24 research outputs found
The importance of hole concentration in establishing carrier-mediated ferromagnetism in Mn doped Ge
In the present work, we have prepared Mn-doped Ge using different annealing
approaches after Mn ion implantation, and obtained samples with hole
concentrations ranging from 10^18 to 2.1x10^20 cm^-3, the latter being the
highest reported so far. Based on the magnetotransport properties of Mn doped
Ge, we argue that the hole concentration is a decisive parameter in
establishing carrier-mediated ferromagnetism in magnetic Ge.Comment: 7 pages, 3 figure
Nonvolatile bipolar resistive switching in Au/BiFeO3/Pt
Nonvolatile bipolar resistive switching has been observed in an Au/BiFeO3/Pt
structure, where a Schottky contact and a quasi-Ohmic contact were formed at
the Au/BiFeO3 and BiFeO3/Pt interface, respectively. By changing the polarity
of the external voltage, the Au/BiFeO3/Pt is switched between two stable
resistance states without an electroforming process. The resistance ratio is
larger than two orders of magnitude. The resistive switching is understood by
the electric field - induced carriers trapping and detrapping, which changes
the depletion layer thickness at the Au/BiFeO3 interface.Comment: 23 pages, 4 figures, accepted by J. Appl. Phy
Decisive role of oxygen vacancy in ferroelectric vs. ferromagnetic Mn-doped BaTiO3 thin films
Single-phase perovskite 5 at.% Mn-doped and undoped polycrystalline BaTiO3
thin films have been grown under different oxygen partial pressures by pulsed
laser deposition on platinum-coated sapphire substrates. Ferroelectricity is
only observed for the Mn-doped and undoped BaTiO3 thin films grown under
relatively high oxygen partial pressure. Compared to undoped BaTiO3, Mn-doped
BaTiO3 reveals a low leakage current, increased dielectric loss, and a
decreased dielectric constant. Ferromagnetism is seen on Mn-doped BaTiO3 thin
films prepared under low oxygen partial pressure and is attributed to the
formation of bound magnetic polarons (BMPs). This BMP formation is enhanced by
oxygen vacancies. The present work confirms a theoretical work from C. Ederer
and N. Spaldin on ferroelectric perovskites [Nature Mat. 3, 849 (2004)] which
shows that the existence of ferroelectricity is incompatible with the existence
of a spontaneous magnetization in Mn-doped BaTiO3 thin films.Comment: 36 pages, 11 figure
Synaptic Plasticity in Memristive Artificial Synapses and Their Robustness Against Noisy Inputs
Emerging brain-inspired neuromorphic computing paradigms require devices that can emulate the complete functionality of biological synapses upon different neuronal activities in order to process big data flows in an efficient and cognitive manner while being robust against any noisy input. The memristive device has been proposed as a promising candidate for emulating artificial synapses due to their complex multilevel and dynamical plastic behaviors. In this work, we exploit ultrastable analog BiFeO3 (BFO)-based memristive devices for experimentally demonstrating that BFO artificial synapses support various long-term plastic functions, i.e., spike timing-dependent plasticity (STDP), cycle number-dependent plasticity (CNDP), and spiking rate-dependent plasticity (SRDP). The study on the impact of electrical stimuli in terms of pulse width and amplitude on STDP behaviors shows that their learning windows possess a wide range of timescale configurability, which can be a function of applied waveform. Moreover, beyond SRDP, the systematical and comparative study on generalized frequency-dependent plasticity (FDP) is carried out, which reveals for the first time that the ratio modulation between pulse width and pulse interval time within one spike cycle can result in both synaptic potentiation and depression effect within the same firing frequency. The impact of intrinsic neuronal noise on the STDP function of a single BFO artificial synapse can be neglected because thermal noise is two orders of magnitude smaller than the writing voltage and because the cycle-to-cycle variation of the current–voltage characteristics of a single BFO artificial synapses is small. However, extrinsic voltage fluctuations, e.g., in neural networks, cause a noisy input into the artificial synapses of the neural network. Here, the impact of extrinsic neuronal noise on the STDP function of a single BFO artificial synapse is analyzed in order to understand the robustness of plastic behavior in memristive artificial synapses against extrinsic noisy input
Reduced leakage current in BiFeO3 thin films with rectifying contacts
BiFeO3 thin films were grown on Pt/c-sapphire substrates by pulsed laser
deposition with different growth rates. With increasing growth rate the leakage
current is decreased and the conduction mechanism changes from bulk-limited
Poole-Frenkel emission to interface-limited Schottky emission. In the present
work, we show that only the growth rate of the BiFeO3 films close to the metal
contacts has to be increased in order to reduce the leakage current and to
observe saturated polarization-electric field hysteresis loops.Comment: 15 pages, 4 figures, accepted by Appl. Phys. Let
Disturbing-free determination of yeast concentration in DI water and in glucose using impedance biochips
Deionized water and glucose without yeast and with yeast (Saccharomyces cerevisiae) of
optical density OD600 that ranges from 4 to 16 has been put in the ring electrode region of six
different types of impedance biochips and impedance has been measured in dependence on the
added volume (20, 21, 22, 23, 24, 25 µL). The measured impedance of two out of the six types of
biochips is strongly sensitive to the addition of both liquid without yeast and liquid with yeast and
modelled impedance reveals a linear relationship between the impedance model parameters and
yeast concentration. The presented biochips allow for continuous impedance measurements without
interrupting the cultivation of the yeast. A multiparameter fit of the impedance model parameters
allows for determining the concentration of yeast (cy) in the range from cy = 3.3 × 107
to cy = 17 ×
107
cells/mL. This work shows that independent on the liquid, i.e., DI water or glucose, the impedance
model parameters of the two most sensitive types of biochips with liquid without yeast and with
liquid with yeast are clearly distinguishable for the two most sensitive types of biochips
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Towards Bacteria Counting in DI Water of Several Microliters or Growing Suspension Using Impedance Biochips
We counted bacterial cells of E. coli strain K12 in several-microliter DI water or in several-microliter PBS in the low optical density (OD) range (OD = 0.05–1.08) in contact with the surface of Si-based impedance biochips with ring electrodes by impedance measurements. The multiparameter fit of the impedance data allowed calibration of the impedance data with the concentration cb of the E. coli cells in the range of cb = 0.06 to 1.26 × 109 cells/mL. The results showed that for E. coli in DI water and in PBS, the modelled impedance parameters depend linearly on the concentration of cells in the range of cb = 0.06 to 1.26 × 109 cells/mL, whereas the OD, which was independently measured with a spectrophotometer, was only linearly dependent on the concentration of the E. coli cells in the range of cb = 0.06 to 0.50 × 109 cells/mL
Engineering interface-type resistive switching in BiFeO3 thin film switches by Ti implantation of bottom electrodes
BiFeO3 based MIM structures with Ti-implanted Pt bottom electrodes and Au top electrodes have been fabricated on Sapphire substrates. The resulting metal-insulator-metal (MIM) structures show bipolar resistive switching without an electroforming process. It is evidenced that during the BiFeO3 thin film growth Ti diffuses into the BiFeO3 layer. The diffused Ti effectively traps and releases oxygen vacancies and consequently stabilizes the resistive switching in BiFeO3 MIM structures. Therefore, using Ti implantation of the bottom electrode, the retention performance can be greatly improved with increasing Ti fluence. For the used raster-scanned Ti implantation the lateral Ti distribution is not homogeneous enough and endurance slightly degrades with Ti fluence. The local resistive switching investigated by current sensing atomic force microscopy suggests the capability of down-scaling the resistive switching cell to one BiFeO3 grain size by local Ti implantation of the bottom electrode
Synaptic Plasticity in Memristive Artificial Synapses and Their Robustness Against Noisy Inputs
Emerging brain-inspired neuromorphic computing paradigms require devices that can emulate the complete functionality of biological synapses upon different neuronal activities in order to process big data flows in an efficient and cognitive manner while being robust against any noisy input. The memristive device has been proposed as a promising candidate for emulating artificial synapses due to their complex multilevel and dynamical plastic behaviors. In this work, we exploit ultrastable analog BiFeO 3 (BFO)-based memristive devices for experimentally demonstrating that BFO artificial synapses support various long-term plastic functions, i.e., spike timing-dependent plasticity (STDP), cycle number-dependent plasticity (CNDP), and spiking rate-dependent plasticity (SRDP). The study on the impact of electrical stimuli in terms of pulse width and amplitude on STDP behaviors shows that their learning windows possess a wide range of timescale configurability, which can be a function of applied waveform. Moreover, beyond SRDP, the systematical and comparative study on generalized frequency-dependent plasticity (FDP) is carried out, which reveals for the first time that the ratio modulation between pulse width and pulse interval time within one spike cycle can result in both synaptic potentiation and depression effect within the same firing frequency. The impact of intrinsic neuronal noise on the STDP function of a single BFO artificial synapse can be neglected because thermal noise is two orders of magnitude smaller than the writing voltage and because the cycle-to-cycle variation of the current–voltage characteristics of a single BFO artificial synapses is small. However, extrinsic voltage fluctuations, e.g., in neural networks, cause a noisy input into the artificial synapses of the neural network. Here, the impact of extrinsic neuronal noise on the STDP function of a single BFO artificial synapse is analyzed in order to understand the robustness of plastic behavior in memristive artificial synapses against extrinsic noisy input