Marianna Batkova

Electrical resistivity and superconductivity of LaB6 and LuB12

Abstract The electrical resistivity of LaB 6 and LuB 12 single crystals with high residual resistivity ratios has been measured down to 1.6 K. The temperature dependence below 30 K, caused by phonon scattering, shows for both materials a T n behaviour with n = 4.2 ± 0.1 for LaB 6 and n = 5.2 ± 0.1 for LuB 12 . From susceptibility measurements the superconducting transition temperature of LuB 12 was determined to be 0.44 K, for the LaB 6 sample with residual resistivity ratio of 160 no transition into the superconducting state was observed down to 5 mK.

Electrical resistivity of doped EuB6 down to 50 mK

Abstract Results of electrical resistivity measurements of C-doped EuB 6 between 50 mK and 300 K are discussed. The attention is concentrated on the high residual resistivity of this material below its magnetic transition temperature. Results have shown that the high resistivity at lowest temperatures originates from the scattering of conduction electrons on mixed magnetic structure of the sample.

AFM-utilizing approach to search for new oxide materials for perspective applications in memristive devices

We demonstrate that memristive devices can be fabricated by tip-induced oxidation of thin metallic films using atomic force microscope. Electrical measurements of such prepared Ti/TiOx/Ti test structures confirmed their memristive behavior and inferred diffusion of oxygen vacancies in the TiOx barrier. Consequent Kelvin probe force microscopy studies provided evidence for the diffusion, as well as for expected oxygen vacancy drift. Time evolution of the space distribution of the vacancies due to the diffusion process revealed minute-scale (at least) retention times of the devices. The work presents technology alternative for fabrication of memristive nanodevices in geometry favouring advantageous scanning probe microscopy studies of their in-barrier processes, as well as widely utilizable approach to search for novel oxide materials for perspective memristive applications. Observation of drift and diffusion processes in Ti/TiO x /Ti memristive devices 2 Four decades ago Chua provided strong arguments [1] that besides the resistor, inductor, and capacitor, there should exist one more fundamental passive circuit element, so-called memristor (shortcut for memory resistor). A characteristic property of this two terminal device is its resistance, which depends on history of the current passing through it resulting in hysteretic current-voltage (I-V ) curves. Although over time many observations of hysteretic I-V curves were reported, connection of this phenomenon with memristive behavior was done only recently by Strukov and coworkers [2], who shoved, that memristance arises naturally in nanoscale systems in which solid-state electronic and ionic transport are coupled under an external bias voltage [2]. As typical representatives of memristive devices can be considered metal/oxide/metal two-terminal nanodevices, and Pt/TiO2/Pt nanodevices (exhibiting bipolar resistive switching) seem to be a prototypical (nano)structure exhibiting memristive behavior. Based on experimental studies of TiO2 junction devices with platinum electrodes it was established that electrical conduction in metal/oxide/metal thin-film devices is controlled by a spatially heterogeneous metal/oxide electronic barrier [3], and memristive electrical switching proceeds by means of the drift of positively charged oxygen vacancies acting as native dopants to form (turn ON) or disperse (turn OFF) locally conductive switches through the electronic barrier [3]. Although metal/oxide/metal type devices exhibiting memristive/resistive switching are intensively investigated for very long time (especially because of their potential use in non-volatile memory applications), and significant progress in the field was done after ”rediscovery” of the memristor [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], there are still many open questions related to local physical and chemical properties in the oxide barrier region [13]. Many of these peculiarities arising at nanoscale could be very effectively studied by various advantageous scanning probe microscopy (SPM) techniques. For instance, Kelvin probe force microscopy (KPFM), which measures contact potential between the sample and a (metal coated) tip of atomic force microscope (AFM) [14], and is obviously used for direct detection of the work-function changes in metals or doping-level changes in semiconductors [14, 15], could provide valuable information about space distribution of oxygen vacancies in the active regions of the nanodevices. Unfortunately, SPM studies of oxide-barrier processes in this kind of nanodevices is a non-trivial task, such as they are as a rule prepared in vertical, stacked geometry with oxide layer between a top and a bottom electrode [3, 7, 16]; thus removing of the ”top” electrode is necessary to enable direct observation of the oxide barrier by SPM [16]. Stacked geometry of the memristive devices has at least one more negative aspect; it requires specific, often very expensive equipment that might be a limitation for widespread use in searching for new materials for perspective memristive applications. Therefore, new approaches enabling simple preparation of memristive devices in geometry suitable for use of SPM-based techniques to study the processes in the oxide barrier are highly desired for faster progress in the field. Interesting technology alternative for definition of oxide patterns with resolution in the nanoscale is local anodic oxidation (LAO) by use of AFM, which is well-established Observation of drift and diffusion processes in Ti/TiO x /Ti memristive devices 3 method for tip-induced oxidation of semiconducting [18] and metallic [17] surfaces. The method was succesfully used to fabricate many nanoelectronic and nanophotonic devices, e.g. metal/insulator/metal (MIM) type nanodevices, single-electron transistor, or a photoconductive switch [19]. Important advantage of this method is lateral planar geometry of devices fabricated this way, which enables realization of studies of the oxidized regions by desired SPM techniques. The aim of this work is to demonstrate that LAO can be effectively used also for fabrication of metal/oxide/metal (MIM) type devices exhibiting memristive behavior. In the presented case study Ti/TiO x /Ti structures were prepared such as TiO2 (an important fraction of TiO x formed in the LAO process [19]), is a prototypical memristive material [13]. Consequently, KPFM was used to perform direct experimental observations of oxygen vacancy movement in the TiO x region. For purposes of this work, MIM type devices on the base of Ti thin films were prepared as follows. First, microbridges (6-8 nm thick, 30-80 μm wide, 100-300 μm long) were deposited by DC magnetron sputtering on glass substrates kept at ambient temperature. The sputtering was done from polycrystalline Ti target at Ar-pressure of 3 mTorr and rate of 0.04 nm/s. A shadow mask was used to define the bridges. Oxide barrier across the bridge was fabricated by LAO using AFM equipped with commercial nanolithography software. The oxidation process was performed in contact mode at ambient conditions with relative humidity between 55 and 60 %. To ensure oxidation up to the substrate, the lines across the bridge were overwritten several times like described in Ref. 17, 20. The width of the formed TiO x lines varied between 400 and 1200 nm. Resistance of the films/microbridges before the oxidation was of order of tens of kΩ; during the oxidation process, while forming Ti/TiO x /Ti structure, the resistance increased typically more than three orders of magnitude. KPFM studies of the Ti/TiO x /Ti structures were done in passive state (with both Ti electrodes grounded), as well as in active state (with bias voltage applied to one of the electrodes while the other one was grounded). For electrical characterization of the devices bias voltage, V , was applied to the prepared structure and the current, I, flowing through it was measured. First electric characterization of the Ti/TiO x /Ti devices was done by measuring the I-V curves at both increasing and decreasing V , at applied triangular excitation voltage signal of a period not longer than a few tens of seconds. Recorded curves (see inset in Fig. 1) show almost symmetric characteristics strongly resembling tunneling between two identical metals. In addition, the curves reveal very small hysteresis, and a small decrease of the slopes, as shown in the inset of Fig. 1. These small, but well detectable changes reveal a change of the device conductance state due to the applied bias and are interpreted as a sign of memristive behavior. Considering a physical model of the memristor [4, 2] a small hysteresis can be adequately interpreted as a consequence of small change of oxygen vacancy distribution in the TiO x region during one period of the applied voltage, what indicates relatively ”fast” ramping of the bias voltage [4, 2]. Exciting the device by positive voltage with sinusoidal modulation and DC offset equal to the amplitude A of the sinusoidal waveform [V (t) = A(1+sin(2πt/T ), where t is time Observation of drift and diffusion processes in Ti/TiO x /Ti memristive devices 4 0 1 2 3 4 5 6 0 50 100 150 200 4

Electrical properties of SmB6 thin films prepared by pulsed laser deposition from a stoichiometric SmB6 target

Abstract Possible existence of topologically protected surface in samarium hexaboride has created a strong need for investigations allowing to distinguish between properties coming from the surface states and those originating in the (remaining) bulk. Studies of SmB6 thin films represent a favorable approach allowing well defined variations of the bulk volume that is not affected by surface states. Moreover, thin films are highly desirable for potential technology applications. However, the growth of SmB6 thin films is accompanied by technology problems, which are typically associated with maintaining the correct stoichiometry of samarium and boron. Here we present feasibility study of SmB6 thin film synthesis by pulsed laser deposition (PLD) from a single stoichiometric SmB6 target. As proved by Rutherford Backscattering Spectrometry (RBS), we succeeded to obtain the same ratio of samarium and boron in the films as that in the target. Thin films revealing characteristic electrical properties of (crystalline) SmB6 were successfully deposited on MgO, sapphire, and glass-ceramics substrates, when the substrates were kept at temperature of 600 °C during the deposition. Performed electrical resistance studies have revealed that bulk properties of the films are only slightly affected by the substrate. Our results indicate that PLD is a suitable method for complex and intensive research of SmB6 and similar systems.

High residual electrical resistivity of carbon doped EuB6

Results of electrical resistivity measurements of carbon doped EuB 6 down to 50 mK are discussed. We have focused our attention on the high residual resistivity of this material below its magnetic transition temperature. Additional measurements and the analysis of results have shown that the high residual resistivity at the lowest temperatures originates from the substantial amount of scattering of the conduction electrons due to the presence of small ferromagnetic and helimagnetic domains in the sample.

Electrical conduction of Ti/TiOx/Ti structures at low temperatures and high magnetic fields

We present results of electrical conduction studies of Ti/TiO x /Ti planar structures prepared by tip-induced local anodic oxidation (LAO) of titanium thin films. The prepared structures have shown almost linear I-V curves at temperatures between 300 K and 30 K, and only slight deviation from linear behaviour at lower temperatures. Electrical conductance of the structures can be adequately explained by a two-channel model where variable range hopping channels and metallic ones coexist in parallel, while a crossover from Mott to Efros-Shklovskii variable-range-hopping conductivity has been observed at decreasing temperature. The magnetoresistance of the studied structures is very small even in magnetic fields up to 9 T. The reported electrical properties of the structures indicate their promising applications as very low heat capacity temperature sensors for cryogenic region and high magnetic fields.