Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E42: Magnetic Materials for Semiconductor SpintronicsFocus
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Sponsoring Units: GMAG DMP DCOMP FIAP Chair: Zeke Johnston-Halperin, Ohio State University Room: 389 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E42.00001: Magnetic Anisotropy in Organic-Based Magnetic Materials Michael Chilcote, Megan Harberts, Yu Lu, Howard Yu, Bodo Fuhrmann, Katrin Lehmann, Andrew Franson, Georg Schmidt, Ezekiel Johnston-Halperin Here, we present the facile synthesis of a new class of organic-based magnetic nanostructures consisting of nanowires of vanadium tetracyanoethylene (V[TCNE]$_{\mathrm{2}})$ that self-assemble along the ridges of a grooved substrate. These nanowires exhibit uniaxial magnetic anisotropy in direct contrast to the isotropic in-plane response of typical thin-films. Furthermore, the magnetic anisotropy persists to the point of re-coalescence of the nanowires into a thin film, suggesting effects beyond simple shape anisotropy are at work. Moreover, isolated films of the V[TCNE] analog vanadium methyl tricyanoethylene carboxylate show a temperature-dependent switch in the easy axis from in-plane to out of plane, again suggesting underlying effects beyond shape anisotropy. These results introduce a new degree of freedom for organic-based magnetism and spintronics, allowing for the engineering of magnetic anisotropy at nanometer length scales in a material that exhibits both robust room-temperature magnetic order and the benefits of low cost, mechanical flexibility, and facile synthesis found in other organic materials. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E42.00002: High Q Factor Microwave Excitations in Organic Ferrimagnet Vanadium Tetracyanoethylene Andrew Franson, Michael Chilcote, Na Zhu, Xufeng Zhang, Ian Froning, Michael Flatt\'e, Hong Tang, Ezekiel Johnston-Halperin Room temperature magnetism in organic based semiconducting materials is an increasingly active area of research due to the growing interest in spintronic devices and next generation magnetoelectronics. Here we present an investigation into the ferromagnetic resonance and spin wave properties of the organic-based ferrimagnetic semiconductor V[TCNE]_x (x$\sim$2, TCNE: tetracyanoethylene). Here we discuss V[TCNE]$_x$ films synthesized by chemical vapor deposition on epitaxially flat, a-plane sapphire substrates that show exceptionally sharp resonant features for both ferromagnetic resonance (FMR) and spin wave resonance studies. Films of 1 $\mu$m thickness exhibit a peak to peak linewidth of 1.0 G in FMR studies and spin wave resonance studies reveal thickness standing wave modes over a frequency range of 1 - 5 GHz with quality factors (Q) in excess of 3200 [APL 109, 082402 (2016)]. Further, we find that extending the film thickness to 10 $\mu$m further enhances the Q to over 8,000 and reveals fine structure within the standing wave spectra. These results establish the versatility and potential of V[TCNE]$_x$ as a building block for future organic-based spintronic and magnetoelectronic devices. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E42.00003: A New Electrically Detected Magnetic Resonance Approach: Spin Dependent Charge Pumping Mark Anders, Patrick Lenahan, Aivars Lelis Electrically detected magnetic resonance (EDMR) studies have provided important insight into semiconductor/insulator interface defects. However, virtually all of these studies involve spin dependent recombination (SDR). Since SDR utilizes a recombination current, it is sensitive only to deep level defects. A new EDMR technique, spin dependent charge pumping (SDCP), overcomes this limitation. In SDCP, a trapezoidal waveform applied to the gate cycles the Fermi level from near the conduction to valence band edges. Interface traps are repeatedly filled and then emptied, creating a current which is sensitive to defects in most of the band gap. The sensitivity of SDCP is very nearly field and frequency independent, allowing for a wide range of resonance field/frequency measurements. SDCP at low resonance frequency allows for: (1) partial separation of spin-orbit coupling and hyperfine effects on magnetic resonance spectra, (2) observation of otherwise forbidden half-field effects which make EDMR, at least in principle, quantitative, and (3) observation of Breit-Rabi shifts in superhyperfine measurements. In addition, a strong SDCP response near zero magnetic field can provide some hyperfine information and EDMR-like detection without the expense and complexity of a resonance spectrometer. We present results on 4H-SiC MOSFETs, but the approach utilized should be widely applicable to other interfaces. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E42.00004: Electrically Detected Study of Variable Range Hopping in Silicon Nitrides Ryan Waskiewicz, Michael Mutch, Patrick Lenahan, Sean King Electrically detected magnetic resonance (EDMR) offers greatly improved sensitivity over conventional electron paramagnetic resonance (EPR) studies in semiconductor/insulator systems; in EDMR measurements, one observes EPR via changes in device currents which are spin-dependent. In our study, we observe EDMR via spin-dependent trap assisted tunneling (SDTAT) via variable range hopping (VRH) through stoichiometric silicon nitride dielectric films. In these films, leakage current effectively changes at resonance. In our study, we have investigated the EDMR response as a function of dielectric electric field and temperature for films of various thicknesses. We believe that these measurements allow us to identify the defects responsible for transport in such these thin films using EDMR and to some extent measure the distances between the defects. The separation between the defects can, at least in principle, be measured using the recently demonstrated half-field EDMR response and we can also count total number of spins responsible for transport through dielectric films. Although we present results only on silicon nitride thin films, we believe that the approach utilized will be widely applicable to other dielectric films in which electronic transport is of interest. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E42.00005: Magnetic field effect on the optoelectronic response of amorphous hydrogenated silicon. Ryan McLaughlin, Dali Sun, Chuang Zhang, Eitan Ehrenfreund, Zeev Valy Vardeny We have studied the magneto-photoluminescence and magneto photoconductivity in amorphous hydrogenated silicon (a-Si:H) thin films and devices as a function of temperature up to field of 5 Tesla. The magnetic field effects (MFE) are interpreted as spin mixing between spin-singlet and spin-triplet charge pairs due to the "delta-$g$" mechanism that is based on the $g$-value difference between the paired electron and hole, which directly affects the rate of radiative recombination and charge carrier separation, respectively. We found that the MFE($B)$ response does not form a Lorentzian (that is expected from the "delta-$g$" mechanism) due to disorder in the film that results in a broad distribution of e-h recombination rates, which could be extracted directly by time-resolved photoluminescence. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E42.00006: Interfacial exchange, magnetic coupling and magnetoresistance in ultra-thin GdN/NbN/GdN tri-layers Yota Takamura, Rafael S. Goncalves, Juan Pedro Cascales, Atilgan Altinkok, Clodoaldo I. L. de Araujo, Valeria Lauter, Jagadeesh S. Moodera Superconducting spin-valve structures with a superconductive (SC) spacer sandwiched between ferromagnetic (FM) insulating layers [Li PRL 2013, Senapati APL 2013, Zhu Nat. Mat. 2016.] are attractive since the SC and FM characteristics can mutually be controlled by the proximity effect. We investigated reactively sputtered GdN/NbN/GdN tri-layer structures with various (SC) NbN spacer thicknesses ($d_{\mathrm{NbN}})$ from superconducting to normal layers. Magnetoresistive behavior similar to GMR in metallic magnetic multilayers was observed in the tri-layers with $d_{\mathrm{NbN}}$ between 5-10 monolayers (ML), where thinner NbN layers did not show superconductivity down to 4.2 K. The occurrence of GMR signal indicates the presence of \textasciitilde a ML of FM metallic layers at the GdN/NbN interfaces. Susceptibility and transport measurements in these samples revealed that the interface layers (ILs) are ferromagnetically coupled with adjacent GdN layers. The thickness of each of the IL is deduced to be about 1.25 ML, and as a result for $d_{\mathrm{NbN}}$ \textless 2.5-ML the two FM layers in the tri-layer were magnetically coupled and switched simultaneously. These findings and interfacial characterization by various techniques will be presented. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E42.00007: Proximity effect and magnetic coupling in ultrathin GdN/NbN/GdN trilayers. Juan Pedro Cascales Sandoval, Yota Takamura, Rafael S. Goncalves, Atilgan Altinkok, Clodoaldo I.L. de Araujo, Valeria Lauter, Jagadeesh S. Moodera In general the coupling between magnetic layers is typically controlled by the RKKY interaction through non-magnetic spacers [1]. The antagonistic character of superconductivity and ferromagnetism has drawn much interest as to how these two states with opposing spin configurations can be controlled by the proximity effect in superconducting spintronic devices [2]. We present magnetization and transport measurements on ferromagnetic insulator (FI) and superconductor (SC) GdN/NbN/GdN trilayer structures, fabricated by reactive sputtering at room temperature. The magnetic and transport behavior of these trilayers dramatically changed in the superconducting state of the NbN spacer. The superconductive state was found to heavily influence the indirect coupling of the FI layers for certain FI/SC/FI thickness combinations. The interplay between magnetism and superconductivity, along with interfacial exchange coupling play major roles on the resulting magnetic coupling which depends on the FI thickness and FI/SC thickness ratio, producing a rich variety of effects. [1] P. Bruno, PRL 67, 2592 (1991). [2] B. Li et al., PRL 110, 097001 (2013), K. Senapati et al., APL 103, 132406 (2013), Y. Zhu et al., Nat. Mat. 2016. \newline [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E42.00008: Observation of large spin splitting in the conduction band of n-type ferromagnetic semiconductor (In,Fe)As Le Duc Anh, Pham Nam Hai, Masaaki Tanaka Ferromagnetic semiconductors (FMSs) both with large spin-split conduction band (CB) and valence band (VB) and with high Curie temperature ($T_{C})$ are highly desired for spintronic devices, which is not yet realized so far. Here, we report the first observation of large spontaneous spin splitting ($\Delta E=$50 meV) in the CB of n-type FMS (In,Fe)As using tunneling spectroscopy in (In,Fe)As-based Esaki diodes. The device structure consists of 50 nm-thick n$^{+}$(In,Fe)As/5 nm-thick InAs/250 nm-thick p$^{+\, }$InAs:Be grown on a p$^{+}$InAs(001) substrate. At small forward bias voltages, electrons tunnel from the (In,Fe)As CB to the p$^{+}$InAs VB, thus the tunneling conductance d$I$/d$V$ probes the density of states of the (In,Fe)As CB. In the d$^{2}I$/d$V^{2}-V$ curves, we clearly observe double-valley features at low temperatures, which evolve into single-valley features at temperatures above $T_{C\, }$of the (In,Fe)As films. This is clear evidence of the spin splitting of the (In,Fe)As CB bottom. We found that the mean-field Zener model also fails to explain consistently the $T_{C}$ and $\Delta E$ of (In,Fe)As. [1] L. D. Anh et al., Nature Communications (2016), arXiv:1609.01379. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E42.00009: Complete demagnetization of (Ga,Mn)As films via electric field Hailong Wang, Jianhua Zhao Electric field ($E$-field) control of magnetism is promising for decreasing the power consumption of information processing and storage. So far, $E$-field manipulation of magnetism such as Curie temperature ($T$c) and coercivity has been reported. However, the electric field effects on magnetism are usually very small, and the previous results are often observed near $T$c. In this work, we first demonstrate the complete demagnetization of a ferromagnet via $E$-field. Considering the limited charge modulation ability of $E$-field restricted by the breakdown voltage of dielectrics, (Ga,Mn)As films featuring lower carrier concentration than ferromagnetic metals were utilized here. Ultrathin (Ga,Mn)As films (\textasciitilde 2 nm) showing well defined ferromagnetism were successfully fabricated by low-temperature molecular-beam epitaxy. This was achieved by combining heavily Mn doping and post-growth annealing. More importantly, a p-type GaAs buffer was critical to maintain the ferromagnetism of ultrathin (Ga,Mn)As by avoiding partial depletion of holes near the interface. Then ionic liquid or solid state ionic gel were used to produce huge interfacial $E$-field, and giant modulation of magnetism including complete demagnetization was realized. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E42.00010: Spectroscopic Observation of Plasmonic Polarons in a Doped Ferromagnetic Semiconductor J.M. Riley, L. Duffy, M. Watson, L. Bawden, F. Caruso, C. Verdi, T. Hesjedal, F. Giustino, M. Hoesch, P.D.C. King Since its discovery in the 1960s, europium monoxide (EuO) has been shown to host a rich array of physical phenomena including the giant magneto-optic Kerr and Faraday effects, anomalous Hall effect, colossal magnetoresistance under doping [1], and a massive, tuneable ferromagnetic metal-insulator transition [2]. The Curie temperature responds sensitively to carrier doping, which can be readily controlled via substitution of Gd for Eu in epitaxial thin films. Here, we use in-situ synchrotron-based angle-resolved photoemission spectroscopy to study the corresponding electronic structure evolution in Eu1-xGdxO grown by molecular beam epitaxy. At low carrier densities, our measured spectral function exhibits signatures of polaron formation due to strong coupling to a bosonic mode. Similar spectral features have recently attracted great attention in polar oxides such as TiO2 [3] and SrTiO3 [4-6] and interfacial systems such as FeSe/SrTiO3 [7]. Unlike these systems, however, we show that it is not electron-phonon, but, rather, electron-plasmon coupling that is the dominant driver of this effect, providing a rare observation of plasmonic polarons.[1] Phys. Rev. B 8, 2316 (1973); [2] Phys. Rev. Lett. 100, 046404 (2008); [3] Phys. Rev. Lett. 110, 196403 (2013); [4] Nat. Comms. 6, 8585 (2015); [5] Nat. Comms. 7, 10386 (2016); [6] Nat. Mat. 15, 835 (2016); [7] Nature 515, 245 (2014) [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E42.00011: Excitations and long-range-order in a crystalline ferromagnetic semiconductor Matthew Stone, vasile Garlea, Beatrice Gillon, Alain Cousson, Andrew Christianson, Mark Lumsden, Stephen Nagler, David Mandrus, Brian Sales We present polarized and unpolarized neutron diffraction measurements as well as inelastic neutron scattering measurements examining the dilute ferromagnetic semiconductor Yb$_{14}$MnSb$_{11}$. We find that the system consists of RKKY ferromagnetic exchange coupled Mn$^{2+}$ sites with nearest and next nearest neighbor exchange interactions dominating the magnetic spectrum. We observe a distribution of negative magnetization density throughout the crystal structure with no significant negative magnetization on any single site. The extended spread of a negative magnetization around each of the Mn ions supports a Kondo screening cloud scenario for Yb$_{14}$MnSb$_{11}$. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E42.00012: Structural and optical properties of 2-dimensional magnetic semiconductor CrPS4 Changgu Lee, Jinhwan Lee, Taeg Yeong Ko, Jung Hwa Kim, Zonghoon Lee, Sunmin Ryu, Byunggil Kang, Hunyoung Bark Atomically thin 2-dimensional semiconducting metal chalcogenides exhibit diverse physical properties depending on their thickness. However, the lack of magnetism in these materials limited their expansion in more immense exploration of their properties and applications. In this work, we report the structural and optical properties of atomically thin chromium thio-phosphate (CrPS4), which is a magnetic semiconductor. By using polarized light, we could find strong anisotropy in its structure with 180 degree period. Comparison with the transmission electron microscopy observation of lattice structure enables us to easily identify the crystalline orientation of the layered structure. The thickness of crystals exfoliated down to monolayer could be further identified through Raman and photoluminescence (PL) spectroscopies. Thickness dependence of positions and intensities of Raman shift peaks were observed depending on the excitation wavelength. The PL spectroscopy and electronic transport measurement results showed its p-type semiconducting behavior with 1.35 eV of electronic bandgap. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E42.00013: Orbital frustration induced unusual ordering in semiconductor alloys Kai Liu, Wanjian Yin, Shiyou Chen, Xingao Gong, Suhuai Wei, Hongjun Xiang It is well known that ternary zinc-blende semiconductors are always more stable in the chalcopyrite (CH) structure than the Cu-Au (CA) structure because CH structure has large Coulomb interaction and reduced strain energy. Surprisingly, an experimental study showed that ZnFeSe$_{\mathrm{2}}$ alloy takes the CA order as the ground state structure, which is consistent with our density function theory (DFT) calculations showing that the CA order has lower energy than the CH order for ZnFeSe$_{\mathrm{2}}$. We reveal that the orbital degree of freedom of high-spin Fe$^{\mathrm{2+}}$ ion (d$^{\mathrm{6}})$ in the tetrahedral crystal field plays a key role in stabilizing the CA order. First, the spin-minority d electron of the Fe$^{\mathrm{2+}}$ ion tends to occupy the $d_{x}_{\mathrm{2-}}_{y}_{\mathrm{2}}$--like orbital instead of the $d_{\mathrm{3}}_{z}_{\mathrm{2-}}_{r}_{\mathrm{2}}$--like orbital because of its large negative Coulomb energy. Second, for a nearest-neighboring Fe$^{\mathrm{2+}}$ pair, two spin-minority d electrons with occupied $d_{x}_{\mathrm{2-}}_{y}_{\mathrm{2}}$--like orbitals in the plane containing the Fe-Fe bond has lower electronic kinetic energy. Both conditions can be satisfied in the CA ordered ZnFeSe$_{\mathrm{2}}$ alloy, while there is an orbital frustration in the CH structure. Our results suggest that orbital degree of freedom provides a new way to manipulate the structure and properties of alloys. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E42.00014: (La,AE)(Zn,TM)AsO (AE $=$ Ba, Sr, Ca; TM $=$ Mn): two-dimensional 1111-type diluted magnetic semiconductors in bulk form Cui Ding, Fanlong Ning We successfully fabricated “1111” type bulk form diluted ferromagnetic semiconductors (DMS) with decoupled carriers and spins doping: (La,AE)(Zn,TM)AsO (AE $=$ Ba, Sr, Ca; TM $=$ Mn), of which the Curie temperature TC up to 40 K. We investigated the individual influence of carriers and local moments on the ferromagnetic ordering. We observed that no ferromagnetic order occurs with (Zn,Mn) substitution in the parent compound LaZnAsO without charge doping, but too much carriers suppresses both Curie temperature and saturation moments. The results of muSR measurements indicate that the ferromagnetic order transition takes place in entire volume, namely, these DMSs are bulk nature. The muSR measurements also show an universal linear trend between the static internal field parameter as and the ferromagnetic Curie temperature TC for 1111-type system and other system of DMSs, which suggests that the exchange interaction supporting ferromagnetic coupling in these systems has a common origin. [Preview Abstract] |
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