Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B2: Magnetism in Semiconductors: New Frontiers |
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Sponsoring Units: GMAG Chair: Scott Crooker, Los Alamos National Laboratory Room: Morial Convention Center LaLouisiane C |
Monday, March 10, 2008 11:15AM - 11:51AM |
B2.00001: Optical Properties of III-Mn-V Ferromagnetic Semiconductors Invited Speaker: We discuss the important role optical studies have played in our understanding of the electronic structure of III-Mn-V ferromagnetic semiconductors. These extensive studies have established the electronic structure is strongly affected by the strength of the exchange between the Mn local moments and the holes they introduce. Particular focus is given to Ga$_{1-x}$Mn$_{x}$As, where spectroscopic studies suggest the metallic state is unconventional. Finally, we will detail our recent experiments into the ultrafast manipulation of magnetism on the nanoscale. This work is in collaboration with D.B. Shrekenhamer, E.J. Singley, D.N. Basov (University of California, San Diego) J. Stephens, S. Mack, R.K. Kawakami, D.D. Awschalom(University of California, Santa Barbara), B.L. Sheu, N. Samarth (Pennsylvania State University), F. Chen, A. Azad, J. O'Hara, A.M. Dattelbaum, G. Montano, S. Crooker, and A.J. Taylor (Los Alamos National Laboratory). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B2.00002: Spin Transport in Ferromagnet-Semiconductor Heterostructures Invited Speaker: Over the last two years, there has been significant progress in the integration of metallic ferromagnets with semiconductors, resulting in devices in which spin-polarized carriers are injected and detected electronically. I will discuss experiments on epitaxial Fe/GaAs Schottky tunnel barrier heterostructures patterned into lateral devices in which the ferromagnetic injection and detection contacts are separated by several microns.[1] The Schottky barrier consists of a highly-doped $n^+$ region ($n^+ \sim 5 \times 10^{18}$~cm$^{-3}$), and the channel of the device is $n$-doped GaAs ($n\sim 2 \times 10^{16 } - 1 \times 10^{17}$~cm$^{-3}$). A non-equilibrium spin polarization generated by electrical injection is detected potentiometrically using the non-local transport technique applied originally to metallic systems. An important aspect of this approach is the observation of spin precession and dephasing in the semiconductor channel (the Hanle effect), allowing for electrical measurements of the spin lifetime and diffusion length. We find a strong non-linear dependence of the spin polarization on the injection bias voltage, which we have investigated by preparing samples with different thicknesses of the $n^+$ region, thus varying the tunnel barrier profile. We find a systematic change in the spin accumulation observed under forward and reverse bias currents as the thickness of the $n^+$ region increases. Other aspects of these devices have also been explored. For GaAs channels that are doped near the metal-insulator transition, the non-equilibrium electron spin polarization leads to dynamic nuclear polarization, which has a profound impact on the electron spin dynamics at low temperatures. Finally, I will discuss some important considerations for applications in which a bias current flows in the detector. [1] X. Lou {\it et al.}, Nature Physics {\bf 3}, 197 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B2.00003: Ultrafast Photoinduced Non-thermal Phenomena in (III, Mn)V Ferromagnetic Semiconductors Invited Speaker: Magnetic materials displaying carrier-mediated exchange interaction are ideal for non-thermal, potentially fast spin manipulation and detection. Prominent examples of such materials are Mn doped III-V semiconductors such as GaMnAs, in which the strong interaction of carriers (holes) and Mn ions results in high transition temperature ferromagnetism. The steady-state magnetooptical/transport measurements reveal rich magnetic memory effects and strong enhancement of ferromagnetism via external stimuli (i.e., light, electrical field or current). However, no timeresolved experiments in (III,Mn)V semiconductors have shown these collective magnetic phenomena, and hence their time scales are completely unknown. In this talk, I will present our recent observations in GaMnAs of: (1) ultrafast enhancement of ferromagnetism via photoexcited transient holes on a 100 ps time scale and (2) femtosecond detection of magnetic memory states. Our measurements reveal new fundamental collective magnetic processes at ultrafast time scales, and identify the critical roles of the Mn-hole correlation in these photo-induced cooperative behaviors. These results constitute the first evidence for \textit{ultrafast}, \textit{non-thermal} manipulation of the spin order in (III,Mn)Vs, which may represent as-yet-undiscovered universal features in all carrier-mediated ferromagnetic materials. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:39PM |
B2.00004: Ferromagnetism and localization in Ga$_{1-x}$Mn$_{x}$As, Ga$_{1-x}$Mn$_{x}$P, and in between Invited Speaker: Because of their potential as both injectors and filters of spin-polarized carriers, ferromagnetic semiconductors may play an important role in spin-based electronics, or \textit{spintronics}. Ferromagnetic semiconductors are formed by the substitution of a relatively small fraction of host atoms with a magnetic species. Ga$_{1-x}$Mn$_{x}$As has been the most thoroughly studied material among these, and ferromagnetism in it arises from hole-mediated inter-Mn exchange. The Curie temperature T$_{C}$ in Ga$_{1-x}$Mn$_{x}$As has been shown to increase with increasing concentration of substitutional Mn acceptors. However, room temperature ferromagnetism in this canonical system has been elusive due to challenges in materials synthesis---namely, raising $x$ while avoiding the formation of second phases or compensating defects. Increasing $p-d$ exchange by modifying the host semiconductor via anion substitution (e.g., replacing As with P) is a significantly less explored route by which T$_{C}$ may be raised. We are investigating the effect of anion substitution in ferromagnetic Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ formed by ion implantation followed by pulsed-laser melting. In the endpoint compound Ga$_{1-x}$Mn$_{x}$P T$_{C}$ is found to vary linearly with $x$, and non-metallic transport is observed for $x$ up to $\sim $4.2{\%}, corresponding to a T$_{C}$ of $\sim $62 K compared to $\sim $112 K for Ga$_{1-x}$Mn$_{x}$As with a similar $x$. Dilution of the endpoint compound Ga$_{1-x}$Mn$_{x}$As with P results in a precipitous decrease in T$_{C}$ to below 60 K for y=2.8{\%}. Remarkably, Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ films undergo a metal-insulator transition between $y$=1.5{\%} and 2.3{\%} even as $x$ is held approximately constant indicating that alloy disorder in the anion sublattice induces hole localization, which in turn may be responsible for a strong suppression of T$_{C}$. Thus, while anion substitution may enhance $p-d$ exchange, localization effects must be considered when developing a suitable picture for ferromagnetism in these materials. [Preview Abstract] |
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