AVS2004 Session SC+MI-MoM: Dilute Magnetic and Ferromagnetic Semiconductors

Monday, November 15, 2004 8:20 AM in Room 304B

Monday Morning

Start	Invited?	Item
8:20 AM	Invited	SC+MI-MoM-1 Heterointerfaces and Magnetism in Ferromagnetic Semiconductor Heterostructures N. Samarth (Pennsylvania State University) We discuss recent experiments that demonstrate how heterointerfaces impact the magnetic properties of heterostructures derived from the "canonical" ferromagnetic semiconductor (Ga,Mn)As. In this material, holes created by the Mn acceptors mediate a ferromagnetic interaction between the Mn ions themselves, and the Curie temperature is determined by a complex interplay between substitutional magnetic ions, interstitial defects and holes. Although as grown epilayers of (Ga,Mn)As typically have Curie temperatures lower than 110 K, post-growth annealing at low temperatures (180 C - 250 C) significantly enhances the ferromagnetic properties, leading to Curie temperatures above 150 K. The first set of experiments examines the effects of capping ferromagnetic Ga1-xMnxAs epilayers with a thin layer of undoped GaAs. We find that the overgrowth of even a few monolayers of GaAs significantly suppresses the enhancement of the ferromagnetism associated with low temperature annealing, suggesting that heterointerfaces have a direct impact on the migration of interstitial defects during post-growth annealing. In the next set of experiments, we demonstrate the first exchange biasing of (Ga,Mn)As by an overgrown antiferromagnet (MnO). Although the exchange bias effect is unambiguous when successful, we also find that the high reactivity between Mn and GaAs affects the ferromagnet/antiferromagnet heterointerface, presenting interesting experimental challenges for the routine achievement of exchange bias in this important spintronic material. This work was carried out in collaboration with K. C. Ku, M. B. Stone, K. F. Eid, P. Schiffer, T. Shih, and C. Palmstrom. Supported by ONR and DARPA.
9:00 AM		SC+MI-MoM-3 Structural and Magnetic Properties of a Magnetic Semiconductor MnGeN₂ Grown by MBE S.H. Cheung, M.L. Harland, V.K. Lazarov (University of Wisconsin, Milwaukee); Y. Zhang (Peking University, China); M. Weinert, M. Gajdardziska-Josifovska (University of Wisconsin, Milwaukee); Z. Gai (Peking University, China); L. Li (University of Wisconsin, Milwaukee) A novel magnetic semiconductor MnGeN₂ was synthesized on 6H-SiC(0001), Al₂O₃(0001), and MgO(111) substrates by plasma assisted molecular beam epitaxy. In situ reflection high-energy diffraction, ex situ atomic force microscopy and transmission electron microscopy (TEM) investigations indicate that the films grown are epitaxial on all three substrates, with the ones on MgO having the best overall quality. Detailed analysis of high-resolution TEM digital diffractograms and convergent beam electron diffraction patterns of the films show that the MnGeN₂ is orthorhombic, and has the following crystallographic orientation relationships with the substrate: MnGeN₂(001)//MgO(111), MnGeN₂(100)//MgO(11-1), and MnGeN₂(210)//MgO(01-1). Investigations by SQUID magnetometry indicate that the magnetic properties of the films can be controlled by the stoichiometry, i.e. Mn/Ge ratio, varying from paramagnetic to ferromagnetic, with the ferromagnetic samples exhibiting a Curie temperature above 300 K.
9:20 AM		SC+MI-MoM-4 Thermal Stability of GaCrN Epitaxial Layers G.T. Thaler, R.M. Frazier, C.R. Abernathy, S.J. Pearton (University of Florida) A number of recent studies have reported the observation of room temperature ferromagnetism in GaMnN. However, this material appears to be thermally unstable during processing at temperatures as low as 500°C unless co-doped with oxygen. For the development of spintronics devices based on GaN, thermal annealing at or above ~700°C is necessary to improve contact resistances and for p-dopant activation. An alternative material that has received some interest of late is GaCrN, which has also been reported to be ferromagnetic at room temperature. However, little is known as yet about the thermal stability of this material and its suitability for integration with GaN device processing technology. In this talk we will discuss the thermal stability of GaCrN and the effect of Cr concentration on both the as-grown magnetic behavior and the magnetic properties as a function of annealing. Epitaxial growth was performed using Gas Source Molecular Beam Epitaxy. Films with magnetic transition temperatures above room temperature were produced for a variety of Cr concentrations, though the signal appeared to maximize around 2-3% Cr, as is the case for GaMnN. Unlike GaMnN, the addition of Cr to GaN produced material that was thermally stable after annealing up to 700°C with little change observed in the magnetic behavior of the GaCrN films. The implications of this stability for device processing and performance will also be discussed. This work was supported by the Army Research office under: ARO-DAAD19-01-1-0701 and by NSF under: ECS-0224203.
9:40 AM		SC+MI-MoM-5 Structural Characterization of GaMnN Thin Films Grown by Chemical Beam Epitaxy (CBE). L.A. Carreno, C. Boney, A. Bensaoula, Z. Zhang (University of Houston) Diluted magnetic semiconductors (DMS) based on Mn doped GaN are intensively investigated for their potential spintronics applications. Ferromagnetism has been demonstrated in Mn-doped implanted p-type GaN, Mn-diffused GaN, and n-type films of GaMnN grown by MBE. Two approaches to understanding the magnetic properties of DMS materials are pursued: one considers these materials as more-or-less random alloys; the second one considers the magnetic atoms forming small clusters that produce the observed ferromagnetism. To clarify these issues we have performed structural analysis of GaMnN thin films grown by CBE using two in-situ time of flight (TOF) ion spectroscopy techniques combined with SARIC trajectory simulations. These were complemented with ex-situ XRD, PL, Raman spectroscopy, and Backscattering/ channeling combined with PIXE. GaMnN has been grown using TEG, NH₃, and solid Mn on sapphire/GaN templates prepared by CBE and MBE. Evolution of the stress for the as grown and annealed thin films has been studied by XRD and Raman. Samples grown on sapphire/GaN templates prepared by CBE show n-type conductivity, those grown on MBE GaN templates are highly resistive. The reactor is fitted with two in-situ TOF techniques, Direct Recoil Spectroscopy (DRS) and Mass Spectroscopy of Recoiled Ions. For structural characterization, azimuthal DRS scans are used to extract the surface periodicity and from that construct models of GaMnN surfaces. Simulations of scattering and recoiling scans for GaMnN surfaces have been performed for different possible lattice locations of Mn in GaN. Similar experiments were performed using Rutherford backscattering/channeling combined with particle induced X-ray emission. DRS confirmed retention of wurtzite crystal structure obtained by RHEED and XRD for Mn concentrations up to 2.5%. Although results have shown mainly substitutional incorporation of Mn atoms at Ga sites, DRS scans also show presence of Mn atoms at interstitial positions.
10:00 AM	Invited	SC+MI-MoM-6 Induced Host Moments and Mn Electronic Structure in Mn-Doped Iii-V Ferromagnetic Semiconductors D.J. Keavney (Argonne National Laboratory); D. Wu, J. Shi (University of Utah); E. Johnston-Halperin, D.D. Awschalom (University of California, Santa Barbara); Y. Cui, L. Li (University of Wisconsin-Milwaukee) We have used soft x-ray magnetic circular dichroism (XMCD) and absorption spectroscopy (XAS) to examine induced host magnetic moments and the local Mn environment in Mn-doped GaAs and GaN. X-ray absorption probes unoccupied states via transitions from deep core levels, thus providing electronic structure information with element specificity. With circularly polarized radiation at the L edges, element specific moments can be detected via their projection onto the Mn 3d and host 4s states, providing a test of predictions made by the carrier-mediated model of ordering. In (Ga,Mn)As, we find small XMCD signals at the onset of the absorption edge for both Ga and As, which we attribute to induced 4s moments. The relative orientations of all three elements are as expected for carrier-mediated coupling, and we estimate that the As moment is larger than the Ga moment. In (Ga,Mn)N, we detect a weak Ga XMCD signal 2-3 eV above the absorption edge of opposite sign to that in (Ga,Mn)As, which may be attributable to Mn 3d tails at the Ga sites. The absence of a Ga 4s moment would suggest a weaker p-d hybridization consistent with the deeper position of the Mn acceptor level. In both systems, XAS shows that Mn is divalent, although with differing amounts of line broadening, suggesting that the Mn 3d localization varies significantly depending on the host. (Ga,Mn)N has a lineshape closer to atomic Mn 2+ than (Ga,Mn)As. These results show that the Mn 3d and valence band electronic structure in doped III-V systems is strongly dependent on the host, and have implications for the degree of p-d hybridization and the coupling mechanism responsible for ferromagnetism. Use of the Advanced Photon Source was supported by the U.S. DOE, Office of Science, Contract No. W-31-109-Eng-38. Work at the Univ. of Utah was supported by ONR/DARPA grant No. N00014-02-10595, at UCSB by ONR/DARPA grant No. N00014-99-1-1096 and AFOSR F49620-02-10036, and at Univ. of Wisconsin by NSF DMR-0094105.
10:40 AM		SC+MI-MoM-8 Intrinsic Versus Extrinsic Nature of Co Doped TiO₂ Diluted Magnetic Semiconductor Thin Films S.R. Shinde, S.B. Ogale, J. Higgins, T. Zhao (University of Maryland); S.E. Lofland (Rowan University); V.N. Kulkarni (University of Maryland); A.J. Millis (Columbia University); S. Das Sarma, R.L. Greene, R. Ramesh, T. Venkatesan (University of Maryland) The issue of Co distribution in TiO₂ (in anatase and rutile forms), a widely studied oxide based diluted magnetic semiconductor (DMS) system, is still controversial. Although all the reported studies have discovered room temperature ferromagnetism in this system, some of the researchers claim that the material is intrinsic, whereas others have found that cobalt forms small clusters and therefore the material has extrinsic origin of ferromagnetism. In our work we have grown (by pulsed laser deposition) and characterized epitaxial thin films of Co:TiO₂ at different growth conditions and Co doping concentrations. We noticed that Co distribution strongly depends on the growth parameters. At lower growth temperature (~700C) there is a limited solubility of Co (up to ~2%) above which nanometer sized Co clusters are formed. When the films are grown in ultrahigh vacuum (10-8 Torr), the films have low resistivity and show the anomalous Hall effect. Although this could be interpreted as a signature of carrier induced DMS nature of these particular films, our detailed magnetic and structural analysis shows the presence of Co nanoclusters in these films. In the magnetization data of these films we observe superparamagnetism with a blocking temperature of 250K. This temperature corresponds to Co particles of 7nm diameter, the presence of which was further confirmed by transmission electron microscopy (TEM). On the other hand, when the films grown at lower temperature are annealed at high temperature (~900C) the clusters dissolve in titanium dioxide matrix leading to an intrinsic DMS with a Curie temperature ~650C. Similar properties are observed for films directly grown at high temperature and no indication of any clustering of Co is observed in TEM. We have also observed electric field induced reversible modulations, in the magnetization of these films in PbZr_0.2Ti_0.8O₃/Co:TiO₂/SrRuO₃ field effect transistor structure.
11:00 AM		SC+MI-MoM-9 Ferroelectric Field Effect on Ferromagnetism in Diluted Magnetic Insulator Anatase Co:TiO₂ T. Zhao, S.R. Shinde, S.B. Ogale, H. Zheng, T. Venkatesan (University of Maryland); R. Ramesh (University of California, Berkeley); S. Das Sarma (University of Maryland); J. Misewich (Brookhaven National Laboratory) Recently considerable success is reported in making a non-magnetic semiconductor ferromagnetic by dilute doping of magnetic impurities. However, the possibilities of extrinsic effects such as dopant clustering, impurity magnetic phases etc., have not been completely ruled out in many systems. In this work we report the first successful implementation of an external electric field modulation of ferromagnetism in an oxide-based DMS anatase Co:TiO₂. An anatase TiO2 layer with 7% Co doping and a ferroelectric PbZr_0.2Ti_0.8O₃ layer were epitaxially grown on a conducting SrRuO₃ buffered LaAlO₃ substrate by pulsed laser deposition. The high-quality of epitaxy and uniform distribution of Co were confirmed by X-Ray diffraction and transmission electron microcopy. The Co:TiO₂ channel grown in this case at a high temperature of 875°C is insulating in nature. The magnetic hysteresis loops of the Co:TiO₂ were measured by superconducting quantum interference device after positive or negative electric poling on PZT. The room temperature saturated magnetic moment clearly shows two stable states which are reversible by switching the ferroelectric polarization. The observed effect, which is about 15% in strength can be modulated over several cycles. This first demonstration of electric field effect in an oxide based diluted ferromagnetic insulator system provides evidence of its intrinsic nature. Furthermore, the ability of electric field modulation of ferromagnetism is very promising for next-generation multi-functional electronic devices. Possible mechanisms for electric field induced modulation of insulating ferromagnetism are discussed. This work was supported by DARPA SpinS program (through US-ONR) and the NSF-MRSEC (DMR 00-80008) at Maryland. The PLD and RBS facilities used in this work are shared experimental facilities (SEF) supported in part under NSF-MRSEC.
11:20 AM		SC+MI-MoM-10 Applications of a Dilute Magnetic Semiconductor Based on AlN R.M. Frazier, G.T. Thaler, J.Y. Leifer, C.R. Abernathy, S.J. Pearton (University of Florida) With the increasing interest in spintronics, many attempts have been made at incorporating spin-based functionality into existing semiconductor technology. One approach, utilizing dilute magnetic semiconductors (DMS) formed via introduction of transition metal ions into III-Nitride hosts, would allow for integration of spin based phenomena into current wide bandgap technology. Further, the use of AlN broadens III-V DMS applications to tunneling devices and UV light emitters. The most evident application of ferromagnetic AlN is as a ferromagnetic tunnel barrier, similar to EuS, but unlike EuS should allow for operation at room temperature. Ion implantation has been shown to be an effective survey method for introduction of various transition metals into AlN. However, it is not a technique which will allow for the development of advanced spin based devices. Such devices will require epitaxial methods of the sort currently used for synthesis of III-Nitride optoelectronics. In this study, one such technique, Gas Source Molecular Beam Epitaxy (GSMBE) has been used to synthesize AlN films doped with Cr and Mn. In the Mn doped films, increasing the V/III ratio corresponded to an increased magnetic signal, indicating an increase in active Mn sites. In the case of both Mn and Cr doped AlN, the magnetic signal was found to depend on the flux of the dopant, and the optimal growth conditions were found. Growth of tunnel devices using AlTMN as a barrier will also be discussed. This work is supported by the Army Research Office under ARO-DAAD19-01-0-0701 and NSF under ECS-0224203.
11:40 AM		SC+MI-MoM-11 Ferromagnetism and Polaron Percolation in Mn_xGe_1-x Dilute Magnetic Semiconductor A.P. Li, J.F. Wendelken, J. Shen (Oak Ridge National Laboratory); J.R. Thompson, H.H. Weitering (Oak Ridge National Laboratory, University of Tennessee) In dilute magnetic semiconductors (DMS), ferromagnetic ordering is carrier mediated. This picture seems to be accepted more or less universally, but the detailed nature of the ferromagnetism varies greatly from system to system. We have studied ferromagnetism and the correlation between transport and ferromagnetism in Mn_xGe_1-x DMS for Mn concentrations up to 9%. By carefully controlling the growth conditions, we obtained precipitate-free Mn_xGe_1-x that exhibits magnetic phase transitions at Tc = 20 K and Tc* = 112 K. The magnetic response to temperature and doping concentration is indicative of a magnetic-polaron percolation transition at Tc [1], which coincides with a metal-insulator transition and Hall-effect sign anomaly. Tc* is the ferromagnetic ordering temperature within isolated polarons which can be determined from a Curie-Weiss plot of the high-temperature magnetic susceptibility. Ferromagnetism in Mn_xGe_1-x DMS reveals a striking analogy with the magnetism of so-called "clustered states" in manganite compounds [2]. [1] A. Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003) [2] G. Alvarez and E. Dagotto, Phys. Rev. B 68, 045202 (2003).