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Transient level crossing of free and bound excitonic magnetic polarons in Cd1−xMnxTe
Physica E 10 (2001) 320–325
www.elsevier.nl/locate/physe
Transient level crossing of free and bound excitonic magnetic polarons in Cd 1−x Mnx Te M. Nogakua; c; ∗ , J.X. Shenb; c , R. Pittinib; c , T. Satob; c , Y. Okab; c a Electrical
Engineering, Miyagi National College of Technology, 48 Nodayama, Medesima Shiote, Natori 981-1239, Japan b RISM, Tohoku University, Katahira 2-1-1, Sendai, 980-8577, Japan c CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract We report an experimental evidence of the existence of free excitonic magnetic polaron (FEMP) states from the study of the dynamics of free and bound excitons in Cd 1−x Mnx Te (x = 0:03) by time-resolved photoluminescence measurements. We found, for the 3rst time, that FEMP are more stable than the polarons formed from bound excitons. The polaron binding energy for the free excitons is evaluated to be 4 meV, which is considerably larger than that of 1 meV found for the magnetic polarons of the bound excitons; these values were determined from the suppression of the excitonic energy relaxation in high magnetic 3elds. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons c 2001 Elsevier Science B.V. All rights reserved. cross in the time-resolved photoluminescence spectra. PACS: 78.47.+p; 71.35.Lk; 71.10.Pn; 71.20.Nr Keywords: Magnetic polarons; Level cross; Excitons
1. Introduction In diluted magnetic semiconductors (DMSs), magnetic elements are contained on the cation site and change the band parameter. Magnetic polarons (MPs) are entities where the band electrons (or the excitons) couple to the localized spins of the magnetic elements ∗ Corresponding author. Electrical Engineering, Miyagi National College of Technology, 48 Nodayama, Medesima Shiote, Natori 981-1239, Japan. Tel.: +81-22-381-0313; fax: +81-22-381-0310. E-mail address: [email protected] (M. Nogaku).
through the exchange interactions [1–19]. This state results in the polarization of the localized spins of the magnetic ions within the Bohr radius of the MP. For the realization of this local polarization of magnetic ion spins, the carriers need to be localized. A convincing experimental proof of the formation of the MP is the 3nite spin-Gip energy of an electron bound to a donor impurity (D0 ) detected at zero magnetic 3eld in the spin-Gip Raman scattering experiment on DMSs [2]. In contrast to this clear demonstration of the bound MP in the D0 state, there are still controversies
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 1 0 8 - 4
M. Nogaku et al. / Physica E 10 (2001) 320–325
about the existence free excitonic magnetic polarons (FEMPs) in which a free exciton polarizes the neighboring magnetic ion spins and is trapped in the exchange potential well. The stability of the FEMP state is still under discussion [2]. We present here a detailed study on the stability of free and bound magnetic polarons (BEMPs) with a detailed analysis of the transient optical spectra of Cd 1−x Mnx Te single crystals. In contrast to the generally accepted picture of a strong BEMP compared to the FEMP, we found that the polaron binding energy of free excitons is much larger than that of bound excitons. Photoluminescence (PL) experiments have been performed on a series of samples with Mn content x ranging from 0 to 0.1. The results are repeatable and consistent with the Mn composition variation. In the following we present only the spectra measured in the samples with x = 0:03 and 0.05. We interpret the observed phenomena by a qualitative model taking into account the self-consistent con3nement of the holes and the spin orientation of Mn ions. 2. Experiments High-purity Cd 1−x Mnx Te single crystals with the Mn composition x = 0– 0.1 were prepared by the vertical Bridgman method. For the optical excitation, we used a wavelength tunable femto-second pulsed laser. The time variation of the excitonic luminescence was recorded by a streak camera at 4:2 K applying magnetic 3elds up to 7 T. 3. Results and discussion In Fig. 1, we show the contour plots and the time-integrated spectra until 5 ns of the time-resolved PL spectra of a Cd 0:97 Mn0:03 Te crystal measured in zero 3eld and in 7 T. In these spectra, two peaks are resolved. One of them is sharp, while the other one is broad and shows a strong transient energy relaxation at zero 3eld. We assign these two peaks to the signals of the (D0 ; X) and the FX, respectively. The transient level crossing between the PL emissions of the FX and (D0 ; X) is observed at 190 ps after the optical excitation. The temporal evolution of the two peaks is displayed in Fig. 1 by solid lines with
321
arrows, which energy are determined by Gaussian curve 3tting at each time resolved spectra, as explained later. With increasing magnetic 3eld, both emission peaks shift to lower energies due to the giant Zeeman splitting of DMSs [1]. From the 3eld dependence of the PL spectra, the eNective g value of the main emission peak is estimated to be geN = 113 (neglecting the eNect the MP). At zero 3eld, both PL signals show clearly a transient energy relaxation (Fig. 1). In contrast, in a 3led of 7 T, the energy relaxation of both peaks vanishes (Fig. 1). As a result, no level crossing between the FX and (D0 ; X) peaks is observed at 7 T. Furthermore, at 7 T the FXs emission peak is observed at 1 meV above the (D0 ; X) peak, similar to the situation occurring in CdTe [16]. This indicates that the anomalous level crossing and the strong FX energy relaxation observed at zero 3eld arises as a consequence of FMP formation. After the FX is trapped in the exchange potential well of the Guctuating Mn spins, the sp–d exchange interaction stabilizes the MP by orienting the Mn spins. This process yields the transient energy relaxation observed at zero 3eld in Fig. 1. In Fig. 2 we show the contour plots and spectra of the time-resolved PL spectra of a Cd 0:95 Mn0:05 Te crystal measured at magnetic 3elds of 0 and 7 T. At zero 3eld, we can also see the two peaks from FX and the (D0 ; X), and the lower energy part of the peak is well de3ned. The energy relaxation of each peaks is larger than that of the x = 0:03 sample. It suggests that the magnetic polaron eNect slightly strong and the ratio of the PL intensities of the FX to the (D0 ; X) peak gradually increases with increasing composition x. At 7 T, we can see one more additional peak at the lower energy side, at about 6 meV from FX, which is considered as the luminescence from (A0 ; X). To analyze the energy relaxation of the two PL peaks, we integrated the transient PL spectra within time intervals of 20 ps and plotted the integrated spectra in Fig. 3 for four time delays. The PL spectra feature a sharp peak of the (D0 ; X) signal superimposed on a broad peak of the FXs recombination. These PL spectra can be decomposed into two-Gaussian peaks. The 3t curves are included in Fig. 3. For each of these two 3tting components, in Fig. 4, we illustrate the time dependence of the 3tting parameter, i.e., the energy of peak, the full width at half maximum (FWHM) and the intensity of 3tting curve as the area. The energy of the sharp peak, displayed
322
M. Nogaku et al. / Physica E 10 (2001) 320–325
Fig. 1. Time-integrated PL spectra in 0 and 7 T (top) and transient PL contour plots (bottom) measured in Cd 0:97 Mn0:03 Te.
Fig. 2. Time-integrated PL spectra in 0 and 7 T (top) and transient 1PL contour plots (bottom) measured in Cd 0:95 Mn0:05 Te.
M. Nogaku et al. / Physica E 10 (2001) 320–325
323
Fig. 4. The 3tting parameter of two Gaussian components. Fig. 3. Two components in the time resolved PL spectra 3tted by two Gaussian peaks in Cd 0:97 Mn0:03 Te.
as a square, slightly decreases as time developes and the FWHM is constant. On the other hand, the energy of broad peak decreases rapidly and the FWHM of this peak decreases. This means that the hot excitons (or electrons) are cooling down as time develops and relax energy by arranging the Mn spin their orbits creating FEMPs. From these results, we assign the sharp and broad peaks observed at zero 3eld to the signals of BEMP for the D0 state and the FEMP, respectively. This is the 3rst observation of the transient level crossing of the BEMP and FEMP levels [20]. The energy relaxation and the energy crossing of the FX and the (D0 ; X) peaks are presented in detail in Fig. 5. The FX peak crosses the (D0 ; X) at the time delay of 190 ps. In the 3rst 50 ps, the peak energy of the FX signal measured at zero 3eld increases slightly and then decreases following a double exponential decay law with the decay constants 1 = 280 ps and 2 = 5300 ps. The initial rise of the transient energy
relaxation is not fully understood. But it is probably not related to the MP formation, since it is detected also in CdTe. The slow energy relaxation component with decay time 2 represents the non-magnetic localization of the FXs in the alloy potential Guctuations and has a very small amplitude (A2 = 0:92 meV). The non-magnetic localization energy measured in a 3eld of 7 T lies below 0:1 meV for both PL peaks. This indicates that the non-magnetic localization in the alloy potential Guctuations is negligibly small in Cd 0:97 Mn0:03 Te. Therefore, the transient PL energy relaxations of 0.8 and 4:2 meV detected at zero 3eld for (D0 ; X) and FX, respectively, have a purely magnetic origin and non-magnetic localization is not required to form a MP. In fact, our value of 4:2 meV for the binding energy of the FMP in Cd 0:97 Mn0:03 Te is larger than the theoretical value of 1 meV [13,14], and is in good agreement with the experimental value of 3:3 meV observed for the x = 0:18 crystals [11]. For (D0 ; X), the time dependence of the PL peak energy follows a single exponential decay law with a long decay time
324
M. Nogaku et al. / Physica E 10 (2001) 320–325
Fig. 5. Time dependence of the PL peak energy in the time-resolved spectra. The upper 3gure was measured in zero 3eld and the lower 3gure at H = 7 T.
1 = 944 ps, indicating that the MP formation process takes more time for BMPs than for FMPs (280 ps for FMPs in Cd 0:97 Mn0:03 Te). From the temperature dependence of the PL spectra of Cd 0:952 Mn0:048 Te crystals, Bukivskii et al. [4] observed a spin splitting of 0:8 meV at 4:2 K for the (D0 ; X) complex. This is in perfect agreement with our results (Fig. 5). At 7 T, no crossing between the (D0 ; X) and FX levels is observed. The mechanism of localization of
the FX in the exchange potential wells created locally by the favorable alignment of the Mn2+ spin Guctuations [2,3] is inhibited because the Mn2+ spins are aligned in the 3eld. In this case, there is no longer the possibility of having a local exchange potential well, as it is occurring at zero 3eld due to the Mn2+ spin Guctuations. Therefore, at 7 T there is no trapping potential anymore for the excitons and the positions of the (D0 ; X) and FX levels are as in CdTe, although with a diNerent energy gap of the host material and a diNerent Zeeman shift of the bands. In fact, at 7 T the FXs emission peak is observed at 1 meV above the (D0 ; X) peak, similar to the situation occurring in CdTe [16]. In summary, we investigated the magnetic polaron formation in Cd 1−x Mnx Te crystals with x = 0:03 and 0.05. We observed the formation of magnetic polarons from the donor-bound excitons with a PL energy relaxation of 0:8 meV. For the magnetic polarons originating from the FXs we observed a binding energy of 4:2 meV. This is observed in a large energy relaxation of the FX peak, which crosses the (D0 ; X) peak at 190 ps after the laser excitation. The small binding energy of the (D0 ; X)-BMP (compared to the FMP) indicates that the strong coupling of the exciton to the donor impurity prevents the hole to be trapped in the localization potential of the Mn spin Guctuations. This situation is described in Fig. 6. From the analysis of the PL energy relaxation, it turns out that the FMP has O i.e. about one half of the a localization radius of 50 A, Bohr radius of the hole in the (D0 ; X)-BMP. Furthermore, we found that non-magnetic localization is not required to form a FMP. Finally, we comment that the magnetic localization of the FMP is not eternal, as it
Fig. 6. Schematic diagram for the (D0 ; X)-BMP and FMP. The thick arrows at the bottom represent the Mn2+ spins.
M. Nogaku et al. / Physica E 10 (2001) 320–325
depends on the orientation of the Mn spins. Therefore, in principle, it does not prevent the FMP to be mobile [21]. References [1] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [2] D. Heiman, P. WolN, J. Warnock, Phys. Rev. B 27 (1983) 4848. [3] K.V. Kavokin, I.A. Merkulov, D.R. Yakovlev, W. Ossau, G. Landwehr, Phys. Rev. B 60 (1999) 16499. [4] P.N. Bukivskii, Yu.P. Gnatenko, A.Kh. Rozhko, O.A. Shigil’chev, Fiz. Tverd. Tela 39 (1997) 527. [5] G. Mackh, W. Ossau, D.R. Yakovlev, A. Waag, G. Landwehr, Phys. Rev. B 49 (1994) 10248. [6] D.R. Yakovlev, K.V. Kavokin, Comments Cond. Mat. Phys. 18 (1996) 51. [7] A. Golnik, J. Ginter, J.A. Gaj, Solid State Phys. 16 (1983) 6073. [8] R. Pittini, J.X. Shen, M.C. Debnath, I. Souma, M. Takahashi, Y. Oka, J. Appl. Phys. 87 (2000) 6454. [9] J.J. Zayhowski, C. Jagannath, R.N. Kershaw, D. Ridgley, K. Dwight, A. Wold, Solid State Comm. 55 (1985) 941. [10] Y. Oka, J.X. Shen, M.C. Debnath, I. Souma, T. Sato, E. Shirado, Proceedings of the 24th International Conference on the Physics of Semiconductors, Jerusalem, 1998.
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[11] S. Takeyama, S. Adachi, Y. Takagi, T. Karasawa, V.F. Aguekian, in: M. ScheQer, R. Zimmermann (Eds.), Proceedings of the 25th International Conference on the Physics of Semiconductors, Berlin 1996, World Scienti3c, Singapore, 1995, p. 429. [12] C. Benoit a la Guillaume, Yu.G. Semenov, M. Combescot, Phys. Rev. B 51 (1995) 14124. [13] J. Miao, T. Stirner, W.E. Hagston, J. Appl. Phys. 81 (1997) 6297. [14] T. Stirner, P. Harrison, W.E. Hagston, J.P. Goodwin, Phys. Rev. B 53 (1996) 2143. [15] P.A. WolN, in: J.K. Furdyna, J. Kossut (Eds.), Semiconductors and Semimetals, Vol. 25, Diluted Magnetic Semiconductors, Semiconductors and Semimetals, Chap. 10, Academic, New York, 1988. [16] Landolt-BRornstein, Vol. III=41B, Springer, Berlin, 1999. [17] T. Dietl, J. Spalek, Phys. Rev. Lett. 48 (1982) 355. [18] S.I. Gubarev, J. Cryst. Growth 101 (1990) 882. [19] J.A. Gaj, R. Planel, G. Fishman, Solid State Comm. 29 (1979) 435. [20] M. Nogaku, M. Takahashi, J.X. Shen, Y. Oka, Extended Abstracts of 5th PASPS, 1999, p. 27. [21] Yu.G. Semenov, V.A. Stephanivoch, Mater. Sci. Forum 182– 184 (1995) 557.
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Transient level crossing of free and bound excitonic magnetic polarons in Cd 1−x Mnx Te M. Nogakua; c; ∗ , J.X. Shenb; c , R. Pittinib; c , T. Satob; c , Y. Okab; c a Electrical
Engineering, Miyagi National College of Technology, 48 Nodayama, Medesima Shiote, Natori 981-1239, Japan b RISM, Tohoku University, Katahira 2-1-1, Sendai, 980-8577, Japan c CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract We report an experimental evidence of the existence of free excitonic magnetic polaron (FEMP) states from the study of the dynamics of free and bound excitons in Cd 1−x Mnx Te (x = 0:03) by time-resolved photoluminescence measurements. We found, for the 3rst time, that FEMP are more stable than the polarons formed from bound excitons. The polaron binding energy for the free excitons is evaluated to be 4 meV, which is considerably larger than that of 1 meV found for the magnetic polarons of the bound excitons; these values were determined from the suppression of the excitonic energy relaxation in high magnetic 3elds. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons c 2001 Elsevier Science B.V. All rights reserved. cross in the time-resolved photoluminescence spectra. PACS: 78.47.+p; 71.35.Lk; 71.10.Pn; 71.20.Nr Keywords: Magnetic polarons; Level cross; Excitons
1. Introduction In diluted magnetic semiconductors (DMSs), magnetic elements are contained on the cation site and change the band parameter. Magnetic polarons (MPs) are entities where the band electrons (or the excitons) couple to the localized spins of the magnetic elements ∗ Corresponding author. Electrical Engineering, Miyagi National College of Technology, 48 Nodayama, Medesima Shiote, Natori 981-1239, Japan. Tel.: +81-22-381-0313; fax: +81-22-381-0310. E-mail address: [email protected] (M. Nogaku).
through the exchange interactions [1–19]. This state results in the polarization of the localized spins of the magnetic ions within the Bohr radius of the MP. For the realization of this local polarization of magnetic ion spins, the carriers need to be localized. A convincing experimental proof of the formation of the MP is the 3nite spin-Gip energy of an electron bound to a donor impurity (D0 ) detected at zero magnetic 3eld in the spin-Gip Raman scattering experiment on DMSs [2]. In contrast to this clear demonstration of the bound MP in the D0 state, there are still controversies
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 1 0 8 - 4
M. Nogaku et al. / Physica E 10 (2001) 320–325
about the existence free excitonic magnetic polarons (FEMPs) in which a free exciton polarizes the neighboring magnetic ion spins and is trapped in the exchange potential well. The stability of the FEMP state is still under discussion [2]. We present here a detailed study on the stability of free and bound magnetic polarons (BEMPs) with a detailed analysis of the transient optical spectra of Cd 1−x Mnx Te single crystals. In contrast to the generally accepted picture of a strong BEMP compared to the FEMP, we found that the polaron binding energy of free excitons is much larger than that of bound excitons. Photoluminescence (PL) experiments have been performed on a series of samples with Mn content x ranging from 0 to 0.1. The results are repeatable and consistent with the Mn composition variation. In the following we present only the spectra measured in the samples with x = 0:03 and 0.05. We interpret the observed phenomena by a qualitative model taking into account the self-consistent con3nement of the holes and the spin orientation of Mn ions. 2. Experiments High-purity Cd 1−x Mnx Te single crystals with the Mn composition x = 0– 0.1 were prepared by the vertical Bridgman method. For the optical excitation, we used a wavelength tunable femto-second pulsed laser. The time variation of the excitonic luminescence was recorded by a streak camera at 4:2 K applying magnetic 3elds up to 7 T. 3. Results and discussion In Fig. 1, we show the contour plots and the time-integrated spectra until 5 ns of the time-resolved PL spectra of a Cd 0:97 Mn0:03 Te crystal measured in zero 3eld and in 7 T. In these spectra, two peaks are resolved. One of them is sharp, while the other one is broad and shows a strong transient energy relaxation at zero 3eld. We assign these two peaks to the signals of the (D0 ; X) and the FX, respectively. The transient level crossing between the PL emissions of the FX and (D0 ; X) is observed at 190 ps after the optical excitation. The temporal evolution of the two peaks is displayed in Fig. 1 by solid lines with
321
arrows, which energy are determined by Gaussian curve 3tting at each time resolved spectra, as explained later. With increasing magnetic 3eld, both emission peaks shift to lower energies due to the giant Zeeman splitting of DMSs [1]. From the 3eld dependence of the PL spectra, the eNective g value of the main emission peak is estimated to be geN = 113 (neglecting the eNect the MP). At zero 3eld, both PL signals show clearly a transient energy relaxation (Fig. 1). In contrast, in a 3led of 7 T, the energy relaxation of both peaks vanishes (Fig. 1). As a result, no level crossing between the FX and (D0 ; X) peaks is observed at 7 T. Furthermore, at 7 T the FXs emission peak is observed at 1 meV above the (D0 ; X) peak, similar to the situation occurring in CdTe [16]. This indicates that the anomalous level crossing and the strong FX energy relaxation observed at zero 3eld arises as a consequence of FMP formation. After the FX is trapped in the exchange potential well of the Guctuating Mn spins, the sp–d exchange interaction stabilizes the MP by orienting the Mn spins. This process yields the transient energy relaxation observed at zero 3eld in Fig. 1. In Fig. 2 we show the contour plots and spectra of the time-resolved PL spectra of a Cd 0:95 Mn0:05 Te crystal measured at magnetic 3elds of 0 and 7 T. At zero 3eld, we can also see the two peaks from FX and the (D0 ; X), and the lower energy part of the peak is well de3ned. The energy relaxation of each peaks is larger than that of the x = 0:03 sample. It suggests that the magnetic polaron eNect slightly strong and the ratio of the PL intensities of the FX to the (D0 ; X) peak gradually increases with increasing composition x. At 7 T, we can see one more additional peak at the lower energy side, at about 6 meV from FX, which is considered as the luminescence from (A0 ; X). To analyze the energy relaxation of the two PL peaks, we integrated the transient PL spectra within time intervals of 20 ps and plotted the integrated spectra in Fig. 3 for four time delays. The PL spectra feature a sharp peak of the (D0 ; X) signal superimposed on a broad peak of the FXs recombination. These PL spectra can be decomposed into two-Gaussian peaks. The 3t curves are included in Fig. 3. For each of these two 3tting components, in Fig. 4, we illustrate the time dependence of the 3tting parameter, i.e., the energy of peak, the full width at half maximum (FWHM) and the intensity of 3tting curve as the area. The energy of the sharp peak, displayed
322
M. Nogaku et al. / Physica E 10 (2001) 320–325
Fig. 1. Time-integrated PL spectra in 0 and 7 T (top) and transient PL contour plots (bottom) measured in Cd 0:97 Mn0:03 Te.
Fig. 2. Time-integrated PL spectra in 0 and 7 T (top) and transient 1PL contour plots (bottom) measured in Cd 0:95 Mn0:05 Te.
M. Nogaku et al. / Physica E 10 (2001) 320–325
323
Fig. 4. The 3tting parameter of two Gaussian components. Fig. 3. Two components in the time resolved PL spectra 3tted by two Gaussian peaks in Cd 0:97 Mn0:03 Te.
as a square, slightly decreases as time developes and the FWHM is constant. On the other hand, the energy of broad peak decreases rapidly and the FWHM of this peak decreases. This means that the hot excitons (or electrons) are cooling down as time develops and relax energy by arranging the Mn spin their orbits creating FEMPs. From these results, we assign the sharp and broad peaks observed at zero 3eld to the signals of BEMP for the D0 state and the FEMP, respectively. This is the 3rst observation of the transient level crossing of the BEMP and FEMP levels [20]. The energy relaxation and the energy crossing of the FX and the (D0 ; X) peaks are presented in detail in Fig. 5. The FX peak crosses the (D0 ; X) at the time delay of 190 ps. In the 3rst 50 ps, the peak energy of the FX signal measured at zero 3eld increases slightly and then decreases following a double exponential decay law with the decay constants 1 = 280 ps and 2 = 5300 ps. The initial rise of the transient energy
relaxation is not fully understood. But it is probably not related to the MP formation, since it is detected also in CdTe. The slow energy relaxation component with decay time 2 represents the non-magnetic localization of the FXs in the alloy potential Guctuations and has a very small amplitude (A2 = 0:92 meV). The non-magnetic localization energy measured in a 3eld of 7 T lies below 0:1 meV for both PL peaks. This indicates that the non-magnetic localization in the alloy potential Guctuations is negligibly small in Cd 0:97 Mn0:03 Te. Therefore, the transient PL energy relaxations of 0.8 and 4:2 meV detected at zero 3eld for (D0 ; X) and FX, respectively, have a purely magnetic origin and non-magnetic localization is not required to form a MP. In fact, our value of 4:2 meV for the binding energy of the FMP in Cd 0:97 Mn0:03 Te is larger than the theoretical value of 1 meV [13,14], and is in good agreement with the experimental value of 3:3 meV observed for the x = 0:18 crystals [11]. For (D0 ; X), the time dependence of the PL peak energy follows a single exponential decay law with a long decay time
324
M. Nogaku et al. / Physica E 10 (2001) 320–325
Fig. 5. Time dependence of the PL peak energy in the time-resolved spectra. The upper 3gure was measured in zero 3eld and the lower 3gure at H = 7 T.
1 = 944 ps, indicating that the MP formation process takes more time for BMPs than for FMPs (280 ps for FMPs in Cd 0:97 Mn0:03 Te). From the temperature dependence of the PL spectra of Cd 0:952 Mn0:048 Te crystals, Bukivskii et al. [4] observed a spin splitting of 0:8 meV at 4:2 K for the (D0 ; X) complex. This is in perfect agreement with our results (Fig. 5). At 7 T, no crossing between the (D0 ; X) and FX levels is observed. The mechanism of localization of
the FX in the exchange potential wells created locally by the favorable alignment of the Mn2+ spin Guctuations [2,3] is inhibited because the Mn2+ spins are aligned in the 3eld. In this case, there is no longer the possibility of having a local exchange potential well, as it is occurring at zero 3eld due to the Mn2+ spin Guctuations. Therefore, at 7 T there is no trapping potential anymore for the excitons and the positions of the (D0 ; X) and FX levels are as in CdTe, although with a diNerent energy gap of the host material and a diNerent Zeeman shift of the bands. In fact, at 7 T the FXs emission peak is observed at 1 meV above the (D0 ; X) peak, similar to the situation occurring in CdTe [16]. In summary, we investigated the magnetic polaron formation in Cd 1−x Mnx Te crystals with x = 0:03 and 0.05. We observed the formation of magnetic polarons from the donor-bound excitons with a PL energy relaxation of 0:8 meV. For the magnetic polarons originating from the FXs we observed a binding energy of 4:2 meV. This is observed in a large energy relaxation of the FX peak, which crosses the (D0 ; X) peak at 190 ps after the laser excitation. The small binding energy of the (D0 ; X)-BMP (compared to the FMP) indicates that the strong coupling of the exciton to the donor impurity prevents the hole to be trapped in the localization potential of the Mn spin Guctuations. This situation is described in Fig. 6. From the analysis of the PL energy relaxation, it turns out that the FMP has O i.e. about one half of the a localization radius of 50 A, Bohr radius of the hole in the (D0 ; X)-BMP. Furthermore, we found that non-magnetic localization is not required to form a FMP. Finally, we comment that the magnetic localization of the FMP is not eternal, as it
Fig. 6. Schematic diagram for the (D0 ; X)-BMP and FMP. The thick arrows at the bottom represent the Mn2+ spins.
M. Nogaku et al. / Physica E 10 (2001) 320–325
depends on the orientation of the Mn spins. Therefore, in principle, it does not prevent the FMP to be mobile [21]. References [1] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [2] D. Heiman, P. WolN, J. Warnock, Phys. Rev. B 27 (1983) 4848. [3] K.V. Kavokin, I.A. Merkulov, D.R. Yakovlev, W. Ossau, G. Landwehr, Phys. Rev. B 60 (1999) 16499. [4] P.N. Bukivskii, Yu.P. Gnatenko, A.Kh. Rozhko, O.A. Shigil’chev, Fiz. Tverd. Tela 39 (1997) 527. [5] G. Mackh, W. Ossau, D.R. Yakovlev, A. Waag, G. Landwehr, Phys. Rev. B 49 (1994) 10248. [6] D.R. Yakovlev, K.V. Kavokin, Comments Cond. Mat. Phys. 18 (1996) 51. [7] A. Golnik, J. Ginter, J.A. Gaj, Solid State Phys. 16 (1983) 6073. [8] R. Pittini, J.X. Shen, M.C. Debnath, I. Souma, M. Takahashi, Y. Oka, J. Appl. Phys. 87 (2000) 6454. [9] J.J. Zayhowski, C. Jagannath, R.N. Kershaw, D. Ridgley, K. Dwight, A. Wold, Solid State Comm. 55 (1985) 941. [10] Y. Oka, J.X. Shen, M.C. Debnath, I. Souma, T. Sato, E. Shirado, Proceedings of the 24th International Conference on the Physics of Semiconductors, Jerusalem, 1998.
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