Spintronics and Quantum Information Processing are currently two central issues in the physics and applications of semiconductors. They can have tremendous impact on future information technology by revolutionizing the way we process and store information.

"Spintronic" devices are those in which the spin degree of freedom of the electron is actively used in addition to, or in place of, the charge (orbital) degrees of freedom. Expected improvements to conventional charge-based devices include nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities. Electron spins in semiconductors have been recognized as the ideal medium on which to encode quantum bits (or "qubits") due to their very long spin lifetimes as demonstrated by recent experiments. It is also expected that electronic, magnetic, and photonic functions can be incorporated into single devices to create spin-based multifunctional devices.

Magnetic III-V semiconductors (e.g., InMnAs and GaMnAs), first grown by Hiroo Munekata in 1989 , have emerged as a prime candidate for these innovative device applications, demonstrating carrier-induced ferromagnetism, sensitive to applied electric fields and light. In this DARPA-SPINS-funded program (grant No. MDA972-00-1-0034, see Review), we are investigating various aspects of optical and terahertz properties of InMnAs, InGaMnAs, and GaMnAs.

 

Project 1: Ultrafast Optical Manipulation of Ferromagnetism

Figure 1.

Successful exploitation and manipulation of spins is desired for these revolutionary device ideas as well as for the solid-state realization of quantum information processing, computation and communications. Because of the carrier-induced nature of ferromagnetism in (III,Mn)V ferromagnets, carrier-density-tuning is the key to the successful manipulation of ferromagnetic order. Here, we have demonstrated ultrafast photoinduced softening (i.e., a transient coercivity decrease) in InMnAs/GaSb heterostructures for the first time (Fig. 1). A large density of spin-polarized transient carriers was created only within the InMnAs magnetic layer using intense 140 fs mid-infrared pulses , and then time-resolved magneto-optical Kerr effect spectroscopy was used to monitor the transient magnetic properties induced by the photogenerated carriers. We discovered that this softening persists only for ~2 ps, after which the coercivity recovered its original value.

We attribute this striking observation to transiently increase ferromagnetic exchange interaction among Mn ions due to the increased carrier density, which originate from carrier-mediated ferromagnetism in (III,Mn)V semiconductors. The sub-picosecond decay and extremely fast recovery of the loop are probably due to the ultrashort carrier lifetime in this system, which arises from low-temperature MBE growth. This unusual phenomenon opens up new possibilities for ultrafast optical manipulation of ferromagnetic order as well as for studying the dynamics of strongly-correlated many-body systems. Finally, an entirely non-thermal scheme for magneto-optical recording is possible based on our observation.

References:

J. Wang, G. A. Khodaparast, J. Kono, A. Oiwa, and H. Munekata, "Ultrafast Optical and Magneto-Optical Studies of III-V Ferromagnetic Semiconductors," Journal of Modern Optics 51, 2771 (2004). (full text)

J. Wang, G. A. Khodaparast, J. Kono, T. Slupinski, A. Oiwa, and H. Munekata, "Ultrafast Softening in InMnAs" (invited paper), Physica E 20, 412 (2004). (full text, erratum)

J. Wang, J. Kono, A. Oiwa, H. Munekata, and C. J. Stanton, "Ultrafast Carrier Dynamics in Ferromagnetic InGaMnAs," Superlattices and Microstructures 34, 563 (2004). (full text)

J. Wang, G. A. Khodaparast, J. Kono, T. Slupinski, A. Oiwa, and H. Munekata, "Ultrafast Photoinduced Softening in a Ferromagnetic Semiconductor," in: Quantum Electronics and Laser Science Conference, OSA Technical Digest (Optical Society of America, Washington DC, 2003), QThB6. (full text)

J. Wang, G. A. Khodaparast, J. Kono, T. Slupinski, A. Oiwa, and H. Munekata, "Ultrafast optical manipulation of ferromagnetic order in InMnAs/GaSb," Journal of Superconductivity 16, 373 (2003). (full text)

 

Project 2: Cyclotron Resonance in InMnAs

Figure 2. Hole cyclotron resonance spectra for ferromagnetic InMnAs/GaSb heterostructures. The transmission of hole-active circularly polarized 10.6 um radiation is plotted as a function of magnetic field at different temperatures. Both samples show two strongly temperature-dependent features. (cond-mat/0207485)

The interaction of free carriers with localized spins plays an important role in a variety of magnetic and many-body phenomena in metals. Carriers in the vicinity of a magnetic ion are magnetized, which in turn leads to an indirect exchange interaction between magnetic ions. The discovery of carrier-induced ferromagnetism in magnetic III-V semiconductors has not only opened up new device opportunities but also provided a novel material system in which to study the physics of itinerant carriers interacting with localized spins. Various theoretical models have been proposed but the microscopic mechanism is still a matter of controversy.

One of the open questions is the nature of the carriers mediating the exchange interaction between Mn ions, i.e., whether they reside in the impurity band (d-like), the delocalized valence bands (p-like), or some type of mixed states. Here we are studying hole cyclotron resonance (CR) in ferromagnetic InMnAs/GaSb heterostructures. Our observations unambiguously demonstrate the existence of delocalized p-like carriers. In addition, to our knowledge, this is the first study of CR in any ferromagnetic system covering temperature ranges both below and above the Curie temperature (Tc). CR is a direct and accurate method for determining the effective masses of carriers (i.e., the curvature of the energy dispersion) and therefore the nature of the carrier states.

In all InMnAs samples studied, we observed two pronounced resonances. Both lines exhibited unusual temperature dependence in their position, intensity, and width. The lower-field resonance showed an abrupt reduction in linewidth with a concomitant decrease in resonance magnetic field slightly above Tc. The higher-field line, which was absent at room temperature, suddenly appeared above Tc, rapidly grew in intensity with decreasing temperature, and became comparable to the lower-field resonance at low temperatures.

References:

Y. H. Matsuda, G. A. Khodaparast, M. A. Zudov, J. Kono, Y. Sun, F. V. Kyrychenko, G. D. Sanders, C. J. Stanton, N. Miura, S. Ikeda, Y. Hashimoto, S. Katsumoto, and H. Munekata, "Ultrahigh-Field Hole Cyclotron Resonance Absorption in InMnAs Films," Physical Review B 70, 195211 (2004). (abstract, full text)

G. D. Sanders, Y. Sun, F. V. Kyrychenko, C. J. Stanton, G. A. Khodaparast, M. A. Zudov, J. Kono, Y. H. Matsuda, N. Miura, and H. Munekata, "Electronic States and Cyclotron Resonance in n -type InMnAs," Physical Review B 68, 165205 (2003). (abstract, full text)

G. A. Khodaparast, J. Kono, Y. H. Matsuda, S. Ikeda, N. Miura, Y. J. Wang, T. Slupinski, A. Oiwa, H. Munekata, Y. Sun, F. V. Kyrychenko, G. D. Sanders, and C. J. Stanton, "High-Field Cyclotron Resonance Studies of InMnAs-Based Ferromagnetic Semiconductor Heterostructures," Physica E 21, 978 (2004). (full text)

G. D. Sanders, Y. Sun, C. J. Stanton, G. A. Khodaparast, J. Kono, D. S. King, Y. H. Matsuda, S. Ikeda, N. Miura, A. Oiwa, and H. Munekata, "Determining Carrier Densities in InMnAs by Cyclotron Resonance," Physica E 20, 378 (2004). (full text; cond-mat/0307653)

Y. Sun, G. D. Sanders, F. V. Kyrychenko, C. J. Stanton, G A. Khodaparast, J. Kono, Y. H. Matsuda, N. Miura, and H. Munekata, "Electron-active Cyclotron Resonance in p-type InMnAs in High Magnetic Fields," Physica E 20, 374 (2004). (full text)

G. D. Sanders, Y. Sun, C. J. Stanton, G. A. Khodaparast, J. Kono, Y. H. Matsuda, N. Miura, T. Slupinski, A. Oiwa, and H. Munekata, "Theoretical and experimental studies of cyclotron resonance in p-type InAs and InMnAs at ultrahigh magnetic fields," Journal of Applied Physics 93, 6897 (2003). (abstract, full text)

G. D. Sanders, Y. Sun, C. J. Stanton, G. A. Khodaparast, J. Kono, Y. H. Matsuda, N. Miura, T. Slupinski, A. Oiwa, and H. Munekata, "Theory of cyclotron resonance and magneto-optics in n- and p-type InMnAs in ultrahigh magnetic fields," Journal of Superconductivity 16, 449 (2003). (full text)

G. A. Khodaparast, M. A. Zudov, J. Kono, Y. H. Matsuda, T. Ikaida, S. Ikeda, N. Miura, T. Slupinski, A. Oiwa, H. Munekata, G. D. Sanders, Y. Sun, and C. J. Stanton, "Cyclotron resonance of electrons and holes in paramagnetic and ferromagnetic InMnAs-based films and heterostructures," Journal of Superconductivity 16, 107 (2003). (full text)

G. A. Khodaparast, J. Kono, Y. H. Matsuda, T. Ikaida, S. Ikeda, N. Miura, T. Slupinski, A. Oiwa, and H. Munekata, "Cyclotron resonance of itinerant holes in ferromagnetic InMnAs/GaSb heterostructures," in: Proceedings of the 26 th International Conference on the Physics of Semiconductors, edited by A. R. Long and J. H. Davies (Institute of Physics Publishing, Bristol, 2003), P320. (full text; cond-mat/0207485)

M. A. Zudov, J. Kono, Y. H. Matsuda, T. Ikaida, N. Miura, H. Munekata, G. D. Sanders, Y. Sun, and C. J. Stanton, "Ultrahigh field electron cyclotron resonance absorption in InMnAs films," Physical Review B 66, 161307(R) (2002). (abstract, full text)

 

Project 3: Infrared and Terahertz Dynamics in Ferromagnetic InGaMnAs

Figure 3.

(In_0.53Ga_0.47)_1-xMn_xAs is a new type of (III,Mn)V ferromagnetic semiconductor having relatively high Curie temperatures (up to ~120 K). It is grown on top of an In_0.53Ga_0.47As nonmagnetic buffer layer on a lattice-matched InP(001) substrate with a substrate temperature of 186 °C. We have measured the picosecond carrier response of (In_0.53Ga_0.47)_0.87Mn_0.13As by two-color pump-probe spectroscopy, using a near-infrared (NIR) beam as the pump and a 5.7 THz beam as a probe. We observed strongly non-exponential decays, especially at high pump fluences and low temperatures, where a pronounced dip developed in the THz differential transmission. This dip disappeared at the highest fluence where a transmission plateau versus time delay was observed. Our band structure calculations suggest that this intriguing behavior may be due to carrier dynamics associated with Gamma-L intervalley scattering.

The top panel in Fig. 3 shows THz differential transmission as a function of time delay for different pump fluences (10-0.1 mJ/cm²) at 40 K. As the fluence is increased, the transmission develops an interesting shoulder at higher fluences which is most pronounced at 4.0 mJ/cm², rising to roughly 10 percent above the low-intensity decay shape and situated at 250 ps. At 10 mJ/cm², the transmission does not begin to decrease until after the shoulder, at about 400 ps. We performed calculations based on a 30 band k × p model using the virtual crystal approximation for the InGaAs alloy. The right panel shows the allowed transitions for an 800 nm pump, originating from each of the three valence bands. The transition from the heavy hole to the conduction band is near the threshold for transfer to the satellite L valley. This allows the G electrons to scatter back and forth from the L valley. This effect may be more pronounced at high pump intensities, giving rise to additional structure in the differential transmission plots.

References:

G. A. Khodaparast, D. C. Larrabee, J. Kono, D. S. King, J. Kato, T. Slupinski, A. Oiwa, H. Munekata, G. D. Sanders, and C. J. Stanton, "Terahertz dynamics of photo-generated carriers in ferromagnetic InGaMnAs," Journal of Applied Physics 93, 8286 (2003). (abstract, full text)

 

Collaborators:

Hiro Munekata, Tokyo Institute of Technology

Lu J. Sham, University of California, San Diego

Chris Stanton, University of Florida