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...

The magnetization reversal in ordered arrays of iron oxide nanotubes of 50 nm outer diameter grown by atomic layer deposition is investigated theoretically as a function of the tube wall thickness, dw. In thin tubes (dw < 13 nm) the reversal of magnetization is achieved by the propagation of a vortex domain boundary, while in thick tubes (dw > 13 nm) the reversal is driven by the propagation of a transverse domain boundary. Magnetostatic interactions between the tubes are responsible for a decrease of the coercive field in the array. Our calculations are in agreement with recently reported experimental results. We predict that the crossover between the vortex and transverse modes of magnetization reversal is a general phenomenon on the length scale considered. PACS numbers: 75.75.+a, 75.10.-b arXiv:1106.2833v1 [cond-mat.mes-hall]

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Magnetic properties of arrays of nanowires produced inside the pores of anodic alumina membranes have been studied by means of vibrating sample magnetometer techniques. In these systems the length of the wires strongly influences the coercivity of the array. A simple model for the coercivity as a function of the geometry is presented which exhibits good agreement with experimental results. Magnetostatic interactions between the wires are responsible for a decrease of the coercive field.

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