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Ultrafast spin dynamics in diluted magnetic semiconductor superlattices
Surface Science 229 (1990) North-Holland
ULTRAFAST
SPIN DYNAMICS
D.D. AWSCHALOM,
145
145-147
IN DILUTED
M.R. FREEMAN,
MAGNETIC
SEMICONDUCTOR
SUPERLATTICES
C. HSU and L.L. CHANG
IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA Received
11 July 1989; accepted
for publication
14 September
1989
Direct time-resolved optical and magnetic studies of charge carriers and ionic spins in diluted magnetic semiconductors are reported. These measurements are made on a series of CdTe/Cd,_,Mn,Te superlattices, in order to probe the spin dynamics in thin magnetic layers by utilizing the overlap of the quantum-confined carrier wavefunctions and the Mn-containing barriers. We discuss representative results for the thinnest (9 A) CdTe quantum wells.
It has recently been demonstrated that magnetic spectroscopy with nearly quantum-limited spin sensitivity [ 1 ] can reveal the quantized energy levels and the relaxation times of optically induced magnetization in diluted magnetic semiconductor superlattices. Similarly, ultrafast luminescence measurements have been used to monitor, in the time domain, the evolution of polarization of photoexcited charge carriers in these systems [ 21. Applied in concert, these two techniques yield a fairly complete picture of the dynamics of the total magnetization of the system, and may be used to study the evolution of such behaviour to lower dimensions and different magnetic states. In the present work, we discuss preliminary electronic and magnetic results of a study of thin magnetic layers. Unlike the earlier measurements, these samples consist of a series of CdTe/Cd, _,Mn,Te superlattices grown by molecular beam epitaxy with non-magnetic quantum wells having thicknesses of approximately 10, 20, and 40 A [ 3 1. The magnetic barriers span a range of thicknesses from 10 to 100 A and contain manganese concentrations between 10% and 38%. The carrier wavefunctions significantly overlap the barriers in these structures, and it is in the regions of overlap that the dominant interactions between the electrons and holes and the magnetic ions take place. In this way optically generated charge carriers can be used to probe the spin dynamics in even the thinnest magnetic layers that it is possible to grow with convincing structural integrity. 0039-6028/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
A DC SQUID probe with direct optical access has been constructed for magnetic spectroscopy in a 1 K top-loading optical cryostat. This probe eliminates the dispersive optical fiber used in our previous studies [ 11, and should enable a clean observation of the onset time of the optically induced magnetization through the use of subpicosecond compressed pulses [ 2 1. A small sample is placed superlattice side down over one half of a thin film gradiometer coupled to the SQUID. The sample is illuminated through the transparent (sapphire or quartz) gradiometer substrate. The excitation beam is from a synchronously pumped two-jet dye laser producing N 1 ps pulses cavity dumped at 17.4 kHz and tunable throughout the red region of the spectrum. It is necessary to use very little optical power ( - 10 PW ), as heating contributes a background signal through both the temperature dependence of the sample magnetization in the residual field trapped in the probe and thermal expansion of the gradiometer. An additional piece of superlattice is mounted on the SQUID probe for the luminescence measurements. In this work the luminescence is collected in an optical fiber and directed to a monochromator or a streak camera with 35 ps time resolution. The DC luminescence signature of our narrowest quantum wells (9 A, with 100 A barriers) is shown in fig. la. The bound state of these wells lies - 40 meV below the bandgap of the magnetic barriers ( 1.97 eV). We note that there is also a significant Stark shift due to binding of the excitons on impur-
D.D. Awschalom et al./Ultrafast spin dynamics in semiconductor superlattices
146
.z
t
1.85
I
I
1.90
1.95
Energy
I
2.00
2.05
(eV)
Fig. 1. (a) The luminescence from the 9 A quantum well, 100 A barrier, 100 period CdTe/Cd,_,Mn,Te superlattice with x=0.23 at T=4 K, for an excitation energy of 2.02 eV. (b) The photoluminescence excitation spectrum for the peak of the line, at E= 1.90 eV.
ities. The photoluminescence excitation spectrum (PLE) displayed as fig. 1b reveals the peak characteristic of resonant excitation at the quantum energy level. Note that in this very narrow non-magnetic quantum well, much of the carrier wavefunction rests in the barrier. The resulting sensitivity to magnetic alloy fluctuations may contribute to the somewhat broader luminescence line and PLE than seen in earlier measurements [ 31. The data of fig. 2 represent the magnetic analog of the PLE. This is the time-integrated magnetic signal arising from excitation with circularly polarized light, and indeed the polarity of the signal is observed to follow the helicity of the pump photons. Of particular significance is the red-shift of the peak of the magnetic spectrum with respect to that of the PLE signal, suggesting that the polarization is more effciently transferred to the Mn ions at low carrier energies. At higher energies, there is more opportunity for spin-flip scattering [2] before the carriers bind into states in which magnetic polarons may develop. The time-integrated magnetic signal is due almost entirely to the ionic magnetization, for two reasons: The spin-lattice relaxation time for the ions is vastly longer than for the electrons and holes, and the polaron formation process is spin-nonconserving [ 41. A time-resolved magnetic spectrum can be ob-
a-o-4le
,
1
2.00
Energy
(eV)
Fig. 2. Time-integrated magnetic spectroscopy at T=4 K for rightand left- circularly polarized excitation beams. The optically induced magnetization peaks slightly below the maximum of the PLE spectrum, at 1.92 eV.
tained by perturbing the system with small probe pulses at variable time delays after the excitation (pump) pulse. At any particular instant in time a probe pulse measures the magneto-optical susceptibility, xop, by inducing a magnetization GM=,yJd, where 61 is the probe intensity. We detect the com-
Time
(nsec)
Fig. 3. Time-resolved magnetic and electronic responses at 4 K following optical excitation at 1.90 eV. The magnetic signal in (a) shows the fast spin-lattice relaxation at this magnetic concentration, which may preclude the development of magnetic polarons. The longer duration of the luminescence decay, (b), confirms that the carrier spins depolarize rapidly in comparison to the excitomc lifetime.
D.D. Awschalom et al./Ultrafast spin dynamics in semiconductor superlattices
ponent of xop modulated by the transient magnetization created by the pump pulse - to first order, this is a measure of the instantaneous magnetization. A representative signal is plotted in fig. 3a, and indicates the fast relaxation of the magnetic spins with extremely good time resolution. In contrast, we show in fig. 3b the luminescence decay following excitation at the same energy. Clearly the electron-hole recombination proceeds at a much slower rate than the magnetic relaxation, demonstrating that the charge carriers are also depolarized very rapidly. Parallel measurements have shown this spin relaxation directly through the luminescence polarization (2 ). In future work this will be compared to the initial transfer of spin from the carriers to the magnetic ions in the femtosecond time domain.
Our MBE effort was supported, Army Research Office.
147
in part, by the I.JS
References
[ 11D.D. Awschalom,
J. Warnock, J.M. Hong, L.L. Chang, M.B. Ketchen and W.J. Gallagher, Phys. Rev. Lett. 62 ( 1989) 199. [ 21 M.R. Freeman, D.D. Awschalom, J.M. Hong and L.L.Chang, Surf. Sci. 228 (1990) 233. [ 31 J.M. Hong, D.D. Awschalom, L.L. Chang and A. Segmiiller, J. Appl. Phys. 63 (1988) 3285. [4] D.D. Awschalom and J. Wamock, Mater. Res. Sot. Symp. Proc. 89 (1987) 71.
ULTRAFAST
SPIN DYNAMICS
D.D. AWSCHALOM,
145
145-147
IN DILUTED
M.R. FREEMAN,
MAGNETIC
SEMICONDUCTOR
SUPERLATTICES
C. HSU and L.L. CHANG
IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA Received
11 July 1989; accepted
for publication
14 September
1989
Direct time-resolved optical and magnetic studies of charge carriers and ionic spins in diluted magnetic semiconductors are reported. These measurements are made on a series of CdTe/Cd,_,Mn,Te superlattices, in order to probe the spin dynamics in thin magnetic layers by utilizing the overlap of the quantum-confined carrier wavefunctions and the Mn-containing barriers. We discuss representative results for the thinnest (9 A) CdTe quantum wells.
It has recently been demonstrated that magnetic spectroscopy with nearly quantum-limited spin sensitivity [ 1 ] can reveal the quantized energy levels and the relaxation times of optically induced magnetization in diluted magnetic semiconductor superlattices. Similarly, ultrafast luminescence measurements have been used to monitor, in the time domain, the evolution of polarization of photoexcited charge carriers in these systems [ 21. Applied in concert, these two techniques yield a fairly complete picture of the dynamics of the total magnetization of the system, and may be used to study the evolution of such behaviour to lower dimensions and different magnetic states. In the present work, we discuss preliminary electronic and magnetic results of a study of thin magnetic layers. Unlike the earlier measurements, these samples consist of a series of CdTe/Cd, _,Mn,Te superlattices grown by molecular beam epitaxy with non-magnetic quantum wells having thicknesses of approximately 10, 20, and 40 A [ 3 1. The magnetic barriers span a range of thicknesses from 10 to 100 A and contain manganese concentrations between 10% and 38%. The carrier wavefunctions significantly overlap the barriers in these structures, and it is in the regions of overlap that the dominant interactions between the electrons and holes and the magnetic ions take place. In this way optically generated charge carriers can be used to probe the spin dynamics in even the thinnest magnetic layers that it is possible to grow with convincing structural integrity. 0039-6028/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
A DC SQUID probe with direct optical access has been constructed for magnetic spectroscopy in a 1 K top-loading optical cryostat. This probe eliminates the dispersive optical fiber used in our previous studies [ 11, and should enable a clean observation of the onset time of the optically induced magnetization through the use of subpicosecond compressed pulses [ 2 1. A small sample is placed superlattice side down over one half of a thin film gradiometer coupled to the SQUID. The sample is illuminated through the transparent (sapphire or quartz) gradiometer substrate. The excitation beam is from a synchronously pumped two-jet dye laser producing N 1 ps pulses cavity dumped at 17.4 kHz and tunable throughout the red region of the spectrum. It is necessary to use very little optical power ( - 10 PW ), as heating contributes a background signal through both the temperature dependence of the sample magnetization in the residual field trapped in the probe and thermal expansion of the gradiometer. An additional piece of superlattice is mounted on the SQUID probe for the luminescence measurements. In this work the luminescence is collected in an optical fiber and directed to a monochromator or a streak camera with 35 ps time resolution. The DC luminescence signature of our narrowest quantum wells (9 A, with 100 A barriers) is shown in fig. la. The bound state of these wells lies - 40 meV below the bandgap of the magnetic barriers ( 1.97 eV). We note that there is also a significant Stark shift due to binding of the excitons on impur-
D.D. Awschalom et al./Ultrafast spin dynamics in semiconductor superlattices
146
.z
t
1.85
I
I
1.90
1.95
Energy
I
2.00
2.05
(eV)
Fig. 1. (a) The luminescence from the 9 A quantum well, 100 A barrier, 100 period CdTe/Cd,_,Mn,Te superlattice with x=0.23 at T=4 K, for an excitation energy of 2.02 eV. (b) The photoluminescence excitation spectrum for the peak of the line, at E= 1.90 eV.
ities. The photoluminescence excitation spectrum (PLE) displayed as fig. 1b reveals the peak characteristic of resonant excitation at the quantum energy level. Note that in this very narrow non-magnetic quantum well, much of the carrier wavefunction rests in the barrier. The resulting sensitivity to magnetic alloy fluctuations may contribute to the somewhat broader luminescence line and PLE than seen in earlier measurements [ 31. The data of fig. 2 represent the magnetic analog of the PLE. This is the time-integrated magnetic signal arising from excitation with circularly polarized light, and indeed the polarity of the signal is observed to follow the helicity of the pump photons. Of particular significance is the red-shift of the peak of the magnetic spectrum with respect to that of the PLE signal, suggesting that the polarization is more effciently transferred to the Mn ions at low carrier energies. At higher energies, there is more opportunity for spin-flip scattering [2] before the carriers bind into states in which magnetic polarons may develop. The time-integrated magnetic signal is due almost entirely to the ionic magnetization, for two reasons: The spin-lattice relaxation time for the ions is vastly longer than for the electrons and holes, and the polaron formation process is spin-nonconserving [ 41. A time-resolved magnetic spectrum can be ob-
a-o-4le
,
1
2.00
Energy
(eV)
Fig. 2. Time-integrated magnetic spectroscopy at T=4 K for rightand left- circularly polarized excitation beams. The optically induced magnetization peaks slightly below the maximum of the PLE spectrum, at 1.92 eV.
tained by perturbing the system with small probe pulses at variable time delays after the excitation (pump) pulse. At any particular instant in time a probe pulse measures the magneto-optical susceptibility, xop, by inducing a magnetization GM=,yJd, where 61 is the probe intensity. We detect the com-
Time
(nsec)
Fig. 3. Time-resolved magnetic and electronic responses at 4 K following optical excitation at 1.90 eV. The magnetic signal in (a) shows the fast spin-lattice relaxation at this magnetic concentration, which may preclude the development of magnetic polarons. The longer duration of the luminescence decay, (b), confirms that the carrier spins depolarize rapidly in comparison to the excitomc lifetime.
D.D. Awschalom et al./Ultrafast spin dynamics in semiconductor superlattices
ponent of xop modulated by the transient magnetization created by the pump pulse - to first order, this is a measure of the instantaneous magnetization. A representative signal is plotted in fig. 3a, and indicates the fast relaxation of the magnetic spins with extremely good time resolution. In contrast, we show in fig. 3b the luminescence decay following excitation at the same energy. Clearly the electron-hole recombination proceeds at a much slower rate than the magnetic relaxation, demonstrating that the charge carriers are also depolarized very rapidly. Parallel measurements have shown this spin relaxation directly through the luminescence polarization (2 ). In future work this will be compared to the initial transfer of spin from the carriers to the magnetic ions in the femtosecond time domain.
Our MBE effort was supported, Army Research Office.
147
in part, by the I.JS
References
[ 11D.D. Awschalom,
J. Warnock, J.M. Hong, L.L. Chang, M.B. Ketchen and W.J. Gallagher, Phys. Rev. Lett. 62 ( 1989) 199. [ 21 M.R. Freeman, D.D. Awschalom, J.M. Hong and L.L.Chang, Surf. Sci. 228 (1990) 233. [ 31 J.M. Hong, D.D. Awschalom, L.L. Chang and A. Segmiiller, J. Appl. Phys. 63 (1988) 3285. [4] D.D. Awschalom and J. Wamock, Mater. Res. Sot. Symp. Proc. 89 (1987) 71.