Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W22: Focus Session: Dilute Magnetic Nitride Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Roland Kawakami, University of California Riverside Room: 324 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W22.00001: Nitrogen defects and ferromagnetism in Cr-doped AlN Bang-Gui Liu It is believed that N defects play important roles in achieving high-temperature ferromagnetism in Cr-doped AlN. We use state-of-the-arts DFT method to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies $V_{N}$. Our total-energy calculations show that the nearest Cr-Cr pair with the two spins in parallel is the most favorable and the nearest Cr-$V_{N}$ pair makes a stable complex. Our formation energies indicate that $V_{N}$ regions can be formed spontaneously under N-poor condition, or Cr-doped regions can be formed under N-rich condition. Hence real samples should be inhomogeneous. Both of the single Cr and $V_{N}$ create filled electronic states in the semiconductor gap of AlN. N vacancies enhance the ferromagnetism by enlarging Cr moment, but reduce the ferromagnetic exchange constants between the spins in the nearest Cr-Cr pairs. These calculated results are in agreement with experimental observations and facts. Phys. Rev. B 78, 195206 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W22.00002: Optical and magnetic properties of Tm-doped doped AlGaN alloys N. Nepal, S.M. Bedair, J.M. Zavada, N.A. El-Masry, D.S. Lee, A.J. Steckl, A. Sedhain, J.Y. Lin, H.X. Jiang Trivalent RE-ions in AlGaN alloys have been shown to emit narrow intra-4$f$ transitions over the entire infra red to ultraviolet (UV) spectral range. Also, unpaired 4$f$-electrons of RE ions can align along an easy axis giving magnetic properties to these RE-doped semiconductors. Thulium is one of the RE element which has special optical and magnetic characteristics. We present optical and magnetic properties of Tm doped Al$_{x}$Ga$_{1-x}$N (0 $\le $ x $\le $ 1) alloys grown by solid-source molecular beam epitaxy. Hysteresis measurements on these alloys show ferromagnetic behavior at room temperature. The measured magnetization was strongly dependent upon the Al content and reached a maximum for x = 0.62. Previous photoluminescence measurements on these films yielded a blue emission at 465 nm with peak intensity at the same Al content. Our experimental results indicate that both optical and magnetic properties are directly correlated with the alloy compositional fluctuation found in undoped Al$_{x}$Ga$_{1-x}$N alloys. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W22.00003: The Failure of LDA and GGA to describe Relative Stability, Electronic Structure and Magnetism of MnN and (Ga,Mn)N Alloys Jennifer Chan, Zhe Liu, Hannes Raebiger, Stephan Lany, Alex Zunger Pure MnN and (Ga,Mn)N alloys are studied using {\it ab initio} generalised gradient approximation +$U$ (GGA+$U$) or hybrid-exchange density functional (B3LYP) methods which predict dramatically different electronic structure, magnetic behavior and relative stabilities compared to local-density calculations. A unique structural anomaly of MnN, in which local-density calculations fail to predict the experimentally observed rocksalt as the ground state, is resolved with GGA+U and B3LYP. The phase-separation of zinc-blende (Ga,Mn)N alloys is examined using a mixed-basis cluster expansion based on the corrected GGA total energies. The predicted asymmetric spinodal phase diagram indicates that (Ga,Mn)N precipitates contain $\sim$5\% or $\sim$50\% Mn at typical growth temperatures. Thus, 100\% pure MnN, that suppresses the Curie temperature, will not be formed. The Curie temperature for the x$_{Mn}$=50\% phase is estimated to be T$_C$=$\sim$300~K indicating that high T$_C$ ferromagnetism in zinc-blende (Ga,Mn)N alloys is due to precipitates. \\ Chan {\it et al.} Phys. Rev. B {\bf 78}, 184109 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W22.00004: Origin and control of ferromagnetism in magnetically doped semiconductors. The case of (Ga,Fe)N Invited Speaker: The comprehensive search for materials exhibiting spintronic functionalities has resulted in the discovery of a number of magnetically doped or nominally undoped wide-band gap semiconductors and oxides showing ferromagnetic features persisting up to high temperatures. In order to shed light on the origin of the high-TC ferromagnetism in these materials systems, we have undertaken studies of MOVPE-grown (Ga,Fe)N, either undoped or co-doped with Si or Mg, combining the magnetic (SQUID and EPR), magnetooptical, and XANES investigation with a thorough structural and chemical characterization (SIMS, TEM, EDS, synchrotron XRD), that provides information on the Fe distribution at the nanoscale. In this talk, we first discuss our quantitative study of the exchange coupling between the spins S = 5/2 localized on the Fe ions and of the effective mass electrons. Our results point to an anomalous p-d exchange splitting of the valence band [1], that we explain in terms of a renormalization of extended states occurring if the impurities perturb strongly the crystal potential. We then show that the Fe ions are incorporated in the nitride matrix in a way giving rise either to a diluted random alloy or to ferromagnetic nanocrystals that aggregate by precipitation or by spinodal decomposition into regions more or less rich in the magnetic component, and that can be controlled by the growth parameters and co-doping with shallow donors and acceptors [2].\\[4pt] [1] W. Pacuski, P. Kossacki, D. Ferrand, A. Golnik, J. Cibert, M. Wegscheider, A. Navarro-Quezada, A. Bonanni, M. Kiecana, M. Sawicki, T. Dietl, Phys. Rev. Lett. 100, 037204 (2008).\\[0pt] [2] A. Bonanni, A. Navarro-Quezada, Tian Li, M. Wegscheider, R.T. Lechner, G. Bauer, Z. Matej, V. Holy, M. Rovezzi, F. D'Acapito, M. Kiecana, M. Sawicki, and T. Dietl, Phys. Rev. Lett. 101, 135502 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W22.00005: Magnetic, structural and optical properties of Mn-based and Cr-based diluted magnetic semiconductors and alloys A. Alsaad We have implemented supercell approach by using local spin density functional theory for Mn-doped GaN, Mn-doped ScN and the linear muffin-tin orbital method to predict the structural and magnetic properties of these novel diluted magnetic semiconductors and their Ga$_{x}$Mn$_{1-x}$N and Sc$_{x}$Mn$_{1-x}$N alloys. The global energy minimum of MnN is obtained for zinc-blende structure. If the compound is compressed by 6 {\%} the energy minimum corresponds to the NaCl structure in disagreement with the experimentally observed a slightly tetragonally distorted rocksalt structure, known as ? phase. The rocksalt structure of CrN at about 8 {\%} lattice expansion becomes stable in the ferromagnetic (FM) state and has a global minimum energy at a lattice constant of 3.9 {\AA}. We have observed an isostructural phase transition for Sc$_{x}$Mn$_{1-x}$N alloys from zince-blende phase to hexagonal phase that occurs at a hydrostatic pressure of 17.5 GPa. Moreover, the structural and optical properties of single crystal CrN/ScN superlattices and Cr$_{1-x}$Sc$_{x}$N alloys are studied in details. We report an isostructural phase transition from wurtzite ($w-$CrN) to hexagonal ($h$-ScN) at a hydrostatic pressure of 21 GPa. We have also used first-principles methods to study the electronic, optical and magnetic properties of MnN and MnAs compounds in the hypothetical cubic zinc-blende phase, a phase in which the two MnN and MnAs binaries have the same local environment as that they have in GaMnN and GaMnAs alloys. We show that MnN exhibits antiferromagnetic (AFM) ground state and MnAs adopts ferromagnetic (FM) ground state. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W22.00006: Role of the Localized Defect States in the Unconventional Magnetism of GaN and ZnO Pratibha Dev, Peihong Zhang The cation defects -vacancies and the appropriate substitutionals - introduce localized defect states chiefly centered around the four surrounding anions in GaN and ZnO. This defect-induced magnetism in these otherwise nonmagnetic semiconductors is studied using ab-initio methods. The defects investigated include the cation vacancy, substitutional acceptors, and acceptor-like defect complexes. The defect states show two opposing attributes -one one hand, they are strongly localized on the anions surrounding the defect site, leading to local magnetic moment formation, while on the other hand, the extended tails of their wavefunctions lead to the long-ranged exchange interaction between the local moments. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W22.00007: Defect induced ferromagnetism in Gd doped GaN Chandrima Mitra, Walter Lambrecht We review various suggested mechanisms for the ferromagnetism in Gd-doped GaN using local spin density approximation supercell calculations. The spin splitting of the conduction band induced by the Gd $s-f$ coupling is found to decrease linearly with Gd concentration and hence colossal magnetic moments cannot be explained by filling this spin-split band with ionized donor electrons. Furthermore we find the Gd-Gd interactions to be antiferromagnetic except in p-type material. Although, Ga vacancies can induce long range interactions and up to three Bohr magneton moments per vacancy in the neutral charge state, we note that these defects only are favorable to form in n-type materials and then should predominantly occur in a $3-$ charge state which has no magnetic moment. N-interstitials are likely to form in conjunction with N vacancies for mid gap Fermi levels consistent with the semi-insulating nature of the samples and have a magnetic moment for the corresponding charge state. We find that Gd in the presence of N interstitials alone prefer antiferromagnetic coupling but in the presend of both N-interstitials and N vacancies prefer ferromagnetic coupling. We find that oxygen tends to seggregate toward an interstitial site near the Gd and in that case can induce a strong ferromagnetic coupling between Gd. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W22.00008: Ferromagnetism in GdN: an antiferromagnet in disguise. Walter R. L. Lambrecht, Chandrima Mitra We analyze the exchange interactions in GdN and Gd pnictides GdX with X=P,As,Sb,Bi using full-potential linearized muffin-tin orbital calculations as well as the linear response approach in the atomic sphere approximation. We show that in GdN, the ferromagnetism arises from the small induced opposite magnetic moments on Gd-d and N-p orbitals. When these form a perfect antiferromagnetic arrangement on the rocksalt lattice, it pins the large magnetic 4f moments in a ferromagnetic arrangement through the on-site f-d coupling. In contrast, in the other pnictides, the AFM-II (111)-ordered state is preferred, in which case there is no moment induced on N. The N\'eel temperatures as well as the Curie-Weiss temperatures extracted from this model are in good agreement with experiment for the pnictides, but the Curie temperature of GdN in this model at only about 10 K strongly underestimates the experimentally observed Curie temperature of about 70 K. Linear response calculations give an alternative view including exchange interactions with empty spheres but give consistent estimates of $T_c$ and also give a reasonable Curie temperature for metallic Gd. We find that adding n-type doping by shifting the Fermi level does not increase $T_c$ in GdN substantially but adding N-vacancies explicitly does. Thus, it seems that defects play a significant role in establishing the Curie temperature of GdN. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W22.00009: Ferromagnetism and Photoluminescence in Rare-Earth doped GaN via Diffusion M. Oliver Luen, N. Nepal, S.M. Bedair, J.M. Zavada, Ei Ei Brown, U. Hommerich, P. Frajtag, N.A. El-Masry Rare-earth doped GaN is attracting attention both as a diluted magnetic semiconductor (DMS) material and for optical devices useful in communications and multi-color semiconductor display technology. GaN's large band gap (3.4 eV) gives rise to optical transparency over a wide spectral range, from the infrared (IR) to the ultraviolet. These properties make it an optimum host for the various emissions that are possible from rare-earth (RE) ions. Recently, rare-earth doped GaN also has demonstrated above room temperature ferromagnetism. In this study, we report the diffusion of RE (Nd, Sm, Gd and Er) into undoped, Mg-doped and Si-doped GaN templates. Room temperature optical and ferromagnetic properties were studied using photoluminescence (PL) and alternating gradient magnetometer, respectively. Ferromagnetic properties show a preference for undoped and n-type GaN. PL spectra exhibit RE ion inner shell transitions in the visible and infrared regions. The mechanisms for above room temperature ferromagnetism and emission intensity related to the RE concentration, is discussed. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W22.00010: Dilute Magnetic and Electronic Properties of Mn$_{x}$Sc$_{(1-x)}$N/ScN(001)/MgO(001) Films Grown by Molecular Beam Epitaxy Costel Constantin, Kangkang Wang, Abhijit Chinchore, Arthur Smith, Han-Jong Chia, John Markert In this study, we report the magnetic and electronic properties of Mn$_{x}$Sc$_{(1-x)}$N films grown by molecular beam epitaxy. Recently, theoretical calculations predicted a Curie temperature above 350 K for ScN films with up to 20{\%} Mn impurity concentrations[1]. The magnetic hysteresis data suggests ferromagnetic behavior for Mn$_{0.03}$Sc$_{0.97}$N and Mn$_{0.15}$Sc$_{0.85}$N films with Curie temperatures of 383 K and 361 K, respectively. Furthermore, the measured electron concentrations for the Mn$_{0.03}$Sc$_{0.97}$N and Mn$_{0.15}$Sc$_{0.85}$N films are 6.51$\times $10$^{19}$ cm$^{-3}$ and 6.17$\times $10$^{19}$ cm$^{-3}$, respectively. These measured carrier concentration agree well with the prediction of Herwadkar \textit{et al.} that ferromagnetism above room temperature in Mn$_{x}$Sc$_{(1-x)}$N should be possible by keeping the electron concentration below 10$^{20}$ cm$^{-3}$. This work is supported by: Seton Hall: University Research Council; Ohio University: DOE-BES Grant No. DE-FG02-06ER46317 and NSF Grant No. 0730257; and UT Austin: NSF Grant Nos. DMR-0605828 and DGE-0549417, Welch Foundation Grant No. F-1191. [1] A. Herwadkar (\textit{et al}.), \textit{Phys. Rev. B }\textbf{\textit{77}}\textit{, 134433 (2008).} [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W22.00011: Vibrational properties of ScN and rare-earth nitrides: theory and and Raman spectra T.R. Paudel, W.R.L. Lambrecht, C. Meyer, H.J. Trodahl, J. Zhang, A.R.H. Preston, S.E. Granville, B.J. Ruck, G.V.M. Willams Frozen phonon calculations are presented for the phonons at $\Gamma$, $L$ and $X$ points in the rare-earth nitrides using the FP-LMTO and LSDA+U. The method is found to be in good agreement with linear response pseudopotential calculations for the closely related ScN. Comparison of the calculated phonon DOS in ScN with the Raman spectra (RS) reported in literature, show that the spectum corresponds to disordered induced first order Raman scattering and emphasizes the zone-boundary modes, in particular the LO(L) mode, because the latter correspond to a breathing mode and has the strongest electron-phonon coupling for above band gap Raman excitation. We present the measured RS for SmN, GdN, DyN, ErN, LuN thin films measured with 633 and 514 nm excitation grown by evaporation of the metals in ultrapure N$_2$ gas. No significant changes were found for GdN below the T$_{\rm c}$, indicating that the mechanism is not spin-disorder related but rather to disorder originated in the presence of N vacancies. The main Raman line in the RE-N is found to correspond to the pure N-like LO(L) mode. [Preview Abstract] |
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