Papers by Heidemarie Schmidt
Frontiers in neuroscience, 2015
Memristive devices are popular among neuromorphic engineers for their ability to emulate forms of... more Memristive devices are popular among neuromorphic engineers for their ability to emulate forms of spike-driven synaptic plasticity by applying specific voltage and current waveforms at their two terminals. In this paper, we investigate spike-timing dependent plasticity (STDP) with a single pairing of one presynaptic voltage spike and one post-synaptic voltage spike in a BiFeO3 memristive device. In most memristive materials the learning window is primarily a function of the material characteristics and not of the applied waveform. In contrast, we show that the analog resistive switching of the developed artificial synapses allows to adjust the learning time constant of the STDP function from 25 ms to 125 μs via the duration of applied voltage spikes. Also, as the induced weight change may degrade, we investigate the remanence of the resistance change for several hours after analog resistive switching, thus emulating the processes expected in biological synapses. As the power consump...
Frontiers in Neuroscience, 2015
Memristive devices present a new device technology allowing for the realization of compact nonvol... more Memristive devices present a new device technology allowing for the realization of compact nonvolatile memories. Some of them are already in the process of industrialization. Additionally, they exhibit complex multilevel and plastic behaviors, which make them good candidates for the implementation of artificial synapses in neuromorphic engineering. However, memristive effects rely on diverse physical mechanisms, and their plastic behaviors differ strongly from one technology to another. Here, we present measurements performed on different memristive devices and the opportunities that they provide. We show that they can be used to implement different learning rules whose properties emerge directly from device physics: real time or accelerated operation, deterministic or stochastic behavior, long term or short term plasticity. We then discuss how such devices might be integrated into a complete architecture. These results highlight that there is no unique way to exploit memristive devices in neuromorphic systems. Understanding and embracing device physics is the key for their optimal use.
Physical Review Letters, 2008
We report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions wi... more We report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom ferromagnetic electrode and Co as a top ferromagnetic electrode prepared by pulsed laser deposition. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al 2 O 3 and tunnelled through the amorphous Al 2 O 3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.
ZnO films with different 3d transition metal doping (Ti, Co, Mn) were prepared by pulsed laser de... more ZnO films with different 3d transition metal doping (Ti, Co, Mn) were prepared by pulsed laser deposition. The occurrence of positive magnetoresistance (MR) was studied in dependence on temperature. Large positive MR was observed in Co-and Mn-doped ZnO films at 5 K. Zn 0.98 Mn 0.02 O films show a positive MR up to ∼ 31% at 5 K. The positive MR for Co-and Mn-doped ZnO films decreases drastically from 5 K to 50 K. Positive MR was also observed in Zn 0.9998 Ti 0.0002 O films at 290 K, while negative MR was observed at 5 K. Clear stripe-like magnetic domain pattern was observed by magnetic force microscopy for the Zn 0.9998 Ti 0.0002 O films, indicating the possible ferromagnetism at room temperature due to Ti 3+ -ions in a polarized spin state.
Thin Solid Films, 2008
We prepared conductive, polycrystalline or amorphous Cu 1.05 Cr 0.89 Mg 0.05 O 2 films on a-plane... more We prepared conductive, polycrystalline or amorphous Cu 1.05 Cr 0.89 Mg 0.05 O 2 films on a-plane sapphire substrates by pulsed laser deposition under different O 2 partial pressure and substrate temperature. Hall measurements were performed to study the majority carrier type in these films. Polycrystalline Cu 1.05 Cr 0.89 Mg 0.05 O 2 is n-type conducting at 290 K, while in amorphous Cu 1.05 Cr 0.89 Mg 0.05 O 2 the type of majority charge carriers changes from electrons to holes at around 270 K. Interestingly, the structure has little influence on the magnetic properties of the films. A clear antiferromagnetic to paramagnetic transition was observed in both polycrystalline and amorphous Cu 1.05 Cr 0.89 Mg 0.05 O 2 films at 25 K. Similar electrical properties to Cu 1.05 Cr 0.89 Mg 0.05 O 2 film were observed for Cu 0.96 Cr 0.95 Mg 0.05 Mn 0.04 O 2 in dependence on the structure, while only paramagnetic without antiferromagnetic ordering was observed down to 5 K. Large negative magnetoresistance of 27% at 20 K was observed at 6 T in amorphous Cu 1.05 Cr 0.89 Mg 0.05 O 2 film. 2. Experimental details Cu 1.05 Cr 0.89 Mg 0.05 O 2 and Cu 0.96 Cr 0.95 Mg 0.05 Mn 0.04 O 2 films were grown from CuCr 0.95 Mg 0.05 O 2 and CuCr 0.93 Mg 0.05 Mn 0.02 O 2 ceramic targets on 10 × 10 mm 2 a-plane ((110) plane)
Solid State Communications, 2006
The ferromagnetism in highly transparent and intrinsically n-type conducting zinc oxide doped wit... more The ferromagnetism in highly transparent and intrinsically n-type conducting zinc oxide doped with 3d transition metals (TM), is predicted to be defect mediated. We investigate the generation of deep defects in n-conducting 1 mm thick ZnO:TM films (TMZCo, Mn, Ti) with a nominal TM content of 0.02, 0.20 and 2.00 at.% grown by pulsed laser deposition on a-plane sapphire substrates using deep level transient spectroscopy. We find that a defect level is generated, independent of the TM content, located 0.31 and 0.27 eV below the conduction band minimum of ZnO:Mn and ZnO:Ti, respectively. Different defect levels are generated in dependence on the Co content in ZnO:Co. This work shows that an optimization of defect-related ferromagnetism in n-conducting ZnO:TM thin films will only be possible if the preparation sensitive formation of deep defects is controlled in the same time. q
Physical Review B, 2007
The magnetoresistance (MR) effect in Co-doped ZnO films prepared by pulsed laser deposition on a ... more The magnetoresistance (MR) effect in Co-doped ZnO films prepared by pulsed laser deposition on a -plane sapphire substrates with electron concentration at 5K ranging from 8.3×1017cm-3to9.9×1019cm-3 has been studied experimentally and theoretically. A large positive MR of 124% has been observed in the film with the lowest electron concentration of 8.3×1017cm-3 , while only a negative MR of -1.9% was
Physical Review B, 2006
The magnetotransport properties ͓magnetoresistance ͑MR͒ and Hall effect͔ of Co-doped ZnO films pr... more The magnetotransport properties ͓magnetoresistance ͑MR͒ and Hall effect͔ of Co-doped ZnO films prepared by pulsed laser deposition have been investigated around the metal-insulator transition ͑MIT͒ as a function of temperature ͑from 5 to 290 K͒ under a maximum magnetic field strength of 6 T. From the MR behavior measured at 5 K we conclude that the MIT occurs at the critical electron concentration n c Ϸ 4 ϫ 10 19 cm −3 . At 5 K we observed positive MR in the insulating regime ͑n Ͻ n c ͒ and negative MR in the metallic regime ͑n Ͼ n c ͒. Furthermore, in the transition regime of the MIT ͑n ϳ n c ͒ negative MR at low magnetic field and positive MR at high field was observed. We consider the critical electron concentration n c as an important material parameter because n c does not depend on film thickness or Co content. The anomalous Hall effect being of importance for future spintronic materials was only clearly observed in Co-doped ZnO with n Ͻ n c .
Journal of Physics D: Applied Physics, 2008
Nd and Mn were codoped into ZnO films which have been grown on a-plane sapphire substrates by pul... more Nd and Mn were codoped into ZnO films which have been grown on a-plane sapphire substrates by pulsed laser deposition with the thickness ranging between 46 and 971 nm. The room temperature resistivity of the codoped films is independent of film thickness. Large positive magnetoresistance and clear anomalous Hall effect were observed at 5 K. Ferromagnetism with clear hysteresis up to 290 K was observed. Codoping is suggested to be an efficient method to introduce energy levels in the ZnO band gap to mediate the electron spins of the magnetic doping ions.
Journal of Physics D: Applied Physics, 2006
... thin films Lars Hartmann, Qingyu Xu, Heidemarie Schmidt, Holger Hochmuth, Michael Lorenz,Chri... more ... thin films Lars Hartmann, Qingyu Xu, Heidemarie Schmidt, Holger Hochmuth, Michael Lorenz,Chris Sturm, Christoph Meinecke and Marius Grundmann ... Phys. Lett. 79 988 [3] Stamenov P, Venkatesan M, Dorneles LS, Maude D and Coey JMD 2006 J. Appl. Phys. 99 08M124 ...
Journal of Applied Physics, 2006
Zn0.90Co0.10O films of different thicknesses (689, 408, 355 nm) doped with 0.5 at. % Al were prep... more Zn0.90Co0.10O films of different thicknesses (689, 408, 355 nm) doped with 0.5 at. % Al were prepared by pulsed laser deposition on a-plane sapphire substrates. At 290 K the resistivity increases drastically with decreasing film thickness, while the electron concentration and mobility decrease. Magnetoresistance (MR) effects were measured in the temperature range of 5-290 K. At low temperature, the positive
Journal of Applied Physics, 2007
Magnetotransport measurements were performed on n-type conducting Co-doped ZnO and Mn-doped ZnO f... more Magnetotransport measurements were performed on n-type conducting Co-doped ZnO and Mn-doped ZnO films prepared by pulsed laser deposition on a-plane sapphire substrates, and positive magnetoresistance (MR) was observed at low temperature. The positive MR ...
Journal of Applied Physics, 2008
The transport properties of phosphorous-doped ZnO thin films, grown by pulsed-laser deposition on... more The transport properties of phosphorous-doped ZnO thin films, grown by pulsed-laser deposition on thermally pretreated hydrothermally grown ZnO single-crystal substrates, are reported. The ZnO:P thin films show very good morphological and structural properties as confirmed by atomic force microscopy ͑AFM͒, high resolution x-ray diffraction, and Rutherford backscattering ͑RBS͒ channeling. Steps of height c / 2 are visible in AFM investigations for all samples. For an oxygen partial pressure of 0.1 mbar, two-dimensional growth was found. RBS channeling of a ZnO:P film shows a minimum yield of 0.034 which is comparable to that of an annealed substrate ͑0.033͒. Hall effect measurements revealed that all films are n-type for the present growth conditions. Peak mobilities of 800 cm 2 / Vs have been observed around 70 K, in line with the high structural quality of the samples. Room-temperature mobility in ZnO:P is up to 170 cm 2 / Vs.
Applied Physics Letters, 2007
The Co valence state and the location of Co dopant atoms in ZnO:Co thin films revealing anomalous... more The Co valence state and the location of Co dopant atoms in ZnO:Co thin films revealing anomalous Hall effect below 50 K have been determined by electron energy-loss spectroscopy ͑EELS͒ measurements and atom location by channeling enhanced microanalysis ͑ALCHEMI͒, respectively. The method of ALCHEMI-EELS to investigate the dopant in materials was brought. It was used to determine that the intrinsic ferromagnetism in ZnO:Co thin films derives from the +2 Co which substitute for Zn atoms at Zn sites in the ZnO matrix. The divalent Co state in ZnO:Co has been unambiguously proven by modeling the corresponding EELS data.
Applied Physics Letters, 2008
ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N 2 atmos... more ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N 2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5 K at 60 kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect ͑AHE͒ was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.
Applied Physics Letters, 2008
We present superparamagnetic clusters of structurally highly disordered Co-Zn-O created by high f... more We present superparamagnetic clusters of structurally highly disordered Co-Zn-O created by high fluence Co ion implantation into ZnO (0001) single crystals at low temperatures. This secondary phase cannot be detected by common x-ray diffraction but is observed by highresolution transmission electron microscopy. In contrast to many other secondary phases in a ZnO matrix, it induces low-field anomalous Hall effect and is thus a candidate for magnetoelectronics applications.
Co-doped ZnO films with Co content above 7% have been grown by pulsed laser deposition on a-sapph... more Co-doped ZnO films with Co content above 7% have been grown by pulsed laser deposition on a-sapphire substrates. Temperature and magnetic field dependent magnetotransport measurements have been performed on samples with electron concentration n around the critical electron concentration nc~4.9×1019 cm-3, where the metal-insulator transition (MIT) occurs. At 5 K we observed positive magnetoresistance (MR) in the insulating range (nnc).
Journal of Applied Physics, 2012
BiFeO 3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO 2 /Si substrates with pulsed ... more BiFeO 3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO 2 /Si substrates with pulsed laser deposition using the same growth conditions, respectively. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interface. However, thin films on Pt/Ti/SiO 2 /Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO 3 thin films on Pt/sapphire and Pt/Ti/SiO 2 /Si substrates are discussed to understand the different resistive switching behaviors.
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Papers by Heidemarie Schmidt