Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P6: Spectroscopy of Novel Magnetic Materials |
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Sponsoring Units: GMAG DMP Chair: H. Dennis Drew, University of Maryland Room: LACC 502A |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P6.00001: Ultrafast Magneto-Optics in Ferromagnetic III-V Semiconductors Invited Speaker: The carrier-density-dependent magnetic properties of Mn-doped III-V semiconductors allow us to explore novel ultrafast optical phenomena. Photo-generated, spin-coherent, and transient carriers can break the equilibrium among charges, spins, phonons, and ferromagnetic order, triggering an array of dynamical phenomena and providing ways to control magnetism optically. Here, we report results of ultrafast magneto- optical studies on ferromagnetic InMnAs and GaMnAs using non- degenerate time-resolved magneto-optical Kerr effect and transient reflectivity spectroscopies. We observe very short carrier lifetimes ($\sim$2 ps) and multi-level decay dynamics, due to low temperature MBE growth and heavily $p$-type magnetic doping. In an InMnAs/GaSb heterostructure, we observe a transient coercivity decrease (“softening”) during the free carrier lifetime. Furthermore, both in InMnAs and GaMnAs, we observe {\it ultrafast demagnetization}, similar to but much more drastic than what has been observed in itinerant ferromegnets. Above a pump fluence of $\sim$10 mJ/cm$^2$, we observe a complete quenching of ferromagnetic order, implying an ultrafast phase transition into a paramagnetic state.\\This work was carried out in collaboration with J. Wang, C. Sun, G.A. Khodaparast, A. Oiwa, H. Munekata, G.D. Sanders, C.J. Stanton, L. Cywinski, and L.J. Sham. We acknowledge support from DARPA (Grant No. MDA972-00-1-0034), ONR (Award No. N000140410657), and NSF (Grant Nos. DMR-0134058, DMR-0325474, and INT-0221704). [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:27PM |
P6.00002: Electrical spin injection from ferromagnetic metals and semiconductors Invited Speaker: Electrical injection, transport, manipulation and detection of spin polarized carriers in a semiconductor are essential requirements for utilizing the spin degree of freedom in a future semiconductor spintronics technology. Electrical injection has been a particularly vexing issue. We describe results using both ferromagnetic semiconductor (FMS) and FM metal contacts, and review the on-going effort to understand the fundamental issues. Spin-LED structures serve as test platforms, and the quantum selection rules provide a quantitative measure of the electron spin polarization, Pspin, produced in an AlGaAs/GaAs QW. The large values of Pspin=85{\%} obtained using semimagnetic ZnMnSe contacts enable detailed analysis of spin scattering by interface defects. True FM materials are preferable as contacts, and FMSs are promising candidates -- their exchange split band edges offer both spin injection and spin-selective transport. An $n$-type FMS is especially attractive. We describe measurements of electrical spin injection from such an n-type FMS, CdCr$_{2}$Se$_{4}$, into AlGaAs/GaAs LEDs, and discuss interface structure and band offsets. FM metals offer many desirable attributes as spin injecting contacts -- high Curie temperatures, low coercive fields and fast switching times -- and the fundamental issue of interface conductivity mismatch can be circumvented by utilizing a tunnel barrier. We have successfully injected polarized electrons from a reverse-biased Fe Schottky contact into AlGaAs/GaAs, with Pspin $>$ 32{\%}. We demonstrate via the Rowell criteria that tunneling is indeed the dominant transport process, and confirm that majority spin electrons are responsible. We compare these data with spin injection using Fe/Al$_{2}$O$_{3}$ contacts into identical structures. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 1:03PM |
P6.00003: Photoemission and magnetic circular dichroism studies of magnetic semiconductors Invited Speaker: Recently, a series of novel ferromagnetic semiconductors have been synthesized using MBE and related techniques and have attracted much attention because of unknown mechanisms of carrier-induced ferromagnetism and potential applications as "spin electronics" devices. Some new materials show ferromagnetism even well above room temperature. Photoemission spectroscopy has been used to study the $d$ orbitals of the dilute transition-metal atoms, mostly Mn, and their hybridization with the host band states [1]. Soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the transition-metal 2$p$-3$d$ absorption edges are useful techniques to study the valence and spin states of the transition-metal atoms. Furthermore, since MCD has different sensitivities to the ferromagnetic and paramagnetic components at different temperatures and magnetic fileds, if the sample is a mixture of ferromagnetic and non-ferromagnetic transition- metal atoms, it can be used to separate the two components and to study their electronic structures. In this talk, results are presented for the prototypical diluted ferromagnetic semiconductor Ga$_{1-x}$Mn$_x$As [2] and the room-temperature ferromagnets Zn$_{1-x}$Co$_x$O and Ti$_{1-x}$Co$_x$O$_2$.\\ I acknowledge collaboration with Y. Ishida, J.-I. Hwang, M. Kobayashi, Y. Takeda, Y. Saitoh, J. Okamoto, T. Okane, Y. Muramatsu, K. Mamiya, T. Koide, A. Tanaka, M. Tanaka, Hayashi, S. Ohya, T. Kondo, H. Munekata, H. Saeki, H. Tabata, T. Kawai, Y. Matsumoto, H. Koinuma, T. Fukumura and M. Kawasaki. This work was supported by a Grant-in-Aid for Scientific Research in Priority Area "Semiconductor nano-spintronics" (14076209) from MEXT, Japan.\\ 1. J. Okabayashi et al., Phys. Rev. B 64, 125304 (2001).\\ 2. A. Fujimori et al., J. Electron Spectrosc. Relat. Phenom., in press.\\ [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:39PM |
P6.00004: Optical Properties of Magnetic Semiconductors Invited Speaker: We have employed Infrared Sprectroscopy (IR) and Ellipsometry to explore the band structure of thin films and digitally doped superlattices of Ga$_{1-x}$Mn$_{x}$As, prepared in the group of D.D. Awschalom (UCSB). These measurements reveal the important role played by the Mn induced impurity band in the band structure and ferromagnetism of Ga$_{1-x}$Mn$_{x}$As. Our IR work on Digital Ferromagnetic Heterostructures reveals a unique ability to tune their optical properties as well as their intrinsic electronic structure without changing the doping/defect level. This work is in collaboration with E.J. Singley, D.N. Basov (University of California, San Diego) J. Stephens, R.K. Kawakami, and D.D. Awschalom(University of California, Santa Barbara). [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 2:15PM |
P6.00005: Ferromagnetic resonance studies of dilute magnetic semiconductors Invited Speaker: We describe ferromagnetic resonance (FMR) measurements on ferromagnetic II$_{1-x}$Mn$_{x}$VI semiconductor alloys in thin film form. These include Ga$_{1-x}$Mn$_{x}$As layers grown by low-temperature molecular beam epitaxy on various buffers used to obtain different strain conditions. The analysis of the FMR provides values of cubic and uniaxial magnetic anisotropy fields -- i.e., those associated with the natural (undistorted) zinc-blende structure, and those arising from strain. Similar studies were also carried out on In$_{1-x}$Mn$_{x}$As, providing analogous information. Finally, we applied the FMR technique to Ga$_{1-x}$Mn$_{x}$As/Ga$_{1-y}$Al$_{y}$As heterostructures modulation-doped by Be. Here it was found that the increase in doping -- in addition to raising the Curie temperature of the Ga$_{1-x}$Mn$_{x}$As layers -- also leads to a significant increase of their uniaxial anisotropy field. The FMR data for modulation-doped heterostructures further show that the effective $g$-factor of Ga$_{1-x}$Mn$_{x}$As is strongly affected by the doping, thus providing a direct estimate of the free hole contribution to the magnetization of Ga$_{1-x}$Mn$_{x}$As. [Preview Abstract] |
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