Femtosecond spin dynamics in magnetically tunable heterostructures

PHYSICA

Physiea B 194--196(1994) 1305-1306 North-Holland

Femtosecond Spin Dynamics in Magnetically Tunable Heterostructures D. A. Tulchinsky, l J. F. Smyth, 1 D. D. Awschalom, l Samarth, 2 H. Luo,3 and J. K. Furdyna 3

t Department of Physics, University of California, Santa Barbara, CA 93106, USA 2Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA 3Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Ultrafast spin dynamics in diluted magnetic semiconductor heterostructures are probed to observe the effects of dimensionality and quantum confinement on spin scattering. Two novel structures, the spin superlattice and the magnetically-coupled double quantum well, exhibit unusual spin behavior. These experiments resolve energydependent electronic and magnetic interactions on a femtosecond time scale in quantum geometries. Investigations of carrier spin dynamics in semiconductor heterostructures are important for understanding the equilibrium and non-equilibrium interactions in confined systems. 1 Despite recent studies, spin relaxation in low dimensions remains poorly understood. Quantum wells and heterostructures containing diluted magnetic semiconductors (DMS) provide an opportunity to explore the interactions between band electrons and magnetic ions in well-defined quantum geometries. DMS materials possess the convenient optical properties of traditional GaAs based systems while also providing magnetic ions that serve as spin scattering centers through carrier-ion exchange. This sp-d exchange interaction appears as a large effective g-factor (~1000) for carriers at low temperatures resulting in large Zeeman splittings in modest applied magnetic fields. We present a series of time resolved photoluminescence (PL) experiments aimed at exploring the effects of quantum confinement and magnetic interactions on carrier spin dynamics. Results from two MBE grown spin engineered systems, the spin superlattice (SSL) and the magnetically-coupled double quantum well (MCDQW), are discussed. The samples are placed in a magneto-optical cryostat in the Faraday configuration and cooled to T ~ 4.5K. Optically pumping with linearly polarized light creates equal populations of spin up and spin down carders. After scattering, the selection rules for carder recombination determine the helicity of the emitted photons. The two circular polarizations are optically resolved, and ultrafast time resolution is achieved using upconversion spectroscopy.2 After exciting the sample with a laser pulse, a second time delayed probe pulse interrogates a time slice of the collected luminescence as the two pass coincidentally through a non-linear crystal. Sumfrequency photons are generated for the

luminescence and probe photons that satisfy the phase matching conditions of the crystal. These photons are collected, energy resolved, and detected by a cooled photomultiplier tube. A mode-locked Ti:sapphire laser is used to achieve a temporal resolution of At ~ 200fs. The first set of heterostructures consist of 10 period SLs made from alternating 100A layers of magnetic and non-magnetic semiconductors with small zero-field band offsets. In an applied field, B, the Zeeman splitting produces a large shift in the band edges of the magnetic semiconductor and small splittings in the non-magnetic semiconductor. When the Zeeman shift in the magnetic layers overcomes the zero-field band offsets, the magnetic layers act as quantum wells for spin down carders and as barriers for spin up carriers. This spin dependent spatially varying potential causes a spatial separation of the carriers in the different layers--the SSL. Our samples, a ZnSe/ZnMnSe (9% Mn) multilayer structure, have been established as SSLs above Bss1~ 0.ST. 3 Spin dynamics as carriers cool to the ground state can be directly probed by time resolving the luminescence polarization, P = (I(t~÷) - I(o-))/(I(c ÷) + I(c-)), versus PL energy. PL detection energies above the ground state probe carders that radiatively recombine before reaching equilibrium, providing a stroboscopic picture of spin relaxation as the carders cool. Fig. la displays the time-resolved polarization for B < Bssl at the ground state and at a higher detection energy. The rise time of the polarization reveals spin scattering times in a QW consistent with sp-d exchange interactions. 4 A remarkable change in exciton dynamics is observed in the SSL regime. Here, no ~- photons from the SSL are detected. Upon excitation, spin polarization occurs nearly instantaneously and is ~100% o~ polarized as shown in Fig. lb. Since the

0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E1233-C

1306

1.0

I

a) •

I

I

I

I

I

1.0 0.8

I

B = 1/4T E = 2.792eV

a~l

I

I

I

I

I

I

I

I

0 6

-

P 0.4 - / o

32 A

f

o.2

0.0 0.0 ~'"Jk: " ' ~ 0 20

I I ~ I 40 60 80 Time (ps) ¢:¢¢--¢¢~¢¢¢¢¢¢¢¢--:¢--¢¢~ 1.0

1.0

~

_

I

-0.2

0

_

I I 1

,.

I 2

I B

I 3

_

I

I 4

-

I 5

(T)

0.5

b) P0.5 -

B=3T

o÷--->

_ 0 . 6 ~ ..

~~up~x

Ci+~

_ 0.4"~

~

l

p

_ 0.0

-1

0

1 2 Time ~s)

3

4

0.0

Fig. 1. (a) Time-resolved polarization at B < B~I for detection energies at the ground state ( I ) and 20meV above the ground state (A). (b) Timeresolved polarization (~) and PL (A) of the SSL. system is excited with linearly polarized light, this indicates an extremely rapid spin flip relaxation and subsequent shift for carriers in the ZnSe layers to the magnetic quantum wells on a time scale much shorter than the exciton lifetimes. A second set of experiments are performed on a series of symmetric 40A wide non-magnetic ZnSe/ZnCdSe double quantum wells (DQWs) coupled by a thin (Lb = 12A, 32A; 24% Mn) ZnMnSe DMS barrier. Incorporating magnetic spins into the barrier allows one to probe the effects of spin-dependent coupling. The giant Zeeman splittings of the barrier's band edges enables the continuous tuning of the barrier's potential for each spin state with magnetic field. The interaction of the confined carriers with the magnetic ions in the barrier can be used to optically explore how the magnetic state of the barrier (i.e., paramagnetic, spin glass, or antiferromagnetic) depends on dimensionality. The zero-field degeneracy of the ground state energy levels of the electron in the DQW is removed by application of a magnetic field. The Zeeman splitting is observed as polarized PL resulting from the unequal spin population caused by rapid spin flip from the higher energy spin up state to the spin

0.0

~

,

-

_]

1.2

_t 0.4

¢r

0.0 4 6 8 Time (ps) Fig. 2. (a) Polarization of the MCDQWs for Lb = 12A and 32A, and for a non-magnetically (NM) CDQW with Lb -- 12A. (b) Time- resolved polarization and PL for a MCDQW with Lb = 12A. 0

2

down state. Fig. 2a shows that a significant static polarization develops with application of a modest external field. Polarization is greatest for the sample with the 12A barrier because of the larger interwell coupling resulting from the smaller barrier. 5 The temporal response of the carriers and polarization is shown in Fig. 2b. The polarization rise time and hence the spin flip interaction time is faster than our resolution. We also find that once an initial carrier polarization is obtained, the polarization remains constant and persists for several radiative lifetimes. Results from a series of experiments on a variety of DMS heterostructures reveal energydependent spin dynamics of photo-excited carriers in both SSLs and MCDQWs. These results should stimulate further investigations of magnetic interactions in 2D systems. This work is supported by NSF grants DMR 92-07567 and DMR 92-08400, and the IBM Corporation. 1. S. Schmitt-Rink, et al., Adv. Phys. 38, 89 (1989). 2. M. Freeman, et al., J. Appl. Phys. 67, 5102 (1990). 3. N. Dai, et al., Phys. Rev. Lett. 67, 3824 (1991). 4. J. F. Smyth, et al., submitted for publication. 5. J. F. Smyth, et al, Phys. Rev. B ~ , 4340 (1992).