Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H35: Focus Session: Spins in Semiconductors -- DMS: Nitrides and Oxides |
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Sponsoring Units: GMAG DMP FIAP Chair: Alberta Bonanni, Johannes Kepler University Room: E145 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H35.00001: The magnetic behaviour of diluted (Ga,Mn)N Alberta Bonanni, Bogdan Faina, Tian Li, Andrea Navarro-Quezada, Andreas Grois, Thibaut Devillers, Wiktor Stefanowicz, Dariusz Sztenkiel, Maciej Sawicki, Tomasz Dietl We report on the magnetic properties of MOVPE (Ga,Mn)N films characterized by TEM, EDS, synchrotron -XRD and -EXAFS, SIMS, Raman and PL. In contrast to our previously studied (Ga,Fe)N [1], no decomposition is found in the Mn-doped samples in the concentration range to $x$ = 1 {\%}. Accordingly, the magnetization measured up to 6 T and down to 2 K for two orientations of the magnetic field can be quantitatively described by assuming more than 95{\%} of Mn to be in the 3+ state, with no evidence for interactions between the Mn spins. Furthermore, we have fabricated modulation doped (Ga,Mn)N/(Al,Ga)N:Mg multilayers accurately replicating the design of the structures in which indications of ferromagnetism characterized by above room temperature $T_{C}$ and large spontaneous magnetization, have been recently reported [2]. Our data do not reproduce those observations. \\[4pt] [1] A. Bonanni et al., Phys. Rev. B 75, 125210 (2007); Phys. Rev. Lett. 101, 135502 (2008); Phys. Rev. B. 79, 195209 (2009). \\[0pt] [2] N. Nepal et al., Appl. Phys. Lett. 94, 132505 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H35.00002: SmN; a zero-moment ferromagnetic semiconductor Joe Trodahl, Ben Ruck, Claire Meyer, Andrew Preston, Bart Ludbrook, Julio Criginski Cezar The rare-earth nitrides condense into the NaCl structure with a slowly-varying lattice constant across the series. We have grown a number of them, as polycrystalline and/or epitaxial films, and so far find them all to be semiconductors with a ferromagnetic phase at low temperature. SmN is especially interesting for its near cancellation between spin and orbital moments, in which the spins order ferro-magnetically below 27 K with a net magnetic moment of order 0.03 $\mu _{B}$/Sm. It is thus a nearly fringe-field free ferromagnetic semiconductor that couples so weakly to an applied field that its coercive field is very large; both of these properties are attractive for spintronics. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H35.00003: Room temperature ferromagnetism and spin polarization in Cr doped InN thin films Ambesh Dixit, C. Sudakar, P. Thapa, R. Panguluri, V. Naik, R. Naik, J. S. Thakur, B. Nadgorny, G. Lawes Understanding the development of ferromagnetism in transition metal doped semiconductors is a topic of great current interest. We studied the properties of chromium doped InN thin films fabricated by RF sputtering (In$_{1-x}$Cr$_{x}$N, where x= 0, 0.02 and 0.05) on c-sapphire substrates as a possible dilute magnetic semiconductor. We carried out detailed structural, optical, electrical, and magnetic characterization of these samples to investigate the interplay of these different materials properties. These films are found to be polycrystalline without any secondary impurity phases. Optical and electrical measurements indicate that these films are highly degenerate and have a carrier concentration of $\sim $ 10$^{20}$ cm$^{-3}$. Most significantly, we find that these films show room temperature ferromagnetism. We present evidence for intrinsic magnetic properties through measurements of the spin polarization. The results are discussed in the context of carrier mediated ferromagnetism. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 9:12AM |
H35.00004: Materials design of dilute magnetic semiconductors based on the control of spinodal decomposition Invited Speaker: Recently, spinodal decomposition phenomena attract much attention in the fabrication of dilute magnetic semiconductors (DMS). Many experimental results indicate that the magnetic properties of DMS are strongly affected by the occurrence of spinodal decomposition [1], thus people are now interested in controlling the magnetic properties of DMS by tuning the spinodal decomposition. In this talk, I will discuss spinodal decomposition in DMS based on the first-principles calculation. The electronic structure of DMS is calculated by using the Korringa-Kohn-Rostoker coherent potential approximation method. Based on the calculated mixing energy I will discuss phase diagrams of DMS systems and their chemical trends. By using the calculated chemical pair interactions between magnetic impurities in DMS, the self-organization of nano-structures in DMS of the nano-structures are simulated by using the Monte Carlo method. The simulation results indicate that we can control super-paramagnetic blocking temperature by optimizing the size of the nano-structures by changing the crystal growth condition [2]. Next, I will propose co-doping method to control solubility limit of magnetic impurities in DMS. From the total energy calculations, it is shown that the solubility of magnetic impurities is strongly enhanced under the existence of interstitial donors [2]. However, due to the compensation of holes by the co-dopants, the ferromagnetism is suppressed. Based on the kinetic Monte Carlo simulations, we propose low temperature annealing method to remove interstitial co-dopants for recovering the ferromagnetism. By combining the co-doping and the low temperature annealing, we can fabricate DMS with high concentration of magnetic impurities which should show high-Tc. This work is based on the collaboration with H. Fujii, L. Bergqvist, P. H. Dederichs and H. Katayama-Yoshida.\\[4pt] [1] A. Bonanni, Semicond. Sci. Technol. 22 (2007) R41.\\[0pt] [2] K. Sato et al., Rev. Mod. Phys. Phys. accepted. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H35.00005: Charge transfer ferromagnetism in defect-ridden oxide films Michael Coey, Plamen Stamenov, Munuswami Venkatesan Ferromagnetic magnetization curve of certain oxide thin films, whether undoped or doped 3$d$ ions, shows a characteristic anhysteretic approach to saturation of the form M $\approx $ M$_{s}$tanh(H/H$_{0})$, which is \textit{independent of temperature} below RT. There is no magnetic ordering of the dopant ions, but ferromagnetism is associated with Stoner splitting of a defect-related impurity band. In a model of ferromagnetic grain boundaries, H$_{0}$ = 0.16 M$_{0}$, where M$_{0}$ is the magnetization of the ferromagnetic regions. Data mining on six oxide systems shows that \textit{no more than 1 -- 2 {\%} of the volume of the films is magnetically ordered}. Charge transfer ferromagnetism arises when the impurity band can be populated from a proximate charge reservoir. Changing electron concentration leads to fulfillment of the Stomer criterion. There is a rich phase diagram as a function of bandwidth $W$, Stoner integral $I$ and band occupancy (which is related to the cost of electron transfer $U)$ with metallic and insulating regions which may be ferromagnetic, half metallic or nonmagnetic, consistent with behavior as a function of electron concentration observed experimentally. Our model of sparse Stoner ferromagnetism with variable electron concentration accounts consistently for the main experimental features of these puzzling materials. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H35.00006: Defect-induced magnetism in oxides Andrea Droghetti, Sri Chaitanya Das Pemmaraju, Stefano Sanvito The so called ``d$^0$ magnets'' are a class of materials which, lacking any magnetic ions with open $d$ or $f$ shells, should in principle not be magnetic, but nevertheless exhibit signatures of room temperature ferromagnetism. The most widely studied class of d$^0$ magnets is that of medium to wide gap oxides. Since ferromagnetism is generally observed in highly defective samples, it is expected that the magnetism must be somehow defect-related. However, at the present, there is no clear explanation of the experimental observations. We then present the results of a detailed research which combines Density Functional Theory (DFT) with model Hamiltonian calculations. In particular we consider whether ferromagnetism might be due to carrier-mediated long-range coupling between magnetic moments residing on cation vacancies or to a Stoner-like splitting of an impurity band in defect-rich regions of the samples. [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H35.00007: Defect pairing and magnetism in C or N-doped MgO and ZnO: a density-functional study Peter Kratzer, Hua Wu, Sung Sakong, Xin-Gao Gong, Matthias Scheffler It is demonstrated that C or N doping recently proposed as a way to create magnetism in otherwise nonmagnetic oxide insulators is curtailed by formation of defect pairs. Our density-functional calculations show that N-N pairing in MgO lowers the energy by 0.4 eV, leading to a nonmagnetic state. C-C pairing is even exothermic by more than 3 eV, and the resultant (C-C)$^{4-}$ molecules with spin=1 couple antiferromagnetically in MgO. However, calculations for C-doped ZnO, when properly treated using the PBE0 hybrid functional, show that the spin-polarized $pp\pi^*$ levels resonate with the host conduction band, which could possibly mediate a long-range ferromagnetic order. Magnetism of open-shell {\it impurity molecules} is proposed as a possible route to $d^0$-ferromagnetism in oxide spintronic materials. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H35.00008: Ferromagnetism in N doped ZnO and MgO Michael Shaughnessy, L.H. Yang, C.Y. Fong We investigate single and multiple dopings of N in ZnO$_{1-x}$N$_x$ and in MgO$_{1-x}$N$_x$ using a first principles density functional theory supercell method for x$<$ 10\%. We find evidence for ferromagnetism and propose a mechanism mediating the interaction. The magnetic coupling between the local moments on the N defects is studied as a function of the distance between N atoms and the role of hole and carrier co-doping is investigated. An RKKY-type interaction is found to mediate coupling, leading to both ferromagnetic and antiferromagnetic couplings for different doping configurations. [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H35.00009: Magnetic properties of multilayered CoO-ZnO Hyeon-Jun Lee, Frances Hellman The magnetic character of CoO/ZnO multilayers is investigated at low temperature (2 - 100K). Multilayers of CoO/ZnO, with varying nominal thickness of antiferromagnetic insulating (111) cubic CoO (0.7 -- 1.6 nm) and semiconducting (002) wurtzite ZnO:Al semiconductor (0.4 -- 2.0 nm), were prepared on c-cut sapphire substrate at 550 C by rf-magnetron reactive sputtering. Magnetic, transport, and magnetotransport measurements were carried out over a temperature range of 2 to 100 K. CoO( $\sim $1.4 nm)/ZnO( $\sim $0.7 nm) shows ferromagnetism with small moment up to 100 K. Up to 10 K there is clear ferromagnetic hysteresis in the out of plane direction and no magnetic hysteresis in-plane. For thicker ZnO, no magnetic properties are observed. We suggest that at low temperature the uncompensated spins in the CoO antiferromagnetic layers are coupled by exchange interaction through the ZnO:Al layer. This research was supported by both DOE and WIN. [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H35.00010: Producing room temperature ferromagnetism by doping non-magnetic Ta ions in transparent conducting anatase TiO$_{2}$ thin film S. Dhar, A. Rusidy, A. Roy Barman, A. Ariando, B.C. Qi, J.B. Yi, Y.P. Feng, K. Yang, J. Ding, A.T.S. Wee, T. Venkatesan, G. Neuber, M. Ruebhausen, Y. Dai, H. Hilgenkamp Anatase TiO$_{2}$ is being explored for fabrication of various spintronic, magneto-optic, opto-electronic, and memory devices. The keys to these technologies are our ability to control the magnetic, and transport properties of the host TiO2. In this work, we present the recent discoveries of room-temperature half-metallic ferromagnetism in non-magnetic Ta-doped anatase TiO$_{2}$ thin films prepared by pulsed laser deposition. Spin-polarized ferromagnetism with large carrier densities has been observed by a combination of SQUID magnetometry, XMCD, XAS, and OMCD. The results show that 90{\%} of the contribution to the ferromagnetism originates from the Ti sites and the remaining 10{\%} from the O sites. The OMCD results supported by band structure calculation, validates the half-metallicity of this ferromagnetic system. These results indicate that the magnetic moments at the Ti$^{3+}$ and Ti vacancy sites, are ordered ferromagnetically by the itinerant carriers via a Ruderman-Kittel-Kasuya-Yosida mechanism. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H35.00011: Room-temperature ferromagnetism of Cu-doped ZnO films probed by soft X-ray magnetic circular dichroism A. Rusydi, T.S. Herng, D.-C. Qi, J.B. Yi, Y.P. Feng, I. Santoso, X.Y. Gao, A.T.S. Wee, J. Ding, J. Berlijn, W. Ku, C. Sanchez-Hanke, K.S. Yang, Y. Dai In this paper, we report the direct evidence of the room temperature ferromagnetism in O-deficient ZnO:Cu films using soft X-ray magnetic circular dichroism (SXMCD) and X-ray absorption (XAS). SXMCD has revealed two distinct features of Cu atoms associated with (1) magnetically ordered Cu ions present only in the oxygen deficient samples, and (2) magnetically disordered \textit{regular} Cu$^{2+}$ ions present in all the samples. These observations indicate that sufficient amount of both oxygen vacancies (V$_{O})$ and Cu impurities are essential to the observed ferromagnetism, and non-negligible portion of Cu impurities are uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic ``indirect double exchange'' model, in which alignment of localized magnetic moment of Cu in the vicinity of the V$_{O}$ are mediated by the large-sized vacancy orbitals. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H35.00012: Spin dynamics in colloidal n-type and Mn$^{2+}$-doped ZnO quantum dots Stefan Ochsenbein, Daniel Gamelin Spins in semiconductor quantum dots (QDs) have been proposed as qubits for quantum computing. We have explored the reversible introduction of additional unpaired electrons into colloidal QDs, and have examined the spin properties of these electrons by electron paramagnetic resonance (EPR) spectroscopies. These experiments have revealed that the added electron resides in the conduction band and is delocalized over the entire QD. EPR linewidth analysis has allowed the transverse spin relaxation time constant, $T_{2}$, to be determined. Reducing the concentration of spin active $^{67}$Zn nuclei in the QDs is shown to increase $T_{2}$ substantially. The spins of Mn$^{2+}$ dopants in ZnO QDs have also been explored. Pulsed EPR experiments show that Mn$^{2+}$ spins additionally interact with nuclear spins of the QD surface capping ligands, despite large spatial separation. This study quantifies the interactions that determine $T_{2}$ of electron and dopant spins in ZnO QDs, and demonstrates manipulation of these interactions by chemical means. These results have bearing on consideration of spins in semiconductor nanostructures for information processing. Relevant references: K. M. Whitaker, S. T. Ochsenbein, V. Z. Polinger, and D. R. Gamelin, J. Phys. Chem. C \textbf{112}, 14331 (2008). W. K. Liu, K. M. Whitaker, A. L. Smith, K. R. Kittilstved, B. H. Robinson, and D. R. Gamelin, Phys. Rev. Lett. \textbf{98}, 186804 (2007). [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H35.00013: Models of Spinodal Decomposition in an Oxide Diluted Magnetic Semiconductor, Zn$_{1-x}$Co$_{x}$O Michael White, Stefan Ochsenbein, Tracy Lovejoy, Marjorie Olmstead, Daniel Gamelin Magnetic-ion-rich nanoscale domains formed by spinodal decomposition have been postulated for many diluted magnetic semiconductors, and are implicated in some materials for the observed ferromagnetic ordering. Colloidal nanocrystals of the ternary alloy wurtzite Zn$_{1-x}$Co$_{x}$O, with $x$ ranging from 0.0 (w-ZnO) to 1.0 (w-CoO), have been synthesized as model systems for the proposed spinodal decomposition of ferromagnetic Zn$_{1-x}$Co$_{x}$O thin films and powders. These enriched phases do not show any signs of ferromagnetism for any value of $x$. Changes in the electronic absorption, magnetic circular dichroism, and X-ray photoelectron (XPS) spectra with $x$ are defined that will allow identification of spinodal decomposition in other Zn$_{1-x}$Co$_{x}$O samples. Optical, magneto-optical, and XPS results are presented for the end member of this series (w-CoO), for the first time, and show this binary oxide to be an indirect-gap charge-transfer insulator with $E_{g} \quad \approx $ 2.3 eV. Reduction of Co$^{2+}$ to Co$^{0}$ by argon ion (Ar$^{+})$ sputtering was found to become markedly more facile with increasing $x$, placing spinodally segregated Zn$_{1-x}$Co$_{x}$O at greater risk of yielding false-positive Co$^{0}$ XPS signals than dilute Zn$_{1-x}$Co$_{x}$O with the same average composition. Reference: White, M.; Ochsenbein, S.; Gamelin, D., \textit{Chem. Mater.}, \textbf{2008}, $20$, 7107. [Preview Abstract] |
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