Ion-carrier electron exchange constants for CdCoSe semimagnetic semiconductor

~

Solid State Co~nunications, Vol. 77, No. 2 pp. 111-114, 1991. Printed in Great Britain

0038-I098/9153.00+.00 Pergamon Press plc

ION-CARRIER ELECTRON EXCHANGE CONSTANTS FOR CdCoSe SEHIMAGNETIC SEMICONDUCTOR M.Nawrockl ÷, F.Hamdani ++ ÷++and J.P.Lascaray Groupe d'Etude des Semlconducteurs , Unlverslte Montpelller II Place E.Batalllon, 34060-Montpelller-CEDEX, France Z. Golackl Institute of Physics, Polish Academy of Sciences, AI.Lotnlkow 32/46, 02-668 Warsaw, Poland J. Depor tes Laboratolre Louis Neel, Grenoble, France (Reclved 29~ October 1990 by H . B a l k a n s k i ) Hagnetoreflectance and magnetization measurements were performed on the recently synthesized semlmagnetic semiconductor Cd 0.052Coo.0,8Se. The results allowed to obtain for the first time both ion-carrler exchange constants Noa - 279-+29 meV and NoB - -1873-+42 meV.

onal. Under the action of dominant, tetrahedral one the free-lon ~F ground state splits into a 4Az orbital singlet and two orbital triplets ~Tz and ~T*. The trigonal term, characteristic of the wurtzite structure splits the 4-fold ground state into two doublets (in the absence 3 of the magnetic field) separated by energy 4of about IK. The energy separation between A2 ground state and ~T2 excited state is about 3300 cm -I and is much larger than the thermal iz energy at the temperature of our experiment . Consequently, the occupations of the 4Tz and 4T2 states are much smaller than that of the ~A2 ground state. The lowest four levels can then be described by an effective spin 3/2 with slightly a n t s o C r o p i c g-factor about 15% higher than the spln-only value. In consequence the CdCoSe system can be regarded as an analog of CdMnSe system with Mn 2÷ para~agnetlc ion with 5/2 spin and g - 2 replaced by Co 2÷ paramagnetic ion with 3/2 spin and g - 2.295_+0.005 measured by ESR 13. Recently g-factor 2.32_+0.04 was obtained also by electron Raman scattering .

The class of diluted magnetic (semlmagnetic) semiconductors (DMS) i.e. II-VI semiconductors with a group II lons randomly substituted by transition metals has been recently extended, after the synthesis of materials containing cobalt ions as the magnetic substltute. The results concerning electron Raman scattering in CdCoSe I magnetic susceptibility in ZnCoSe 2 ' 3 ' ZnCoS and CdCoS,CdCoSe , spln-glass properties of ZnCoS*, neutron scattering in ZnCoS, ZnCoSe 5 6 and ZnCoTe , magnetoreflectance in ZnCoSe have recently been published. The results obtained show that properties of Co- based DMS are similar to those of most studied Mn-based DMS. However the values of parameters describing magnetic interactions as well as energetic structure under influence of magnetic field are different. In this paper the magnetoreflectance data for recently obtained semimagnetic semiconductor CdCoSe are presented. The main purpose of this study is to evaluate values of exchange constants, describing the interaction between localized moments of the magnetic cobalt ions and band carriers. The wurtzite crystallographic structure of CdCoSe up to x - .085 was confirmed by x-ray diffractlon analysis 3. Analogically to the case of CdMnSe 7's'g we can suppose that the band structure of CdCoSe is the same as that of the host crystal CdSe, eventually with a small modification of band parameters. The properties of Co 2÷ ion in CdCoSe and its influence on the magnetic susceptibility of this material were discussed in details by Lewlcki at el. 3. When introduced into the crystal, Co 2. ion is subjected to a crystal field consisting of two terms - tetrahedral and trig-

EXPERIMENTAL The CdCoSe crystals were grown by modified Bridgman method. The cobalt mole fraction x-0.048 was checked by microprobe analysis. The single crystals were oriented, cut into parallelepipeds, then polished and etched in a 1% solution of bromine in methanol. The samples were placed in a superconducting magnet and immersed in superfluid helium. The temperature measured by the vapor pressure was 1.8K. The maximum field of the magnet was 5.5T. The magnetoreflectance measurements were performed using a standard experimental setup in both Faraday and Voigt configuration, with c- axis of sample parallel to the magnetic field. The magnetization measurements were carried out by an extraction method in magnetic field up to 6.5T and temperature 1.8K.

T h i s work was p a r t l y s u p p o r t e d by t h e p r o g r a m CPBP-OI.04 o f P o l i s h Academy o f s c i e n c e s . ÷ On l e a v e f r o m I n s t i t u t e o f E x p e r i m e n t a l P h y s i c s , U n i v e r s i t y o f Warsaw, P o l a n d . On l e a v e f r o m UDTS Haut C o m m i s s a r i a t a l a Recherche Algiers, A1gerla. "*+Laboratolre assocI~ au Centre National de la Recherche Scientlflque.

Figures la and magnetoreflectance iii

RESULTS Ib show spectra

representative for both

112

CdCoSe SEMIMAGNETIC ,

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Vol. 77, NO. 2

SEMICONDUCTORS

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in magnetic field _ at 1.8K. a) Faraday configuration, a ,G -circular polarization. b) Volgt configuration, ~-llnear polarization. experimental configurations. In Faraday configuration excitons A and B are visible and progressive splitting of a + and #- components can be observed. In Volgt configuration the exciton B splitting into two components in . polarization is seen. The reflectance structure related to the C-exclton was not detected. The energy of A-exclton transition in o + and G- polarization can be well determined. The precision of the determination of the B-exclton transition energy is smaller because of the weakness of the reflectance structure and, in the case of o component, because of the overlap and crossing of A and B-exclton peaks. The Structure for B-exclton transition in . polarization is masked by a high background level and overlap of two ~ components. However determination of the transition energy is possible. The magnetic field dependence of the energetic position of reflectance peaks is presented in Fig.2. The splitting of exclton levels vary subllnearly with magnetic field, however the saturation is not reached up to 5.5T. For B-exclton transition the shift of the center of gravity of two ~ components in Volgt configuration and of o ,o components in Faraday configuration, characteristic of the wurtzite type DMS is to be noted. T h e magnetization results are presented in Fig.3. The experimental data have been fitted to the modified Brlllouin function I* M - M..B3/2[3gpH/2k(T+To)] (i) where B3/2 is Brillouin function of spin 3/2, T.~f- T+To is an effective temperature, H is the magnetic field, M. is a fitting parameter, smaller than the theoretical saturation mag-

1950 1930

~EE1910 1890 w 1870 x= T = 1850

4.8~ 1.8K

e-ao - -

........ I ........ '~' ....... ~ ....... '~ ....... '~' ....... MAGNETIC FIELD (T)

Flg.2.Energy of the A and B excitons vs magnetic fleld.The lines are the fit to Eq.(2).

netizatlon tu and is related to the So parameter describing saturation value of the average spin component along magnetic field; Ms- gpNoxSo. Magnetization MJ is in ~emu/g], g - 2.295 is the g-factor of the Co ion, ~ is the Bohr magneton, No is the number of cation sites per gram, k is the Bolzman constant and x is the cobalt mole fraction. DISCUSSION The analysis of the magnetoreflectance resuits is possible on the basis of complete

Vol.

CdCoSc SEHIMAGNETIC

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113

SEMICONDUCTORS

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Fig.3.Magnetlzation vs magnetic field. line is calculated from the fit to Eq.(1).

The

Hamiltonlan, describln~ the band structure of wurtzite type DMS 7'8~°'15. It consists of original wurtzlte Hamiltonlan and an exchange term, describing interaction between localized Co 2+ ions and band electrons. Taking into account the possibility of an anisotropy of the exchange interaction for the valence band the energies of the allowed transitions in the magnetic field parallel to the 10. c-axls+of crystal are Ea~ - Eo ; A - (A1 - 62)/2 ± B~ (2a) Es~ - Eo ± A ± (B= - Bx)/2 - E~ (2b) Ec- - Eo ± A ± (B= - B.)/2 + E(2c) for Faraday configuration and Es~ - Zo ; A ± (B, - B , ) / 2 - E ~ (2d) Ec~ - Eo ± A ± (B, - Bx)/2 + E(2e) for Voigt configuration, where F~-(At-A2)/2 is th$ energy of the A-exclton transition for H-O, 2 E - ~ [ (Az-A=)±(Bx+B,) ]2+8-a3z} I/2, a*, &z, A3 wurtzlte band parameters describing the crystal field splitting and the spln-orbit coupllng11.A priorl these parameters can be slightly dlfferent for CdCoSe and CdSe host crystal. A, B=, B= are related to the exchange constants N ~ for conduction band and NoBx and No~z for valence band: 2A- N ~ . x . < S z > and 2Bx(.)- NoBx(z)-x. Where denotes the thermal average of the Co 2+ spin component along the external magnetic field. Using eq.(1) a difference between Noa and No@x can be obtained. This value is well defined because of the most accurate determination of A-exciton transition energy. From Eq. (2a), using A and Bx definition the splitting between o and o- components of A-exclton is EA-- EA*- (Noa - NoB).x. (6) in Fig.4 the splitting of A-exciton in Faraday configuration is plotted as a function of x. determined from magnetlzatlon measurements. The slope of a straight llne, calculated by least-square procedure gives No(a-~x)-2162±48 meV. It should be noticed that A-exclton splitting is really proportional to the mean spin value. However this procedure can provide neither the values of Noa and NoB, separately nor the values of the remaining parameters. The complete set of band parameters and exchange constants can be ob~alned from a

*

1000

Fig.4.Splltting of the A-exciton line vs the mean value of Co2÷spln along the magnetic field per unit cell.

fit of expressions (2a-e) to the complete magnetoreflectance data (A,B,C-exclton). Unfortunately in our case the transitions for Cexclton were not observed. In consequence, because of high correlation of fitted parameters the attempt to find all of them would not be realistic. We have limited ourselves to Noa, No~x and No,z, assuming that eventual change of At,A2 and A3 is small enough and we have used its values for CdSe 23. The fit of the experimental results of reflectance under the mentioned assumption and for experimental magnetization data is presented in Fig.2. It seems that a persisting difference between experimental data and calculations for B-exclton energies in ~ polarization is related to a small change in A', values. The obtained exchange constant values are (in meV): N~-275±30 No~x--1870±42 No~,--1925±42 Taking into account the precision of No(a-B=) determination one note the consistence of results obtained by two procedures. The difference between NoBx and NoB, is comparable to the error and it seems reasonable to take a common exchange constant value without anlsotropy. It can be noticed however that a trend in anisotropy I s the same in the case of CdCoSe like for CdMnSe T M and CdMnS le, i.e. NoBx < NOB,. Under assumption of isotropy of valence band exchange constant the fit gives Noa - 279±29 meV NoB - -1873±42 meV The conduction band exchange constant can be compared to N ~ - 320 meV obtained from electron Raman scattering* where the magnetization was not measured but calculated using the assumption of random distribution of cobalt ions in crystal and dependence of magnetization on magnetic field given by modified Brillouln function of spin 3/2. The exchange constants of CdCoSe can be compared to these of CdMnSe. The Noa value in CdCoSe is about 10% greater. More important is a factor 1.5 between NoB value in CdMnSe (-1238 meV I°) and CdCoSe(-1873 meV). A similar tendency was observed for Co z+ and Mn 2+ Ions in ZnSe host crystal ~. This difference can be interpreted using a theoretical approach ,7.16 where

114

CdCoSc~ SEMIMAGNETIC SEMICONDUCTORS

the exchange interaction for a valence band consists of two terms. The first, ferromagnetic, is related to the potential exchange between band and d-electrons active for conduction and valence band. The second one, anciferromagnetic, is related to the hybridization between band electrons and d-electrons of the paramagnetlc ion. This mechanism, inactive for conduction band electrons for symmetry reasons is dominant for valence band electrons. The important increase in No~ value could then be attributed to the increase of the hybridization term for Co2+ion in CdSe. This Increase can be related as well to the change in the hybridization parameter as co the difference in position of ion d-levels relatively to the top of the 2÷ 2+ v a l e n c e b a n d b e t w e e n Co a n d Hn c a s e s . The s i m i l a r hybridization factor describes the superexchange interaction between magnetic 18 i o n s . I f t h e i n c r e a s e o f No~ i s r e l a t e d to the increase of hybridization term the increase of the nearest neighbor exchange integ r a l J s h o u l d be o b served. This idea is qualitatively s u p 2 o r t e d b y T a b l e 1, w h e r e t h e r e Z+ 2+ 2+ sults for Co ,Fe and Mn are presented.

Noa(meV) No$(meV) J/k(K)

CdCoSe

Table 1 CdFeSe

279 -1873 373

25020 -14502° 1921

CdMnSe 2611° -1238 ~° 7.622

The high J value for CdCoSe should also to influence the magnetic field dependence of magnetization. ~le experimental data of magnetization show significant deviation from the fitted curve, calculated using modified Brillouin function of spin 3/2. However the obtained values of fit parameter So can provide the indica-

vOl. 77, NO. 2

tion on the formation of Co2+ion clusters with strong antlferromagnetic ordering, thus producing no magnetic moment. The So obtained from the fit ~s by 7% smaller and a fraction of isolated CoZ+spins is smaller also than that calculated under assumption of random distribution 2+ of Co ions. The To value which reflects the strength of the interaction between more dls2+ Cant Co neighbors is in CdCoSe (4.7K) three times greater than those of CdMnSe with 5% of manganese (1.48K lg) Ic suggests that antlferromagnetic interactions in CdCoSe are stronger than in CdMnSe in consistence with results of maEnetoreflectance and susceptibility results. CONCLUSION Simultaneous measurements of magnetoreflecCanoe and magnetization on CdCoSe provided the exchange constants for magnetic ion-band carrier interaction. The first time obtained No~--1873 meV value is much higher than in the case of Fe 2. and Mn2÷ions in the same host crystal. This effect can be related to the increase of the hybridization term in the valence band electron- ion exchange interaction. The quantitative discussion is difficult because the data on the Co z* level position and proper calculation of the exchange interaction for CdCoSe are not available at the moment. However the relation between valence band ion- carrier exchange c o n s t a n t and ion-ion exchange as well as the high value of effective temperature in modified Brillouin function fitting magnetization d a t a supports explanatfon of NoB enhancement by increasing of hybridization term. - We are grateful Co J.Gaj critical reading of the manuscript.

Acknowledgment

for

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