DOI: 10.1007/s00339-005-3393-z Appl. Phys. A 81, 1545–1549 (2005) Materials Science & Processing Applied Physics A p. garc´ ıa-mochales 1, ∗ s. pel ´ aez 1 p.a. serena 1, ✉ e. medina 2 a. hasmy 2 Breaking processes in nickel nanocontacts: a statistical description 1 Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cient´ ıficas, Cantoblanco, 28049 Madrid, Spain 2 Centro de F´ ısica, Instituto Venezolano de Investigaciones Cient´ ıficas, Apdo. 21827, Caracas 1020A, Venezuela Received: 11 March 2005/Accepted: 23 August 2005 Published online: 28 September 2005 • © Springer-Verlag 2005 ABSTRACT In this work we perform a statistical study of fa- vorable atomic configurations of nickel nanocontacts during their stretching at 4 K and 300 K. Nanowire breaking events are simulated using molecular dynamics (MD) where atomic in- teractions are represented with state-of-the-art embedded atom (EAM) interatomic potentials. The full determination of atomic positions during the contact evolution allows determination of the evolution of the minimum-cross section S m during stretch- ing. By accumulating many breaking traces, we built minimum cross-section histograms H( S m ). These simulated histograms reveal the presence of preferential geometrical arrangements during the nanocontact breaking, showing that no remarkable differences should appear between the low (4 K) and room temperature (300 K) situations. These results show that differ- ences observed between low and room temperature experimen- tal Ni conductance histograms, are not caused by the different structural evolution and, that therefore, other phenomena are involved. PACS 81.07.Lk; 68.65.-k; 73.63.Rt; 31.15.Qg 1 Introduction Metallic wires with diameters of the order of a few nanometers (nanowires) are key systems for both basic sci- ence and future development of nanoelectronic devices [1]. Electron transport through metallic nanowires presents ballis- tic features below room temperature since the electron elastic mean free path is larger than the characteristic nanocontact di- mensions. Furthermore, well defined quantized propagating modes appear in nanowires with diameters of the order of few Fermi wavelengths (λ F ), due to the quantization of the elec- tron motion associated with transversal confinement. In such limits, the electric conductance G is well described within the scattering matrix formalism through the Landauer formula G = G 0 ∑ N n=1 T n , where G 0 = 2e 2 /h is the conductance quantum (where e and h are the electron charge and Planck’s constant, respectively), T n is the transmission probability as- sociated with the n-th channel or mode, and N is the number ✉ Fax: +34 91 372 0623, E-mail: pedro.serena@icmm.csic.es ∗ Present address: Depto. de F´ ısica de la Materia Condensada, C–III, Universidad Aut´ onoma de Madrid, Cantoblanco 28049 Madrid, Spain of propagating or opened modes (those with energies below the Fermi energy, E F ) [2]. There are several experimental approaches for obtaining metallic nanowires [1], although methods based on scanning tunneling microscopy (STM) [3–6] and mechanically con- trollable break junctions (MCBJ) [7–9] have been widely used due to their sub nanometric accuracy. Furthermore, methods based on “table-top” experiments (by separating macroscopic wires) [10], using electron-beam irradiation in- side a transmission electron microscope (TEM) [11, 12], or applying electrochemical methods [13, 14] have been pro- posed as candidates to fabricate metallic nanowires. The electrical characterization of a metallic nanowire is generally done by measuring its conductance G as a func- tion of the nanowire elongation during its rupture process. In this way, a standard experiment acquires a conductance trace formed by those conductance values associated with a sequence of different nanowire configurations. Since each conductance trace presents its own features, it is difficult to extract representative information from it. A deeper un- derstanding is obtained by means of the so-called conduc- tance histogram H(G ) [6] which is built by adding hundreds of independent conductance traces. In general, these con- ductance histograms present well defined peaks associated with preferred conductance values which reflect the pres- ence of conductance quantization [9], or the existence of energetically favorable atomic arrangements [15–18]. Con- ductance histograms constitute a standard tool to analyze metallic nanowires, although it is very difficult to interpret them, since they simultaneously include mechanical as well as information of ionic and electronic origin. This com- plexity increases for polyvalent metals since several elec- tronic transport channels per atom are involved [19, 20]. For instance, aluminum conductance histograms obtained at 4K [15] and room temperature [17] show well defined peaks at conductance values close to integer values of G 0 , although three channels per atom are involved in electron transport. The analysis of H(G ) becomes even more intricate in magnetic nanowires due to the presence of a new degree of freedom, the electron spin. In particular, nickel nanowires have been studied for a long time [5, 6, 21–29] due to their promising applications. The first study where a conductance histogram was shown [6] also reported the existence of a Ni