HIGH FIELD STUDY OF HYPERFINE INTERACTIONS AND COVALENCY EFFECTS IN EUROPIUM-MONOCHALCOGENIDES

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HIGH FIELD STUDY OF HYPERFINE

INTERACTIONS AND COVALENCY EFFECTS IN EUROPIUM-MONOCHALCOGENIDES

Ch. Sauer, U. Köbler, W. Zinn, G. Kalvius

To cite this version:

Ch. Sauer, U. Köbler, W. Zinn, G. Kalvius. HIGH FIELD STUDY OF HYPERFINE INTER-

ACTIONS AND COVALENCY EFFECTS IN EUROPIUM-MONOCHALCOGENIDES. Journal de

Physique Colloques, 1974, 35 (C6), pp.C6-269-C6-274. �10.1051/jphyscol:1974640�. �jpa-00215797�

JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 12, Tome

35,

Ddcembre 1974, page C6-269

HIGH FIELD STUDY OF MYPERFINE INTERACTIONS

AND COVALENCY EFFECTS IN EUROPIUM-MONOCHALCOGENIDES

Ch. SAUER, U.

KOBLER,

W. ZINN

Institut fiir Festkorperforschung der Kernforschungsanlage Julich, D-5170 Julich, Germany and

G. M. KALVIUS

Physik Department der Technischen Universitat Munchen D-8046 Garching, James-Franck-Str., Germany

Rksume. - Les champs hyperfins et les deplacements isomkriques de la transition de 21,6 keV du noyau IslEu ont kt6 ktudies dans des champs magnetiques externes jusqu'ti 13 T dans 1'6tat ferro- magnetique sature des monochalcogCnides d'europium. Pour la phase antiferromagnktique du EuTe le champ hyperfin a la valeur de - 25,6 T et differe de - 5,O T du champ hyperfin dans la phase ferromagnetique, ce qui est dii aux contributions du champ transferre des voisins Eu. Les champs hyperfins dans l'btat ferromagnetique valent respectivement BI = - 30,5, - 33,0, - 32,8 et

-

30,6 T pour EuO, EuS, EuSe et EuTe. On les compare avec la grandeur R = g(S

+

^{1) qui est}

relike au moment magnetique ti = g p ~ S de E u ~ + . Elle a BtC determinke par des mesures de magnkti- sation et vaut 9,72,9,32,9,28 et 9,69 pour la m&me sequence que precedemment. Pour une configu- ration ideale 4f7-8S7/2 de l'ion Eu2+ on devrait avoir BI = - 34,2 T et R = 9,O. Nous suggkrons que les deviations observks rksultent des adjonctions orbitales, que nous supposons varier entre EuO et EuTe en relation avec la position du niveau 4f dans le schkma des bandes d'knergie.

Abstract. - The hyperfine fields and isomer shifts of the 21.6 keV transition of 1 5 lEu have been studied in external magnetic fields up to 13 T in the ferromagnetic saturated state of the Europium- monochalcogenides. For the antiferromagnetic phase of EuTe the h. f. field is - 25.6 T and differs by

-

5.0 T from the h. f. field in the ferromagnetic phase due to the transferred field contributions of the Eu-neighbours. The h. f. fields, BI, in the ferromagnetic state should be compared with the value of R = g(S

+

1) which is derived from the Euz+ magnetic moment, p = g p ~ S, as determined by magnetization measurements. The values measured in EuO, EuS, EuSe, and EuTe are BI = - 30.5, - 33.0, - 32.8, - 30.6 T, and R = 9.72, 9.32, 9.28, 9.69, respectively. For a pure 4f7-8S7/2 confi- guration of the Euz+-ion the values of Br and R are expected to be - 34.2 T and 9.0. The deviations are probably caused by orbital admixtures which are assumed to vary between EuO and EuTe in relation to the location of the 4f-level within the energy bands.

1. Introduction. - The Europium-Monochalco- genides EuO, EuS, EuSe, and EuTe form a series of cubic semiconducting magnetic materials. Their phy- sical properties have been investigated intensively using various methods (see e. g. P. Wachter [1] for

a

recent review). In particular, the ferromagnets EuO and EuS (T, = 69 K and 16.6 K, respectively) may be considered the best existing examples of Heisenberg ferromagnets. EuSe shows a metamagnetic behaviour, i. e. a complex antiferromagnetic structure between 2.8 K and the ordering temperature (TN = 4.6 K), a ferromagnetic phase between 1.8

K

and 2.8

K,

and the MnO-type antiferromagnetic structure below 1.8

K.

EuTe also forms the MnO-spin structure below T, =

9.6 K.

The magnetic phase transition into the ferromagnetic state occurs at applied external fields of

about 0.6 tesla (1 T =

1

Vsm-2 = 10 kG) and

8

T for EuSe and EuTe, respectively.

Most physical properties, such as the exchange constants

(J,

and J,) 121, the width of the energy-band gap (E,) and bonding character [I], or the isomer shift [3], vary in a monotonic and systematic way with the lattice constant (a,) which is smallest for EuO and largest for EuTe. The magnetic hyperfine interactions, however, as deduced from numerous Mossbauer-effect and n. m. r. studies using I5'Eu- and 153Eu-resonances, do not vary likewise systematically. In a previous paper [4] the influence of the transferred hyperfine interactions from nearest and next nearest Europium- neighbours has been demonstrated and used to explain the n. m. r. data of the different magnetic phases of metamagnetic EuSe. For EuTe no such data have been

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974640

available and only extrapolations could be carried out using the values of EuSe.

It is the aim of the present investigation to provide the needed data for EuTe by Mossbauer spectroscopy of the 21.6 keV transition in 15'Eu in high external fields up to 13 T. In order to ensure a good systematic comparison, similar data were also collected for well characterized samples of the other Eu-monochalco- genides. The hyperfine fields of the saturated ferro- magnetic state were all obtained by extrapolating the high field data to zero applied field. For comparison, a high field magnetization study has been carried out using the same samples. The present data are also com- pared with the results of recent high pressure experi- ments on EuO and EuS [5].

2.

Experimental. -

Chalcogenide samples (single- crystals or polycrystalline bulk material) from different sources and grown by different techniques were used for this investigation. The selection of good samples was mainly done on the basis of precise magnetization measurements. A compilation of relevant data is given in table I.

The magnetization measurements have been carried out using a Faraday balance system (sensitivity p) with both the magnetic field and the field gradient variable up to B,

⁼

8 T and gradB,

⁼

0.1 T/cm, respectively. From the M(B,,

T)

dependence mea- sured both in the ferro- and paramagnetic regime

(2 <

T

< 250 K) we derived the Curie-constant,

C =

N&

^g2

S(S + 1)/3 k, the Curie-Weiss-tempera- ture,

8,,

and the saturation magnetization,

M(0,O)

⁼ M , ⁼ NpB

gS .

The ratio R

=

3 kC/(p, M,)

=

g(S + 1) is indepen- dent of the number of Eu2+-cations per unit volume (N).

R has been determined here with sufficient precision to establish a systematic behaviour within this series of compounds. The measured values of R in good samples of the chalcogenides show deviations up to 8 % from the theoretical value R,

=

2.(7/2 + 1)

=

9, which is expected for a pure 'S,,, - ground state of the Eu2+-ion.

Samples for the Mossbauer measurements were prepared by grinding the selected materials into par- ticles with a mean diameter of 5 pm. The powdered material was then diluted in a ratio of about 1

:

200 with outgased graphite powder, and pressed into sealed acrylic absorber holders of 20 mm diameter. All these procedures were carried out in a pure argon atmo- sphere. The absorber thickness was 10-15 mg/cm2 of 151Eu. The absorber was located in the center of the bore (2" /a) of a Nb3Sn superconducting coil system capable of producing fields up to 13 T. The source was 350 mCi of 151Sm,03. Both, source and absorber were kept at a temperature of 4.2 K. An additional NbTi superconducting coil allowed the compensation of the stray field of the main solenoid at the source which was moved sinusoidally by 25 Hz. The data were analyzed with a least squares fitting routine using a superposition of Lorentzian lines and taking into account t h e , (fit11 or partial) polarization in the absorber spectrum. In the analysis of the hyperfine fields it has been assumed that the chalcogenide par- ticles are roughly spherical and widely separated from each other. Then, after ferromagnetic saturation of all particles is obtained, the positive external field acts

Characterization of the samples Compound

-

Sample No.

Source Cryst. State Method Tc(K)

Q(K)

g(S + 1)

AM(Bo

= 10

T)

M(0) (%I

EuO

-

15

("1

S. C.

M 69.6 75.0 9.72

EuS EuSe

- -

66 72

("1

^{( b )}

S. C. S.

C.

S M

16.6

^-

18.0 8.2

9.32 9.28

(")

ETH Ziirich (Dr. E. Kaldis, Dr. 0. Vogt).

( b )

MIT Lincoln Lab. (Dr. T. B. Reed).

(')

KFA Jiilich, Inst. f. Festkorperforschung (K. Fischer).

s. c. single crystal.

S

=

Grown by sublimation.

M

=

Melt-grown with excess europium.

V(I)

=

Vapor-grown by iodine-transport.

(*) =

see also ref. [6].

undoped

-

384

(a)

(*I

S. C.

V(I)

-

- 4.0 9.69

-

+ ^2.5

EuTe

I-doped - 919

(")

S.

^C.

V(I!

+ - 9.0

-

HIGH FIELD STUDY OF HYPERFINE INTERACTIONS AND COVALENCY EFFECTS C6-27 1

fully on the nuclei in addition to the negative hyperfine field.

3.

Results. -

In figure 1 two 151Eu-Mosssbauer effect spectra of EuS measured at nearly zero and at maximum external field are shown as an example. In figure 2 the effective magnetic fields, Be,,, as deduced from the measured 151Eu-Mossbauer spectra are

FIG. 1. - Mossbauer effect spectra at 4.2 K of EuS (sample no 66 of table I) with and without an external field Bo applied.

The solid line is the best fit to the data points.

EXTERNAL FIELD Bo

-

FIG.

2. - Effective magnetic field, Beff, deduced from the Mossbauer effect spectra as a function of the external field, Bo.

Extrapolations to zero temperature are indicated by horizontal bars. The experimental errors are given by the size of the data points. The arrow pairs QJ. and

tt

label the antiferromagnetic

and ferromagnetic ordered state, respectively.

plotted as a function of the external field, B,. Be,, is the sum of three contributions, namely the hyperfine field, B,, the dipolar field Bd, and the external field, B,

:

With S-state ions such as Eu2+ the hyperfine field at a lattice site

i

is given by

B ~ , i = A ~ , i . < S z > i + C A,,i.< S z > j . (la)

i f i

The first term arises mainly from the core s-electron polarization caused by the average z-component,

<

^S,

>i, of the atom's own 4f-spin. The second term accounts for all transferred hyperfine interactions originating from the electron spin polarization pro- duced by the Eu2+-neighbours having the average spin components <

S,

>j.

The dipolar field contribution, B, in eq. (I), reduces in a ferromagnet of cubic lattice symmetry like the Eu-monochalcogenides to

:

(ND is the demagnetization factor, the value 3 stems from the Lorentz field,

p,

is the vacuum permeability and M(T) is the magnetization). For spherical sample shape one finds in particular, N ,

=

3 and thus B,

=

0 which leads to Be,,

=

B, + B,. For a ferromagneti- cally saturated sample (i. e. M(T)

⁼ M,)

the values of B, and Bd will both reach saturation and then Be,, will vary linearly with the external field B,. Hence, in the Be,, vs. B,-plot of figure 2 the slope is expected to be equal to one. In fact, small deviations from the theore- tical slope are observed for all samples investigated.

They indicate the presence of small additional field contributions, AB,,,, which are positive and propor- tional to B, i. e.

:

ABeff

=

ai.Bo.

The value of ABeff seems to depend on sample imperfections and at B,

=

10 T we obtain values of ABe,,/B,(0) between 5 x l o p 3 and lo-' (see also Table I). The connection between ABefs and lattice imperfections is supported by comparing the data of doped and undoped EuTe in figure 2 and table I. The probable correlation between AB,,,

⁼

ai.Bo and the additional para- or diamagnetic contributions, AM

=

Axi. H,, as they appear in high-field magnetiza- tion measurements could not be established unambi- gously at present (see Table I).

Despite the small deviation of the slopes of the Be,,

vs.

B,

curves from one at high external fields their

extrapolation to zero external field should

give

the

correct value, B,(O), for the hyperfine field in the ferro-

magnetically saturated state. Generally, with samples

consisting of small ferromagnetic particles, like the EuO

and EuS absorbers described above, the effective field,

B,,(O), observed at zero applied field will deviate from

BI(0) by the mean dipolar field &(o) within the sample

according to eq.

(1)

and

(lb).

&(o) is determined by the

mean demagnetization factor x,, which depends on

both, the geometry of the sample and the domain

structure within the individual absorber particles.

~ i n c e K ( 0 ) is difficult to estimate, it is usually neglected.

This may at least partially account for the scatter in the Beff(0) data reported by different authors in the literature. As can be seen from figure 2, the differences, Bef,(0) - B,(O), are 1.5 T for EuO and 0.8 T

(*)

for EuS. This agrees fairly well with estimates of ~ ~ ( 0 ) for our sample geometry. It is also consistent with an increase of the linewidth by about 20 % observed in the Mossbauer spectra at high external fields. This is expected for ferromagnetic saturated particles if their individual shapes deviate from the spherical form. EuSe and EuTe form antiferromagnetic structures at 4.2 K, where Bd vanishes and, hence, for zero external fields one must have B;:~(O)

=

B['(o). The observed diffe- rences

can only arise from differences in the second term of eq. (la), i. e. the different contributions of the transferred h. f. fields in the antiferromagnetic and ferromagnetic spin state, respectively. We follow a previous extensive discussion of these transferred fields [4] by considering only the contributions of the 12 nearest neighbours, AB,, and the 6 next nearest neighbours, AB2, of the f. c. c. Eu-sublattice of the EuX-compounds. Then eq. ( l a ) can be approximated for the ferromagnetic state by

For the antiferromagnetic MnO-type structure one finds

The difference between the hyperfine fields as shown in figure 2 can now be expressed in the form

In figure 3 the results for BI' ^and

^B/'

as deduced from figure 2 are plotted as a function of the measured isomer shifts (aIs). For the purpose of comparison and for the discussion of the systematics of the hyperfine fields in the Eu-monochalcogenides we have also included in figure 3 the results of a recent high-pressure Mossbauer effect study of EuO and EuS

151,

the hyper- fine field of the Eu-configuration 4f7 as deduced by means of ENDOR-techniques

[7]

and the values R

=

g(S

i-

1) following from our magnetization mea- surements. The relevant data of BT', B?', ABI and are all compiled in table 11.

4.

Discussion.

- The results of the present high- field study on the h. f, splitting in the Eu-chalcogenide

(*) Note that for EuS the measured value B ~ P E (0, 4.2 K ) without an external field had to be extrapolated first to its zero temperature value, Beff(O, O), as indicated by the arrow in figure 2.

S

-

ELECTRON DENSITY INCREASING

-

I

ISOMER SHIFT

-

mn

0

W

FIG. 3. - Summary of the results on R = g(S

+

¹⁾ (see Table I) as deduced from magnetization measurements (a) and of the hyperfine data (b) obtained from figure 2. The symbols used to label the different EuX-compounds and their spin structures are the same as in figure 2. Included are the high-pressure results of reference [5]. Both results are plotted as a function of the isomer shift. The s-electron density decreases monotonically with the lattice constant (see Table 11). The slope angles a, B, and y are

discussed in the text.

Z

LL E

compounds are compiled in figure 3. They confirm the previously discussed [4] non-monotonic behaviour of the h. f. fields for the identical saturated spin struc- tures. In particular, between EuTe and EuSe the h. f.

fields for both, the ferromagnetic and antiferromagnetic state, vary as a function of the isomer shift (resp. of the lattice constant) in the same way as exhibited by the high-pressure data of EuO and EuS

[ 5 ] .

That is

:

B, increases in absolute magnitude with decreasing lattice constant or increasing s-electron density. Between EuS and EuO, however, the h. f. fields decreases in absolute magnitude despite the further increasing s-electron densities (i, e. decreasing lattice constant). A possible reason for this behaviour is brought out by a compari- son with our data on R

⁼

g(S + 1) as plotted in figure 3a. The data on R exhibit the very same variation over the Eu-chalcogenide series as the B, data shown in figure 3b. The deviations of R from the value expect- ed for a pure 4f7

-

's,,, groundstate of

EU",

i. e.

R, =

9, suggest the existence of orbital contributions being largest at the extreme cases (i. e. for EuTe and EuO), and smallest in between (i. e. for EuSe and EuS). In the well-known energy band scheme of the Eu-chalcogenides the 4f7-states are located nearest to the 5d-conduction band states in EuO, but nearest to

- -

_'-0.- ^{87 kbor}

w - 4 0 ^{I I} I

-13.0 -12.0 mrnlsec -11.0

HIGH FIELD STUDY OF HYPERFINE INTERACTIONS AND COVALENCY EFFECTS C6-273

Hyper-ne data and lattice constants of the magnetically saturated Eu-monochalcogenides

Lattice

B/

(0)

B/

"0)

61s

constant

Compound

- ( T )

- ^(TI

- (TI

-

(mmis)

- ^a0

(A)

-

EuO - 30.5 + ^{0.5 -}

^{- -}

^{11.92 + 0.1 5.141}

- 30.77 (1)

(")

EuS - 33.0

f

0.5

^-

-

-

12.56 + 0.1 5.968

- 3 3.54(0)

(*)

EuSe - 32.8

f

0.5

-

26.2

t- 0.25 [4]

- (6.6 + 0.5)

-

12.74 rt 0.1 6.195 EuTe - 30.6

k 0.5 -

25.6 + ^0.3

^-

^(5.0 rt 0.5)

-

12.85 f 0.1 6.598

(*)

NMR measurements [8].

the (p, s)-valence band states in EuTe (see ref. [I]).

Hence the orbital admixtures to the 4f7-configuration which could cause the increase of both

R

and B, are likely to have preferred d-character with EuO, but predominant p-character with EuTe. As a consequence of this configuration mixing being different in character, magnitude, and sign, the radial extension, width, and overlap of the 4f-, 5d- and 5s-electrons of the Euf

^+-

cations change in various ways. These assumptions are consistent with suggestions recently used by U. Klein

et al. [5] to explain the variation of the h. f. fields and

isomer shifts of EuO and EuS under pressure. The application of pressure on the order of 50 kbar reduces the lattice constant as well as the energy gap between the 4f- and Sd-bands and between the valence and conduction bands. In particular, the proposed admix- ture of p- and d-orbital contributions into the 4f7- configuration would cause a reduction of the screening of the 5s-electrons of the Eu2

+

-ion which in fact should be largest for EuO. The increasing 5d-admixture then would account for both, the observed increase of the s-electron densities between EuTe and EuO, and the unexpected large positive contribution to B, as seen in EuO. Consistent with this explanation is the trend towards positive values for the two exchange constants,

J ,

and J , which has recently been demonstrated experi- mentally [2]. Therefore, an additional term

independently. In addition, the transferred hf-fields

AB,

and AB, will have to be deduced separately for EuO and EuS, in order to determine

BLri.

The change in sign of

J , between EuS and EuO [2] prohibits a

direct comparison. Further interesting features appa- rent from this investigation are the different slopes of the B, versus a,, dependences marked as

a,

P and

y

in figure 3. The slope is largest for the ferromagnetic phase of EuTe, i. e.

y z -

25 Tlmmls, and smallest for the antiferromagnetic EuTe phase, i. e.

p ^z

^-

5.5 T/mm/s. The difference,

y -

B, is supposed to be due to the cancellation of all nearest neighbour contributions to

B,

in the antiferromagnetic MnO- structure [4]. It would be interesting in this connection to investigate the slope under pressure, in order to determine its deviation from the value of roughly

a = -

7 T/mm/s deduced for the ferromagnetic EuS and EuO.

5. Conclusion. - The present high-field Mossbauer effect and magnetization study has shown that it may be possible to correlate the systematic behaviour of the h. f. data in the series of the Europium-monochalco- genides to orbital admixtures of the Eu2+-4f 7-8S7,2 configuration. These admixtures are presumably con- trolled mainly by the well established different bonding character and by the location of the 4f-states within the energy band schemes within the chalco- genide series.

describing the orbital contribution, should be included

in eq. (la) and (2) besides the predominant Fermi

Acknowledgments. -

We are indebted to contact term, ASJi <

^S,

> arising from the core Dr. E. Kaldis and Dr. 0 . Vogt (ETH Zurich), to s-electron polarization

[9].

The expectation value Dr. T. B. Reed (MIT Lincoln Lab.), and to

<

L,

> can in principle be derived from the observed Mr. K. Fischer of our institute for providing us with

deviation of R from R,. For a quantitative determina- the materials used in this study. One of us (GMK)

tion, however, the g-factor in R must be measured would like to thank the IFF Jiilich for its hospitality.

Ch. S'AUER, U. YOBLER, W. ZINN AND G. M. KALVIUS

References

[I] WACHTER, P., CRC-Critical Reviews in Solid State Sciences 3 (1972) 189.

[2] ALS-NIELSEN, J., DIETRICH, 0. W., KUNNMANN, W. and PASSELL, L., Phys. Rev. Lett. 27 (1971) 741 (see also BNL Report 18284).

[3] GERTH, G., KIENLE, P. and LUCHNER, K., Phys. Lett. 27A (1968) 557.

[4] ZINN, W., J. Physique Collq. 32 (1971) Cl-724.

[5] KLEIN, U. F., WORTMANN, G. and KALVIUS, G. M., Proceed.

ICM 73, Vol. I11 (1973) 149 ; see also.

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

(( Volume Dependence of Hf-Interactions ^D, J. Physique Collq. 35 (1974) C6-139.

[6] VITINS, J., WACHTER, P., Solid State Conmun. 13 (1973) 1273.

[7] BAKER, J. M. and WILLIAMS, F. I. B., PYOC. R. SOC. A 267 (1962) 283.

[8] ARONS, R. R., LUTGEMEIER, H., BOHN, H. G., this laboratory, unpublished results.

[9] FREEMAN, A. J. and WATSON, R. E., Phys. Rev. 127 (1962) 2058.