Impact of Magnetic Coupling and Density on STT-MRAM Performance

TL;DR

This paper models intra- and inter-cell magnetic coupling in STT-MRAMs, analyzing their effects on performance and retention, and proposes a coupling factor to optimize array density while maintaining device reliability.

Contribution

It introduces an analytical model for magnetic coupling in STT-MRAMs and proposes the inter-cell magnetic coupling factor Psi to optimize array density.

Findings

Psi=2% maximizes array density with negligible performance impact

Inter-cell magnetic coupling causes variations in switching time at low voltages

Marginal degradation of data retention due to magnetic coupling

Abstract

As a unique mechanism for MRAMs, magnetic coupling needs to be accounted for when designing memory arrays. This paper models both intra- and inter-cell magnetic coupling analytically for STT-MRAMs and investigates their impact on the write performance and retention of MTJ devices, which are the data-storing elements of STT-MRAMs. We present magnetic measurement data of MTJ devices with diameters ranging from 35nm to 175nm, which we use to calibrate our intra-cell magnetic coupling model. Subsequently, we extrapolate this model to study inter-cell magnetic coupling in memory arrays. We propose the inter-cell magnetic coupling factor Psi to indicate coupling strength. Our simulation results show that Psi=2% maximizes the array density under the constraint that the magnetic coupling has negligible impact on the device's performance. Higher array densities show significant variations in average switching time, especially at low switching voltages, caused by inter-cell magnetic coupling, and dependent on the data pattern in the cell's neighborhood. We also observe a marginal degradation of the data retention time under the influence of inter-cell magnetic coupling.