Hybrid normal metal/ferromagnetic nanojunctions for domain wall tracking

Hybrid normal metal/ferromagnetic, gold/permalloy (Au/Py), nanojunctions are used to investigate magnetoresistance effects and track magnetization spatial distribution in L-shaped Py nanostructures. Transversal and longitudinal resistances are measured and compared for both straight and 90° corner sections of the Py nanostructure. Our results demonstrate that the absolute change in resistance is larger in the case of longitudinal measurements. However, due to the small background resistance, the relative change in the transversal resistance along the straight section is several orders of magnitude larger than the analogous longitudinal variation. These results prove that hybrid nanojunctions represent a significant improvement with respect to previously studied all-ferromagnetic crosses, as they also reduce the pinning potential at the junction and allow probing the magnetization locally. In addition, unusual metastable states with longitudinal domain walls along Py straight sections are observed. Micromagnetic simulations in combination with a magnetotransport model allow interpretation of the results and identification of the observed transitions

Magnetic scanning gate microscopy of CoFeB lateral spin valve

Devices comprised of CoFeB nanostructures with perpendicular magnetic anisotropy and non-magnetic Ta channel were operated in thermal lateral spin valve (LSV) mode and studied by magnetotransport measurements and magnetic scanning gate microscopy (SGM). Due to the short spin diffusion length of Ta, the spin diffusion signal was suppressed, allowing the study of the contribution from the anomalous Nernst (ANE) and anomalous Hall effects (AHE). The magnetotransport measurements identified the switching fields of the CoFeB nanostructures and demonstrated a combination of AHE and ANE when the devices were operated in thermally-driven spin-injection mode. Modified scanning probe microscopy probes were fabricated by placing a NdFeB magnetic bead (MB) on the apex of a commercial Si probe. The dipole magnetic field distribution around the MB was characterized by using differential phase contrast technique and direct measurement of the switching field induced by the bead in the CoFeB nanodevices. Using SGM we demonstrate the influence of localized magnetic field on the CoFeB nanostructures near the non-magnetic channel. This approach provides a promising route towards the study of thermal and spin diffusion effects using local magnetic fields

Tunnel junction based memristors as artificial synapses

Thomas A, Niehörster S, Fabretti S, et al. Tunnel junction based memristors as artificial synapses. Frontiers in Neuroscience. 2015;9: 241.We prepared magnesia, tantalum oxide and barium titanate based junction structures and investigated their memristive properties. The low amplitudes of the resistance change in these types of junctions are the major obstacle for their use. Here, we increased the amplitude of the resistance change from 10% up to 100%. Utilizing the memristive properties, we looked into the use of the junction structures as artificial synapses. We observed analogs of longterm potentiation, long-term depression and spike-time dependent plasticity in these simple two terminal devices. Finally, we suggest a possible pathway of these devices towards their integration in neuromorphic systems for storing analog synaptic weights and supporting the implementation of biologically plausible learning mechanisms

Current induced resistance change of magnetic tunnel junctions with ultra-thin MgO tunnel barriers

Ultra-thin magnetic tunnel junctions with low resistive MgO tunnel barriers are prepared to examine their stability under large current stress. The devices show magnetoresistance ratios of up to 110 % and an area resistance product of down to 4.4 ohm micrometer squared. If a large current is applied, a reversible resistance change is observed, which can be attributed to two different processes during stressing and one relaxation process afterwards. Here, we analyze the time dependence of the resistance and use a simple model to explain the observed behavior. The explanation is further supported by numerical fits to the data in order to quantify the timescales of the involved phenomena

Memristive Tunnelkontakte

Krzysteczko P. Memristive tunnel junctions. Bielefeld (Germany): Bielefeld University; 2010.The thesis is divided in three chapters. In the first chapter, we reveal that magnetic tunnel junctions fit in the recently emerged field of memristors and memristive systems. We describe how this 'hot topic' emerged from the combination of two research fields and take a step further by including magnetic systems. The somewhat illustrative data presented in this chapter has been chosen to demonstrate the most striking features of memristive magnetic tunnel junctions. Not only the smooth switching between two well defined resistive states is presented, but also - and this is the novelty introduced by magnetic electrodes - additional states due to spin-dependent tunneling. At the end of the first chapter, the limits of the applicability of the memristor theory are discussed. In the second chapter, the resistive switching is identified as the physical mechanism responsible for the memristivity of magnetic tunnel junctions. First, the main models and theories proposed in the literature are summarized. The data presented thereafter has been chosen to give a complete phenomenological picture of the memristivity of magnetic tunnel junctions. Based on this experimental evidence, a model is developed particularly suited to describe the physical mechanisms responsible for the memristivity of magnetic tunnel junctions. The third chapter deals with neural networks and synapses in particular. In the beginning, a brief introduction to biological neural networks is given. The experimental results needed for a comparison of memristive magnetic tunnel junctions with biological synapses are provided in the middle part of this chapter. Finally, the astonishing equivalency of both systems is revealed by a discussion of the measurements on memristive magnetic tunnel junctions and by comparison with recent results from the neuroscientific literature

Magnetoresistance of Co nanoconstrictions fabricated by means of electron beam lithography

Krzysteczko P, Dumpich G. Magnetoresistance of Co nanoconstrictions fabricated by means of electron beam lithography. PHYSICAL REVIEW B. 2008;77(14): 144422.Co nanowires of T-shaped geometry with and without nanoconstrictions are fabricated by means of electron beam lithography (EBL) followed by electron beam evaporation. We have succeeded in minimizing the width of the nanoconstriction, i.e., the nanocontact, down to 6 nm. In a subsequent second EBL process, gold contact leads are attached as close as possible to the nanoconstrictions, which allows us to measure the magnetoresistance (MR) of the sample as a function of the nanocontact width. We found that the MR of the T-shaped nanowires without a nanoconstriction can exclusively and quantitatively be explained by the anisotropic magnetoresistance of the Co nanowires. This allows us to separate the MR contribution of the nanoconstriction from the total MR of the sample quantitatively. We find that the resistance of the nanoconstriction is independent of the width of the nanocontact, whereas the corresponding MR contributions fluctuate depending on the various shapes of the nanoconstriction

Improved reliability of magnetic field programmable gate arrays through the use of memristive tunnel junctions

Münchenberger J, Krzysteczko P, Reiss G, Thomas A. Improved reliability of magnetic field programmable gate arrays through the use of memristive tunnel junctions. JOURNAL OF APPLIED PHYSICS. 2011;110(9): 96105.Since the recent successful implementation of the long-hypothesized memristor, its use in neuronal computing and in the reproduction of biological neural networks has gained increasing attention. In addition to the development of these new applications, the growing number of devices with memristive properties is promising to improve already established technologies. Herein, we use the recently reported memristance in magnesium-oxide-based magnetic tunnel junctions to improve the error tolerance in magnetic random access memory and magnetic field programmable logic arrays. (C) 2011 American Institute of Physics. [doi:10.1063/1.3660521

The Memristive Magnetic Tunnel Junction as a Nanoscopic Synapse-Neuron System

Krzysteczko P, Münchenberger J, Schäfers M, Reiss G, Thomas A. The Memristive Magnetic Tunnel Junction as a Nanoscopic Synapse-Neuron System. Advanced Materials. 2012;24(6):762-766