Out-of-equilibrium electronic transport properties of a misfit cobaltite thin film

Abstract

We report on transport measurements in a thin film of the 2D misfit Cobaltite Ca3Co4O9. Dc magnetoresistance measurements obey the modified variable range hopping law expected for a soft Coulomb gap. When the sample is cooled down, we observe large telegraphic-like fluctuations. At low temperature, these slow fluctuations have non Gaussian statistics, and are stable under a large magnetic field. These results suggest that the low temperature state is a glassy electronic state. Resistance relaxation and memory effects of pure magnetic origin are also observed, but without aging phenomena. This indicates that these magnetic effects are not glassy-like and are not directly coupled to the electronic part. [5]. Furthermore, there is a possibility of frustrated magnetic interactions in a kagome lattice in the CoO 2 layers [6]. Ferrimagnetic or very weak ferromagnetic-like properties are also observed at low temperature [7]. In addition, µ + SR experiments have been interpreted with the presence of a spin density wave at low temperature [8], giving a clue for a gap opening near the Fermi level leading to the observed "Fermi liquid"-insulator transition. Nevertheless, the low temperature transport behavior is not really understood, specially because the ground state of the system is quite complex. The eventual link between all these observations and the measured transport properties has to be clarified. The above mentioned results suggest that low dimensionality, frustration and disorder, together with strong electronic correlations, can be important features for the physical properties of the Ca 3 Co 4 O 9 system. Since they are known to reinforce fluctuations and metastability, one can expect a strong influence on the transport properties. To our knowledge, this has not been verified by the usual macroscopic transport measurements. Because of the statistical averaging, the discrete nature of the underlying "mesoscopic" processes can be masked in a bulk sample but revealed in a smallarea system. In this case, the transport measurements can be used as an efficient probe to bring some light on the ground states of the system [9].In this paper, we report a study of DC magnetotransport properties and of resistance fluctuations in a microbridge of a Ca 3 Co 4 O 9 thin film at low temperature. We particularly focus on the origin (magnetic or electronic) of these resistance fluctuations and relaxation.A 2000Å epitaxial thick film of Ca 3 Co 4 O 9 was used for the measurements. The film was deposited on (0001) Al 2 O 3 using the pulsed laser deposition technique. The details of the optimization, growth conditions and structural characterizations have been described previously [10]. Prior to the transport measurements, a silver layer was firstly deposited via thermal evaporation onto the film, and secondly a gold layer via RF sputtering. Microbridges (L = 200 µm × W = 50 µm) were then patterned with UV photolithography and argon ion etching. Thin aluminum contact wires were finally used to connect the areas to the electrodes w...

Properties of expitaxial Ca3Co4O9thin films fabricated by chemical solution method

“…The positive values of the Hall coefficient indicate hole transport. The calculated hole carrier concentration of the Ca 3 Co 4 O 9 film is about 6.8 × 10 20 cm −3 , quite consistent with the reported values [13,18]. We also measured the temperature dependence resistivity curves of the film with different applied current densities.…”

“…Figure 5 shows the temperature dependence of the Seebeck coefficient for the MOD Ca 3 Co 4 O 9 film on c-sut sapphire substrates. The room temperature S of the film is about 130 µV K −1 , which is also comparable to those of the single crystal (∼125 µV K −1 ) and the best PLD thin film samples (126-130 µV K −1 ) [12,13,16].…”

“…For many TE technological applications such as DNA microarrays, fibre-optic switches, thermochemistry-on-a-chip and microelectrothermal systems, high quality thin films are required [10]. At present, the pulsed laser deposition (PLD) method is the most widely used technique for growth of high quality Ca 3 Co 4 O 9 thin films [11][12][13]. However, this technique is expensive and requires a vacuum system and complex equipment, which limits the TE applications of 1 Author to whom any correspondence should be addressed.…”

Growth of Ca3Co4O9 films: Simple chemical solution deposition and stress induced spontaneous dewetting

“…[1][2][3] The CCO349 is built up by stacking along the c-axis of triple rock salt-type layer Ca 2 CoO 3 and single CdI 2 -type CoO 2 layers, which both have monoclinic symmetry with identical a, c, and ␤, but different b parameters ͑for the Ca 2 CoO 3 layer, a = 4.8376 Å, c = 10.833 Å, ␤ = 98.06°, and b 1 = 4.5565 Å, while for the CoO 2 layer, b 2 = 2.8189 Å͒. 4 As for the fabrication of CCO349 films, which can be used in various applications 5 such as thermochemistry-on-achip, biothermoelectric chips, active cooling for microelectronic processors, and localized cooling/heating at points of interest, the commonly used method is pulsed laser deposition ͑PLD͒, [6][7][8] and there have few reports about the chemical solution deposition ͑CSD͒ route, 9 which is a simple, low-cost method for large-area films. Moreover, in the processes of preparing films, the interfacial strain in the films is a key parameter controlling many physical properties, which has stimulated considerable efforts towards understanding the relaxation mechanisms.…”

Electrical, magnetic and magnetotransport properties of Na and Mo doped Ca3Co4O9 materials

“…[1][2][3] In this family of cobalt oxides, mist-layered Ca 3 Co 4 O 9 is especially interesting due to its structural and chemical stabilities at relatively higher temperatures, 4 unusual thermoelectric properties: coexistence of a large Seebeck coefficient and a low metallic-like electrical resistivity, 5 and ferrimagnetic or very weak ferromagnetic-like properties at low temperatures. 6 In conventional TE materials, a large Seebeck coefficient is associated with low carrier concentrations, typically in the range of $10 19 cm À3 . 7 However, the carrier concentration in the Ca 3 Co 4 O 9 system can reach up to $10 21 cm À3 and yet it exhibits a signicantly large Seebeck coefficient, in the range $100-150 mV K À1 at 300 K. The unusual thermoelectric properties of cobaltites may be attributed to the entropy of localized spins, 8 the electronic correlations, 9 spin and charge frustrations, 10 and large effective mass in the vicinity of Mott metal-insulator transition.…”