Researchers from the FOM Foundation and Eindhoven University of Technology have successfully made a ‘magnetic domain-wall ratchet’ memory, a computer memory that is built up from moving bits of magnetised areas. This memory potentially offers many advantages compared to standard hard disks, such as a higher speed, lower electricity consumption and much longer life. Using concentrated ion bundles the researchers have influenced the magnetic wires the bits move through, and they have successfully controlled bits at the nanometer scale and subsequently constructed a new memory.
The bits in a nanowire can be conceptualised as areas that can have two possible magnetic directions, a 0 or a 1. Usually all of the bits are simultaneously set at either 0 or 1 during the construction as they reverse like compass needles. The researchers have now demonstrated that bits can be coherently transferred without the information they contain being lost. This method of magnetic data transport is radically different from that in current computers, where rotating magnetic disks are mechanically moved to address data.
Ion irradiation creates an asymmetric potential or ‘ratchet’ for the main walls (visualised as light-yellow spheres). The bit with a magnetic coating is shifted one position to the left by sequentially positioning a field upwards and downwards
By cleverly varying how the ions are fired across a nanowire, a repeating, saw-tooth-shaped energy landscape is created. This asymmetric saw tooth is crucial: it forces a domain wall, the boundary between bits, to move in a single direction under a variable magnetic field. Due to the variable magnetic field the force on the domain wall continually reverses and this is alternately pushed over the incline and then subsequently pushed back against the sharp edge (see Figure 1). After one cycle of the magnetic field two domain walls are pushed up by exactly one position. This net transfer of a bit would be impossible without saw-tooth potential! Figure 2 also shows the first experiment with a circular magnetic wire: two domain walls move under the influence of a variable magnetic field in an anticlockwise direction. By using this circle the domain walls can always rotate and the bits are retained. This one-way traffic of the domains is a movement comparable to that of a rattle or ‘ratchet’.
In a 0.5 nm-thick cobalt ring with a diameter of 12 μm, a bit (black) is coherently transferred in a variable magnetic field (~16 mT) in an anti-clockwise direction.
The movement of magnetic domain walls can be used to build a device known as a shift register, which has applications in memory and logic circuits. However, the application of magnetic domain wall shift registers has been hindered by geometrical restrictions, by randomness in domain wall displacement and by the need for high current densities or rotating magnetic fields. Here, we propose a new approach in which the energy landscape experienced by the domain walls is engineered to favor a unidirectional ratchet-like propagation. The domain walls are defined between domains with an out-of-plane (perpendicular) magnetization, which allows us to route domain walls along arbitrary in-plane paths using a time-varying applied magnetic field with fixed orientation. In addition, this ratchet-like motion causes the domain walls to lock to discrete positions along these paths, which is useful for digital devices. As a proof-of-principle experiment we demonstrate the continuous propagation of two domain walls along a closed-loop path in a platinum/cobalt/platinum strip.