Electrical Transport in Ultrathin NdNiO 3 Films
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Electrical Transport in Ultrathin NdNiO3 Films Megan Campbell and Ashutosh Tiwari* Nanostructured Materials Research Laboratory University of Utah, Department of Materials Science & Engineering *E-mail: [email protected] ABSTRACT Electrical transport properties in ultrathin NdNiO3 films grown on single crystal LaAlO3 (001) substrate were characterized. Films with thicknesses ranging from 0.6 nm to 12 nm were grown using a pulsed laser technique. Four probe resistivity as a function of temperature measurements indicated a strong dissipation of strain effects from 0.6 nm to 6 nm as well as the presence of defects in the 12 nm sample. A proposed mechanism of kinetically stable glassy phase formation explains the time dependence of the resistivity in both cooling and heating cycles. INTRODUCTION Electrical transport in strongly correlated transition metal oxides has been a subject of great scientific interest over the last several decades. Many interesting properties such as, high temperature superconductivity [1, 2], colossal magnetoresistance [3, 4] and multi-ferroelectricity [5, 6] have been observed in these systems. Typical examples of such materials are perovskite oxides like YBCO, LSMO, YMnO3 and BiFeO3. NdNiO3 is another very interesting, but less well understood, member of this family [7]. It possesses orthorhombic perovskite structure and behaves as a metal at high temperatures, as an insulator at low temperatures and undergoes a metal-insulator (M-I) transition in the intermediate temperature range. The M-I transition in NdNiO3 is understood to arise due to the closing of charge-transfer gap between the oxygen 2p and Ni 3d upper Hubbard band. Specifically, in the low temperature phase, the Fermi level lies in the charge transfer gap resulting in an insulating behavior. However, as the temperature is increased, the energy bands get broadened and at the M-I transition temperature, TM-I, they start overlapping, resulting in a metallic phase. The crystallographic unit cell of perovskite oxides (ABO3) comprises of corner sharing BO6 octahedra forming a three dimensional network. In the case of cubic perovskites, BO6 octahedra are arranged regularly at the nodes of a simple cubic lattice forming B-O-B bond angles of 1800 (Figure 1 (a)) [8]. In the case of orthorhombic perovskites such as NdNiO3, NiO6 octahedra are tilted and Ni-O-Ni angles are less than 1800 (Figure 1 (b)). On decreasing the size of atom A in orthorhombic perovskites, the B-O-B angles can be much lower than 1800. Based on neutron diffraction experiments, it has been demonstrated that the charge-transfer M-I transition in NdNiO3 is related to changes in the Ni-O-Ni bond angle [9]. The more the BO6 octahedra are able to rotate in orthorhombic perovskites, the higher the TM-I. [10] When external pressure is applied, the Ni-O-Ni bond angles straighten and TM-I decreases [11, 12]. Obradors et al. [13] showed that on applying external hydrostatic pressure, the TM-I in NdNiO3 can be decreased with a rate dTM-I/dP= -4.8 K/kbar. In another study on thin f
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