Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM

• Published in: arXiv.org
• Main Authors: Santos, Tiffany S, Mihajlovic, Goran, Smith, Neil, J -L Li, Carey, Matthew, Katine, Jordan A, Terris, Bruce D
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 04.08.2020
• Subjects: Critical current density; Damping; Diameters; Domain walls; Magnetic saturation; Magnetic switching; Magnetization; Physics - Applied Physics; Physics - Materials Science; Physics - Mesoscale and Nanoscale Physics; Random access memory; Thermal stability; Thickness; Torque
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2008.01343

The critical current density \(J_{c0}\) required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (STT-MRAM) cell is proportional to the product of the damping parameter, saturation magnetization and thickness of the free layer, \(\alpha M_S t_F\). Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of sub-monolayer thickness, we have reduced \(t_F\) while minimizing \(\alpha\) and maximizing \(M_S\), ultimately leading to lower \(J_{c0}\) while maintaining high thermal stability. Bottom-pinned MRAM cells with device diameter in the range of 55-130 nm were fabricated, and \(J_{c0}\) is lowest for the thinnest (1.2 nm) FLs, down to 4 MA/cm\(^2\) for 65 nm devices, \(\sim\)30% lower than 1.7 nm FLs. The thermal stability factor \(\Delta_{\mathrm{dw}}\), as high as 150 for the smallest device size, was determined using a domain wall reversal model from field switching probability measurements. With high \(\Delta_{\mathrm{dw}}\) and lowest \(J_{c0}\), the thinnest FLs have the highest spin-transfer torque efficiency.