Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V33: Focus Session: Theory of Spin Phenomena in Semiconductors |
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Sponsoring Units: GMAG FIAP DMP Chair: Michael Flatte, University of Iowa Room: Morial Convention Center 224 |
Thursday, March 13, 2008 11:15AM - 11:51AM |
V33.00001: Design of dilute magnetic semiconductors with room temperature ferromagnetism by controlling spinodal decompostion Invited Speaker: Kazunori Sato Owing to the recent development of the first-principles method for calculating magnetic properties of dilute magnetic semiconductors (DMS), it has been recognized that the magnetic percolation effect is disastrous to the high temperature ferromagnetism in DMS in particular for low concentrations [1]. The exchange interactions calculated from first-principles are strong for nearest neighbors, but those interactions are short ranged and can not play an important role for realizing high- $T_{\rm C}$ because the solubility of magnetic impurities into DMS is too low to achieve magnetic percolation. To overcome this difficulty and realize room temperature ferromagnetism, we focus on the spinodal decomposition in DMS, and suggest that by controlling the spinodal decomposition high blocking temperature can be realized leading to ferromagnetic behaviour at high temperature [2]. We calculate electronic structure of DMS from first-principles by using the Korringa- Kohn-Rostoker coherent potential approximation (KKR-CPA) method. Then, chemical pair interactions and magnetic exchange interactions between magnetic are calculated. We use the Monte Carlo techniques to simulate spinodal decomposition of DMS and to estimate the magnetic properties of them [3]. The computer simulations for the magnetization process of the decomposition phases indicate that we can control super-paramagnetic blocking temperature by optimizing the size of the clusters by changing the crystal growth condition. This simulation suggests the material design of high blocking temperature DMS by controlling the spinodal decomposition [2].As another approach for realizing high-Tc DMS we propose co-doping method to increase solubility limit of transition metal impurities in DMS [4]. This work is based on the collaboration with H. Katayama-Yoshida and T. Fukushima. \newline [1] L. Bergqvist et al, Phys. Rev. Lett. 93, 137202 (2004), K. Sato et al., Phys. Rev. B 70, 201202 (2004) \newline [2] K. Sato et al., Jpn. J. Appl. Phys. 46, L682 (2007) \newline [3] K. Sato et al., Jpn. J. Appl. Phys., 44, L948 (2005), T. Fukushima et al., Jpn. J. Appl. Phys., 45, L416 (2006) \newline [4] K. Sato et al., Jpn. J. Appl. Phys. 46 L1120 (2007) [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V33.00002: Defect-induced intrinsic magnetism in wide-gap III-nitrides Peihong Zhang, Pratibha Dev, Yu Xue Cation vacancy induced intrinsic magnetism in GaN and BN is investigated by employing density functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of 3 $\mu_B$ with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wavefunctions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect induced or mediated collective magnetism in these otherwise nonmagnetic $sp$ systems. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V33.00003: Design of Colossal Solubility of Magnetic Impurities for Semiconductor Spintronics by the Co-doping Method Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida Based on first-principles calculations, we propose co-doping method for increasing solubility of magnetic impurities in dilute magnetic semiconductors (DMSs). The concentration dependences of the mixing energy of DMS, such as (Ga,Mn)N, (Ga,Cr)N, (Ga,Mn)As, and (Zn,Cr)Te, show large convexity and these systems have a tendency toward spinodal decomposition. By introducing compensating impurities into these DMS, the mixing energy shows gradual transition from convex to concave concentration dependence resulting in negative mixing energy of magnetic impurities. This observation suggests that the co-doping method dramatically increases the solubility of magnetic impurities in DMS, thus high concentration doping of magnetic impurities into DMS becomes possible. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V33.00004: Suppresion of ferromagnetism in Zn$_x$Cr$_{1-x}$Te: A DFT study Juarez L.F. Da Silva, Gustavo M. Dalpian, Su-Huai Wei The possibility to control the charge and spin degrees of freedom independenlty in diluted magnetic semiconductos (DMS) provides an opportunity in the realization of spintronic devices. Therefore, there is a great desire to understand the physical parameters that control $T_{\rm C}$ in DMS. In this talk, we will report a density functional theory study of Zn$_x$Cr$_{1-x}$Te and Ga$_x$Cr$_{1-x}$As, which show interesting carrier dependence of the $T_{\rm C}$. We found that the stability of ferromagnetism (FM), which is calculated with respect the anti-ferromagnetic (AFM) phase, can be controlled by $p$- and $n$-type co-doping. For randomly distributed Cr, the maximum FM stability is found to be at $\approx1.25$ holes per magnetic ion. Furthermore, we studied the formation of magnetic Cr precipitates by analysing the binding energy of two Cr atoms in the host semiconductors. Our results indicate that a simple Coulomb binding picture cannot provide a consistent explanation for the formation of Cr-rich aggregates. Instead, we show that the variation of Cr-Cr binding energy can be discribed quite well using the band coupling model [G. M. Dalpian, S.-H. Wei, X. G. Gong, A. J. R. da Silva, and A. Fazzio, Solid State Commun. {\bf 138}, 353 (2006)]. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V33.00005: Eight-band model of the dilute magnetic semiconductor GaMnAs A. Nili, M. A. Majidi, J. Moreno, P. Kent, M. Jarrell, R. S. Fishman The ordered state of GaAs doped with Mn is studied using a self-consistent Dynamical Mean Field treatment parameterized with the LDA band structure of the parent material. The resulting model is an eight-band k$\cdot$p model including the heavy and light valence bands, the split-off valence band and the conduction band. The interaction between randomly distributed magnetic ions and itinerant charge carriers is modeled with a modified double-exchange coupling, which includes the attractive potential between holes and charged magnetic ions. Since our method is not perturbative we can treat both strong and weak coupling regimes. We calculate the transition temperature, the temperature dependence of the local magnetization, the density of states and the spin polarization of the holes. We compare our results with more simplified models. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V33.00006: Spin relaxation of two-dimensional excitons in a strong magnetic field O. Olendski, T. V. Shahbazyan We study spin relaxation of quantum well excitons in a perpendicular magnetic field. In a strong field, the dominant relaxation mechanism for an optically-excited magnetoexciton is phonon-assisted spin-orbit transition of the constituent electron or hole. We show that in a quantum well the exciton spin relaxation rate is quadratic in magnetic field in contrast to the fifth power dependence in a quantum dot. We calculate relaxation rates for excitons in GaAs quantum wells excited by both left and right-polarized light and find that they are much slower than those in quantum dots due to the dominant contribution of exciton kinetic energy in the final state. We also find that the relaxation rate is very sensitive to the applied in-plane electric field that causes exciton drift in the ground state. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V33.00007: Theory of xchange splittings of bands in diluted magnetic semiconductors Tomasz Dietl, Cezary Sliwa A series of recent photoemission and magnetooptical experiments, particularly on magnetically doped nitrides and oxides, but also on (Ga,Mn)As, points to the limiting understanding of the physics governing the sp-d exchange interaction in these systems. We have developed theory of band splittings in diluted magnetic semiconductors taking into consideration a possibility that the magnetic ion can trap a hole. We have found that the exchange coupling between the bound and delocalized carriers has actually a similar magnitude to that characterizing the sp-d exchange interaction [1]. Furthermore, our results demonstrate that the presence of the corresponding bound state itself renormalizes extended states in a spin-dependent fashion [2]. We show that these two effects can explain the unexpected sign and magnitude of the apparent s- d and p-d exchange integrals determined by magnetooptical studies carried out for (Ga,Mn)As [3] as well as for (Zn,Co)O, (Ga,Mn)N, and (Ga,Fe)N [4].\newline [1] C. Sliwa, T. Dietl, cond-mat/arXiv:0707.3542. [2] T. Dietl, cond-mat/0703278. [3] J. Szczytko et al., Solid State Commun. 99, 927 (1996); M. Poggio et al., Phys. Rev. B 72, 235313 (2005). [4] W. Pacuski et al., ibid. 73, 035214 (2006); ibid. 76, 165304 (2007); arXiv:0708.3296. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V33.00008: Computational Nano-materials Design for Spin-Currents Control in Semiconductor Nano-spintronics Hiroshi Katayama-Yoshida, Tetsuya Fukushima, Van An Dinh, Kazunori Sato We design the different exchange mechanism like Zener's double exchange, Zener's p-d exchange and super-exchange in dilute magnetic semiconductors (DMS) by ab initio calculations. We obtain a universal trend for the exchange interactions [1]. We show that self-organized spinodal nano-decomposition (Dairiseki- Phase) offers the functionality to have high Curie temperatures[2]. We show that spinodal nano-decomposition under layer-by-layer crystal growth condition (2D) leads to quasi-one dimensional nano-structures (Konbu-Phase) with highly anisotropic shape and high $T_{\rm C}$[2]. We design a spin-currents- controlled 100 Tera bits/icnh$^2$, Tera Hz switching, and non- volatile MRAM without Si-CMOS based on Konbu-Phase [2]. In addition to the conventional Peltier effect, we propose a colossal thermoelectric-cooling power based on the adiabatic spin-entropy expansion in a Konbu-Phase [3]. [1] B. Belhadaji et al., J. Phys.-Condens. Matter, 19 (2007) 436227. [2] H. Katayama-Yoshida et al., Phys. stat. sol. (a) 204 (2007) 15. [3] H. Katayama-Yoshida et al., Jpn. J. Appl. Phys. 46 (2007) L777. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V33.00009: Effect of Fermi level on Microstructure and Magnetism in (Ga,Mn)N Alloys Jennifer Chan, Zhe Liu, Stephan Lany, Alex Zunger GaN doped with Mn has been shown experimentally to exhibit either ferro- or antiferro- magnetic behavior, the results varying considerably depending on the microstructure of the sample. Indeed, the electronic structure and magnetic properties appear to be heavily dependent upon growth conditions and ordering of the material. We used {\it ab-initio} calculations to investigate the magnetism of various ordered structures of (Ga,Mn)N with respect to Mn composition. The results show that high-spin states with the spins on the Mn aligned in parallel (HS-FM) are stable at low Mn composition ($<$0.5\%) but not at high Mn composition ($>$0.5\%). Instead, for high Mn composition, low-spin states (LS-FM) or states where the spins on the Mn are anti-parallel (FI) are found. Interestingly, upon raising the Fermi level, the HS-FM states are stable for all Mn compositions and their formation enthalpies lower with respect to the neutral Fermi level case indicating that electron doping leads to enhanced Mn solubility and hence charged microstructure. The short and long range ordering, tendency for clustering and magnetic properties will be investigated. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V33.00010: First-Principles Study on Electronic Structure of TiO$_2$-Based Dilute Magnetic Semiconductors Hidetoshi Kizaki, Masayuki Toyoda, Kazunori Sato, Hiroshi Katayama-Yoshida We investigate the electronic structure in rutile-TiO$_2$-based dilute magnetic semiconductors (DMS) within self-interaction- corrected local density approximation (SIC-LDA). These results are compared with those calculated within standard LDA. Although the calculated band-gap energy and energetic position of Ti 3{\it d} bands are different in the LDA and the SIC-LDA, half-metallic density of states is predicted in transition- metal-doped TiO$_2$ for both methods. While the LDA calculations predict high-spin state only for Fe-doped one, in the SIC-LDA calculations high-spin state is realized in V-, Cr- and Mn-doped one and low-spin state is realized in Fe- and Co- doped one. However, the absorption and soft x-ray magnetic circular dichroism measurements in (Ti$_{0.97}$, Co$_{0.03}$)O$_ {2-\delta}$ indicate the Co$^{2+}$ high-spin state in the ${\it D}_{2h}$-symmetry crystal field at the Ti site. These experimental results do not agree with our calculated results. We will discuss the origin of the discrepancy between the theoretical predictions and the experimental observations. In addition, we will discuss the ferromagnetism in TiO$_2$-based DMS. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V33.00011: Ferromagnetism in GaN:Gd: A density functional theory study Lei Liu, Peter Yu, Z.X. Ma, Sam Mao First principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the Generalized Gradient Approximation (GGA) of the density functional theory (DFT) with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic $p-d$ coupling is found to be over two orders of magnitude larger than the$ s-d$ exchange coupling. The experimental colossal magnetic moments and room temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4$f$ spins via $p-d$ coupling involving holes introduced by intrinsic defects such as Ga vacancies. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V33.00012: Thermodynamics of carrier-mediated magnetism in semiconductors A.G. Petukhov, L. Makinistian, S.C. Erwin, R. Abolfath, I. Zutic We propose a model of carrier-mediated ferromagnetism in semiconductors that accounts for the temperature dependence of the carriers\footnote{A. G. Petukhov, I. Zutic, and S. C. Erwin, Phys. Rev. Lett. {\bf 99}, 257202 (2007)}. The model permits analysis of the thermodynamic stability of competing magnetic states, opening the door to the construction of magnetic phase diagrams. As an example we analyze the stability of a possible reentrant ferromagnetic semiconductor, in which increasing temperature leads to an increased carrier density, such that the enhanced exchange coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised. We apply this approach to studying thermodynamic fluctuations of magnetization in small systems such as bound magnetic polarons and magnetic nanoislands. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V33.00013: Electronic structure and magnetism of Fe$_{4-x}$Mn$_{x}$N compounds Li Chen The electronic structure and magnetism of Fe$_{4-x}$Mn$_{x}$N compounds have been studied by a periodic quantum-mechanical calculation based on density functional theory. The results show that a ferrimagnetic ordered phase is stable when Fe is substituted by Mn on cube corner sites, whereas the antiferromagnetic phase is energetically favored when Mn substitutes for Fe on face-centered sites. Mn atom concentration and the substitutional sites have significant influence on the exchange coupling. We predict covalent bonds between face-centered Fe or Mn 3d and N 2p orbitals. In contrast, bonding between the atoms at cube corners and face-centered sites is mainly ionic or metallic. [Preview Abstract] |
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