Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Magnetism in Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Connie Li, Naval Research Laboratory Room: 104 |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G8.00001: Insulating origin of the layered antiferromagnetic semiconductor (LaO)MnPn (Pn$=$P, As, Sb) Koichi Takase, Yasuhiro Morosawa, Tadataka Watanabe, Yoshiki Takano The layered oxypnictides (LaO)MnPn are antiferromagnetic semiconductors. Considering a high spin magnetic state (s$=$5/2) of the Mn3d electrons, this system seems to be a Mott insulator. In this study, we have investigated the origin of an insulating character of (LaO)MnPn by carrier doping. The carrier doping is attempted by the fluorine substitution and deficient oxygen. Huge reductions of the absolute value of the electrical resistivity are found in all samples. Especially, the Sb system shows metallic characters in a highly doped concentration. If this system is a Mott insulator, the magnetic ordered phase should disappear in the metallic sample. However, ferromagnetic impurities increase with increase concentration and it hinders evaluation of the magnetic properties. From these results, these systems are robust against carrier doping. Only in the Sb system with relatively large Mn-Mn distances, the antiferromagnetic interaction is slightly weaker than those of P and As cases, the carrier doping might change the character from insulating to metallic. If this scenario is true, the origin of insulator should not be an usual band insulator but a Mott insulator. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G8.00002: Magneto-Optical and Time Resolved Spectroscopy in Narrow Gap MOVPE Grown Ferromagnetic Semiconductors M. Meeker, B. Magill, M. Bhowmick, G.A. Khodaparast, S. McGill, C. Feeser, B.W. Wessels, D. Saha, G.D. Sanders, C.J. Stanton We report on magneto-optical at high magnetic fields and time resolved studies, that provide insight into the band structure, time scales, and the nature of the interactions in ferromagnetic InMnAs and InMnSb grown by MOVPE. By probing the dynamical behavior of the nonequilibrium carriers and spins, created by intense laser pulses, we gain valuable information about different scattering mechanisms and observe the sensitivity and tunability of the carrier and spin dynamics to the initial excitation energy. Theoretical calculations are performed using an 8 band $k\cdotp$ model including non-parabolicity, band-mixing, and the interaction of magnetic Mn impurities with itinerant electrons and holes. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G8.00003: What is the valence of Mn in GaMnN? Ryky Nelson, Tom Berlijn, Juana Moreno, Mark Jarrell, Wei Ku Motivated by the potential high Curie temperature of GaMnN [1], we investigate the controversial Mn-valence in this diluted magnetic semiconductor. From a first-principles Wannier functions analysis [2] of the high energy Hilbert space we find unambiguously the charge state of Mn to be close to $2+$ ($d^5$), but in a mixed spin configuration with average magnetic moments of 4 $\mu_B$. Using more extended Wannier orbitals to capture the lower-energy physics, we further demonstrate the feasibility of both the effective $d^4$ description (appropriate to deal with the local magnetic moment and Jahn-Teller distortion), and the effective $d^5$ description (relevant to study long-range magnetic order). Our derivation highlights the general richness of low-energy sectors in interacting many-body systems and the generic need for multiple effective descriptions, and advocates for a diminished relevance of atomic valence measured by various experimental probes.\\[4pt] [1] Dietl, T., H. Ohno, and F. Matsukura, Phys. Rev. B 63, 195205 (2001).\\[0pt] [2] W. Ku et al., Phys. Rev. Lett. 89, 167204 (2002). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G8.00004: TBD Invited Speaker: Tomasz Wojtowicz |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G8.00005: Interfacial exchange coupling in Fe/(Ga,Mn)As bilayers Abdel Khalq Alsmadi, Y. Choi, D.J. Keavney, K.F. Eid, B.J. Kirby, X. Liu, J. Leiner, K. Tivakornsasithorn, M. Dobrowolska, J.K. Furdyna We have carried out a systematic studies of magnetic order and coupling in Fe/(Ga,Mn)As bilayers using superconducting quantum interference device magnetometry, polarized neutron reflectometry, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism. Our results clearly show that Fe/(Ga,Mn)As bilayers display exchange coupling at the interface. Contrary to recent reports [e.g., F. Maccherozzi et al., Phys. Rev. Lett. 101, 267201 (2008)], a ferromagnetic coupling between the magnetic moment of the Mn ions and the moment of the Fe overlayer is observed. Furthermore, our element-specific data indicate that an ultrathin Mn-rich interfacial (Ga,Mn)As layer directly in contact with the Fe film is strongly coupled to the Fe layer, showing nearly identical coercive fields as the Fe layer, while the coercive fields of the bulk (Ga,Mn)As further from the Fe layer are distinctly weaker. We argue that the exchange coupling strength between Fe and Mn at the interface and throughout the (Ga,Mn)As layer is a function of the Mn concentration in the system, possibly arising from the diffusion of Mn interstitials during the MBE growth. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G8.00006: Valence band ordering restored by the p-d exchange interaction in GaMnAs Iriya Muneta, Hiroshi Terada, Shinobu Ohya, Masaaki Tanaka In ferromagnetic GaMnAs, it is predicted that the Mn impurity level or impurity band (IB) is formed in the band gap by the strong $p$-$d$ exchange interaction with the hybridization [1], which is consistent with the recent result of x-ray photoemission unveiling the disordered IB [2]. Although the hybridization might be expected to result in the disordered or merged valence band (VB), it was found that VB is almost unchanged by Mn doping in GaAs [2-4]. In order to understand the bandstructure and ferromagnetism of GaMnAs induced by the strong exchange interaction, we measured the VB ordering by using the resonant tunneling spectroscopy in the GaMnAs quantum-well (QW) double-barrier hetrerostructures with the Mn content $x$ varied from 0.4\% to 2.3\%. In $x<1\%$ (paramagnetic), the $d^2I/dV^2$ oscillations weaken as $x$ increases, which shows that VB merges with the {\it paramagnetic} IB and becomes disordered. However, the oscillations are restored at the onset of the ferromagnetism ($x>1\%$) and become stronger as $x$ increases. Our results show that the strong exchange interaction does not fluctuate VB but forms the disordered IB. [1] Krstaji\'c et al., PRB (2004). [2] Kobayashi et al., arXiv:1302.0063 (2013). [3] Ohya et al., Nat. Phys. (2011). [4] Muneta et al., APL (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G8.00007: Itinerant Magnetism and the Ferromagnetic Quantum Critical Point in Fe(Ga,Ge)$_3$ David J. Singh FeGa$_3$ is a tetragonal semiconductor with a band gap of $\sim$0.5 eV and interesting thermoelectric properties. It shows diamagnetic behavior but when modestly electron doped by Ge, a ferromagnetic quantum critical point emerges and the ground state becomes a ferromagnetic metal. We present first-principles calculations showing that the magnetism can be readily explained in an itinerant picture without the need for preexisting moments in the semiconducting state and without the need for correlation terms. We also present Boltzmann transport calculations of the thermopower. Itinerant magnetism implies strong coupling between the electrons at the Fermi energy that control transport and the magnetism. Thus, FeGa$_3$ may be a particularly interesting material near a quantum critical point. We find that the ferromagnetic state is half-metallic over a substantial composition range. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G8.00008: Quantitative chemical and structural ordering of Heusler Co$_{\mathrm{x}}$Mn$_{\mathrm{y}}$Ge$_{\mathrm{z}}$ (111) epitaxial films Frank Tsui, Brian Collins, Liang He, Yong Chu Heusler alloys are attractive spintronic materials, owing to the predicted half-metallicity and their compatibility with epitaxial semiconductor heterostructures. Chemical defects have been suggested as the cause of low spin-polarizations measured in these materials. We report a systematic investigation into the structural and chemical ordering of Co$_{\mathrm{x}}$Mn$_{\mathrm{y}}$Ge$_{\mathrm{y}}$ films grown epitaxially on Ge (111) substrates, as a function of composition near the Heulser Co$_{2}$MnGe stoichiometry. X-ray diffraction experiments show that the structural ordering is extremely sensitive to the Co-Mn atomic ratio with the best ordering occurring at compositions rich in Ge, i.e. off the Heulser stoichiometry. A new multi-edge anomalous diffraction technique has been employed to measure the elemental occupancy of the lattice sites. The measurements and analysis reveal that the dominant chemical defect is Mn-Ge site swapping with no detectable Co-Mn swapping, at variance with the predictions based on density functional theory. The observed shift for the most ordered composition from that of the bulk has been attributed to epitaxial constraints. The finding provides impetus for exploring spin polarization at off-stoichiometric compositions. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G8.00009: Annealing-induced enhancement of ferromagnetism and nano-particle formation in ferromagnetic-semiconductor GeFe Yuki Wakabayashi, Yoshisuke Ban, Shinobu Ohya, Masaaki Tanaka Ge-based ferromagnetic semiconductor GeFe is a promising material for future Si-based spintronic devices because of the high-quality single crystallinity and good compatibility with Si. However, its Curie temperature ($T_{C})$ is currently at the highest 170 K. In this study, we investigate the annealing effect on GeFe in order to enhance the ferromagnetism. The Ge$_{0.895}$Fe$_{0.105}$ thin film was epitaxially grown on a Ge(001) substrate by low-temperature molecular beam epitaxy. Then, post-growth annealing was carried out. We have analyzed GeFe films both crystallographically and magnetically by using transmission electron microscopy, transmission electron diffraction, energy-dispersive X-ray spectroscopy, magnetic circular dichroism, and superconducting quantum interference device. We have successfully increased the $T_{C}$ of Ge$_{0.895}$Fe$_{0.105}$ up to $\sim$ 220 K while keeping a single ferromagnetic phase when the annealing temperature was lower than 500$^{\circ}$C. In contrast, when annealed at 600$^{\circ}$C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high $T_{C}$ nearly up to room temperature, were formed in the film. Both types of films have a flat surface (roughness of 2-5 MLs), and thus they are promising for Si-based spin devices. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G8.00010: Investigation of Room temperature Ferromagnetism in Mn doped Ge Leyla Colakerol Arslan, Burcu Toydemir, Aykut Can Onel, Merve Ertas, Hatice Doganay We present a systematic investigation of structural, magnetic and electronic properties of MnxGe1$-$x single crystals. Mn$_{\mathrm{x}}$Ge$_{\mathrm{1-x}}$ films were grown by sequential deposition of Ge and Mn by molecular-beam epitaxy at low substrate temperatures in order to avoid precipitation of ferromagnetic Ge-Mn intermetallic compounds. Reflected high energy electron diffraction and x-ray diffraction observations revealed that films are epitaxially grown on Si (001) substrates from the initial stage without any other phase formation. Magnetic measurements carried out using a physical property measurement system showed that all samples exhibited ferromagnetism at room temperature. Electron spin resonance indicates the presence of magnetically ordered localized spins of divalent Mn ions. X-ray absorption measurements at the Mn L-edge confirm significant substitutional doping of Mn into Ge-sites. The ferromagnetism was mainly induced by Mn substitution for Ge site, and indirect exchange interaction of these magnetic ions with the intrinsic charge carriers is the origin of ferromagnetism. The magnetic interactions were better understood by codoping with nonmagnetic impurities. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G8.00011: Two-carrier Analysis of a- and m-ZnO Thin Films W.C. Hsieh, Q.Y. Chen, P.V. Waderkar, H.C. Huang, Y.F. Cheng, C.F. Chang, K.H. Liao, S.Y. Lai, H.H. Ko, Q.J. Lin, W.Y. Lin, H.W. Seo, C.H. Liao, H.H. Liao, L.W. Tu, N.J. Ho, D. Wijesundera, W.K. Chu Thin films of a- and m-plane oriented ZnO have been produced, respectively, on r- and m-sapphire substrates, all demonstrating reasonable crystalline qualities as judged by the X-ray diffractornetry. Contrary to most reported, the samples all demonstrated p-type charge carriers determined by Hall measurement using a Quantum Design PPMS system. The I-V curves of All-ZnO p-n junctions all demonstrate the characteristic nature of diodes. These two measurements provide unambiguous evidences of p-type behaviors. However, there are some irregularity in Hall measurement and magneto-resistivity. In order to understand the origin, we conducted a two-carrier analysis of the Hall data taken over a wide range of temperatures (T) and magnetic fields (B). The dependence of Hall resistivity and MR on B as T and the existence of hysteresis, we speculate, possibly reflect the complex atomic defects and their mobile nature in the otherwise largely perfect crystalline lattices. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G8.00012: (Zn,Co)O alloyed magnetic semiconductor: giant magnetization and experimental determination of band structure Guolei Liu Magnetic semiconductor exhibits both ferromagnetism and semiconductor properties. Since the magnetic dopants tend to aggregate in, the thermo-dynamical miscibility of transition metals in semiconductors is in fact extremely low (generally <15\%, in diluted region). The miscibility of transition metals exceeding diluted region is challenge for material science and future spintronic applications. In this paper, we have been grown (Zn,Co)O thin films by oxygen plasma-assisted molecular beam epitaxy. The Co context in these films can be up to 45\%, which we defined as alloyed magnetic semiconductor (AMS). The measurements of RHEED, XRD and in. situ. XPS indicated that (Zn, Co)O films are spinodal decomposition phase with ZnO wurtzite lattice. The saturated magnetization at room temperature was remarkably enhanced with increasing the Co content. It reach 530emu/cm${3}$ at x = 45\% which corresponds to the average magnetic moment 1.6$\mu_{B}$ per Co. The electronic band structure of (Zn,Co)O films were determined by angle resolved photoemission. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G8.00013: Magnetoresistance of ZnO and SnO$_{2}$ diluted magnetic oxide thin films Christer Akouala, Sandhyarani Punugupati, Raj Kumar, Yi-Fang Lee, Jagdish Narayan, Justin Schwartz, Frank Hunte Robust ferromagnetism (FM) at room temperature (RT) in semiconductor materials is critical to the development of spintronic devices making use of both charge and spin of electrons. In this work, magnetotransport techniques were used to study the electronic and magnetic properties of pulsed laser-deposited ZnO and SnO$_{2}$ diluted magnetic semiconductor films on sapphire substrates. Results from thin films with high structural quality showed both semiconductor and ferromagnetic characteristics. Magnetization measurements of Co-doped ZnO showed clear hysteretic behavior indicative of ferromagnetic films even at room temperature. Measurements of resistance versus temperature and magnetic field showed consistency with semiconductor characteristics. All samples were n-type with magnetoresistance (MR) behavior being strongly dependent on carrier concentration.. The presence of magnetic ions significantly affects the scattering of the conduction electrons in cobalt-doped samples resulting in different MR behavior from undoped samples. Correlation of transport characteristics with magnetization will be discussed in the context of carrier-mediated ferromagnetism in diluted magnetic semiconductors. [Preview Abstract] |
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