1
|
Quessab Y, Xu JW, Ma CT, Zhou W, Riley GA, Shaw JM, Nembach HT, Poon SJ, Kent AD. Tuning interfacial Dzyaloshinskii-Moriya interactions in thin amorphous ferrimagnetic alloys. Sci Rep 2020; 10:7447. [PMID: 32366864 PMCID: PMC7198596 DOI: 10.1038/s41598-020-64427-0] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2019] [Accepted: 04/15/2020] [Indexed: 11/26/2022] Open
Abstract
Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMI). Further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. Tuning DMI is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMI in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the CoGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. The DMI thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. Our work opens new paths for controlling interfacial DMI in ferrimagnets to nucleate and manipulate skyrmions.
Collapse
Affiliation(s)
- Y Quessab
- Center for Quantum Phenomena, Department of Physics, New York University, New York, New York, 10003, USA.
| | - J-W Xu
- Center for Quantum Phenomena, Department of Physics, New York University, New York, New York, 10003, USA
| | - C T Ma
- Department of Physics, University of Virginia, Charlottesville, Virginia, 22904, USA
| | - W Zhou
- Department of Physics, University of Virginia, Charlottesville, Virginia, 22904, USA
| | - G A Riley
- Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado, 80305, USA
- Center for Memory and Recording Research, University of California San Diego, La Jolla, California, 92093, USA
| | - J M Shaw
- Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado, 80305, USA
| | - H T Nembach
- Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado, 80305, USA
- JILA, University of Colorado, Boulder, Colorado, 80309, USA
| | - S J Poon
- Department of Physics, University of Virginia, Charlottesville, Virginia, 22904, USA
| | - A D Kent
- Center for Quantum Phenomena, Department of Physics, New York University, New York, New York, 10003, USA
| |
Collapse
|
2
|
Abstract
We present a general geometrical approach to the problem of escape from a metastable state in the presence of noise. The accompanying analysis leads to a simple condition, based on the norm of the drift field, for determining whether caustic singularities alter the escape trajectories when detailed balance is absent. We apply our methods to systems lacking detailed balance, including a nanomagnet with a biaxial magnetic anisotropy and subject to a spin-transfer torque. The approach described within allows determination of the regions of experimental parameter space that admit caustics.
Collapse
Affiliation(s)
- D Pinna
- Department of Physics, New York University, New York, New York 10003, USA
| | - A D Kent
- Department of Physics, New York University, New York, New York 10003, USA
| | - D L Stein
- Department of Physics, New York University, New York, New York 10003, USA; Courant Institute of Mathematical Sciences, New York University, New York, New York 10012, USA; and NYU-ECNU Institutes of Physics and Mathematical Sciences at NYU Shanghai, 3663 Zhongshan Road North, Shanghai, 200062, China
| |
Collapse
|
3
|
Mans DRA, Kent AD, Hu RV, Lai A. Fat EJ, Schoone GJ, Adams ER, Rood EJ, Alba S, Sabajo LOA, Lai A Fat RF, de Vries HJC, Schallig HDFH. Monitoring the response of patients with cutaneous leishmaniasis to treatment with pentamidine isethionate by quantitative real-time PCR, and identification ofLeishmaniaparasites not responding to therapy. Clin Exp Dermatol 2015; 41:610-5. [DOI: 10.1111/ced.12786] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/22/2015] [Indexed: 11/29/2022]
Affiliation(s)
- D. R. A. Mans
- Anton de Kom University of Suriname; Paramaribo Suriname
| | - A. D. Kent
- Anton de Kom University of Suriname; Paramaribo Suriname
| | - R. V. Hu
- Dermatological Service; Ministry of Health; Paramaribo Suriname
| | - E. J. Lai A. Fat
- Department of Dermatology; Academic Hospital Paramaribo; Paramaribo Suriname
| | | | - E. R. Adams
- Royal Tropical Institute; Amsterdam The Netherlands
| | - E. J. Rood
- Royal Tropical Institute; Amsterdam The Netherlands
| | - S. Alba
- Royal Tropical Institute; Amsterdam The Netherlands
| | - L. O. A. Sabajo
- Dermatological Service; Ministry of Health; Paramaribo Suriname
| | - R. F. Lai A Fat
- Department of Dermatology; Academic Hospital Paramaribo; Paramaribo Suriname
| | - H. J. C. de Vries
- Department of Dermatology; Academic Medical Center; Amsterdam The Netherlands
- Center for Infection and Immunology Amsterdam (CINIMA); University of Amsterdam; Amsterdam The Netherlands
- STI Outpatient Clinic; Public Health Service of Amsterdam (GGD Amsterdam); Amsterdam The Netherlands
| | | |
Collapse
|
4
|
Backes D, Macià F, Bonetti S, Kukreja R, Ohldag H, Kent AD. Direct Observation of a Localized Magnetic Soliton in a Spin-Transfer Nanocontact. Phys Rev Lett 2015; 115:127205. [PMID: 26431016 DOI: 10.1103/physrevlett.115.127205] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/03/2015] [Indexed: 06/05/2023]
Abstract
We report the direct observation of a localized magnetic soliton in a spin-transfer nanocontact using scanning transmission x-ray microscopy. Experiments are conducted on a lithographically defined 150 nm diameter nanocontact to an ultrathin ferromagnetic multilayer with perpendicular magnetic anisotropy. Element-resolved x-ray magnetic circular dichroism images show an abrupt onset of a magnetic soliton excitation localized beneath the nanocontact at a threshold current. However, the amplitude of the excitation ≃25° at the contact center is far less than that predicted (⪅180°), showing that the spin dynamics is not described by existing models.
Collapse
Affiliation(s)
- D Backes
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA
| | - F Macià
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA
- Grup de Magnetisme, Departament de Física Fonamental, Universitat de Barcelona, Barcelona 08028, Spain
| | - S Bonetti
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sandhill Road, Menlo Park, California 94025, USA
- Department of Physics, Stanford University, Stanford, California 94305, USA
| | - R Kukreja
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sandhill Road, Menlo Park, California 94025, USA
- Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
| | - H Ohldag
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA
- Stanford Synchrotron Radiation Laboratory, 2575 Sandhill Road, Menlo Park, California 94025, USA
| | - A D Kent
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA
| |
Collapse
|
5
|
Kukreja R, Bonetti S, Chen Z, Backes D, Acremann Y, Katine JA, Kent AD, Dürr HA, Ohldag H, Stöhr J. X-ray Detection of Transient Magnetic Moments Induced by a Spin Current in Cu. Phys Rev Lett 2015; 115:096601. [PMID: 26371670 DOI: 10.1103/physrevlett.115.096601] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/09/2015] [Indexed: 06/05/2023]
Abstract
We have used a MHz lock-in x-ray spectromicroscopy technique to directly detect changes in magnetic moment of Cu due to spin injection from an adjacent Co layer. The elemental and chemical specificity of x rays allows us to distinguish two spin current induced effects. We detect the creation of transient magnetic moments of 3×10^{-5}μ_{B} on Cu atoms within the bulk of the 28 nm thick Cu film due to spin accumulation. The moment value is compared to predictions by Mott's two current model. We also observe that the hybridization induced existing magnetic moments at the Cu interface atoms are transiently increased by about 10% or 4×10^{-3}μ_{B} per atom. This reveals the dominance of spin-torque alignment over Joule heat induced disorder of the interfacial Cu moments during current flow.
Collapse
Affiliation(s)
- R Kukreja
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
- Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Stanford, California 94305, USA
| | - S Bonetti
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
- Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, California 94305, USA
| | - Z Chen
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
- Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, California 94305, USA
| | - D Backes
- Physics Department, New York University, 4 Washington Place, New York, New York 10003, USA
| | - Y Acremann
- Laboratorium für Festkörperphysik, ETH Zürich, HPF C 5, Otto-Stern-Weg 1, Zürich 8093, Switzerland
| | - J A Katine
- HGST, a Western Digital Company, 3403 Yerba Buena Road, San Jose, California 95135, USA
| | - A D Kent
- Physics Department, New York University, 4 Washington Place, New York, New York 10003, USA
| | - H A Dürr
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
| | - H Ohldag
- Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
| | - J Stöhr
- Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
| |
Collapse
|
6
|
Wohlgenannt M, Flatté ME, Harmon NJ, Wang F, Kent AD, Macià F. Singlet-to-triplet interconversion using hyperfine as well as ferromagnetic fringe fields. Philos Trans A Math Phys Eng Sci 2015; 373:rsta.2014.0326. [PMID: 25987575 PMCID: PMC4455723 DOI: 10.1098/rsta.2014.0326] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 04/01/2015] [Indexed: 06/04/2023]
Abstract
Until recently the important role that spin-physics ('spintronics') plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several per cent and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity/electroluminescence values in the absence of an applied field. We briefly review a model based on fringe-field-induced polaron-pair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance and magnetoelectroluminescence.
Collapse
Affiliation(s)
- M Wohlgenannt
- Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242, USA
| | - M E Flatté
- Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242, USA
| | - N J Harmon
- Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242, USA
| | - F Wang
- Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242, USA
| | - A D Kent
- Department of Physics, New York University, 4 Washington Place, New York, NY 10003, USA
| | - F Macià
- Department of Physics, New York University, 4 Washington Place, New York, NY 10003, USA
| |
Collapse
|
7
|
Abstract
Spin torque nano-oscillators (STNO) are nanoscale devices that can convert a direct current into short wavelength spin wave excitations in a ferromagnetic layer. We show that arrays of STNO can be used to create directional spin wave radiation similarly to electromagnetic antennas. Combining STNO excitations with planar spin waves also creates interference patterns. We show that these interference patterns are static and have information on the wavelength and phase of the spin waves emitted from the STNO. We describe a means of actively controlling spin wave radiation patterns with the direct current flowing through STNO, which is useful in on-chip communication and information processing and could be a promising technique for studying short wavelength spin waves in different materials.
Collapse
|
8
|
Gopman DB, Liu H, Kent AD. A flux-coupled ac/dc magnetizing device. Rev Sci Instrum 2013; 84:065101. [PMID: 23822372 DOI: 10.1063/1.4807696] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance.
Collapse
Affiliation(s)
- D B Gopman
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA.
| | | | | |
Collapse
|
9
|
Subedi P, Vélez S, Macià F, Li S, Sarachik MP, Tejada J, Mukherjee S, Christou G, Kent AD. Onset of a propagating self-sustained spin reversal front in a magnetic system. Phys Rev Lett 2013; 110:207203. [PMID: 25167444 DOI: 10.1103/physrevlett.110.207203] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/08/2013] [Indexed: 06/03/2023]
Abstract
The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical instability that ignites self-sustained rapid relaxation along a deflagration front that propagates at a constant subsonic speed. Using a trigger heat pulse and transverse and longitudinal magnetic fields, we investigate and control the crossover between thermally driven magnetic relaxation and magnetic deflagration in single crystals of Mn(12)-acetate.
Collapse
Affiliation(s)
- P Subedi
- Department of Physics, New York University, New York, New York 10003, USA
| | - S Vélez
- Grup de Magnetisme, Departament de Física Fonamental, Universitat de Barcelona, Barcelona 08028, Spain
| | - F Macià
- Department of Physics, New York University, New York, New York 10003, USA
| | - S Li
- Department of Physics, City College of New York, CUNY, New York, New York 10031, USA
| | - M P Sarachik
- Department of Physics, City College of New York, CUNY, New York, New York 10031, USA
| | - J Tejada
- Grup de Magnetisme, Departament de Física Fonamental, Universitat de Barcelona, Barcelona 08028, Spain
| | - S Mukherjee
- Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA
| | - G Christou
- Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA
| | - A D Kent
- Department of Physics, New York University, New York, New York 10003, USA
| |
Collapse
|
10
|
Gopman DB, Bedau D, Kent AD. A digitally configurable measurement platform using audio cards for high-resolution electronic transport studies. Rev Sci Instrum 2012; 83:054701. [PMID: 22667635 DOI: 10.1063/1.4709498] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
We report on a software-defined digitally configurable measurement platform for determining electronic transport properties in nanostructures with small readout signals. By using a high-resolution audio analog-to-digital/digital-to-analog converter in a digitally compensated bridge configuration we significantly increase the measurement speed compared to established techniques and simultaneously acquire large and small signal characteristics. We characterize the performance (16 bit resolution, 100 dB dynamic range at 192 kS/s) and demonstrate the application of this measurement platform for studying the transport properties of spin-valve nanopillars, a two-terminal device that exhibits giant magnetoresistance and whose resistance can be switched between two levels by applied magnetic fields and by currents applied by the audio card. The high resolution and fast sampling capability permits rapid acquisition of deep statistics on the switching of a spin-valve nanopillar and reduces the time to acquire the basic properties of the device - a state-diagram showing the magnetic configurations as function of applied current and magnetic field - by orders of magnitude.
Collapse
Affiliation(s)
- D B Gopman
- Department of Physics, New York University, New York, New York 10003, USA
| | | | | |
Collapse
|
11
|
Quddusi HM, Ramsey CM, Gonzalez-Pons JC, Henderson JJ, del Barco E, de Loubens G, Kent AD. On-chip integration of high-frequency electron paramagnetic resonance spectroscopy and Hall-effect magnetometry. Rev Sci Instrum 2008; 79:074703. [PMID: 18681725 DOI: 10.1063/1.2957616] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
A sensor that integrates high-sensitivity micro-Hall effect magnetometry and high-frequency electron paramagnetic resonance spectroscopy capabilities on a single semiconductor chip is presented. The Hall-effect magnetometer (HEM) was fabricated from a two-dimensional electron gas GaAsAlGaAs heterostructure in the form of a cross, with a 50 x 50 microm2 sensing area. A high-frequency microstrip resonator is coupled with two small gaps to a transmission line with a 50 Omega impedance. Different resonator lengths are used to obtain quasi-TEM fundamental resonant modes in the frequency range 10-30 GHz. The resonator is positioned on top of the active area of the HEM, where the magnetic field of the fundamental mode is largest, thus optimizing the conversion of microwave power into magnetic field at the sample position. The two gaps coupling the resonator and transmission lines are engineered differently--the gap to the microwave source is designed to optimize the loaded quality factor of the resonator (Q<or=150) while the gap for the transmitted signal is larger. This latter gap minimizes losses and prevents distortion of the resonance while enabling measurement of the transmitted signal. The large filling factor of the resonator permits sensitivities comparable to that of high-quality factor resonant cavities. The integrated sensor enables measurement of the magnetization response of micron scale samples upon application of microwave fields. In particular, the combined measurement of the magnetization change and the microwave power under cw microwave irradiation of single crystal of molecular magnets is used to determine of the energy relaxation time of the molecular spin states. In addition, real-time measurements of the magnetization dynamics upon application of fast microwave pulses are demonstrated.
Collapse
Affiliation(s)
- H M Quddusi
- Physics Department, University of Central Florida, Orlando, Florida 32816-2385, USA
| | | | | | | | | | | | | |
Collapse
|
12
|
de Loubens G, Kent AD, Krymov V, Gerfen GJ, Beedle CC, Hendrickson DN. High frequency EPR on dilute solutions of the single molecule magnet Ni(4). J Appl Phys 2008; 103:7B910-7B9103. [PMID: 19479002 PMCID: PMC2685210 DOI: 10.1063/1.2834447] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/13/2007] [Accepted: 10/30/2007] [Indexed: 05/21/2023]
Abstract
Dilute frozen solutions of the single molecule magnet Ni(4) (S=4) have been studied using 130 GHz electron paramagnetic resonance (EPR). Despite the random orientation of the molecules, well defined EPR absorption peaks are observed due to the strong variation of the splittings between the different spin states on magnetic field. Temperature dependent studies above 4 K and comparison with simulations enable identification of the spin transitions and determination of the Hamiltonian parameters. The latter are found to be close to those of Ni(4) single crystals. No echo was detected from Ni(4) in pulsed experiments, which sets an upper bound of about 50 ns on the spin coherence time.
Collapse
|
13
|
Kent AD, Jones SE, Yannarell AC, Graham JM, Lauster GH, Kratz TK, Triplett EW. Annual patterns in bacterioplankton community variability in a humic lake. Microb Ecol 2004; 48:550-560. [PMID: 15696388 DOI: 10.1007/s00248-004-0244-y] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/03/2003] [Accepted: 02/27/2004] [Indexed: 05/24/2023]
Abstract
Bacterioplankton community composition (BCC) was monitored in a shallow humic lake in northern Wisconsin, USA, over 3 years using automated ribosomal intergenic spacer analysis (ARISA). Comparison of ARISA profiles of bacterial communities over time indicated that BCC was highly variable on a seasonal and annual scale. Nonmetric multidimensional scaling (MDS) analysis indicated little similarity in BCC from year to year. Nevertheless, annual patterns in bacterioplankton community diversity were observed. Trends in bacterioplankton community diversity were correlated to annual patterns in community succession observed for phytoplankton and zooplankton populations, consistent with the notion that food web interactions affect bacterioplankton community structure in this humic lake. Bacterioplankton communities experience a dramatic drop in richness and abundance each year in early summer, concurrent with an increase in the abundance of both mixotrophic and heterotrophic flagellates. A second drop in richness, but not abundance, is observed each year in late summer, coinciding with an intense bloom of the nonphagotrophic dinoflagellate Peridinium limbatum. A relationship between bacterial community composition, size, and abundance and the population dynamics of Daphnia was also observed. The noted synchrony between these major population and species shifts suggests that linkages across trophic levels play a role in determining the annual time course of events for the microbial and metazoan components of the plankton.
Collapse
Affiliation(s)
- A D Kent
- Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706, USA
| | | | | | | | | | | | | |
Collapse
|
14
|
Graham JM, Kent AD, Lauster GH, Yannarell AC, Graham LE, Triplett EW. Seasonal dynamics of phytoplankton and planktonic protozoan communities in a northern temperate humic lake: diversity in a dinoflagellate dominated system. Microb Ecol 2004; 48:528-40. [PMID: 15696386 DOI: 10.1007/s00248-004-0223-3] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/07/2003] [Accepted: 04/06/2004] [Indexed: 05/03/2023]
Abstract
Species diversity and richness, and seasonal population dynamics of phytoplankton, planktonic protozoa, and bacterioplankton sampled from the epilimnion of Crystal Bog in 2000, were examined in order to test the hypothesis that these groups' diversity and abundance patterns might be linked. Crystal Bog, a humic lake in Vilas County, Wisconsin, is part of the North Temperate Lakes Long-Term Ecological Research Site. Phytoplankton and planktonic protozoa were identified and enumerated in a settling chamber with an inverted microscope. Bacterial cells were enumerated with the use of fluorescence 4', 6'-diamidino-2-phenylindole (DAPI)-staining procedures, and automated ribosomal intergenic spacer analysis (ARISA) was used to assess bacterioplankton diversity. Bacterial cell counts showed little seasonal variation and averaged 2.6 x 10(6) cells/mL over the ice-free season. Phytoplankton and planktonic protozoan numbers varied by up to two orders of magnitude and were most numerous in late spring and summer. Dinoflagellates largely dominated Crystal Bog throughout the ice-free period, specifically Peridiniopsis quadridens in the spring, Peridinium limbatum in summer, and Gymnodinium fuscum and P. quadridens in fall. Brief blooms of Cryptomonas, Dinobryon, and Synura occurred between periods of dinoflagellate domination. The dominant dinoflagellate, Peridinium limbatum, was calculated to have a growth rate of 0.065 day(-1) and a doubling time of 10.7 days. Heterotrophic nanoflagellates (HNFs) were a consistent component of the planktonic protozoa; seasonal patterns were determined for three genera of HNFs (Monosiga, Bicosoeca, and Desmarella moniliformis). Three genera of ciliates (Coleps, Strobilidium, and Strombidium) comprised the greater part of the planktonic protozoa in Crystal Bog. The number of species of planktonic protozoa was too low to calculate a diversity index. Shannon-Weaver diversity indices for phytoplankton and bacterioplankton in the epilimnion followed very similar seasonal patterns in this lake, supporting the hypothesis that in freshwaters, diversity patterns of these groups are linked.
Collapse
Affiliation(s)
- J M Graham
- Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706, USA.
| | | | | | | | | | | |
Collapse
|
15
|
Ozyilmaz B, Kent AD, Sun JZ, Rooks MJ, Koch RH. Current-induced excitations in single cobalt ferromagnetic layer nanopillars. Phys Rev Lett 2004; 93:176604. [PMID: 15525101 DOI: 10.1103/physrevlett.93.176604] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/08/2004] [Indexed: 05/24/2023]
Abstract
Current-induced excitations in Cu/Co/Cu single ferromagnetic layer nanopillars ( approximately 50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current-induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin-wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.
Collapse
Affiliation(s)
- B Ozyilmaz
- Department of Physics, New York University, New York, New York 10003, USA
| | | | | | | | | |
Collapse
|
16
|
del Barco E, Kent AD, Yang EC, Hendrickson DN. Quantum superposition of high spin states in the single molecule magnet Ni4. Phys Rev Lett 2004; 93:157202. [PMID: 15524933 DOI: 10.1103/physrevlett.93.157202] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2004] [Indexed: 05/24/2023]
Abstract
Quantum tunneling of the magnetization in a single molecule magnet has been studied in experiments that combine microwave spectroscopy with high sensitivity magnetic measurements. By monitoring spin-state populations in the presence of microwave radiation, the energy splittings between low lying superpositions of high-spin states of single molecule magnet Ni4 (S=4) have been measured. Absorption linewidths give an upper bound on the rate of decoherence. Pulsed microwave experiments provide a measure of energy relaxation time, which is found to increase with frequency.
Collapse
Affiliation(s)
- E del Barco
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA
| | | | | | | |
Collapse
|
17
|
Yannarell AC, Kent AD, Lauster GH, Kratz TK, Triplett EW. Temporal patterns in bacterial communities in three temperate lakes of different trophic status. Microb Ecol 2003; 46:391-405. [PMID: 12904915 DOI: 10.1007/s00248-003-1008-9] [Citation(s) in RCA: 113] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/16/2003] [Accepted: 03/21/2003] [Indexed: 05/24/2023]
Abstract
Despite considerable attention in recent years, the composition and dynamics of lake bacterial communities over annual time scales are poorly understood. This study used automated ribosomal intergenic spacer analysis (ARISA) to explore the patterns of change in lake bacterial communities in three temperate lakes over 2 consecutive years. The study lakes included a humic lake, an oligotrophic lake, and a eutrophic lake, and the epilimnetic bacterial communities were sampled every 2 weeks. The patterns of change in bacterial communities indicated that seasonal forces were important in structuring the behavior of the bacterial communities in each lake. All three lakes had relatively stable community composition in spring and fall, but summer changes were dramatic. Summertime variability was often characterized by recurrent drops in bacterial diversity. Specific ARISA fragments derived from these lakes were not constant among lakes or from year to year, and those fragments that did recur in lakes in different years did not exhibit the same seasonal pattern of recurrence. Nonetheless, seasonal patterns observed in 2000 were fairly successful predictors of the rate of change in bacterial communities and in the degree of autocorrelation of bacterial communities in 2001. Thus, seasonal forces may be important structuring elements of these systems as a whole even if they are uncoupled from the dynamics of the individual system components.
Collapse
Affiliation(s)
- A C Yannarell
- Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706, USA
| | | | | | | | | |
Collapse
|
18
|
Ozyilmaz B, Kent AD, Monsma D, Sun JZ, Rooks MJ, Koch RH. Current-induced magnetization reversal in high magnetic fields in Co/Cu/Co nanopillars. Phys Rev Lett 2003; 91:067203. [PMID: 12935107 DOI: 10.1103/physrevlett.91.067203] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/17/2002] [Indexed: 05/24/2023]
Abstract
Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (approximately 100 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At 4.2 K an abrupt and hysteretic increase in resistance is observed at high current densities for one polarity of the current, comparable to the giant magnetoresistance effect observed at low fields. A micromagnetic model that includes a spin-transfer torque suggests that the current induces a complete reversal of the thin Co layer to alignment antiparallel to the applied field--that is, to a state of maximum magnetic energy.
Collapse
Affiliation(s)
- B Ozyilmaz
- Department of Physics, New York University, New York, New York 10003, USA
| | | | | | | | | | | |
Collapse
|
19
|
del Barco E, Kent AD, Rumberger EM, Hendrickson DN, Christou G. Symmetry of magnetic quantum tunneling in single molecule magnet Mn12-acetate. Phys Rev Lett 2003; 91:047203. [PMID: 12906693 DOI: 10.1103/physrevlett.91.047203] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2003] [Indexed: 05/24/2023]
Abstract
The symmetry of magnetic quantum tunneling has been studied in the prototype single molecule magnet Mn12-acetate using a micro-Hall effect magnetometer and superconducting high field vector magnet system. An average crystal fourfold symmetry is shown to be due to local molecular environments of twofold symmetry that are rotated by 90 degrees with respect to one another, confirming that disorder which lowers the molecule symmetry is as important to magnetic quantum tunneling. We have studied a subset of these lower (twofold) site symmetry molecules and present evidence for a Berry phase effect consistent with a local twofold symmetry.
Collapse
Affiliation(s)
- E del Barco
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA.
| | | | | | | | | |
Collapse
|
20
|
Bokacheva L, Kent AD, Walters MA. Crossover between thermally assisted and pure quantum tunneling in molecular magnet Mn12-acetate. Phys Rev Lett 2000; 85:4803-4806. [PMID: 11082656 DOI: 10.1103/physrevlett.85.4803] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/19/2000] [Indexed: 05/23/2023]
Abstract
The crossover between thermally assisted and pure quantum tunneling has been studied in single crystals of high spin (S = 10) uniaxial molecular magnet Mn12 using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow ( approximately 0. 1 K) or broad ( approximately 1 K) temperature interval depending on the magnitude of the field transverse to the anisotropy axis.
Collapse
Affiliation(s)
- L Bokacheva
- Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA
| | | | | |
Collapse
|
21
|
Kent AD, Wojtasiak ML, Robleto EA, Triplett EW. A transposable partitioning locus used to stabilize plasmid-borne hydrogen oxidation and trifolitoxin production genes in a Sinorhizobium strain. Appl Environ Microbiol 1998; 64:1657-62. [PMID: 9572932 PMCID: PMC106211 DOI: 10.1128/aem.64.5.1657-1662.1998] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Improved nitrogen-fixing inoculum strains for leguminous crops must be able to effectively compete with indigenous strains for nodulation, enhance legume productivity compared to the productivity obtained with indigenous strains, and maintain stable expression of any added genes in the absence of selection pressure. We constructed a transposable element containing the tfx region for expression of increased nodulation competitiveness and the par locus for plasmid stability. The transposon was inserted into tetA of pHU52, a broad-host-range plasmid conferring the H2 uptake phenotype. The resulting plasmid, pHUTFXPAR, conferred the plasmid stability, trifolitoxin production, and H2 uptake phenotypes in the broad-host-range organism Sinorhizobium sp. strain ANU280. The broad applications of a transposon conferring plasmid stability are discussed.
Collapse
Affiliation(s)
- A D Kent
- Department of Bacteriology, University of Wisconsin-Madison 53706, USA
| | | | | | | |
Collapse
|
22
|
Welch RA, Bauer ME, Kent AD, Leeds JA, Moayeri M, Regassa LB, Swenson DL. Battling against host phagocytes: the wherefore of the RTX family of toxins? Infect Agents Dis 1995; 4:254-72. [PMID: 8665089] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The RTX family of bacterial exotoxins is a group of related cytolytic proteins produced by a wide variety of gram-negative human and animal pathogens. While diverse in their associated diseases and in their target cell specificities, there remain several themes common to RTX toxins, including genetic organization, structural and functional features, and effects on target cells. In this review, we summarize and discuss the genetics, regulation, epidemiology, structure/function relationships, and in vivo and in vitro activities of the best characterized RTX toxins, and speculate on their roles in pathogenesis and their use in immunotherapy.
Collapse
Affiliation(s)
- R A Welch
- Department of Medical Microbiology and Immunology, University of Wisconsin-Madison Medical School 53706, USA
| | | | | | | | | | | | | |
Collapse
|
23
|
|
24
|
Kent AD, Shaw TM, von Molnár S, Awschalom DD. Growth of High Aspect Ratio Nanometer-Scale Magnets with Chemical Vapor Deposition and Scanning Tunneling Microscopy. Science 1993; 262:1249-52. [PMID: 17772647 DOI: 10.1126/science.262.5137.1249] [Citation(s) in RCA: 108] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
A combination of chemical vapor deposition and scanning tunneling microscopy techniques have been used to produce nanometer-scale, iron-containing deposits with high aspect ratios from an iron pentacarbonyl precursor both on a substrate and on the tunneling tip itself. The structure and composition of the resulting nanodeposits were determined by transmission electron microscopy and high spatial resolution Auger electron spectroscopy. Either polycrystalline, relatively pure, body-centered-cubic iron or disordered carbon-rich material can be deposited, depending on the bias conditions of the tip sample junction and the precursor pressure. Two mechanisms of decomposition are inferred from the growth phenomenology.
Collapse
|
25
|
Renner C, Kent AD, Niedermann P, Fischer O, Lévy F. Scanning tunneling spectroscopy of a vortex core from the clean to the dirty limit. Phys Rev Lett 1991; 67:1650-1652. [PMID: 10044209 DOI: 10.1103/physrevlett.67.1650] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
|
26
|
Triscone J, Fischer O, Brunner O, Antognazza L, Kent AD, Karkut MG. YBa2CU3O7/PrBa2Cu3O7 superlattices: Properties of Ultrathin superconducting layers separated by insulating layers. Phys Rev Lett 1990; 64:804-807. [PMID: 10042082 DOI: 10.1103/physrevlett.64.804] [Citation(s) in RCA: 72] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
|
27
|
Kent AD, Maggio-Aprile I, Niedermann P, Fischer O. Direct measurements of the effects of inhomogeneities on the normal-state transport properties of YBa2Cu3O7-x thin films. Phys Rev B Condens Matter 1989; 39:12363-12366. [PMID: 9948094 DOI: 10.1103/physrevb.39.12363] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
|
28
|
Oh B, Char K, Kent AD, Naito M, Beasley MR, Geballe TH, Hammond RH, Kapitulnik A, Graybeal JM. Upper critical field, fluctuation conductivity, and dimensionality of YBa2Cu3O7-x. Phys Rev B Condens Matter 1988; 37:7861-7864. [PMID: 9944093 DOI: 10.1103/physrevb.37.7861] [Citation(s) in RCA: 182] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/11/2023]
|
29
|
Kent AD, Kapitulnik A, Geballe TH. Hc2 spectroscopy of geometrical effects in La-S films. Phys Rev B Condens Matter 1987; 36:8827-8830. [PMID: 9942720 DOI: 10.1103/physrevb.36.8827] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/11/2023]
|
30
|
Sun JZ, Webb DJ, Naito M, Char K, Hahn MR, Hsu JW, Kent AD, Mitzi DB, Oh B, Beasley MR, Geballe TH, Hammond RH, Kapitulnik A. Superconductivity and magnetism in the high-Tc superconductor YBaCuO. Phys Rev Lett 1987; 58:1574-1576. [PMID: 10034474 DOI: 10.1103/physrevlett.58.1574] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
|