HAL Id: jpa-00218478
https://hal.archives-ouvertes.fr/jpa-00218478
Submitted on 1 Jan 1979
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.
VOLUME DEPENDENCE OF Sn2+-TRANSFERRED HYPERFINE FIELDS IN EUROPIUM
MONOCHALCOGENIDES
M. Abd Elmeguid, G. Kaindl
To cite this version:
M. Abd Elmeguid, G. Kaindl. VOLUME DEPENDENCE OF Sn2+-TRANSFERRED HYPERFINE
FIELDS IN EUROPIUM MONOCHALCOGENIDES. Journal de Physique Colloques, 1979, 40 (C2),
pp.C2-310-C2-312. �10.1051/jphyscol:19792109�. �jpa-00218478�
JOURNAL DE PHYSIQUE Colloque C2, suppl&ment au n o 3, Tome 40, mars 1979, page C2-3 10
VOLUME
D E P E N D E N C E O F sn2+-TRANSFERREDHYPERFI NE F1 ELDS IN
EUROPIUM
MONOCHALCOGENI DES
M. Abd Elmeguid and G. ~aindl*AbteiZmg flir Physik und Astronomic, Ruhr-UniversitBt Bochwn, 0-4630 Bochwn, Germany
* ~ n s t i t u t fiir Atom- und F e s t k o ~ e r p h y s i k , Freie U n i v e r s i t a t Berlin, 0-1000 Berlin, & m y
Rdsum6.- Les champs hyperfins transferds B sur des impuretds sn2+ dans les composGs magn6tique- ment ordonnds EuS et EuTe ont 6t6 mesurds ekhgonction de la pression jusque 6 4 kbar par spectrom6- trie Gssbauer de l l 9 ~ n . La valeur absolue de Bth croTt beaucoup plus vite dans EuS que dans EuTe quand le volume diminue. Les r6sultats sont discugbs sur la base d'un modsle dlBchange 5 deux pa- rametres.
Abstract.-The transferred hyperfine fields Bthf at sn2+ impurity atoms in magnetically ordered EuS and EuTe were measured as a function of pressure up to 64 kbar by '19sn ~gssbauer spectroscopy. In EuS the magnitude of B was observed to increase much fasterwith decreasing volume than in EuTe.
The results are discus%6 in terms of a two-parameter exchange model.
h e magnetic properties of the europium chal- cogenides EuCh (with Ch : 0, S, Se, and Te) have been studied extensively in the past /1,2/. Due to their simple NaC1-structure and the spin-only magne- tism of E U ~ + they are considered model substances of Heisenberg magnets. When described in a two-parame- ter exchange model both the ferromagnetic nn exchan- ge constant J1 and the antiferromagnetic superexchan- ge constant J 2 have been shown to vary in a systema- tic way over the chemical series.131 and as a func- tion of interatomic distance 14, 51 : when the lattice parameter is reduced by external pressure J1 increases strongly while J2 shows only a weak in- crease.
Recently, Bykovetz has studied transferred hyperfine fields at ll9sn impurity atoms at substi- tutional cation sites in EuS, EuSe, and EuTe well below the respective magnetic ordering temperatures /6/. The systematic variations of the nn transferred field B1 and particularly of the nnn supertransfer- red field B2 over the chemical series were found to differ quite drastically from those observed for Ji and J 2 in the pure Eu chalcogenides / 3 / . We report here on a high-pressure MEssbauer study 0 6 sn2+
transferred hyperfine fields in EuS and EuTe, aimed at a comparison with existing data on pure Eu chal- cogenides 151.
The experiments were performed with a high- pressure setup with BrC anvils allowing ~Essbauer transmission experimnts to be performed at liquid- helium temperature with low gamma ray energies. A similar setup has been described in the literature / 7 / . A superconducting lead manometer / 8 / was
employed for in-situ pressure measurements.
The substituted samples containing about 1 at% lL9sn (enriched to 92 2) were prepared by solid- state reaction between 1 1 9 ~ n ~ h (with Ch = S or Te) and the pure EuS and EuTe, respectively. The EU'+
contaminations of the samples were checked by 15'~u Mgssbauer spectroscopy and found to be less than 2%
in either case. A 2-mCi CaSn03 source was used for the l l '~n Mijssbnuer experiments.
Some typical spectra of Euo.ggSno.olS at 4.2 K and at various pressures are shown in figure 1. The increase of the magnetic hyperfine splitting with pressure is clearly visible. The relative line in- tensities observed are strongly influenced by a tex- ture of the absorber. Similar ll9sn spectra were recorded for Euo.qsSno.olTe at 4.2 K and 1.5 K.
The pressure dependences of the effective hy- perfine field Beff and of the isomer shift S were derived from spectra taken at 4.2 K (EuS) and at
1.5 K (EuTe), respectively. While Beff varies appre- ciably with pressure, the observed changes of S are small and will not be discussed here (EuS : dS/dP = +0.003(1) mm/s per kbar); EuTe : dS/dP = +0.001(1) m / s per kbar). The effective sn2+ hyperfine fields listed in table I were extrapolated to 0 K assuming proportionality between Beff and the host magneti- zation /g/ and using the known pressure coefficients of the magnetic ordering temperatures 15, 10, 1 l/.
In the present case Beff is made up of two contributions : Beff = Bthf + Bdip. The dipolar fields Bdip in Eu chalcogenides are of the order of 0.4 T and were taken from reference 1 6 1 . In the last column of table I the tranferred hyperfine fields
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792109
Bthf are listed. In a two-parameter exchange model only nn B1 and nnn B2 contributions to Bthf are con- sidered. In EuS (ferromagnetic) these fields add up to Bthf = 12B1 + 6B2, while in EuTe (antiferromagne- tic, MnO-type) only the nnn shell, containing 6 E U ~ + ions, contributes : Bthf = -6B2. With Bthf being negative in EuS a positive sign has to be assumed for EuTe /6/.
The magnitudes of Bthf are plotted in figure 2 as a function of the lattice parameter. To this purpose values of the low-temperature compressibi- lities of EuS ( K = 0.9x10-~ kbar-l) and of EuTe
( K = 1 . ~ X I O - ~ kbar-l) were extracted from referen-
ces /10/ and /12/. The dotted line of figure 2 con- nects the antiferromagnetic B (equal to -6B2) of
thf
EuTe and EuSe and includes a linear extrapolation to EuS.
+ +
present results4
Fig. 1 : ' 1 9 ~ n MSssbauer spectra of Euo.ggSno.olS at 4.2 K and at various pressures. The solid lines are the results of least-squares fits of six Lorent- zian lines to the data.
Table I : Effective Sn2+ fields B ff in EuS and EuTe extrapolated to 0 K, and derfved values for the transferred hyperfine fields B
thf' P
(kbar) Be ff Bthf
(T) (T)
Eu0.99Sn0. O I S
0 -5.26(5) -5.7 8(8)
20(2) -6.34(6) -6.87(8)
41 ( 3 ) -6.90(6) -7.44(8)
64(4) -7.81(6) -8.36 (8)
Euo.ssSno.o~Te
0 4.21 (7) 3.88(9)
21 (2) 4.87(8) 4.54(10)
38(3) 5.30(9) 4.96(11)
54(4) 5.51(11) 5.16(1 )
-
-\
A Bykovetz 119761,
L +(lkborm 20 kbar
I
Fig. 2 : Magnitudes of Sn2+ transferred hyperfine fields Bchf (extrapolated to 0 K) as a function of the lattice constant. The ambient-pressure results of Bykovetz /6/ are also included. The dotted line represents a linear extrapolation of the antiferro- magnetic Bthf to EuS (marked by X).
0 I
-
U 6.0.-
&a
C 2,
v 5.0 E b
-
cE
4a-From its slope the volume dependence of Bthf, caused by chemical variation of the anions, is derived : (dBthf/dln~)ch = -3.6 T. From the slopes of the so- lid lines in figure 2 values for the volume depen- dences of the magnitudes of Bthf induced by pressu- re are obtained : (dBthf/dlnV) = -17(2)T for EuTe,
P
and (dBthf/dlnV) = -45(4)T for EuS. These pressure- P
induced changes of Bthf are by factors of 4.7 (EuTe) and 12.5 (EuS), respectively, larger than(dBthf/
dlnV)ch, supporting a view where the increase of
- \ 4
0 kbor- *...
'... -..0
kbor..
A....'... '".... 'y ....,\
0 kbar
-
EuS EuSe EuTe
I , , l , , ,
- -
the lattice parameter within the chalcogen series reflects mainly the different anion radii.
In pure Eu chalcogenides the nnn E U ~ + trans- ferred field
~t~
decreases from EuTe to EuSe with the chemically-induced decrease of the lattice pa- rameter /13/, while it increases slightly under external pressure 151. On the other hand, the Sn2+transferred field B? increases from EuTe to EuSe and increases quite drastically with external pres- sure (see Fig.2). Obviously, is more sensitive to changes of the lattice parameter than ':B and less to changes in covalency. The decrease of
5.8 6 0 62 6 L 6.6
Lottlce c w t o n t [A)
C2-312 JOURNAL DE PHYSIQUE
from EuTe to EuSe is then interpreted by covalency changes while the observed increase of "!B is domi- nated by "chemical" pressure. This different beha- viour of
BP
andBE^
is due to the different orbi- tals involved in the supertransfer mechanism via the ligand p orbitals : while for Eu the quite extended 5d orbitals are most important /3/ the transfer to the sn2+ sites should be dominated by the 5s elec- trons of sn2+. It should be mentioned here, that the 21.6 keV " ~ E U isomer shift exhibits an almost identical behaviour as B? as a function of pressu- re and chemically-induced changes of the latticeReferences
/l/ Methfessel, S. and Mattis, D.C., in "Handbuch der Physik" (Springer-Verlag, Berlin) 1968, Vol. 18/1.
/2/ Wachter, P., Crit. Rev. Solid State Sci.
3
(1972) 189.
/3/ Kasuya, T., IBM J. Res. Dev. (1970) 214.
/ 4 / Schwob, P., Phys. Kond. Mater.
10
(1969) 186./5/ Klein, U.F., Moser, J., Wortmann, G. and Kal- vius, G.M., Physica 86-88B (1977) 118.
/6/ Bykovetz, N., Solid State Conrmun.~(1976) 143.
/7/ Schilling, J.S., Klein, U.F. and Ho1zapfel.W.B.
Rev. Sci. Instrum.
45
(1974) 1353.parameter /13/. This may be due to the fact that the /8/ Eichler, A. and Wittig, J.,Z. Angew. Phys.
2
observed changes of
sEU
are mainly caused by a (1968) 319.compression of the Eu 5s2 shell /13/. /9/ Zinn, W., J. blagn. Mat.3 (1976) 23.
found EuTe
.
/10/ Shrivastava, V.C. and Stevenson, R., Can. J.
The volume dependence of Bthf at sn2+ was
Phys.
5
(1968) 2703.to be about 3 times larger in EuS than in
/l]/ Schwob, P. and Vogt, O.,Phys.Lett. (1967) This indicates quite appreciable nn contribu- 242.
tions B1 to Bthf in EuS, in contrast to the discus- /l21 Levy, F. and Wachter. P., Solid State Conunun-B sion in reference /6/. A more quantitative separa- (1970) 183.
tion of the B1 and B p contributions and their volu- /l31 Klein, U.F., Wortmann, G. and Kalvius, G.M., J. Magnet. Magn. Mat.
2
(1976) 50.me dependences must wait for Sn2+ field measurements in EuTe and EuSe in externally applied magnetic fields and with external pressure.
Acknowledgement.- The authors gratefully acknowledge valuable discussions with S. Methfessel, H. Micklitz.
and W. Zinn. This work was partly supported by the Sonderforschungsbereich 161 of the Deutsche For- schungsgemeinschaft.
