INVESTIGATION OF MAGNETIC STRUCTURES AND PHASE TRANSITIONS IN NiS2 BY 61Ni-MÖSSBAUER SPECTROSCOPY

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INVESTIGATION OF MAGNETIC STRUCTURES AND PHASE TRANSITIONS IN NiS2 BY

61Ni-MÖSSBAUER SPECTROSCOPY

G. Czjzek, J. Fink, H. Schmidt, G. Krill, F. Gautier, M. Lapierre, Cédric Robert

To cite this version:

G. Czjzek, J. Fink, H. Schmidt, G. Krill, F. Gautier, et al.. INVESTIGATION OF MAGNETIC STRUCTURES AND PHASE TRANSITIONS IN NiS2 BY 61Ni-MÖSSBAUER SPECTROSCOPY.

Journal de Physique Colloques, 1974, 35 (C6), pp.C6-621-C6-623. �10.1051/jphyscol:19746134�. �jpa- 00215748�

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, Ddcembre 1974, page C6-621

INVESTIGATION OF MAGNETIC STRUCTURES AND PHASE TRANSITIONS IN NiS, BY ~i-MOSSBAUER SPECTROSCOPY

G. CZJZEK, J. FINK, H. SCHMIDT Institut fiir Angewandte Kernphysik Kernforschungszentrum Karlsruhe (GFR)

G. KRILL, F. GAUTIER, M. F. LAPIERRE and C . ROBERT Laboratoire de Structure Eiectronique des Solides,

UniversitC Louis-Pasteur, Strasbourg, France

RksumB. - Les ktudes des interactions hyperfines magnktiques dipolaires et klectriques quadru- polaires aux noyaux ⁶1Ni dans NiS 1 , 9 3 de la structure cubique de pyrite dans la rtgion de tempkratures 4,2 K < T < 50 K ont mene aux rksultats suivants. Entre 30,9 K et T N = 445 K les moments magnttiques des ions nickel sont alignks parallbles aux axes locaux de symktrie trigonale. Au-dessous de 30,9 K nous trouvons deux sites de nickel distingub par des angles diffkrents entre la direction du moment magnetique et l'axe local de symktrie. Pour a peu prQ 50 % des ions nickel la direction du moment commence a changer a (30,9 f 0,l) K, pour les autres

a

(30,2 f. 0,l) K. Dune analyse des champs hyperfins magnktiques au sens des interactions dipale-dipale anisotropes et des rapports des constantes de couplage quadrupolaire nous dkrivons des angles de 77O et de 39O entre les moments magnktiques et les axes de symktrie pour les deux sites. Qualitativement les mgmes rksultats sont obtenus pour NiS1,96 et N ~ S Z , ~ ~ dans la region de basses tempkratures avec des petits changements de tempkratures critiques et des parametres d'interactions hyperfines. I1 parait, pourtant, que T N croit rapidement avec la teneur de soufre. Une distribution statique des moments magnktiques ou des effets de relaxation mkne a un klargissement significatif des raies d'absorption entre 50 K et T N .

Abstract. - Measurements of the magnetic dipole and electric quadrupole interactions at 61Ni in NiS ⁹³with cubic pyrite structure in the temperature region 4.2 K

<

T < 50 K have yielded the following results. Between 30.9 K and TN = 44.5 K the magnetic moments on Ni ions are aligned parallel to the local trigonal symmetry axis. Below 30.9 K we find two nickel sites distinguished by different angles between the direction of the magnetic moment and the local symmetry axis. For about 50 % of the nickel ions the moment direction begins to change at (30.9 f 0.1) K, for the other ones at (30.2 & 0.1) K. From an analysis of the magnetic hyperfine fields in terms of aniso- tropic dipoledipole interactions and from the ratios of the effective quadrupole constants we derive values of 770 and 390 for the angles between magnetic moments and symmetry axes at the two sites.

Qualitatively the same results are obtained for NiS1.96 and NiS2.00 in the low-temperature region with small changes of critical temperatures and hyperfine-interaction parameters. However, T N appears to increase rapidly with increasing sulfur content. Either a static distribution of magnetic moments or relaxation effects lead to a significant broadening of the absorption lines in the temperature region between 50 K and T N .

Recent studies of NiS, with cubic pyrite structure by neutron diffraction [I] and by measurements of magnetic susceptibility and electrical resistivity [2, 31 have established the occurrence of two magnetic phase transitions in this compound for compositions slightly deficient in sulfur content. Near the composition NiS,,, antiferromagnetic order is found below

TN

-

40-50 K, and the material becomes weakly ferromagnetic below T,

-

30-31 K. For both phases, the magnetic moment arrangements have not yet been determined.

We have now investigated the temperature dependence of hyperfine interactions a t 61Ni nuclei in a sample with composition NiS,,,, in the temperature range 4.2 K I T

<

⁵⁰K by Mossbauer spectroscopy.

The experimental results are summarized in figure 1.

Some preliminary results have also been obtained with samples of NiS,.,, and NiS2.00, prepared by high-pressure synthesis with nickel enriched t o 75

%

61Ni. X-ray diffraction and susceptibility measurements of all samples gave results in agreement with those reported previously [2,3]. The apparatus, experimental techniques, and data analysis procedures were the same as described in [4].

For T 2 45 K (paramagnetic phase) we find a slightly broadened single line. By comparison with the unsplit absorption spectrum of a NiV (14

%)

absorber, taken with the same source immediately after the spectrum of NiS,.,,, we deduce a quadrupole coupling constant

I

vQ

I

⁼

I

e2 qQ/h

I

= (8

+

^{3) MHz.}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746134

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C6-622 G. CZJZEK, J, FINK, H. SCHMIDT, G. KRILL, F. GAUTIER, M. F. LAPIERRE AND C. ROBERT

100.

8 0 -

X

60-

LO-

2 0 -

FIG. 1 . -Temperature dependence of hyperfine interaction parameters in NiS1.93. antiferromagnetic phase, x a-sites, 0 8-sites. (a) Magnetic hyperfine field, (b) effective quadrupole coupling constant, (c) fraction of 61Ni-nuclei occupying 8-sites.

For 31 K 5 T 5 44 K (antiferromagnetic phase) the 61Ni nuclei experience a magnetic hyperfine field H,, whose temperature dependence is quite well described by the molecular-field approximation with T, =44.5 K and

I

^Hhf(T⁼⁰⁾

¹

⁼47 kG. The effective quadrupole coupling constant, v? = '+(3 cos2 8 - l).e2 qQ/h, where 8 is the angle between the direction of Hhf and the principal axis of the field-gradient tensor, is given by v? = (- 5.8

+

0.2) MHz, independent of temperature.

From the fact that we find the same magnetic hyperfine field and the same,value of v? for all 61Ni nuclei in conjunction with the results of neutron diffraction [I]

we conclude that the magnetic moment on any nickel ion is parallel to that (Ill)-direction which is the local trigonal symmetry axis. That is, 8 = 0, and the experimental value of v? is equal to va = e2 qQ/h.

At (30.9

+

0.1) K a discontinuity of the slope of the Hhf-vs.-T-curve is observed for about 50

%

of the 61Ni nuclei (p-sites), whereas the field on the other nuclei (a-sites) remains practically constant down to (30.2 f 0.1) K. For temperatures below 30.2 K the field on all 61Ni nuclei deviates from the curve extra- polated from the antiferromagnetic phase (Fig. la).

However, in the entire temperature region below

30.9 K, we find two nickel sites with different values of Hhf (Fig. la) and of v? (Fig. Ib). The effective quadrupole coupling constants change gradually between 30.9 K and 29 K. Below 29 K we find the values

V Q , eff = (- 2.8

+

0.1) MHz, x$,> = (2.4 +0.2)MHz, independent of temperature. Between about 29 K and 25 K the fraction of 61Ni nuclei in p-sites decreases gradually from about 50

%

to about 25

%

(Fig. Ic).

The changes of Hhf near 30 K could be caused by similar changes of the values of the nickel moments.

However, this would lead to corresponding changes of the intensities of magnetic powder diffraction lines which have not been reported in the literature.

Therefore it seems more likely that the observed effects are caused by rotations of the magnetic moments in conjunction with anisotropic hyperfine interactions.

The Hamiltonian for the magnetic hyperfine interaction in a coordinate system with the z-axis parallel to the direction of the magnetic moment can be written in the form :

H,,,,,, = ~,,,~l[Ao

+ t

AD. (3 cos2 8 - I)] S,. I, - - $ pnuC1 AD. sin 8.cos 0. S,. (I,

+

I-) (1) with the usual notation for the nuclear spin operators I,, I+ = I,

+

iIy, I- = I, - iIy and the electronic spin operator S,. The angle 8 is again that between the moment direction and the symmetry axis. The quantity A, is determined by the isotropic part of the hyperfine interaction : the contact term and the interaction with the orbital moment. The anisotropic part, A,, originates from the dipole-dipole interaction between electronic and nuclear spins. The orbital term may also contribute to the anisotropy if the electronic g-tensor is anisotropic [5]. As EPR measurements on nickel ions in Fe,-,Ni,S2 with the same structure have shown that in this case g, is very nearly the same as g, [6], we assume that the anisotropy in NiS, can be ascribed entirely to the dipole-dipole interaction.

Treating the anisotropic part of the Hamiltonian, eq. (I), as a perturbation to second order, we can express the magnetic hyperfine interaction in terms of an effective magnetic hyperfine field, given by

As no significant structural changes have been observed in the critical temperature region, we analyze our results assuming changes only of the moment directions, that is of the angle 8, not of the intrinsic hyperfine interaction parameters A,, AD and vQ. From a comparison of the experimental values of the hyperfine fields

~ l ' f ' f ,

and H;& at 4.2 K with the value extra- polated from the antiferromagnetic phase, which we set equal to H$' (8 = 0) = - A,.

<

^S,

> .

^[I

⁺

^A,/A,]

and using the ratio

eff eff

v ~ , , / v Q , ~ = (3. C O S ~ 8, - 1)/(3.cos2 Os - 1) ⁼- 1.16

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INVESTIGATION OF MAGNETIC STRUCTURES AND PHASE TRANSITIONS C6-623 we obtain the values 8, ⁼390, OD ⁼770, both with an

uncertainty of about 10

%.

The same values for these angles are obtained from a comparison of

vca

^and

vZB

with the value of vQ measured in the antiferromagnetic phase. This agreement lends support to the assumptions involved in the derivation.

For the hyperfine interaction parameters we derive the values :

with A,/A, < 0. The dipole-dipole term A, is closely related to the contribution of the valence electrons to the electric-field gradient, both quantities being proportional to

<

(3 cos2 8 - l)/r3

>

151. Using

<

^S,

>

⁼1 . 2 , ~ ~ as given by Hastings and Corliss [I], we derive :

I <

(3cos28 - l)/r3

1

⁼^3.6^x1 0 2 4 ~ m - 3 .

For the quadrupole coupling constant we have to account for the screening factor (1 - R). As no calculation of the value of R for Ni2+ is known to us, we estimate it from a comparison with iron to be about 0.3. Then we obtain

<

(3 cos2 8 - l)/r3

>

⁼- 1.5 x 1 0 2 4 ~ m - 3 . The discrepancy between these two results may have

several causes : (a) an anisotropic contribution of the electronic orbital moment, (b) a larger value of the nickel moment than that given by Hastings and Corliss, (c) a positive lattice contribution to the electric field gradient, partially compensating the gradient due to thevalence electrons, and ( d ) a different radial distribution of spin and charge densities.

In the samples of composition NiS,.,, and NiS,.,, we find essentially the same phenomena as in NiS,.,, in the temperature region below 45 K. With increasing sulfur content, the critical temperature for the moment rotation increases slightly to about 34 K for NiS,.,,.

The magnetic hyperfine fields at 4.2 K decrease only by a few percent. As the bulk magnetization decreases very rapidly with increasing sulfur content in this concentration region, the directions of the magnetic moments must change to an antiferromagnetic arran- gement.

The NCel temperature increases rapidly with the sulfur content. In NiS,.,, we found so far a magnetic splitting up to 70 K. At the same time, the absorption lines are broadened strongly for T 2 50 K. This can be due either to a static distribution of magnetic moments or to relaxation effects.

References

[I] HASTINGS, J. M. and CORLISS, L. M., IBM J. Res. Develop. COEY, J. M. D. and BRUSETTI, R., to be published in

14 (1970) 227. J. Physique.

121 GAUTIER, F., KRILL, G., LAPIERRE, M. F. and ROBERT, C., [5] JOHNSON, C. E., in Hyperfine Structure and Nuclear Radia- Solid State Commun. 11 (1972) 1201. tions, Matthias, E. and Shirley, D. A,, eds. (North- [3] GAUTIER, F., KRILL, G., LAPIERRE, M. F. and ROBERT, C., Holland Amsterdam, 1968) p. 226.

J. Phy.~. C : Solid State Phys. 6 (1973) L 320. [6] CHANDLER, R. N. and BENE, R. W., Phys. Rev. B 8 (1973) [4] FINK, J., CZJZEK, G., SCHMIDT, H., RUEBENBAUER, K., 4979.