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We have investigated the equilibrium states of ferromagnetic single wall nanotubes by means of atomistic Monte Carlo simulations of a zig-zag lattice of Heisenberg spins on the surface of a cylinder. The main focus of our study is to determine how the competition between short-range exchange (J) and long-range dipolar (D) interactions influences the low temperature magnetic order of the nanotubes as well as the thermal-driven transitions involved. Apart from the uniform and vortex states occurring for dominant J or D, we find that helical states become stable for a range of intermediate values of γ = D/J that depends on the radius and length of the nanotube. Introducing a vorticity order parameter to better characterize helical and vortex states, we find the pseudo-critical temperatures for the transitions between these states and we establish the magnetic phase diagrams of their stability regions as a function of the nanotube aspect ratio. Comparison of the energy of the states obtained by simulation with those of simpler theoretical structures that interpolate continuously between them, reveals a high degree of metastability of the helical structures that might be relevant for their reversal modes.

We investigate the effects of the competition between exchange (J) and dipolar (D) interactions on the magnetization reversal mechanisms of ferromagnetic nanotubes. Using first atomistic Monte Carlo (MC) simulations for a model with Heisenberg spins on a cylindrical surface, we compute hysteresis loops for a wide range of the γ = D/J parameter, characterizing the reversal behavior in terms of the cylindrical magnetization components and the vorticity parameter along the tube length. For γ's close to the value for which helical (H) states are stable at zero applied field, we show that the hysteresis loops can occur in four different classes that are combinations of two reversal modes with well-differentiated coercivities with probabilities that depend on the tube length and radius. This variety in the reversal modes is found to be linked to the metastability of the H states during the reversal that induce different paths followed along the energy landscape as the field is changed. We further demonstrate that reversal by either of the two modes can be induced by tailoring the nanotube initial state so that vortices with equal or contrary chirality are formed at the ends, thus achieving low or high coercive fields at will without changing γ. Finally, the results of additional micromagnetic simulations performed on tubes with similar aspect ratio show that dual switching modes and its tailoring can also be observed in tubes with more microscopic dimensions. arXiv:1904.05116v1 [cond-mat.mtrl-sci]

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A methodology is implemented to deform the surface of a magnetorheological elastomer (MRE) exposed to an external magnetic field by means of data matrix manipulation of the surface. The elastomer surface is created randomly using the Garcia and Stoll method to realize a nonuniform morphology similar to that found in real MREs. Deformations are induced by means of the translations of the magnetic particles inside the elastomer, under the influence of a uniform magnetic field, generating changes in the surface roughness. Our model computes these deformations using a three-dimensional Gaussian function bounded at 2 standard deviations from its mean value, taking as the standard deviation value the radius of the particle that causes the deformation. To find the regions deformed by the particles, we created a methodology based on the consultation, creation and modification of a system of matrices that control each point of the random surface created. This methodology allows us to work with external files of initial and subsequent positions of each particle inside the elastomer, and allows us to manipulate and analyze the results in a smoother and faster way. Results were found to be satisfactory and consistent when calculating the percentage of surface deformation of real systems.