
Affolter, J., M. Tesei, H. Pastoriza, C. Leemann, and P. Martinoli. "Observation of Isinglike critical fluctuations in frustrated Josephson junction arrays with modulated coupling energies." Physica C 369 (2002): 313–316.
Abstract: We report the results of ac sheet conductance measurements performed on fully frustrated square arrays of Josephson junctions whose coupling energy is periodically modulated in one of the principal lattice directions. Such systems are predicted to exhibit two distinct transitions: a lowtemperature Isinglike transition triggered by the proliferation of domain walls and a hightemperature transition driven by the vortex unbinding mechanism of the BeresinskiiKosterlitzThouless (BKT) theory. Both the superfluid and dissipative components of the conductance are found to exhibit features which unambiguously demonstrate the existence of a double transition whose properties are consistent with the IsingBKT scenario.



Alejandro, G., L. B. Steren, H. Pastoriza, D. Vega, M. Granada, J. C. Rojas Sánchez, M. Sirena, and B. Alascio. "Magnetoresistance effect in (La, Sr)MnO_3 bicrystalline films." Journal of Physics: Condensed Matter 22, no. 34 (2010): 346007.
Abstract: The angular dependence of the magnetoresistance effect has been measured on bicrystalline La 0.75 Sr 0.25 MnO 3 films. The measurements have been performed on an electronically lithographed Wheatstone bridge. The study of the angular dependence of both the magnetoresistance and the resistance of singlecrystalline and grainboundary regions of the samples allowed us to isolate two contributions of lowfield magnetoresistance in manganites. One of them is associated with the spinâ€“orbit effect, i.e.Â the anisotropic magnetoresistance of ferromagnetic compounds, and the other one is related to spindisorder regions at the grain boundary. Complementary xray diffraction, ferromagnetic resonance and low temperature magnetization experiments contribute to the characterization of the magnetic anisotropy of the samples and the general comprehension of the problem.



André, M.  O., M. Polichetti, H. Pastoriza, and P. H. Kes. "Vortex dynamics in 2HNbSe$_2$ containing a periodic channel structure." Physica C 338 (2000): 179–188.
Abstract: We have measured the resistivity and magnetic AC susceptibility of 2HNbSe single crystals containing alternating 2 stripes of irradiated and nonirradiated regions of columnar defects created by heavyion bombardment. Without applied magnetic field, the sample undergoes a doublestep transition into the superconducting state, each step corresponding to the transitions in the irradiated and nonirradiated regions, respectively. For fields smaller than half of the matching field and upon increasing the temperature, the onset of flux motion in the nonirradiated channels occurs, when the applied stress due to the electrical current equals the shear stress at the channel edges, while depinning in the irradiated stripes occurs at higher temperature. The weak amplitude dependence of the shear process suggests that it takes place at the melting transition. We observed only a singlestep transition at DC magnetic fields larger than half the matching field, because pinning by the columnar defects in the irradiated stripes is much less effective, and consequently the shear stress at the channel edges is strongly reduced. The comparison between the shear stress deduced from I curves and the theoretical value $?max = AC66$ yields a value for the constant Α, which is in good agreement with theoretical predictions.



Antonio, D., and H. Pastoriza. "Nonlinear Dynamics of a Micromechanical Torsional Resonator: Analytical Model and Experiments." Journal of Microelectromechanical Systems 18, no. 6 (2009): 1396–1400.
Abstract: Mechanical microoscillators have been valuable tools for magnetic measurements of microscopic samples. This paper presents an analytical model for the dynamics of an electrostatically actuated torsional oscillator and validates it experimentally. While, at low excitations, the system is well described by a damped linear oscillator, at higher oscillation amplitudes, a nonlinear regime is observed. Nonlinearity is originated exclusively by the electrostatic driving and detection and can be tuned by modifying the excitation or detection bias voltages. The parameters of the analytical model are obtained from the device dimensions and material properties. No empirical or fitting parameters are needed except for the quality factor, which is extracted from the linear resonance curve. The proposed model can be a valuable and straightforward tool for the design and analysis of many other similar devices based on electrostatically actuated mechanical resonators.



Antonio, D., M. I. Dolz, and H. Pastoriza. "Magnetization measurement of single La0.67Ca0.33MnO3 nanotubes in perpendicular magnetic fields using a micromechanical torsional oscillator." Journal of Magnetism and Magnetic Materials 322, no. 5 (2010): 488–493.
Abstract: Using a silicon micromechanical resonator as a sensitive magnetometer, the authors have studied both experimentally and theoretically the magnetic behavior of two isolated ferromagnetic nanotubes of perovskite La0.67Ca0.33MnO3. The article investigates the specific configuration where a magnetic field H is applied perpendicular to the magnetic easy axis of an isolated nanotube characterized by an uniaxial anisotropy constant K. In this situation, the magnetization M reduces the effective elastic constant kM of the resonator. This softening of the mechanical system is opposed to the hardening effect of M observed in a previous work, where H was applied parallel to the easy axis. Moreover, in this magnetic field configuration two distinct magnetization regimes are manifested, depending on the magnitude of H. For H[not double greaterthan sign]2K/M the magnetization is almost parallel to the applied magnetic field and for H[double lessthan sign]2K/M it is almost parallel to the easy axis of the nanotube. At a certain value of H there is a sharp transition



Antonio, D., M. I. Dolz, and H. Pastoriza. "Micromechanical magnetometer using an allsilicon nonlinear torsional resonator." Applied Physics Letters 95, no. 13 (2009): 133505–3.
Abstract: In this work, a micromagnetometer employing a nonlinear torsional resonator with a high quality factor Q is presented experimentally. Oscillatory rotation of a conducting plate in the sensed magnetic field H induces eddy currents that dissipate energy. Due to the nonlinear response of the oscillator, the resulting mechanical damping originates frequency shifts in the resonance curve that depend on H. Nonlinearity results from the electrostatic detection, which introduces high order electrical spring constants. The device is fabricated with a standard silicon process and does not incorporate ferromagnetic materials. An analytical nonlinear model that correctly describes the device is also introduced.



Antonio, D., P. Julian, P. Mandolesi, and H. Pastoriza. "Amplificador de transimpedancia criogénico para detección capacitiva de MEMS." In Actas de la Escuela Argentina de Microelectrónica, Tecnología y Aplicaciones: Trabajos Breves, 49–51. Vol. 1., 2007.



Bolecek, N. R. C., M. I. Dolz, H. Pastoriza, M. Konczykowski, C. J. van der Beek, A. B. Kolton, and Y. Fasano. "Excess of topological defects induced by confinement in vortex nanocrystals." Physical Review B 96, no. 2 (2017): 024507.
Abstract: We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the firstorder transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this â€œhealinglengthâ€ distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Braggglass phase. This suggests that the healing length may be a key quantity to model confinement effects in the firstorder transition of extremely layered vortex nanocrystals.



Cejas Bolecek, N. R., A. B. Kolton, M. Konczykowski, H. Pastoriza, D. Dominguez, and Y. Fasano. "Vortex matter freezing in Bi2Sr2CaCu28 samples with a very dense distribution of columnar defects." Physical Review B 93, no. 5 (2016): 054505.
Abstract: We show that the dynamical freezing of vortex structures nucleated at diluted densities in Bi2Sr2CaCu2O8 samples with a dense distribution of columnar defects, Bâˆ¼10âˆ’2BÎ¦ with BÎ¦=5kG, results in configurations with liquidlike correlations. We propose a freezing model considering a relaxation dynamics dominated by doublekink excitations driven by the local stresses obtained directly from experimental images. With this model we estimate the relaxation barrier and the freezing temperature. We argue that the lowfield frozen vortex structures nucleated in a dense distribution of columnar defects thus correspond to an outofequilibrium nonentangled liquid with strongly reduced mobility rather than to a snapshot of a metastable state with divergent activation barriers as, for instance, expected for the Boseglass phase at equilibrium.



Cejas Bolecek, N. R., M. I. Dolz, A. Kolton, H. Pastoriza, C. Jvander Beek, M. Konczykowski, M. Menghini, G. Nieva, and Y. Fasano. "Geometrical Confinement Effects in Layered Mesoscopic Vortex Matter." Journal of Low Temperature Physics 179, no. 12 (2015): 35–41.
Abstract: We study the geometrical confinement effect in Bi 2 Sr 2 CaCu 2 O 8+Î´ mesoscopic vortex matter with edgetosurface ratio of 7â€“12 %. Samples have inplane square and circular edges, 30 Î¼ m widths, and âˆ¼ 2 Î¼ m thickness. Direct vortex imaging reveals the compact planes of the structure align with the sample edge by introducing topological defects. The defect density is larger for circular than for square edges. Molecular dynamics simulations suggest that this density is not an outofequilibrium property but rather determined by the geometrical confinement.

