Abstract
A long piece of magnetic material shaped as a central cylindrical wire (diameter d=50 nm) with two wider coaxial cylindrical portions (diameter D=90 nm and thickness t=100 nm) defines a bimodulated nanowire. Micromagnetism is invoked to study the equilibrium energy of the system under the variations of the positions of the modulations along the wire. The system can be thought of as composed of five independent elements (3 segments and 2 modulations) leading to 2^5=32 possible different magnetic configurations, which will be later simplified to 4. We investigate the stability of the configurations depending on the positions of the modulations. The relative chirality of the modulations has negligible contributions to the energy and they have no effect on the stability of the stored configuration. However, the modulations are extremely important in pinning the domain walls that lead to consider each segment as independent from the rest. A phase diagram reporting the stability of the inscribed magnetic configurations is produced. The stability of the system was then tested under the action of external magnetic fields and it was found that more than 50 mT are necessary to alter the inscribed information. The main purpose of this paper is to find whether a prototype like this can be complemented to be used as a magnetic key or to store information in the form of firmware. Present results indicate that both possibilities are feasible.
Highlights
A long piece of magnetic material shaped as a central cylindrical wire with two wider coaxial cylindrical portions defines a bimodulated nanowire
In order to synthesize cylindrical nanowires there are mainly two ways: template-free[18,19] or templateassisted[20,21,22] methods. This last method based on self-organized porous membranes has made notable advances, allowing us to dream of the design proposed by Parkin in 2008 of a racetrack memory[23]. This memory is based on the fact that information can be stored in a solid-state device through magnetic domains separated by magnetic domain walls
The idea is that the position of these magnetic domain walls can be precisely controlled through pinning centers, which can be generated by varying the composition of the nanowire[28,29,30] or by introducing geometrical inhomogeneities[20,24,31,32,33,34,35,36,37,38,39], such as modulations in its diameter during the synthesis process
Summary
This last method based on self-organized porous membranes has made notable advances, allowing us to dream of the design proposed by Parkin in 2008 of a racetrack memory[23] This memory is based on the fact that information can be stored in a solid-state device through magnetic domains (areas where magnetic moments point in a defined direction) separated by magnetic domain walls (areas where magnetic moments vary from the direction of one domain to the other). Diameter modulations of the nanowire effectively allow controlling the domain wall positions since they locally reduce the magnetostatic and exchange energy in the different cross-sectional p arts[40,41,42,43,44,45,46,47,48]. We propose here permalloy as the material of study, this is merely by simplicity, bearing in mind that the search for properties in nanowires of other materials is a very actual research field[50]
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