Periodic Reporting for period 3 - SMART DESIGN (Spin-orbit mechanism in adaptive magnetization-reversal techniques, for magnetic memory design)
Reporting period: 2018-10-01 to 2020-03-31
Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) has been identified by the ITRS as the best candidate. STT-MRAM memory cell is formed by two ferromagnetic layers (“fixed” and “free” layer”) separated by an insulator - a magnetic tunnel junction (MTJ). The information is stored in the magnetization of the “free” layer. The magneto-resistance of the MTJ depends on the orientation of the free layer. For reading, a low bias voltage is applied while for writing, a larger voltage is required; The electric current, spin polarized by the fixed layer, carries spin angular momentum to the free layer (hence the name Spin Transfer Torque - STT). Inherent to its working principle, this memory element has high density, good scaling and low consumption. However, during the writing process, the voltage can damage the thin insulating layer, limiting the speed as well as the endurance.
We use an alternative way to switch the magnetization, not by spin transfer from a ferromagnet, but by transferring angular momentum directly from the crystal lattice. The Spin-Orbit Torque (SOT) occurs in materials with large spin orbit coupling that lack structural inversion symmetry. From a practical perspective, the most significant difference is related to the geometry of the current injection. Instead of passing the current vertically, like in STT, the SOT switching relies on in-plane current injection. This improves the reliability of the memory element since there is no electric stress on the tunnel barrier during the writing phase.
Based on this difference, we propose to use the liberty allowed by the in-plane current injection to design novel magnetization switching schemes. We have discovered that the lateral shape of the devices, as defined by lithography during the device fabrication, can be used to determine the SOT induced switching of the magnetization. Magnetic objects having different shape will react differently to the electric current. For example, depending on their shape, two such objects can switch in opposite directions when subjected to the same current pulse.
Our current efforts are threefold: (i) we develop new materials with efficient SOT; (ii) we design novel geometries that lead to original switching schemes, and study the magnetization dynamics in such structures using magneto-optical imaging; (iii) we are building a near-field optical microscope to probe the magnetization dynamics with high spatial (50nm) and temporal resolution (30ps).