The electron spin gives access to additional device functionalities that are not exploited in traditional charge-based devices, and that find use in a variety of application fields. In this section we focus on the information and communication technologies (ICT) where spin-based effects are the key to hard disk read heads, non-volatile memories, RF oscillators, logic devices, etc. Our work on the use of nanomagnetism for bioelectronic systems is outlined in the section on magnetic actuators, (see section Magnetic actuators).
Spin torque oscillators
When electrons flow through a magnetic material, they experience a force that tries to align their spin with the magnetization of the material. As a reaction, the electrons exert a force back on the magnetic material, which is called the spin torque. When the current density is large enough (at present typically 106-107 A/cm2), and when the electron spins have been polarized by a first magnetic layer, this spin torque can be used to manipulate the magnetization of a second magnetic layer without the need for external magnetic fields. This can be used to reverse the magnetization, e.g. in spin-torque based magnetic random access memories (MRAM), or to drive a sustained precession of the magnetization, leading to spin-torque based RF oscillators. As the total current is directly proportional with the device area, the performance and power consumption of these devices scales very well towards nanoscale devices.
Our work at imec focuses on spin-torque oscillators in either a nanopillar P20462, P20585, P18745, P21145 or a point contact configuration C20681, C21885, P21882, C21895, C21886, C20680, C21884, C21896, C20691, C20690. These oscillators cover the important frequency range from a few 100MHz (vortex oscillations in point contact devices) to more than 10GHz (nanopillars). We have previously shown the pure current-driven operation of these oscillators, as well as the rapid modulation of the operating frequency.
Figure 1: Schematic configurations of nanopillar and nanocontact devices.
Our work has work continued towards better understanding and control of the oscillation modes in these devices. In nanopillars we demonstrated oscillations of both the free magnetic layer and the reference magnetic layer P20462. Our work on point contacts was done within the framework of the SpinSwitch project RP177 and focused on the so-called vortex oscillations. Here a magnetic vortex is formed that gyrates around the electrical current injected through the point contact. In a joined effort with the Université Paris-Sud we have revealed the vortex nucleation mechanism consisting of a spontaneous formation of a close vortex-antivortex pair, followed by their separation over a characteristic energy barrier, and the ejection of the antivortex from the system P21882.
Figure 2: Left: magnetization configuration of a vortex-antivortex pair. Right: Energy landscape of a vortex-antivortex pair as function of their mutual distance. Curves are shown for different values of the current through the point contact.
Noise performance, with phase noise dominating the linewidth of the oscillations C20680, and showed how reversal of the vortex core polarity can account for the frequency hopping that is observed under certain operating conditions C21886.
Domain wall devices
Nanowires or nanostripes made of ferromagnetic materials are another candidate for spin-based information storage or transport. A bit of information can be stored in the magnetic domain wall (DW) between adjacent regions with opposite magnetization directions. The domain wall can be moved by an applied magnetic field, or by a current through the stripe. At small applied fields the DW velocity increases linearly with the applied field. This regime can be characterized by a domain wall mobility. At higher fields the internal geometry of the domain wall becomes instable and oscillates between multiple conformations. This leads to an oscillatory forward and backward motion of the domain wall that results in a very low average velocity and that is known as Walker breakdown.
Within the framework of the Dynamax project RP42, imec has developed new techniques to increase the domain wall mobility and the maximum velocity before breakdown. So far the material of choice for domain wall devices has been Ni80Fe80 (also known as permalloy). This soft ferromagnetic material is popular since it exhibits low pinning of domain walls. We have demonstrated a new material, NiMnSb that exhibits similar properties, but that has in addition a very low magnetic damping constant α. This results in a very low dissipation and a domain wall mobility that is 2 to 5 times larger than for Ni80Fe80, allowing device operation at smaller fields and thus smaller power consumption D22140.
A second breakthrough is the introduction of a built-in transverse magnetic anisotropy in the material, which is e.g. possible by simply applying a transverse magnetic field during the deposition of the material. We showed that this influences the domain wall instabilities that cause Walker breakdown, by modifying the internal energy of the domain wall. This leads to an increase in both the domain wall mobility and the maximum domain wall velocity that can be obtained before breakdown. In addition we identified a novel regime where the velocity saturates above the breakdown field rather than collapsing to zero D22140. This can lead to more robust devices that can be operated closer to their maximum performance point.
Figure 3: Micromagnetic simulations of the domain wall velocity as function of the applied external field. With an additional transverse anisotropy (red) the mobility (initial slope) and maximum velocity are larger than without anisotropy (black). The right panel shows a novel regime where the velocity does not drop to zero beyond the breakdown field.