Journal of Magnetism and Magnetic Materials 54-57 (1986) 1033-1035

OPTICAL O B S E R V A T I O N OF 3 d - B A N D W I D T H I N C o ( S x S e t_ x)Z S Y S T E M

K a t s u a k i S A T O

Facul(v of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan

T a d a s h i S E K I G U C H I * a n d T o m o n a o M I Y A D A I

Faculty of Science, Hokkaido Uniuersity, Sapporo 060, Japan

1033

Optical dielectric functions c'(co) and ~"(~) of Co(SxSe 1 x)2 (0.86 < x <1.00 and x = 0.0) obtained from reflectivity spectra were simulated by Drude term and Lorentzian terms, from which it is found that the band-width of the 3d eg-band increases with the Se composition, which is consistent with the theory of itinerant magnetism.

There have been numerous studies on the cobalt dichalcogenide solid solution system, CoS2:CoSe 2 as a typical example for the detailed studies of itinerant magnetism in narrow bands [1 5]. CoS 2 has been known to be an itinerant ferromagnet with T c = 116 K and a saturation moment of 0.85~B per Co, whereas CoSe 2 an exchange-enhanced and temperature-induced para- magnet. In the solid solution system ferromagnetic order is rapidly lost with increasing Se concentration, and metamagnetism appears. Such behavior has been ex- plained in terms of the broadening of 3d eg-band due to increase of covalency with Se content [2,6]. However, there have been no direct observations of the band- broadening. The purpose of this study is to get the evidence for the band-broadening from optical observa- tions.

Single crystals of COS2, CoSe 2 and their solid solu- tions Co(SxSe I x)2 (0.86 < x < 0.965) have been pre- pared by chemical vapor transport using chlorine as a transporting agent [4]. The compositions x have been determined by using the lattice parameters deduced from the X-ray diffraction with the aid of the lattice parameter data reported in ref. [1] **.

Reflectivity spectra of CoS2:CoSe 2 system were mea- sured for photon energies between 0.2 and 4.4 eV. Technical details for reflectivity measurement have been published elsewhere [7]. The results are shown in fig. l a - e by solid curves. These spectra have common spec- tral features: sharp decrease in reflectivity in the low energy region which has been attributed to intraband transitions [8] and two dominant peaks around 1 and 3.2 eV. The 1 eV peak seems to become broader and weaker upon going from CoS 2 to CoSe 2. In order to discriminate 1 eV peak from overlapping intraband

* Present address: Medical Engineering Lab., Toshiba Corpo- ration, Otawara, Tochigi 329-26, Japan.

** There exist some ambiguities in determination of the com- position x: It is argued from ferromagnetic resonance stud- ies that there exist differences in the composition between the surface region and the bulk region in these solid solu- tions [11].

contributions we have calculated dielectric functions using Kramers -Kron ig relations.

The dielectric spectra were then simulated by Drude formula for intraband contribution and by Lorentzian terms for interband contributions with transition en- ergies around 0.5, 1.1, 1.6 and 3.2 eV.

In fig. 2a and b are shown dielectric spectra simu- lated by the above-mentioned calculations for two ex- treme cases, i.e. CoS 2 and CoSe 2. Reflectivity spectra of the CoS2:CoSe 2 system are then reproduced by using the dielectric spectra thus obtained and are shown by dotted curves in fig. 1. The fits are satisfactory for energy region between 0.4 and 3 eV.

By subtracting the Drude terms from the calculated c"(0~), interband contributions are estimated and are

iod

8O )... I.-

> 6O I - (,.) ILl

u.I n..

2 0

"~, Co(S, Se,-,)z \

~ ~ , ~ = 0 ICo_Se2I

0 i ~ ~ 4 PHOTON ENERGY(eV)

80

~o

) -

4 O ~ b- u

2 0 w

w 0 n-

Fig. 1. Reflectivity spectra of CoS 2:CoSe 2 system. Solid curves are experimental spectra and dotted curves those fit by Drude and Lorentzian terms.

0 3 0 4 - 8 8 5 3 / 8 6 / $ 0 3 . 5 0 © El sev ie r Sc ience Pub l i she r s B.V.

1034

,o I

-I0

-20

-30

-40

-50

-60

-70

K. Sato et al. / 3 d - b a n d width in Co(S, Se 1 - x)z system

-I0

-20

-30

L' -40

-50

/a) CoS~ 40

- - EXP.

. . . . . CAL. 50 to

, , / , ~ . ~20

° _.__ ~10

~ ' ~ 5 i i

,o 50

~-0

5O

20

I0

5 0 2 5 4

PHOTON ENERGY (eV)

Fig. 2. Real and imaginary parts of dielectric functions for (a)CoS 2 and (b)CoSe 2. Solid curves denote those obtained from the experiment, while dotted ones those given by simulation.

plotted in fig. 3 for energies between 0.2 and 4 eV, from which it is found that the dominant peak around 1.1 eV becomes weaker and broader as one traverses from CoS 2 to CoSe 2. The most prominent peak around 1 eV can be decomposed into a dominant peak at about 1.1 eV and a shoulder near 1.6 eV. The half value of the 1.1 eV peak width together with the measured Curie tem- perature are plotted in fig. 4. This figure clearly shows a correlation between the band width and the ordering temperature.

It has been elucidated that the peak of c " ( ~ ) ob- served around 1 eV in CoS 2 spectrum is associated with

20 16 Co(SxSe t-x)=,

8

o / L - - /

0 i i i

O0 I 2 5 4

PHOTON E N E R G Y ( e V ) Fig. 3. Spectra of imaginary part of dielectric function without the Drude (intraband) term. Each curve is shifted by 4 from the other for the sake of clarity.

0.5

5 0 . 4

0.3

-~15o q

E

,v 5 0

/ o - - - q~- - - - - ~

Co(S x Sel-x)2

0.95 0'.9 0 C ~ 8 5 / 0 o S 2 x CoSe2

Fig. 4. Variation of the half band width of the 1.1 eV compo- nent with Se content. Variation of T~. is also shown for com- parison.

the optical transition from the filled t Zg band to the empty portion of eg conduction band [8,12]. Since t2g states have essentially a non-bonding nature they can be assumed to show no substantial variation of band width on increasing covalency by Se-substitution. Variation in the width and the intensity of e" peak can therefore be attributed to the variation in those of the final states, i.e. empty portion of eg conduction band. According to the theory of itinerant magnetism [9] ferromagnetic ordering occurs only when the product of Density of States, DOS, at the Fermi energy p(cF) and electronic correlation energy U takes much larger value than unity, and this is the case in CoS 2 where DOS is large at cv. When S is substituted by Se, DOS becomes less intense as seen in fig. 3, which lowers the magnetic ordering temperature.

Our result is also consistent with the interpretation of Curie-Weiss type susceptibility of the system in terms of Moriya theory [10]: One of the favorable conditions for the appearance of temperature-induced local moment is that the local DOS is flat around the Fermi energy. The fact that the local moment is induced at much lower temperature in CoSe 2 than in CoS 2 is accounted for by our experimental results that DOS curve of the former is flatter than that of the latter.

Moreover, recent theoretical study by Takahashi and Tano [5] revealed that metamagnetic behavior in this system can be explained by increase in the band width relative to the effective Coulomb interaction brought about by Se substitution. This is also consistent to our result.

In conclusion, we have obtained the direct evidence for 3d eg band-broadening in CoS2:CoSe 2 system from the optical studies, which supports interpretation of salient magnetic properties of this system in terms of the theory of itinerant magnetism.

K. Sato et al. / 3 d - b a n d width in Co(SxSe I _ .02 sys tem 1035

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