$\mathrm{Hf}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$ is an attractive alternative free-layer structure for the realization of high-thermal-stability magnetoresistance random access memory (MRAM) because of its enhanced perpendicular magnetic anisotropy (PMA). Writing energy, which is equally crucial for MRAM, is determined not just by its PMA, but also by Gilbert damping and is so far unknown for this system. These parameters are particularly important in ultrathin ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ $(t_{\mathrm{CFB}}<1.3\mathrm{nm})$ for implementation in MRAM. Within this short thickness range, we find that the measured damping varies dramatically from approximately 0.026 to approximately 0.012. This strong thickness-dependent damping is attributed to extrinsic spin-pumping effects as revealed by the large spin-mixing conductance of approximately ${10}^{20}{\mathrm{m}}^{-2}$. It is also found through first-principles calculations that the intrinsic damping in this system will be larger in comparison to bulk ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$. We also establish that a previously excluded higher-order anisotropy term is crucial in the accurate determination of interfacial PMA in $\mathrm{Hf}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$. Unlike similar perpendicular heterostructures, this system provides wide tunability in damping, opening up opportunities for niche applications in spintronics.

• Received 2 May 2018
• Revised 22 July 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.044057