MAGNETO-OPTIC KERR EFFECT AND ELECTRONIC STRUCTURE OF Fe1/3TaS2

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MAGNETO-OPTIC KERR EFFECT AND

ELECTRONIC STRUCTURE OF Fe1/3TaS2

J. Wijngaard, J. Dijkstra, R. de Groot, H. Feil, C. Haas

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplbment au no 12, Tome 49, dbcembre 1988

MAGNETO-OPTIC KERR EFFECT

AND

ELECTRONIC STRUCTURE OF

J. W. Wijngaard (I), J. Dijkstra (2), R. A. De Groot (3), H. Feil (2) and C. Haas (I)

(I) Laboratory of Inorganic Chemistry, Materials Science centre, University of Groningen, Nijenborgh 16, 9747

AG Groningen, The Netherlands

(') Philips Reserarch Labs., Eindhoven, The Netherlands

(3) Research Institute for Materials, University of Nijmegen, The Netherlands

Abstract. - Fel13TaS2 is a ferromagnetic intercalation compound. Band structure calculations show various deviations from the rigid band model. We suggest that the measured Kerr spectrum is due to four transitions with paramagnetic line shape from states situated near the Fermi level to higher lying Ta-states.

Introduction

2H-TaSz is a layer compound build of S-T*S sandwiches. Each sandwich consists of hexagonal closepa- cked layers of Ta and S. Ta is coordinated by a trig- onal prism of S. Bonding in the sandwiches is pre- dominantly covalent, while the sandwiches are weakly bonded to each other by the Van der Waals interaction. In the space between the sandwiches, the Van der Waals gap, a variety of atoms and molecules can be intercalated, among which 3d transition metal atoms (V, Cr, Mn, Fe, Co, Ni). In the structure only the c-axis is elongated during intercalation. Around intercalant concentrations 1/3 and 1/4 ordered

&

x

&

and 2

x

2 superstructures are observed [I]. The intercalated 3d ions have a trigonally-distorted octahedral coordination by S.

cal Wave (ASW) method [4]. Scalar relativistic effects were included [5]. The used muffin tin sphere radii are:

1 373

.&

for Ta and 1 772

A

for S in 2H-TaS2, 1 756

A

for S, 1 137

.&

for Ta and Fe, and an empty sphere in Fel13TaSz. The empty spheres are placed a t the octahedral positions between the sandwiches that are not occupied by Fe.

The density of states (DOS) of 2H-TaSz and the partial DOS of the contributing atoms are given in figure 1. The band around an energy of -13 eV is the S 3s-band, the band from -7 to -112 eV has predom- inantly S-3p character. The Ta dq-band lies a t the Fermi level and is split off from the res3 of the d-bands that ly a t higher energy. The gap between the S 3p- and Ta dq-bands is just closed.

In this paper we report band structure calculations

of Fell3TaS2 and magnet~optic Kerr effect measure- E~

ments of Fe0.2sTaSz. The cell parameters of this compound are a = 5.763

A

and c = 12.277

A.

In Fel/3TaS2

there are two different kinds of Ta atoms, one with and ₀

_-

(b)Ta

-

one without a direct Fe neighbour.

Feo.z~TaS2 was synthesized by heating together

proper amounts of the elements at 850 OC for ten days. _-15 _-10 _{- 5} ₀ ₅ After repowdering there was heated again for ten days. E n e r g y ( e U )

Single crystals were grown by iodine vapour transport

Fig. 1. - The DOS of TaSz and partid DOS of the con- in a temperature gradient from 950 t o 800 OC during _tributing_atoms.

15 days. X-ray diffraction showed the

6

x

6

su- perstructure. Feo.zsTaS2 is ferromagnetic with a mea-

sured Curie temperature of 70 K. Fe has its magnetic In figure 2 the DOS and the partial DOS of the ma- moment perpendicular to the layers [2]. jority

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and minority

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electrons of the contributing Electronic s t r u c t u r e

The electronic structure of TaSz intercalates is usu- ally described by the rigid band model [3]. According to this model the only change during intercalation is a shift of the Fermi level to higher energies due t o charge transfer from the intercalant to Ta. Reflectivity measurements [3] show that this model is not sufficient.

Band structure calculations were performed for

atoms are given for ferromagnetic FellaTaSz. In the Fe 3d-band we see clearly a t2,-e, splitting. The TaS2 part strongly resembles that of 2H-TaSz. A remarkable difference is the increased S 3p/Ta 5d gap, which is at variance with the rigid band model. The reason for this is the charge transfer from Fe to Ta, which results in more electropositive Ta inducing a larger Ta-S ionic- ity. The d: band is more than half filled due to charge transfer from Fe to Ta and is slightly spin polarized due 2H-TaS2 and Fel/3TaSz using the Augmented Spheri- to Fe-Ta covalency. Band structure calculations show

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JOURNAL DE PHYSIQUE

-15 -10 -5 0 5

E n e r g y ( e U )

Fig. 2.

-

The DOS of Fel/3Tas2 and partial DOS of the contributing atoms.

holes at

r

in the Brillouin zone. There are 0.04

t

and 0.10

1

holes per formula unit in the, Ta d: band. For each Ta atom there is a transfer of 0,86 electrons from Fe to Ta. This results in a formal valency of Fe 2.58'.

The magnetic moment is calculated from the num- ber of electrons in the Wigner Seitz spheres. For each Fe we find 4.75 spin up and 1.15 spin down electrons giving a magnetic moment of 3.6 p~ per atom. To- gether with a contribution from the host lattice of 0.75 p~ this gives 4.35 pg. The calculated value is not too different from the measured value of 3.86 pg [2].

Magneto-optic K e r r effect

We measured the polar magneto-optic Kerr effect (MOKE) of Feo.zsTaSz from 1.5 to 5 eV using a polarization modulation Kerr spectrometer. Because of the very high coercitivity at 4 K all measurements were carried out at 40 K. The single crystal sample was sat- urated in a magnetic field of 4T and measured without an applied field. The remanence was high enough to allow this procedure which eliminates the Faraday effect of the windows of the cryostat.

The Kerr spectrum is given in figure 3. In Feo.zsTaS2 we expect possible contributions of Fe and Ta atoms to the MOKE. The d-d transitions on Fe are parity forbidden and are therefore not expected t o a give a large contribution. Intervalence charge transfer transitions from one Fe to another are not likely to be strong because of the great distance between the Fe ions. We therefore suggest that transitions from states situated

% I I I

L

Q 2 3 Y 5

Y P h o t o n energy (eU>

Fig. 3.

-

The Kerr rotation cpk and ellipticity ~k of

Feo.zsTaSz.

at the Fermi level (Fe-3d or Ta

-

5d:) to higher lying Ta 5d-states are responsible for the Kerr effect. Strictly speaking we should inspect the dielectric ten- sor element E , ~ but in our case inspection of the Kerr

spectrum showed that it is not due >to double transitions with diamagnetic line shape 161. We suggest that the Kerr spectrum between 2 and 5 eV is caused by four single transitions with paramagnetic line shape, centered at 2.5, 3.1, 3.8 and 4.2 eV. For Ta in a trig- onal prismatic coordination a crysta!,splitting of the 5d-states is expected into a, e' and e levels. Taking spin-orbit coupling into account the e" -level splits into

two levels (mj = f 512 and m j = f 3/2) separated by 0.8 eV. The e" -level splits into two levels (mj = f 312 and m j =

f

112) separated by 0.4 eV. The large value of the Kerr rotation is probably related directly to the spin polarization of the states at the fermi level, in combination with the large spin orbi't splitting of the unoccupied Ta 5d-states.

Acknowledgments

We thank H. Weitering for the preparation of the crystal.

[I] Parkin, S. S. P. and F'riend, R.

IH.,

Philos. Mag.

B

41 (1980) 65.

[2] Dijkstra, J., Thesis (1988).

[3] Parkin, S. S. P. and Beal, A. R., Philos. Mag. B 42 (1980) 627.

[4] Williams, A. R., Kubler, J. and Ctelatt, G. D., Jr.,

Phys. Rev.

B

19 (1979) 6094.

[5] Methfessel, M. and Kubler, J., J. Phys. F 12

(1982) 141.

[6] Suits, J. C., IEEE T m s . Magn. M a g 8, no 1