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• Wire-tube nanostructures. • Hysteresis loops. • Barkhausen jumps. We have studied the magnetostatic interactions between two parallel cylindrical nanostructures with wire-tube morphology as a function of their interaxial separation and the magnetic field applied using micromagnetic simulations. We find that the hysteresis curves of the system can exhibit one, two or three Barkhausen jumps as a function of the separation between the nanostructures and the angle at which the external magnetic field is applied, which are related to the reversal of the magnetic moments of certain segments of the nanostructures. In addition, by studying the averages of magnetization we obtain that for angles less than or equal to 60 degrees, the nanostructures reverse their magnetization through the nucleation and propagation of domain walls (vortex for the tube segment and transverse for the wire segment), while for higher angles the nanostructures revert their magnetization by quasi-coherent rotation. [ABSTRACT FROM AUTHOR]

An analytical expression for the energy of Neel skyrmions in ultra-thin nanodots considering exchange, uniaxial anisotropy, Dzyaloshinskii-Moriya, and dipolar contributions has been obtained. In particular, we have proposed for the Neel skyrmion, a general ansatz for the magnetization component perpendicular to the dot, given by m z ( r ) = [ 1 - ( r / R s ) n ] / [ 1 + ( r / R s ) n ] , where R s is the radius of the skyrmion and n is an even integer number. As proof of concept, we calculate the energy of a Neel skyrmion in an ultra-thin Co/Pt cylinder, and we find that the dipolar contribution cannot be neglected and that both Dzyaloshinskii-Moriya interaction and anisotropy play an important role to stabilize the skyrmion. Additionally, we have obtained a good agreement between our analytical calculations and previously published micromagnetic simulations for n = 10 . For this reliable value of n , we have obtained that for a Dzyaloshinski Moriya constant D = 5.5 (mJ/m 2 ), it is possible to stabilize a Neel skyrmion for K u in the range 0.4 (MJ/m 3 ) < K u < 1.3 (MJ/m 3 ), whereas for K u = 0.8 ( MJ / m 3 ) , the skyrmion stabilizes for 5.0 (mJ/m 2 ) < D < 6.0 (mJ/m 2 ). Thus, this analytical equation can be widely used to predict stability ranges for the Neel skyrmion in spintronic devices. [ABSTRACT FROM AUTHOR]

Ni 81 Fe 19 O y thin films with different thicknesses were obtained by atomic layer deposition. After a thermal reduction process, Ni 81 Fe 19 O y films were converted to their strong Ni 81 Fe 19 ferromagnetic phase, usually known as Permalloy. The different thicknesses were obtained by considering 500, 1000, 1500 and 2000 supercycles, which are defined as a cycle of NiCp 2 /O 3 followed by another cycle of FeCp 2 /O 3 . The samples were characterized before and after the thermal reduction process with respect to their crystallinity, morphology and magnetism. In particular, the thermally reduced samples exhibit holes whose sizes increase significantly as the thickness of the samples also increases, affecting their magnetic behaviour. As Permalloy is widely used in many applications, its synthesis through atomic layer deposition is expected to introduce comparative advantages over other synthesis processes, opening up new possibilities for nanoelectronic devices. [ABSTRACT FROM AUTHOR]

The magnetization reversal mechanisms in soft/hard multisegmented nanowires have been investigated using numerical simulations. In most of the studied systems the magnetization reversal process starts at the ends. However in short soft/hard/soft nanowires, the magnetization reversal process starts at the center of the wire, generating an unusual behavior of the coercivity as a function of the length. [ABSTRACT FROM AUTHOR]

• Fe 2 O 3 thin films can be obtained at 200 °C by atomic layer deposition. • Fe 2 O 3 films can be reduced to Fe 3 O 4 films by annealing in H 2 /Ar at 400 °C. • Different thicknesses were obtained considering from 500 to 2000 cycles of FeCp 2 /O 3. • Controlling the thickness of Fe 3 O 4 films allows adjusting their magnetic properties. Hematite (α-Fe 2 O 3) thin films with different thicknesses between 4 and 16 nm were synthesized by atomic layer deposition, employing 500 to 2000 cycles of FeCp 2 /O 3. After a thermal reduction process, under a controlled atmosphere of hydrogen, it was possible to convert hematite to magnetite (Fe 3 O 4). SEM images of the iron oxide thin films showed that they are macroscopically homogeneous, although the magnetic measurements by MOKE indicated that the samples should present holes that are responsible for pinning the domain walls, raising the coercivity to values ​​that vary between 0.3 and 0.7 kOe. XRD revealed the presence of the Fe 3 O 4 phase by identifying the main peaks. The signal obtained by Raman spectroscopy was identified at 667 cm−1, which suggests the presence of the phonon mode A1g of Fe 3 O 4 in the reduced sample. Controlling the thickness of Fe 3 O 4 thin films allows adjusting their magnetic properties, coercivity and remanence, so that they can be used in potential technological applications. [ABSTRACT FROM AUTHOR]

• Non-monotonic coercivity and remanence as a function of diameter for Py and Ni NTs. • Wave reversal mode could be identified by the area enclosed in the hysteresis curve. • New phase diagram allows determining the stability of the wave reversal mode. The wave reversal mode is a magnetization reversal mechanism that appears in ferromagnetic nanotubes of certain geometric parameters when an external magnetic field is applied perpendicular to their axes. The distinctive feature of this mode is that leads to well-defined S-shaped hysteresis curves. In order to gain insight into the stability of this latter effect, we have performed micromagnetic simulations for permalloy and nickel nanotubes obtaining a non-monotonic behavior for coercivity as well as for remanence as a function of nanotube diameter for both materials. Motivated on these latter intriguing results, we found that measuring the area that encloses the hysteresis curve is a novel and simple strategy to identify the appearance of the wave reversal mode. An additional contribution of this work is the proposal of a new magnetic phase diagram that allows determining the stability of this reversal mechanism as a function of the geometric and magnetic parameters of the tubes. [ABSTRACT FROM AUTHOR]

• Introducing a diameter modulation on a magnetic nanowire changes the coercive field. • Change the position of the nanowire modulation allows tuning the coercive field. • Modulated magnetic nanowires can be used for magnetic memories. Magnetic nanowires of diameter d with a concentric cylindrical modulation of larger diameter D can lead to the production of devices with a variety of coercive fields. Besides D , the modulation thickness t and its position z along the axis of the wire are varied in the vicinity of values usually reported in the experimental literature. It is shown that the system can be understood as the combination of a long wire (with its usual properties for large aspect ratios), a short wire with lower coercive field and a wider modulation with a tendency to produce an early vortex configuration that softens the whole system from the magnetic point of view. The triggering mechanism to achieve the reversal at different coercive fields is studied by means of snapshots of the configurations just prior to the reversal itself. Altogether, upon spanning the parameter space (D , t , and z) a variety of hysteresis curves are obtained and a large range of coercive fields are shown to be possible. Micromagnetic simulations by means of Mumax3 solving the Landau-Lifshitz-Gilbert equation provide the theoretical framework of this work. Possible extensions of this work are also mentioned. [ABSTRACT FROM AUTHOR]

• Permalloy nanowires of 30 nm in diameter and 1 μm in length obtained by AC electrodeposition. • Coercivity and remanence decrease with increasing angles at which the external field is applied. • Magnetostatic interaction modifies the shape of hysteresis curves of simulated Py NW arrays. • Py NW arrays reverse magnetization by nucleation and propagation of transverse domain walls. • Jumps in magnetization observable in hysteresis curves are proportional to the number of simulated Py NWs. This work contributes to the understanding of angular dependence of the magnetic properties of Permalloy nanowire arrays by means of a comparative analysis between experiment and micromagnetic simulations. On the one hand, Permalloy nanowires of 30 nm in diameter and 1 μm in length were synthesized by AC electrodeposition inside mesoporous alumina templates. Magnetic experimental measurements show that both coercivity and remanence decrease monotonously as the angle, at which the external field is applied, increases. On the other hand, the comparison between the simulation of a nanowire array and that performed for one single nanowire reveals that the magnetostatic interactions between the nanowires strongly modifies not only the shape of the hysteresis curve but also the coercivity and remanence values. Besides, simulations also disclose that the nanowires reverse their magnetization through the nucleation and propagation of transverse domain walls. Moreover, the simulation of an array of nanowires also discloses that the hysteresis curve exhibits noticeable jumps in the magnetization that are proportional to the number of nanowires that reverse their magnetization in the array. This latter could provide an interesting hint to control the performance of magnetic nanodevices. [ABSTRACT FROM AUTHOR]

• We have performed micromagnetic simulations of Ni and Py nanotubes as a function of the external magnetic field. • A new striking reversal mechanism, which we have called wave reversal mode (W), has been identified. • This phenomenon describes unusual behaviors in nanotubes when the field is applied perpendicular to its axis. • A sharp increase in remanence is an unequivocal signal that the system reverses according to this mechanism. We have investigated the magnetic properties of 14 nm thick and 1 μm long nickel and permalloy nanotubes with external diameters of 40 and 100 nm as a function of the angle θ at which the external magnetic field is applied. Our results show that the coercivity of 40 nm diameter nickel nanotubes follows a non-monotonic behavior from θ = 0° up to θ = 60°, while that corresponding to permalloy displays an increasing monotonic trend at the same angular range. At θ = 90°, both materials evidence a sharp drop of the coercivity to zero, indicating that the reversal mechanism has changed to a pseudo-coherent rotation. On the other hand, nickel and permalloy nanotubes with 100 nm in diameter exhibit a similar angular dependence of the coercivity, reversing their magnetization through the nucleation and propagation of vortex domain walls for angles lower than 75°. For θ = 90°, a novel striking mechanism, the wave reversal mode (W), arises. This phenomenon leads to an unusual S-type shape in the hysteresis curves at those given parameters, which is until now an effect that has not been reported for these nanostructures. [ABSTRACT FROM AUTHOR]