The magnetic switching of ferromagnetic nanotubes is investigated as a function of their geometry. Two independent methods are used: Numerical simulations and analytical calculations. It is found that for long tubes the reversal of magnetization is achieved by two mechanism: The propagation of a transverse or a vortex domain wall depending on the internal and external radii of the tube.During the last decade, interesting properties of magnetic nanowires have attracted great attention. Besides the interest in their basic properties, there is evidence that they can be used in the production of new devices. More recently magnetic nanotubes have been grown 1,2,3,4 motivating a new research field. Magnetic measurements, 3 numerical simulations 4 and analytical calculations 5 on such tubes have identified two main states: an in-plane magnetic ordering, namely the fluxclosure vortex state, and a uniform state with all the magnetic moments pointing parallel to the axis of the tube. An important problem is to establish the way and conditions for reversing the orientation of the magnetization. Although the reversal process is well known for ferromagnetic nanowires, 6,7,8,9,10 the equivalent phenomenon in nanotubes has been poorly explored so far in spite of some potential advantages over solid cylinders. Nanotubes exhibit a core-free magnetic configuration leading to uniform switching fields, guaranteeing reproducibility, 4,5 and due to their low density they can float in solutions making them suitable for applications in biotechnology (see [1] and refs. therein).Let us consider a ferromagnetic nanotube in a state with the magnetization M along the tube axis. A constant and uniform magnetic field is then imposed antiparallel to M. After some delay time the magnetization reversal (MR) will start at any end. MR or magnetic switching can occur by means of different mechanisms, depending on the geometrical parameters of the tube. In this paper we will focus on the reversal process by means of two different but complementary approaches: numerical simulations and analytical calculations. Their mutual agreement sustains the results reported in this study.Numerical Simulations. Geometrically, tubes are characterized by their external and internal radii, R and a respectively, and height, H. It is convenient to define the ratio β ≡ a/R, so that β = 0 represents a solid cylinder and β → 1 correspond to a very narrow tube. The internal energy, E, of a nanotube with N magnetic moments can be written aswhere E ij is the dipolar energy given by E ij = µ i · µ j − 3(µ i ·n ij )(µ j ·n ij ) /r 3 ij , with r ij the distance between the magnetic moments µ i and µ j ,μ i the unit vector along the direction of µ i andn ij the unit vector along the direction that connects µ i and µ j . J ij = J is the exchange coupling constant between nearest neighbors and J ij = 0 otherwise. E a = − N i=1 µ i · H a is the contribution of the external magnetic field. In this paper we are interested in soft magnetic materials, in which case anisotropy can be s...

In the Edwards-Anderson model of spin glasses with a bimodal distribution of bonds, the degeneracy of the ground state allows one to define a structure called backbone, which can be characterized by the rigid lattice (RL), consisting of the bonds that retain their frustration (or lack of it) in all ground states. In this work we have performed a detailed numerical study of the properties of the RL, both in two-dimensional (2D) and three-dimensional (3D) lattices. Whereas in 3D we find strong evidence for percolation in the thermodynamic limit, in 2D our results indicate that the most probable scenario is that the RL does not percolate. On the other hand, both in 2D and 3D we find that frustration is very unevenly distributed. Frustration is much lower in the RL than in its complement. Using equilibrium simulations we observe that this property can be found even above the critical temperature. This leads us to propose that the RL should share many properties of ferromagnetic models, an idea that recently has also been proposed in other contexts. We also suggest a preliminary generalization of the definition of backbone for systems with continuous distributions of bonds, and we argue that the study of this structure could be useful for a better understanding of the low temperature phase of those frustrated models.

No abstract

Jamming and percolation of square objects of size k × k (k 2-mers) isotropically deposited on simple cubic lattices have been studied by numerical simulations complemented with finite-size scaling theory. The k 2-mers were irreversibly deposited into the lattice. Jamming coverage θ j,k was determined for a wide range of k (2 ≤ k ≤ 200). θ j,k exhibits a decreasing behavior with increasing k, being θ j,k→∞ = 0.4285(6) the limit value for large k 2-mer sizes. On the other hand, the obtained results shows that percolation threshold, θ c,k , has a strong dependence on k. It is a decreasing function in the range 2 ≤ k ≤ 18 with a minimum around k = 18 and, for k ≥ 18, it increases smoothly towards a saturation value. Finally, a complete analysis of critical exponents and universality has been done, showing that the percolation phase transition involved in the system has the same universality class as the 3D random percolation, regardless of the size k considered.

We study the efficiency of parallel tempering Monte Carlo technique for calculating true ground states of the Edwards-Anderson spin glass model. Bimodal and Gaussian bond distributions were considered in two and three-dimensional lattices. By a systematic analysis we find a simple formula to estimate the values of the parameters needed in the algorithm to find the GS with a fixed average probability. We also study the performance of the algorithm for single samples, quantifying the difference between samples where the GS is hard, or easy, to find. The GS energies we obtain are in good agreement with the values found in the literature. Our results show that the performance of the parallel tempering technique is comparable to more powerful heuristics developed to find the ground state of Ising spin glass systems.