Thomas Windbacher was born in Mödling, Austria, in 1979. He studied physics at the Technische Universität Wien, where he received the degree of Diplomingenieur in October 2006. He joined the Institute for Microelectronics in October 2006 and finished his doctoral degree on engineering gate stacks for field-effect transistors in 2010. From 2010 until the beginning of 2012 he worked as a patent attorney candidate in Leoben. In March 2012 he rejoined the Institute for Microelectronics, where he currently works on the modeling and simulation of magnetic device structures.
Magnetization Orientation Encoded Spin-Transfer Torque for an Ultra Dense Shift Register
Nowadays there are two big obstacles hindering further progress in CMOS device technology: Energy dissipation due to leakage and the energy required to copy information between memory and processors. Even though cutting the power of unused circuits reduces the leakage dissipation to zero, it results in the loss of locally stored information. Thus, it must be copied back from memory when the circuit is turned on again. This unfortunately degrades the already strained bandwidth available between memory and processors.
Using nonvolatile elements, which can serve as memory and information processing units, is an attractive path to overcoming these limitations in future computation environments. We therefore proposed nonvolatile circuits based on magnetic flip-flops and a nonvolatile shift register (see Fig. 1) that extensively exploit this feature. Due to the avoidance of signal conversion between the CMOS and the magnetic domain, as well as the inherent nonvolatility of the employed magnetic layers, the proposed circuits exhibit instant-on capabilities, a highly regular structure, a very small layout footprint and a considerable reduction in the actual information transported.
For the copy operation of the shift register, we proposed sending an unpolarized current through a portion of the read flip-flops' free layer to create a spin polarized magnetization orientation encoded current and pass it to a subsequent free layer. There, it exerts a spin-transfer torque and switches the subsequent layer, with the aid of a second clocked spin-transfer torque.
In order to test the feasibility of the operational procedure, we reduced the shift register to two flip-flops (see Fig. 2) and studied all possible input and output combinations via extensive simulation. We found that the switching behaves exactly as required for the proposed copy operation. Thus, the feasibility of the operation, as well as the operability of the shift register, are demonstrated.
Fig. 1: The non-volatile shift register consists of two rows of non-volatile flip-flops that are arranged in two distinct levels. Each free layer of a flip-flop overlaps in two regions with its neighbors on the other level. The polarizer stack in the middle of the free layers is used for the application of the clock signals.
Fig. 2: For this study, the n-Bit shift register from Fig. 1 is reduced to two adjacent flip-flops. For the copy operation, an unpolarized current is sent through Free Layer 2. The polarized current, which has then been encoded with the spin orientation from Free Layer 2, enters Free Layer 1, where the spin-transfer torque acts on the magnetization of the layer. The current pulse through the clock polarizer stack generates a second spin-transfer torque aiding the copy operation by either damping or enforcing the switching of the magnetization in Free Layer 1.
Fig. 3: In order to copy the magnetization orientation stored in the upper layer into the lower layer, an unpolarized current is traversed through their overlapping regions. The then with the spin orientation from the upper layer encoded polarized current enters the lower layer, where it creates a spin-transfer torque that acts on the magnetization of the lower layer. The current pulse through the clock polarizer stack (right box) generates a second spin-transfer torque aiding the copy operation by either damping or enforcing the switching of the magnetization in the lower layer.