Laves phase structure (MgCu, type) and exhibit an easy axis of magnetization parall$ to a [ I l l ] direction. In TmCo, however the difference in anisotropy energies between the [ I l l ] and [loll axes is very small.

Mossbauer studies on RE(Fel - , C O ~ , ternaries with RE = Y, Ce and Ho have already been reported. In the Y case [4] the easy axis of magnetization changes from [ I l l ] to [loll at x

--

0.2 and remains in this direction with further increasing x values. The Moss- bauer spectra of the Ce(Fe,-,Cox), system [5] show a pronounced influence of the local environment on the hyperfine interactions at the iron sites, resulting in broadened spectra. The Ho(Fe, -,Cox), compounds magnetize along a [OOl] direction throughout the whole range of composition [5, 61.

2. Experimental.

-

The intermetallic compounds were prepared by induction melting in a water cooled copper boat under a purified argon atmosphere. In order to avoid evaporation losses 1

%

Tm excess was added. The as cast samples were annealed for 14 days at 700 OC. The phase purity was checked by thermoma- gnetic analysis. The lattice constants were examined by X-ray diffraction measurements using CrK a-

radiation. All samples show the same crystalline structure (MgCu, type). The lattice constants decrease with increasing Co concentration from

a = 7.240 f 0.005

A

for TmFe, to a = 7.121

+

0.005

A

for TmCo,. A positive deviation from Vegard's law is observed. The Curie points starting with Tc = 560 K for TmFe, increase with Co content, a maximum at

x = 0.3 being obtained (Tc = 635 K). followed by a rapid decrease to Tc = 7 K for TmCo, [3].

The variation of the magnetic moment is similar to that of other Re(Fe, Co), compounds, the concentration dependence of the magnetic 3d-moment showing also a maximum at x = 0.3. A detailed description of the bulk magnetic measurements is given elsewhere [I]. Mossbauer spectra were recorded at T = 77 K and T = 295 K using a 57C0 source diffused into a palladium matrix mounted on a conventional constant acceleration drive system with a nitrogen-flow cryostat. The velocity was periodically calibrated with natural iron- and a-Fe,O, absorbers. The isomer shift is measured relative to the Co(Pd) source. The shift of this source relative to natural iron at 3 0 0 K was 0.185 A 0.003 mm s-l. The Mossbauer spectra were fitted by a least squares analysis assuming a superpo- sition of Lorentzian lines [7, 81.

3. Results and discussion.

-

Mossbauer measurements have been performed on nine samples of Tm(Fe,-,Cox), with 0

<

x

<

0.8 at room temperature as well as at liquid nitrogen temperature. In figure 1 typical Mossbauer spectra at T = 77 K are shown. The spectra of compounds with x

<

0.5 can easely be fitted assuming an easy axis of magnetization parallel to a [ I l l ] direction, resulting in two sextetts with intensity ratio 3 : 1, corresponding to two diffe-

(3)

C6-586 G . WIESINGER

VELLOC J T Y ( M M I S I

FIG. 1. - Mossbauer spectra of some T m ( F e ~ - ~ C o ~ ) z compounds measured at T = 77 K.

+,

experimental points ;

solid curves, computer fits.

rent angles between the magnetic axis and the axes of the uniaxial electric field gradient tensor [EFG) [9]. The observed difference of the hyperfine fields in TmFe, agrees with the result obtained from dipole field calculations (AH,,, = 21 kG, AH,,,, = 18.5 kG)

if one assumes that Tm possesses its free-ion magnetic moment gJ. However, bulk magnetic measurements [I] and neutron diffraction data [3] indicate that the Tm moment is lowered by approximately a factor 2. In table I the hyperfine parameters which were experimentally determined are listed. The effective hyperfine fields

(Kff)

have a concentration dependence similar to that observed in other RE(Fe,Co), compounds. An increase of AHeff = 20 kG is obtained between x = 0 and x = 0.3.

The quadrupole coupling constant was determined by computing Mossbauer spectra for any combination of a magnetic field and an electric field gradient with a subsequent least squares fit [7]. The assumption was made that the EFG remains uniaxial although Fe atoms are replaced by Co atoms at random. This procedure could be performed because the shape of the spectra remains essentially unchanged for 0

<

x

<

0.5. Only the line width increases by approximately 10

%.

However the spectra could still be analysed by assuming Lorentzian lines. In contrast to the replacement of iron by a non-magnetic element (e. g. Al) [lo] the replacement of Fe by Co leads to a smooth change of the hyperfine fields indicating a rather long-range magnetic interaction.

An attempt has been made to estimate the range of the influence of the local environment by computing hypothetical Mossbauer spectra on the basis of the assumption that Fe and Co atoms are randomly distributed at the transition metal sites. The probabi- lity that an iron atom in a compound with Co concentration

x

will have n i neighbours in its first j coordination shells is then given by a binomial distribution. The hyperfine fields at the iron sites with different environment can be computed using the formula [ l l ] :

No

is the hyperfine field in TmFe,, the AHi are the shell coefficients. An analogous equation is used for the isomer shift. Quadrupol coupling has been neglected for simplicity. Only the first three shells have been

(4)

MOSSBAUER EFFECT STUDIES ON Tm(Fe, C O ) ~ COMPOUNDS C6-587

taken into account. The lest agreement with the experimental determined spectra is obtained with the follow- ing AHi values : AH, = 2.2, AH, = 1.7, AH, = 1.3 kG/Co neighbour. The small change of AHi indicates an interaction of the Fe atoms with the local environment within a range of more than the first three coordination shells. A sudden decrease of the AHi to zero seems unreasonably. A range of seven coordination shells is estimated. The isomer shift due to a single cobalt nearest neighbour is approximately 0.005 mm s-l, the second- and third-neighbour shift could not be determined because of a large uncertainty. However it seems to be of comparable magnitude. Because of the variation of the hyperfine field the AHi-values change sign. For x < 0.3 additional Co neighbours increase the iron hyperfine field, for

x

>

0.3 a decrease os observed.

The concentration dependence of He,, which follows

closely that of the magnetic 3d moment can be explain- ed by filling the spin-up and spin-down d-subbands with the extra electron of the Co atom.

The average isomer shift is not very strongly concentration dependent. It increases slightly (becomes more positive) with Co concentration. A maximum is obtained at x = 0.5. The variation of the isomer shift is a consequence of a reduced s-electron density at the Fe57 nuclei, an increasing number of electrons in the d-band and a change in the lattice parameter. When analysing spectra of compounds with x

>

0.5 difficulties arise because of the line broadening. At first sight it seems that the easy axis has changed. However, a careful analysis reveals that the weaker spectrum is still present, but can hardly be separated from the background. The number of superimposed hyperfine fields has increased to such a number that

the assumtion of two spectra with Lorentian lines is no longer valid. A second difficulty arises because of the

relative low Curie temperature of the samples. There are already iron atoms present which have no or only a very weak hyperfine field at T = 77 K. It was attempt- ed to take the deviation from Lorentzian line shape into account when performing the fitting procedure. However the hyperfine parameters for the spectra with

x = 0.6, 0.7, 0.8 show a greater uncertainty. To avoid these difficulties measurements at T = 4 K are in progress. An attempt was made to estimate the intensity of the subspectrum with nearly zero hyperhe field. It is strongly temperature dependent. At 77 K

it seems that Fe atoms with no Fe nearest neighbours show no hyperfine field. Altough the shape of the spectra is changed by line broadening it can be concluded that [ I l l ] remains the easy axis of magnetization. This is confirmed by crystalline electric field calculations using a single ion model, which will be published seperately. The difference in the anisotropy energies between [ I l l ] and [loll axis is decreasing with Co concentration but the energy in the [ I l l ] axis remains always minimal [I].

4. Conclusion.

-

Taking the concentration dependence of the hyperfine field at the 57Fe nuclei in Tm(Fe, -,CoJ, we conclude that a long-range metallic character is present. In agreement with crystalline electric field calculations using a single ion model it is found that the easy axis of magnetization remains parallel to the [ I l l ] direction.

Acknowledgements. - The author is indepted to G. Hilscher and H. Rais for communication of results prior to publication.

References

[I] RAIS, H., Thesis, University of Technology, Vienna (1976). [6] VAN DER KRAAN, A. M. and GUBBENS, P. C. M., J. Physique

[2] NESBITT, E. A., WERNICK, J. H., CORENZWIT, E., J. Appl. Colloq. 35 (1974) C 6-469.

Phys. 30 (1959) 365. [7] KUNDIG, W., Nuel. Instrum. Meth. 48 (1967) 219. [8] KUNDIG, W., N d . Instrum. Meth. 75 (1969) 336. 131 DEPORTES, J., GIGNOUX, D. and GIVORD, F., Phys- Stat. _[g]_WERTHEI~,_G._K.,_jACCARINO,_V._and_{WERNICK, J.}_H,,

Solid (b) 64 (1974) 29. _Phys._Rev.₁₃₅_{(1964) A 151.}

[41 G U I M A ~ E S , A P. and BUNBTJRY, D. St. P., J. Phys. F : [lo] MALETTA, H., CRECELIUS, G. and ZINN, W., J. Physique

Metal Phys. 3 (1973) 885. Colloq. 35 (1974) C 6-279.

[5] LONGWORTH, G. and HARRIS, I. R., J. Less-Comm. Met. [ i l l WERTHEIM, G. K., BUCHANAN, D. N. E. and WERNICK, J. H.,