High-pressure Mössbauer study of hyperfine interactions in magnetically ordered europium chalcogenides: EuO, EuS, EuTe

Journal of Magnetism and Magnetic Materials 0 North-Holland Publishing Company

HIGH-PRESSURE MijSSBAUER MAGNETICALLY

3 (1976)

50-54

STUDY OF HYPERFINE INTERACTIONS

ORDERED EUROPIUM CHALCOGENIDES:

IN

EuO, EuS, EuTe

U.F. KLEIN, G. WORTMANN and G.M. KALVIUS Physik Department,

Technische

Universitbl Miinchen, D-8046 CarchinK, W.-Germany

The effect of pressure on the isomer shift S and the magnetic hyperfine field B was investigated in the divalent europium chalcogenides at 4.2 K by means of the 21.6 keV Mossbauer resonance of ‘s’Eu. The variation of the lattice parameter with pressure causes a quite different change in hyperfine interactions than the one observed upon varying the anion within the chalcogenide series. In EuTe no change in the magnetic ordering behaviour has been observed up to 57 kbar.

1. Introduction

The divalent europium chalcogenides EuX (X = 0, S, Se, Te) are particularly suitable subjects for a study of magnetic semiconductors due to their simple NaCl structure and their pure spin magnetism. The magnetic ordering varies from ferromagnetism (EuO, EuS) to metamagnetism (EuSe) and antiferromagnetism (EuTe). This behaviour reflects the dependence of the exchange integrals Jl and J2 on the interatomic distances. [l-3]. High-pressure Mossbauer experiments reveal simultaneous information on the change in charge and spin density with the lattice parameter. They can be used, when ccmpared with the results of a chemical variation of the lattice parameter, to separate the contributions of the different anions to the hyperfine parameters. We report here on high-pressure investigations on the isomer shifts and magnetic hyperfine fields in EuO, and EuS and EuTe, which were performed in the course of a systematic study of the volume dependence of hyperfine interactions in magnetically ordered systems of europium [4,5],

2. Experimental techniques The high-pressure measurements on EuO and EuTe were performed at 4.2 K using the 2 1.6 keV Mossbauer resonance of 151Eu. The results on EuS have been reported earlier [6]. The (polycrystalline) ab-

LEAD

MANOMETER

ELECTRICAL

LEADS

GASKET

Fig. 1. Schematical

representation

of the high pressure

cell.

sorber is pressurized in a high-pressure cell, which is schematically shown in fig. 1. The specially shaped hollows of the B,C anvils provide sufficient transmission of the gamma radiation. The pressure profile at the absorber is determined in situ by a “lead-manometer”. The principal operation of this type of highpressure cell has been outlined in ref. [7]. In the present study, however, two concentric supporting rings (see fig. 1) prevent a radial flowing of the absorber material and yield a quasi-hydrostatic pressure with gradients less than 10%. In fig. 2, the arrangement of the high-pressure clamp (containing the high-pressure

U.F. Klein, G. Wortmann and G.M. KaluiuslHigh-pressure

Mbsshauer study of hyperfine

interactions

t

51

EuO

Okbor 6 --291(11T s --

I Fig. 2. Experimental urements at helium

+-..-setup for high-pressure temperature.

‘15N2, “,n,,Yc

DETECTOR

Mijssbauer

meas-

cell) within the helium cryostat is shown. The source (200 mCi lslSm in Sm203) is brought as near as possible to the absorber and is moved sinusoidally by an electromagnetic drive unit via a stainless steel rod. The Mijssbauer spectra were fitted by a sum of Lorentzians corresponding to the magnetically split 21.6 keV transition of 151Eu with the g-factor ratio given in ref. [8]. Small contaminations of the EuO absorber by trivalent Eu oxide were taken into account by adding a single line resonance near zero velocity to the hyperfine pattern.

3. Results Mbssbauer spectra of EuO at various pressures are shown in fig. 3. For EuO, EuS and EuTe we found a pressure induced increase of the isomer shift S with an initial slope of (&S/W)T = 5 .O, 11 .O and 14.8 (1 O-3 mm/s per kbar), respectively. The (negative) magnetic hyperfine field B in ferromagnetically ordered EuO

970

-40

I

I

I

-30

-20

I-ig. 3. MGssbauer

-10

spectra

0

of tu0

10

20

at various

30 4 ” [mm/sel

pressures

and EuS increases in magnitude with a slope of @B/S’), = -3.4 and --4.3 (10e2 T per kbar). Antiferromagnetically ordered EuTe shows no variation of B within the experimental error. A more conclusive presentation of the results can be obtained when the pressure variations of S and B are plotted as a function of the lattice parameter CI.The compressibilities (aln a/aQ were taken from different authors [9-l 11. The pressure induced variation of S as given in

52

U.F. Klein, G. Wortmann and G.M. Kalvius/High-pressureM6ssbauer study o f hyperfine interactions

11.5 S

-

87kbar

\

437kbar 2kbar ~Okbar

- 12£

%47kbar

~57kbar

~33kbar -

12.5

-

13.0

~Okbar EuO

~'.o

5'~

It

60

l

lattice constant [~,]

6'5

4.1. Isomer shifts

I

4C

B [T) 87kbar ~ 47kbar

-35

X

~:~

o O k b a r

÷

47kbar 32kbar

÷

~Okbar - 30

f i 1 i

57k~/ar[~

-25

EuO l

- 20

510

4. Discussion

EuS EuSe EuTe

I

Fig. 4. Pressure induced variations of the isomer shift of the 21.6 keV line of Is1Eu as a function of the lattice parameter. The values at ambient pressure are taken from ref. [ 13 ].

-

fig. 4 shows nearly the same slope with the lattice parameter a for EuO, EuS and EuTe: (aS/O I n a)T = --18 (2), --20 (1) and - 2 0 (3) mm/s, respectively. The various slopes of the hyperfine fields B with lattice parameter are shown in fig. 5. For comparison, the hyperfine fields o f EuSe and EuTe in the ferromagnetic state, as obtained from high-field measurements by Saner et al. [13], are also included. The measurements of Schwob [14] and preliminary results obtained in the present investigation indicate that the N6el temperature in EuTe is nearly pressure independent.

Okbar

EuS EuSe EuTe i

I

515 60l lattice constant [~,]

I .

61,5

Fig. 5. The magnetic hyperfine yields B at the Eu nucleus as a function of the lattice parameter. Full and open circles refer to hyperfine yields in ferromagnetic state, the triangle and squares correspond to the antiferromagnetic state in EuSe and EuTe, respectively. The values at ambient pressure are taken from ref. [ 13] (the hyperfine fields are extrapolated to the saturation values).

The pressure and volume dependence o f isomer shifts in compounds have been reviewed recently [4,15,16]. In the case of Eu 2+ the change o f electron density at the Eu nucleus can arise from the following mechanisms: (i) A compression o f the s-like part o f the valence electrons which remain partially (as a function of covalency) at the Eu ions. (ii) A compression of closed inner shells, primarily of the 5s 2 shell. (iii) A promotion o f a 4 f electron into the conduction band. Though the covalency changes considerably from EuO the EuTe [17], the volume coefficient of the isomer shift * is nearly the same in all three compounds. This leads us to the conclusion that mechanism (i) is of little importance for the volume coefficient o f S. Mechanism (iii) is considered to be responsible for the recently observed strong temperature dependence of the isomer shift in some divalent europium intermetallics [18]. In the europium chalcogenides, however, a promotion of a 4f electron into the conduction band does not occur in the pressure range used in the present work [11,12]. We therefore conclude that the dominant contri* The volume coefficient of S, which is more convenient for a comparison with other systems [4,5] is simply given for small volume changes by aS]a In V ~- aS~3 a In a.

U.F. Klein, G. I¢ortmann and G.M. Kalvius/High-pressure M6ssbauer study of hyperfine interactions

bution is mechanism (ii). This argument is further supported by the observation that the volume coefficients of S in Eu 3+ compounds are of the same magnitude (at maximum 30% smaller) as in the Eu 2÷ chalcogenides [4]. The pressure induced variations of S have a slope of aSia lna Ip ~ - 1 9 mm/sec for all the chalcogenides measured. The variation of S with the lattice parameter within the chemical series is much smaller and gives a value of as/a In a Ichem. ~--4 mm/s from fig. 4. From this it is evident that the increase of the lattice parameter within the chalcogenide series reflects mainly the different anion radii [2]. The small variation of S within the chemical series can be considered due to a "lattice pressure" which increases slightly from EuTe to the most ionic EuO. It is interesting in this connection that the variation of S with pressure as given above agrees surprisingly well with the variation of S found earlier in some binary and ternary Eu 2+ oxides, sulfides, and selenides [19]. Although one cannot conclude that the ionic radius of Eu 2÷ changes proportional to the lattice parameter in different chalcogenides, the nearly equal volume coefficients of S suggest at least that the Wigner Seitz cell of the Eu 2÷ ions undergoes with pressure in all investigated compounds the same proportional variation in respect to the volume of the unit cell.

4.2. Hyperfine fieMs The observed magnetic hyperfine field B inthe Eu chalcogenides results from different origins, which have been discussed extensively in the last few years [13,20-22]. They can be written in a simplified form as

B = B c +B n + B d . B e is the main contribution to B and arises from the corepolarization of the s-electron shells by the own S = 7/2 spin state of the 4f 7 electrons (B e ~. - 3 4 T). The nearest and nextnearest Eu neighbours account for the transferred hyperfine fields B 1 and B2, respectively. In the ferromagnetic state, these fields add to B n = B 1 + B2, where in the antiferromagnetic (MnOtype) ordering only the next-nearest neighbour shell contributes: B n = - B 2. It was recently demonstrated by a high-field study that the transferred fields con-

53

tribute considerably to B. In EuTe, for example, B 1 + 2B 2 was found to be - 5 T [13].B d stands for the dipole fields, which are small in comparison to B e and B n [13,22] and will be neglected. In spite of several serious efforts within the last years, the variation of B within the series of the Eu chalcogenides is still not full understood [13,20,21]. The hyperfine field B exhibits, in contrast to most other physical and chemical parameters, no systematic and monotonic variation with the lattice constant. This is also demonstrated by the fact that for EuO and EuS the pressure variation of B does not follow the trend within the chemical series (see fig. 5). A recent interpretation of magnetization measurements [21 ] excludes orbital admixtures to the S = 7/2 state of the 4f-electrons. The variation of B within the chamical series must thus be attributed to changes in the polarization of the outer shells of the Eu ions, e.g. the 5 s2 shell and the valence electrons. We shah give in the following a simple interpretation of the observed variation of B with pressure in terms o f J 1 and J2, the exchange integrals of the Eu ion with the first and second neighbour shell, respectively [ 1 - 3 ] . The transferred hyperfine field B1 originates from the direct overlap between nearest neighboured Eu •ions. The ferromagnetic exchange integral J1 corresponds to this overlap. It is wellknown that J1 depends sensitively on the Eu-Eu distance, which has been evidenced by the steep increase of the Curie temperatures in EuO and EuS with pressure [9,14]. The supertransferred fields B 2 result from the nextnearest neighboured Eu shell ions via the p-orbitals of the chalcogen ligands. B 2 should increase with the covalency of the ligands (which increases from EuO to EuTe) since it is connected with the antiferromagnetic exchange integral J2" It is known that J2 depends only weakly on the lattice parameter [ 1,23]. Considering the different origins of the transferred hyperfine fields B n in the ferromagnetic and antiferromagnetic state, one can explain the observed variation of B with pressure as follows: (i) In ferromagnetic EuO and EuS the increase (in magnitude) of the negative hyperfine field B is mainly caused by the rapidly increasing J l overlap. A possible mechanism for the increase of B is an overlap induced reduction of the positive spin-polarization of the 5s 2 shell [24]. (A compression of the 5s 2 shell

54

U.F. Klein, G. Wortmann and G.M. Kalvius/High-pressure MOssbauer study of hyperfine interactions

has been used above to explain the pressure dependence of the isomer shifts.) The supertransferred fields B 2 are assumed to be less sensitive to the lattice constant and contribute by a nearly constant amount to B. (ii) In antiferromagnetic EuTe, all contributions from the first cation shell cancel each other. The transferred field B 2 remains practically unchanged in analogy to the behaviour o f J 2 and the N6el temperature [14,23]. (The compression of the 5s 2 electron shell should produce in this case an almost constant spin polarization.) This discussion may be regarded as oversimplified, since it connects only phenomenologically the observed variation o r b with the exchange integrals J1 and J2- It is evident that theoretical calculations of charge and spin densities in the Eu-chalcogenides as a function of the lattice parameter are urgently needed for a more profound discussion. Such work is presently on the way [25]. Finally, it should be mentioned that the theoretical models which describe the magnetic ordering in the Eu chalcogenides predict a change from antiferromagnetism to ferromagnetism when the lattice constant undergoes a certain value (a < 6.2 A). It can be seen from fig. 5 that such a transition (which should result in a strong increase o r B ) has not yet occurred for EuTe within the pressure range presently applied.

Acknowledgement We gratefully acknowledge the fruitful discussions with Prof. W. Zinn and Dr. Ch. Sauer. We are indebted to Dr. E. Kaldis (ETH Zfirich), Mr. K. Fischer OFF JiJlich) and Dr. H. Pink (Siemens Forschungslabor Mfinchen) for providing us with the materials used in this study. This work was supported by the Deutsche Forschungsgemeinschaft.

References [1] S. Methfessel, Z. angew. Phys. 8 (1965) 414. [2] T. Kasuya, IBM J. Res. Develop. 14 (1970) 214. [3] P. Wachter, CRC-Critical Reviews in Sol. State Sciences 3 (1972) 189. [41 G.M. Kalvius, U.F. Klein and G. Wortmann, J. Physique Collq. 35, C6 (1974) 139. [5[ U.F. Klein, G. Wortmann and G.M. Kalvius, Sol. State Comm. 18 (1976) 291. [6] U.F. Klein, G. Wortmann and G.M. Kalvius, Proc. ICM-73, IV, (Publ. House Nauka, Moscow 1974) p. 149. [7] J. Schilling, U.F. Klein and W.B. Holzaptel, Rev. Sci. Instr. 45 (1974) 1353. [8] G. Crecelius and S. Hiifner, Phys. Lett. 30A (1969) 124. [9] D.B. McWhan,P.C. Souers and G. Jura, Phys. Rev. 143 (1966) 385. [10] F. Levy and P. Wachter, Sol. State Comm. 8 (1970) 183. [11] A. Jayaraman, A.K. Singh, A. Chatterjee and S. Usha Devi, Phys. Rev. B 9 (1974) 2513. [12] P. Wachter, Sol. State Comm. 7 (1969) 693. [13] Ch. Sauer, U. K6bler, W. Zinn and G.M. Kalvius, J. Physique Collq. 35, C6 (1974) 269. [14] P. Schwob, Phys. kondens. Materie 10 (1969) 186. [15] D.L. Williamson, preprint, in: M6ssbauer Isomer Shifts, Ed. G.K. Shenoy and F.E. Wagner (North-Holland, Amsterdam, 1976). [ 16] W.B. Holzapfel, CRC-Critical Reviews in Sol. State Science 5 (1975) 89. [17] G. Gerth and P. Kienle, K. Luchner, Phys. Lett. 27A (1968) 557. [18] E.R. Bauminger, I. Felner, D. Froindlich, D. Levron, I. Nowik, S. Ofer and R. Yanowsky, J. Physique Collq. 35 C6 (1974) 62. [19] O. Berkooz, J. Phys. Chem. Solids 30 (1969) 1763. [20] W. Zinn, J. Physique Collq. 32, CI (1971) 724. [21] U. K6bler and K.J. Fischer, Z. Physik B 20 (1975) 391. [221 N. Bykovetz, Sol. State Comm. 18 (1976) 143. [23] S.J. Cho, Phys. Rev. 157 (1967) 632. [24] R.E. Watson and A.J. Freeman, in: Hyperfine Interactions, Eds. A.J. Freeman and R.B. Frankel (Academic Press, N.Y., 1967). [25] E. Byron, D.E. Ellis and A.J. Freeman, Bull. Am. Soc. 20 (1975) 291.