
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.



Aragón Sánchez, J., R. Cortés Maldonado, N. R. Cejas Bolecek, G. Rumi, P. Pedrazzini, M. I. Dolz, G. Nieva, C. J. van der Beek, M. Konczykowski, C. D. Dewhurst et al. "Unveiling the vortex glass phase in the surface and volume of a typeII superconductor." Communications Physics 2, no. 1 (2019): 143.
Abstract: Orderdisorder transitions between glassy phases are common in nature and yet a comprehensive survey on the entailed structural changes is challenging since the constituents are in the microscale. Vortex matter in typeII superconductors is a model system where some of these experimental challenges can be tackled. Samples with point disorder present a glassy transition on increasing the density of vortices. A glassy yet quasicrystalline phase, the Bragg glass, nucleates at low densities. The vortex glass stable at high densities is expected to be disordered, however its detailed structural properties remained experimentally elusive. Here we show that the vortex glass has large crystallites with inplane positional displacements growing algebraically and shortrange orientational order. Furthermore, the vortex glass has a finite and almost constant correlation length along the direction of vortices, in sharp contrast with strong entanglement. These results are important for the understanding of disorderdriven phase transitions in glassy condensed matter.



Aragón Sánchez, J., R. Cortés Maldonado, M. I. Dolz, N. R. CejasBolecek, C. J. van der Beek, M. Konczykowski, and Y. Fasano. "Direct visualization of local interaction forces in Bi2Sr2CaCu2O8+Î´ vortex matter." Materials Today: Proceedings 14 (2019): 34–37.
Abstract: We study the local vortexvortex interaction force fi of the structure frozen during a fieldcooling process in an electronirradiated Bi2Sr2CaCu2O8+Î´ sample. We compute this magnitude from snapshots of the vortex structure obtained via magnetic decoration experiments at various fields H in the same sample. Since the observed structures correspond to the equilibrium ones frozen at Tâˆ¼Tirr(H)[1], at this temperature the local modulus of fi roughly equals the local pinning force at the decorated surface of the sample. We estimate the most probable local pinning force from the mode value of the fi(r)distribution, fpm. We found that fpm grows algebraically with H and in electronirradiated samples is 5020% smaller than for samples with columnar defects.



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., 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.



Dolz, M. I., W. Bast, D. Antonio, H. Pastoriza, R. D. J. Sánchez Curiale, and A. G. Leyva. "Magnetic Behavior of Single La_{0.67}Ca_{0.33}MnO$_3$ Nanotubes: Surface and Shape Effects." J. Appl. Phys. 103 (2008): 083909.
Abstract: We report magnetization experiments in two magnetically isolated ferromagnetic nanotubes of perovskite Lai_{0.67}Ca_{0.33}MnO$_3$. The results show that the magnetic anisotropy is determined by the sample shape, although the coercive field is reduced by incoherent magnetization reversal modes. The temperature dependence of the magnetization reveals that the magnetic behavior is dominated by grain surface properties. These measurements were acquired using a silicon micromechanical oscillator working in its resonant mode. The sensitivity was enough to measure the magnetic properties of these two samples with a mass lower than 14 pg and to obtain for the first time the magnetization loop for one isolated nanotube.



Dolz, M. I., N. R. C. Bolecek, J. Puig, H. Pastoriza, G. Nieva, J. Guimpel, C. J. van der Beek, M. Konczykowski, and Y. Fasano. "Enhancement of penetration field in vortex matter in mesoscopic superconductors due to Andreev bound states." Physical Review B 100, no. 6 (2019): 064508.
Abstract: We study the field for the penetration of a first vortex, HP, for vortex matter nucleated in micronsized samples with edges aligned along the nodal and antinodal directions of the dwave superconducting order parameter of Bi2Sr2CaCu2O8âˆ’Î´. Here we present evidence that the HP for vortex matter nucleated in mesoscopic samples with edges parallel to the nodal direction is larger than for the antinodal case, âˆ¼72% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce an anomalous Meissner current that increases the BeanLivingston barrier for vortex penetration.



Dolz, M. I., A. B. Kolton, and H. Pastoriza. "Direct determination of the collective pinning radius in the cuprate superconductor Bi2 Sr2 CaCu2 O8+ delta." Physical Review B 81, no. 9 (2010): 092502.
Abstract: We study the onset of the irreversible magnetic behavior of vortex matter in micronsized Bi2Sr2CaCu2O8+δ single crystals by using silicon microoscillators. We find an irreversibility line appearing well below the thermodynamic Braggglass melting line at a magnetic field which increases both with increasing the sample radius and with decreasing the temperature, paradoxically implying the existence of a reversible vortex solid. We show that at this irreversibility line, the sample radius can be identified with the crossover length between the Larkin and the randommanifold regimes of the vortexlattice transverse roughness. Our method thus allows to determine, as a function of temperature and applied magnetic field, the minimum size of a vortex system that can be collectively pinned, or the socalled threedimensional weak collective pinning Larkin radius, in a direct way.



Dolz, M. I., and H. Pastoriza. "Dependence of vortex phase transitions in mesoscopic Bi2Sr2CaCuO8 superconductor at tilted magnetic fields." Journal of Physics: Conference Series 150, no. 5 (2009): 052044.
Abstract: A micron sized single crystal of the superconductor Bi2Sr2CaCuO8 was studied using silicon mechanical microoscillators at various tilt angles of the dc magnetic field with respect to the c axis of the sample. Different phases of the vortex matter were detected by measuring changes in the value and sign of the oscillator resonant frequency variation with temperature. We could explain the change in the sign of this variation at high temperatures as the transition from the 2D liquid of decoupled pancakes to a reversible 3D vortex lattice. The data indicates that this transition only depends on the magnetic field perpendicular to the superconducting layers while the dissipation involved in this process depends on the component parallel to them.

