Heidemarie Schmidt

Room temperature ferromagnetism in ZnO films due to defects

ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 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 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) 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.

Room temperature ferromagnetism in carbon-implanted ZnO

Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [H. Pan et al., Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e., ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.

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 carrier trapping and detrapping, which changes the depletion layer thickness at the Au/BiFeO3 interface.

Plasticity in memristive devices for spiking neural networks.

Memristive devices present a new device technology allowing for the realization of compact non-volatile 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.

Room temperature ferromagnetism in Mn-doped ZnO films mediated by acceptor defects

Mn-doped ZnO films with preferred c-axis growth orientation were prepared by pulsed laser deposition under N2 atmosphere on a-plane sapphire substrates. Large positive magnetoresistance amounting to 60% was observed at 5K. Clear anomalous Hall effect was observed at 20K. Ferromagnetism with Curie temperature higher than 290K has been observed, and a deep acceptor trap due to Zn vacancies with a thermal activation energy amounting to 0.815eV has been detected by deep-level transient spectroscopy. For comparison, only paramagnetism was observed in Mn-doped ZnO films with donor traps prepared under O2 atmosphere. Their results clearly demonstrate that the ferromagnetism in Mn-doped ZnO originates from the parallel alignment of magnetic moments mediated by acceptor defects.

Decisive role of oxygen vacancy in ferroelectric versus 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)] that shows that the existence of ferroelectricity is incompatible with the existence of a spontaneous magnetization in 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)] that shows that the existence of ferroelectricity is incompatible with the existence of a spontaneous magnetization in Mn-doped BaTiO3 thin films.

Deep acceptor states in ZnO single crystals

The authors report the observation of both acceptor- and donorlike defects in ZnO by deep level transient spectroscopy. The observation is facilitated by using a p-n junction allowing the injection of holes and electrons. The junction is realized by implanting a n-conducting ZnO wafer grown by pressurized melt growth with nitrogen ions. The authors found the commonly observed donorlike defects E1 and E3 and two acceptorlike defects A2 and A3, as well as a broad acceptorlike defect band. The thermal activation energies of A2 and A3, were determined to be about 150 and 280meV, respectively.

Spin Manipulation in Co-Doped ZnO

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 Al2O3 and tunnelled through the amorphous Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.

Defects in hydrothermally grown bulk ZnO

Hydrothermally grown bulk ZnO (Tokyo Denpa) was investigated using junction-capacitance spectroscopy on silver oxide Schottky contacts (barrier height of 1.20eV, ideality factor of 1.04). Two main shallow defects, T1 and T2, with thermal activation energies of 13 and 52meV, respectively, were identified. Two closely lying, deep defect levels E3∕E3′ at approximately 320meV below the conduction band were found in higher concentrations (mid-1014cm−3) than the shallow donors. 4K photoluminescence showed dominant emission from excitons bound to three neutral donors, aluminum, hydrogen, and an unassigned impurity, with donor binding energies close to the thermal activation energy of T2.

Pseudopotential band structures of rocksalt MgO, ZnO, and Mg1−xZnxO

The electronic properties of the rocksalt group-II oxides MgO and ZnO are investigated by means of the empirical pseudopotential method. Using a simple empty core model potential and experimentally known low-temperature transition energies of rocksalt MgO and wurtzite ZnO, we obtained cationic model potential parameters for Mg and Zn atoms, respectively. Making use of the transferability of ionic model potential parameters, we obtained one single set of anionic model potential parameters for the O atom. The electronic properties of the Mg1−xZnxO alloy system are investigated by means of the virtual crystal approximation for x<0.5 in the rocksalt phase.