The ability to control the magnetization direction in ferromagnetic elements is critical to magnetic storage. In present devices, an external magnetic field is used for this purpose.
Schematic illustration of the experimental zigzag permalloy wire (grey) attached to electrical contacts (yellow). When the ferromagnetic wire is subjected to a small magnetic field (H) and a current pulse is introduced, the magnetization in the central part (blue arrows) is reversed (red arrows). (Image: RIKEN)
The use of currents is a promising alternative to magnetic fields. According to Yoshihiko Togawa of RIKEN, magnetic storage devices with high recording densities require a large magnetic field to reverse the magnetization, which causes undesirable interference with neighboring magnetic bits and degrades the device performance. “Spin-polarized current induces the magnetization reversal just in the magnetic bit where the current flows,” he explains.
It is well known that when a spin-polarized current flows through a wall, which separates two domains with opposite magnetization, the flowing electrons can affect the magnetization by the so-called spin torque effect. This results in moving the domain wall and consequent reversing the magnetization in one of the domains. In addition, scientists have predicted that even when magnetization is uniform, current pulses can nucleate domain walls, move them and eventually reverse the magnetization direction. However, few experiments in wires have been done.
Now, Togawa and colleagues have studied a zigzag permalloy nanowire attached to large pads used as electrical contacts. They verified that when no external magnetic field is applied, the probability for a current pulse to switch the magnetization is very low. However, as soon as a small field is applied in the direction opposite to the magnetization, the probability rapidly increases and reaches 100% for a magnetic field of just 3.8 Oe.
The researchers have found that the right combination of current and magnetic field is needed for the reversal to occur (Fig. 1). For example, if the field is applied parallel to the magnetization the reversal probability is extremely low.
Although a magnetic field is still necessary for the reversal to occur, this is much lower than the field needed to reverse the magnetization without the use of current pulses. “We can perform magnetization reversal in [targeted] bits using current, which is free from any interference … due to large magnetic field,” says Togawa. “We believe that [our] method is effective and advantageous [for] storage device integration.”