Temperature-dependent analysis of retention and write characteristics of STT-MRAM cells

Abstract

Spin-transfer torque magnetic random-access memory (STT-MRAM) is being investigated for automotive applications. However, to ensure reliable operation across the wide temperature ranges required in automotive systems, it is important to understand the thermal variation of data retention and switching characteristics, such as thermal stability factor (Δ), critical switching current (IC), and STT efficiency. We investigated Δ over a range of −25°C–150°C, and IC from −25°C to 25°C across devices with four different storage layer (SL) thicknesses. The Δ values were extracted using the macrospin (MS) and the domain wall nucleation and propagation (DW) models. Both models well explain the decreasing trend of Δ as a function of temperature. Δ decreases by 60%–70% and 63%–65%, respectively, for MS and DW model when the temperature is increased from −25 to 150°C. It is also observed that at higher temperatures, Δ converges for all SL thicknesses to a similar value, indicating stronger thermal sensitivity for thicker layers. As a result, the blocking temperature is larger for thicker SL. The Gilbert damping constant (α), domain wall width (δw), and domain wall energy density (σw) of the devices were also extracted using the DW model. The rapid reduction in σw with temperature suggests a transition in magnetization reversal mechanism from the DW model to multi-domain nucleation and propagation at higher temperatures. This indicates that the magnetization reversal process is highly dependent on operating thermal conditions. IC and STT-switching efficiency decrease with increasing temperature, predominantly due to the reduction in Δ. In addition, IC also reduces upon increasing the SL thickness, which is attributed to a decrease in α. The practical switching efficiency (κ) is also analyzed, which is crucial for benchmarking STT-MRAM performance under application-specific thermal conditions.