- Email: [email protected]
Hg1-xCoxSe mixed crystals: new semimagnetic semiconductor
Journal of Magnetismand Magnetic Materials 140-144 (1995) 2037-2038
~ i ~ Journalof magnetism
, i ~ and magnelic ~ malerlals
ELSEVIER
H g l _ x C o x S e mixed crystals: new semimagnetic semiconductor W. S z u s z k i e w i c z a,* M. Arciszewska a B. Witkowska a, C. Julien b M. Balkanski b a Institute of Physics, Polish Academy of Sciences, Al. Lotnik6w 32/46, 02-668 Warsaw, Poland b Laboratoire de Pysique des Solides de l'Universite Pierre et Marie Curie (U.R.A. 154), 4, pl.Jussieu, 75252 Paris, France
Abstract Results of infrared reflectivity (in the phonon and plasma edge spectral regions) and magnetic susceptibility investigations performed on Hg I _xCoxSe samples corresponding to a wide range of the crystal compositions (0 < x < 0.05) are presented and discussed. The evidence of an acceptor character of Co impurity is shown. Antiferromagnetic interactions between Co 2÷ are confirmed, effective exchange integral is estimated to be J d _ J k a = (--61.3 + 6.7) K.
Semimagnetic Semiconductors (SMSC) or Diluted Magnetic Semiconductors (DMS) are compounds containing transition metal or rare earth ions [1]. Mn, Fe and Co were mostly used as such ions till now for II-VI compounds. It has been found that the effective exchange integrals Jd-a are negative for Co-based compounds (which indicates an antiferromagnetic interaction between Co 2+ ions). The values of these integrals for Co-based wide gap SMSC are few times larger than those for Mn-based [2]. Hg~_xCoxSe is a recent Co-based narrow gap SMSC. It has inverted band structure in the whole range of compositions investigated. Results of transport, magnetization and magnetic susceptibility measurements performed on crystals with x < 0.01 have been published previously [3,4]. In particular, from low-temperature magnetization and magnetic susceptibility data the effective exchange integral - 26 K < J d _ J k B < -- 19 K has been estimated [4]. Above 80 K a ferromagnetic interaction (probably caused by the presence of Co-rich clusters) was found. It has been also suggested that x = 0.01 corresponds to the solubility limit of Co in HgSe. In this work infrared reflectivity and magnetic susceptibility data have been obtained and analyzed in the wider crystal composition range 0 < x < 0 . 0 5 . Hgl_xCoxSe mixed crystals were grown in the Institute of Physics, PAS, by a modified Bridgman method. The crystal structure quality was verified by electron microscope and X-ray diffraction measurements. The crystal composition has been determined by SEM microprobe and EDXRF method. Obtained values have been confirmed by an optical method based on the composition dependence of the frequency of
* Corresponding author. Fax: +48-22-430926; email: [email protected].
TO phonon-related structure in the infrared reflectivity spectra [5]. Determined TO phonon frequency composition dependence is shown in Fig. 1. Solid solutions were formed for the whole Hgl_xCoxSe composition range under investigation so Co solubility limit in HgSe could not be smaller than 5%. The plasma edge position has also been studied as a function of the composition and of temperature. The results demonstrated non-monotonical behavior of the freecarrier concentration with increasing amount of paramagnetic ions (in disagreement with previous data [3,4]). Doping HgSe with Co significantly decreases the free-carrier concentration (and plasma edge frequency) for x < 0.005 (Fig. 2). Further doping with Co increases the electron concentration. For x = 0.01 the plasma edge position becomes close to that for x = 0, further Co content increase shifts the plasma edge towards higher frequencies. Decrease of the plasma edge frequency is accompanied by some reduction of the plasma damping parameter value. The observed phenomena do not support the hypothesis about the resonant donor character of Co impurity in HgSe
135 t . . . . . . .
1331
~ 131~ ~f ["
0.0
1 t
t 0.01
0.02 0.03 0.04 Co content (x)
0.05
Fig. 1. Compositiondependence of TO-phonon frequency determined from the reflectivity spectra (taken at T = 4.5 K using a Fourier spectrometerwith resolutionof 2 cm- 1). The straight line represents the least-squaresfit of the data.
0304-8853/95f$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01212-1
.zi
132L
2038
W. Szuszkiewiczet al. / Journal of Magnetism and Magnetic Materials 140-144 (1995) 2037-2038
(such conclusion can be found in Refs. [3,4], where a Co 2+ level energy value of 0.15 or 0.11 eV, respectively, has been suggested). As checked by us, after annealing the electron concentration strongly decreases and the estimated Fermi energy value is below the expected resonant Co donor level energy [6]. Our reflectivity data (plasma edge position) confirms these findings. Such behavior is in contradiction to the known properties of other resonant donors in HgSe, namely Fe impurity. Moreover, the corresponding slight electron mobility enhancement observed for Hgl_xCo~Se in a narrow range of low Co content [3,4] could be explained in our opinion by the particular kind of imperfections (decreasing number of native defects, creating the acceptor-donor pairs etc.) introduced for low Co content. Our data clearly show that Co impurity acts as an acceptor for x < 0.005. Such result of doping with Co gradually disappears with further increase of Co content in the mixed crystal (probably due to the influence of created CoSe 2 precipitates [7] and a related increasing number of lattice defects). The ac magnetic susceptibility was measured in the temperature range 60-295 K using a standard mutual-inductance technique. The applied field amplitude was 40 Oe and the frequency was 75 Hz. The absolute value of the susceptibility per unit mass was obtained by comparison with a standard Er20 3 sample. The diamagnetic contribution for HgSe, Xd = --0.3 × 10 -6 e m u / g , and the contribution caused by the sample holder were subtracted from the data. The experimental accuracy of the susceptibility measurements was better than 3%. The sample temperature was determined with an accuracy better than 2% by using a copper-constantan thermocouple. The magnetic susceptibility data obtained for Hgo.97 Coo.03Se sample are shown in Fig. 3. The magnetic susceptibility per unit mass follows the Curie-Weiss law with the negative Curie-Weiss temperature indicating a dominant antiferromagnetic exchange interaction. From the best fit of the Curie-Weiss law to the data the values 0 = ( - 5 5 . 2 + 6) K and C m = (3.1 + 0.2) × 10 -4 emu K / g have been obtained. Using the mean-field approximation the effective
0
I00
200
300
400
500
600
Wavenumber (cm"1) Fig. 2. Composition dependence of the plasma edge (experimental conditions the same as for Fig. 1).
12
,
•
,
•
,
•
,
•
,
-
,
-
,
10
~
4
2
0
-50
8
58
100
150
288
250
300
Temperature(K) Fig. 3. Inverse magnetic susceptibility of Hg0.97Co0.03Se as a function of temperature. The straight line represents the leastsquares fit of the Curie-Weiss law to the data (parameters in text).
exchange integral Ja_d/kB value of (-- 61.3 ___6.7) K was determined. This is about three times higher than that previously reported [4]. Moreover, the obtained Ja-d value is noticeably higher than those reported till now for other Co-based SMSC. The anomalous high and not presently understood value of effective exchange integral determined by us demonstrates the clear need of further investigations of Hgl_xCoxSe magnetic properties in spite of difficulties they could involve. Acknowledgements: This work has been partially supported by CEC Grant CIPA351OCq920790, by KBN Grant No. 2 P302 199 06 and by Grant NSF/PAN-92-113. References [1] Semiconductors and Semimetals, vol. 25, eds. J.K. Furdyna and J. Kossut (Academic Press, Boston, 1988). [2] A. Twardowski, H.J.M. Swagten and W.J.M. de Jonge, in: Semimagnetic Semiconductors and Diluted Magnetic Semiconductors, eds. M. Averous and M. Balkanski, vol. 55 (Plenum, New York, 1991) p. 253; A. Lewicki, A.I. Schindler, J.K. Furdyna and T.M. Giebultowicz, in: Diluted Magnetic Semiconductors, ed. M. Jain (World Scientific, Singapore, 1991) p. 409. [3] I.M. Tsidilkovskii, N.K. Lerinman, L.D. Sabirzyanova, S. Yu. Paranchich and Yu.S. Paranchich, Phys. Status Solidi (b) 171 (1992) 153; I.M. Tsidilkovskii, N.K. Lerinman, L.D. Sabirzyanova, S.Yu. Paranchich and Yu.S. Paranchich, Soy. Phys. Semicond. 26 (1992) 1062. [4] M. Averous, C. Fau, S. Charar, M. El Kholdi, V.D. Ribes, J. Deportes and Z. Golacki, Solid State Commun. 84 (1992) 479; M. Averous, C. Fau, S. Charar, M. El Kholdi, V.D. Ribes and Z. Golacki, Phys. Rev. B 47 (1993) 10261. [5] W. Szuszkiewicz. E. Dynowska, S. Miotkowska, B. Witkowska, C. Julien and M. Balkanski, Nukleonika (1994, in press). [6] E. Grodzicka, W. Szuszkiewicz and B. Witkowska, unpublished. [7] J. Kachniarz, E. Dynowska, S. Miotkowska, B. Witkowska and W. Szuszkiewicz, Proc. XVI Conf. Appl. Cryst., Cieszyn 1994 (Silesian University, 1995).
~ i ~ Journalof magnetism
, i ~ and magnelic ~ malerlals
ELSEVIER
H g l _ x C o x S e mixed crystals: new semimagnetic semiconductor W. S z u s z k i e w i c z a,* M. Arciszewska a B. Witkowska a, C. Julien b M. Balkanski b a Institute of Physics, Polish Academy of Sciences, Al. Lotnik6w 32/46, 02-668 Warsaw, Poland b Laboratoire de Pysique des Solides de l'Universite Pierre et Marie Curie (U.R.A. 154), 4, pl.Jussieu, 75252 Paris, France
Abstract Results of infrared reflectivity (in the phonon and plasma edge spectral regions) and magnetic susceptibility investigations performed on Hg I _xCoxSe samples corresponding to a wide range of the crystal compositions (0 < x < 0.05) are presented and discussed. The evidence of an acceptor character of Co impurity is shown. Antiferromagnetic interactions between Co 2÷ are confirmed, effective exchange integral is estimated to be J d _ J k a = (--61.3 + 6.7) K.
Semimagnetic Semiconductors (SMSC) or Diluted Magnetic Semiconductors (DMS) are compounds containing transition metal or rare earth ions [1]. Mn, Fe and Co were mostly used as such ions till now for II-VI compounds. It has been found that the effective exchange integrals Jd-a are negative for Co-based compounds (which indicates an antiferromagnetic interaction between Co 2+ ions). The values of these integrals for Co-based wide gap SMSC are few times larger than those for Mn-based [2]. Hg~_xCoxSe is a recent Co-based narrow gap SMSC. It has inverted band structure in the whole range of compositions investigated. Results of transport, magnetization and magnetic susceptibility measurements performed on crystals with x < 0.01 have been published previously [3,4]. In particular, from low-temperature magnetization and magnetic susceptibility data the effective exchange integral - 26 K < J d _ J k B < -- 19 K has been estimated [4]. Above 80 K a ferromagnetic interaction (probably caused by the presence of Co-rich clusters) was found. It has been also suggested that x = 0.01 corresponds to the solubility limit of Co in HgSe. In this work infrared reflectivity and magnetic susceptibility data have been obtained and analyzed in the wider crystal composition range 0 < x < 0 . 0 5 . Hgl_xCoxSe mixed crystals were grown in the Institute of Physics, PAS, by a modified Bridgman method. The crystal structure quality was verified by electron microscope and X-ray diffraction measurements. The crystal composition has been determined by SEM microprobe and EDXRF method. Obtained values have been confirmed by an optical method based on the composition dependence of the frequency of
* Corresponding author. Fax: +48-22-430926; email: [email protected].
TO phonon-related structure in the infrared reflectivity spectra [5]. Determined TO phonon frequency composition dependence is shown in Fig. 1. Solid solutions were formed for the whole Hgl_xCoxSe composition range under investigation so Co solubility limit in HgSe could not be smaller than 5%. The plasma edge position has also been studied as a function of the composition and of temperature. The results demonstrated non-monotonical behavior of the freecarrier concentration with increasing amount of paramagnetic ions (in disagreement with previous data [3,4]). Doping HgSe with Co significantly decreases the free-carrier concentration (and plasma edge frequency) for x < 0.005 (Fig. 2). Further doping with Co increases the electron concentration. For x = 0.01 the plasma edge position becomes close to that for x = 0, further Co content increase shifts the plasma edge towards higher frequencies. Decrease of the plasma edge frequency is accompanied by some reduction of the plasma damping parameter value. The observed phenomena do not support the hypothesis about the resonant donor character of Co impurity in HgSe
135 t . . . . . . .
1331
~ 131~ ~f ["
0.0
1 t
t 0.01
0.02 0.03 0.04 Co content (x)
0.05
Fig. 1. Compositiondependence of TO-phonon frequency determined from the reflectivity spectra (taken at T = 4.5 K using a Fourier spectrometerwith resolutionof 2 cm- 1). The straight line represents the least-squaresfit of the data.
0304-8853/95f$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01212-1
.zi
132L
2038
W. Szuszkiewiczet al. / Journal of Magnetism and Magnetic Materials 140-144 (1995) 2037-2038
(such conclusion can be found in Refs. [3,4], where a Co 2+ level energy value of 0.15 or 0.11 eV, respectively, has been suggested). As checked by us, after annealing the electron concentration strongly decreases and the estimated Fermi energy value is below the expected resonant Co donor level energy [6]. Our reflectivity data (plasma edge position) confirms these findings. Such behavior is in contradiction to the known properties of other resonant donors in HgSe, namely Fe impurity. Moreover, the corresponding slight electron mobility enhancement observed for Hgl_xCo~Se in a narrow range of low Co content [3,4] could be explained in our opinion by the particular kind of imperfections (decreasing number of native defects, creating the acceptor-donor pairs etc.) introduced for low Co content. Our data clearly show that Co impurity acts as an acceptor for x < 0.005. Such result of doping with Co gradually disappears with further increase of Co content in the mixed crystal (probably due to the influence of created CoSe 2 precipitates [7] and a related increasing number of lattice defects). The ac magnetic susceptibility was measured in the temperature range 60-295 K using a standard mutual-inductance technique. The applied field amplitude was 40 Oe and the frequency was 75 Hz. The absolute value of the susceptibility per unit mass was obtained by comparison with a standard Er20 3 sample. The diamagnetic contribution for HgSe, Xd = --0.3 × 10 -6 e m u / g , and the contribution caused by the sample holder were subtracted from the data. The experimental accuracy of the susceptibility measurements was better than 3%. The sample temperature was determined with an accuracy better than 2% by using a copper-constantan thermocouple. The magnetic susceptibility data obtained for Hgo.97 Coo.03Se sample are shown in Fig. 3. The magnetic susceptibility per unit mass follows the Curie-Weiss law with the negative Curie-Weiss temperature indicating a dominant antiferromagnetic exchange interaction. From the best fit of the Curie-Weiss law to the data the values 0 = ( - 5 5 . 2 + 6) K and C m = (3.1 + 0.2) × 10 -4 emu K / g have been obtained. Using the mean-field approximation the effective
0
I00
200
300
400
500
600
Wavenumber (cm"1) Fig. 2. Composition dependence of the plasma edge (experimental conditions the same as for Fig. 1).
12
,
•
,
•
,
•
,
•
,
-
,
-
,
10
~
4
2
0
-50
8
58
100
150
288
250
300
Temperature(K) Fig. 3. Inverse magnetic susceptibility of Hg0.97Co0.03Se as a function of temperature. The straight line represents the leastsquares fit of the Curie-Weiss law to the data (parameters in text).
exchange integral Ja_d/kB value of (-- 61.3 ___6.7) K was determined. This is about three times higher than that previously reported [4]. Moreover, the obtained Ja-d value is noticeably higher than those reported till now for other Co-based SMSC. The anomalous high and not presently understood value of effective exchange integral determined by us demonstrates the clear need of further investigations of Hgl_xCoxSe magnetic properties in spite of difficulties they could involve. Acknowledgements: This work has been partially supported by CEC Grant CIPA351OCq920790, by KBN Grant No. 2 P302 199 06 and by Grant NSF/PAN-92-113. References [1] Semiconductors and Semimetals, vol. 25, eds. J.K. Furdyna and J. Kossut (Academic Press, Boston, 1988). [2] A. Twardowski, H.J.M. Swagten and W.J.M. de Jonge, in: Semimagnetic Semiconductors and Diluted Magnetic Semiconductors, eds. M. Averous and M. Balkanski, vol. 55 (Plenum, New York, 1991) p. 253; A. Lewicki, A.I. Schindler, J.K. Furdyna and T.M. Giebultowicz, in: Diluted Magnetic Semiconductors, ed. M. Jain (World Scientific, Singapore, 1991) p. 409. [3] I.M. Tsidilkovskii, N.K. Lerinman, L.D. Sabirzyanova, S. Yu. Paranchich and Yu.S. Paranchich, Phys. Status Solidi (b) 171 (1992) 153; I.M. Tsidilkovskii, N.K. Lerinman, L.D. Sabirzyanova, S.Yu. Paranchich and Yu.S. Paranchich, Soy. Phys. Semicond. 26 (1992) 1062. [4] M. Averous, C. Fau, S. Charar, M. El Kholdi, V.D. Ribes, J. Deportes and Z. Golacki, Solid State Commun. 84 (1992) 479; M. Averous, C. Fau, S. Charar, M. El Kholdi, V.D. Ribes and Z. Golacki, Phys. Rev. B 47 (1993) 10261. [5] W. Szuszkiewicz. E. Dynowska, S. Miotkowska, B. Witkowska, C. Julien and M. Balkanski, Nukleonika (1994, in press). [6] E. Grodzicka, W. Szuszkiewicz and B. Witkowska, unpublished. [7] J. Kachniarz, E. Dynowska, S. Miotkowska, B. Witkowska and W. Szuszkiewicz, Proc. XVI Conf. Appl. Cryst., Cieszyn 1994 (Silesian University, 1995).