[Paper Review] Low Gilbert Damping Constant in Perpendicularly Magnetized W/CoFeB/MgO Films with High Thermal Stability

Dustin M. Lattery, Delin Zhang+3 more|arXiv (Cornell University)|Sep 21, 2017

Magnetic properties of thin films32 references3 citations

TL;DR

This study investigates the Gilbert damping constant (α) in W/CoFeB/MgO films with perpendicular magnetic anisotropy (PMA), using time-resolved magneto-optical Kerr effect to measure α across varying post-annealing temperatures. A minimum α of 0.016 is achieved at 350 °C, attributed to competing effects of CoFeB crystallization and interfacial dead-layer growth, with enhanced thermal stability confirmed by comparison to Ta-based reference films.

ABSTRACT

Perpendicular magnetic materials with low damping constant and high thermal stability have great potential for realizing high-density, non-volatile, and low-power consumption spintronic devices, which can sustain operation reliability for high processing temperatures. In this work, we study the Gilbert damping constant (α) of perpendicularly magnetized W/CoFeB/MgO films with a high perpendicular magnetic anisotropy (PMA) and superb thermal stability. The α of these PMA films annealed at different temperatures is determined via an all-optical Time-Resolved Magneto-Optical Kerr Effect method. We find that α of these W/CoFeB/MgO PMA films decreases with increasing annealing temperature, reaches a minimum of α = 0.016 at an annealing temperature of 350 °C, and then increases to 0.024 after post-annealing at 400 °C. The minimum α observed at 350 °C is rationalized by two competing effects as the annealing temperature becomes higher: the enhanced crystallization of CoFeB and dead-layer growth occurring at the two interfaces of the CoFeB layer. We further demonstrate that α of the 400 °C-annealed W/CoFeB/MgO film is comparable to that of a reference Ta/CoFeB/MgO PMA film annealed at 300 °C, justifying the enhanced thermal stability of the W-seeded CoFeB films.

Motivation & Objective

• To determine the temperature dependence of the Gilbert damping constant (α) in perpendicularly magnetized W/CoFeB/MgO films.
• To identify the mechanisms governing α variation with post-annealing temperature.
• To evaluate the thermal stability of W-seeded CoFeB films relative to conventional Ta/CoFeB/MgO structures.
• To optimize α and thermal stability for application in high-density, low-power spintronic devices.

Proposed method

• Time-resolved magneto-optical Kerr effect (TR-MOKE) was used to measure the Gilbert damping constant (α) in W/CoFeB/MgO films.
• Samples were post-annealed at temperatures ranging from 300 °C to 400 °C to study thermal effects on α.
• The damping constant was extracted from the decay of the magnetization dynamics following ultrafast optical excitation.
• Comparative measurements were performed on a Ta/CoFeB/MgO reference film annealed at 300 °C.
• The role of interfacial dead-layer formation and CoFeB crystallization was analyzed to explain the α trend with annealing temperature.
• Perpendicular magnetic anisotropy (PMA) was maintained across all annealing conditions to ensure consistent magnetic orientation.

Experimental results

Research questions

• RQ1 How does the Gilbert damping constant (α) in W/CoFeB/MgO films vary with post-annealing temperature?
• RQ2 What are the dominant microscopic mechanisms responsible for the observed α dependence on annealing temperature?
• RQ3 How does the thermal stability of W/CoFeB/MgO films compare to that of conventional Ta/CoFeB/MgO films?
• RQ4 Can a low α value be simultaneously achieved with high thermal stability in W-seeded PMA films?
• RQ5 What is the optimal annealing temperature for minimizing α while maintaining structural and magnetic integrity?

Key findings

• The Gilbert damping constant (α) in W/CoFeB/MgO films decreases with increasing annealing temperature up to 350 °C, reaching a minimum value of α = 0.016.
• At 400 °C, α increases to 0.024, indicating a degradation of damping properties due to enhanced interfacial dead-layer formation.
• The minimum α at 350 °C results from a balance between improved CoFeB crystallization (reducing α) and growing interfacial dead layers (increasing α).
• The α value of the 400 °C-annealed W/CoFeB/MgO film is comparable to that of a Ta/CoFeB/MgO film annealed at 300 °C, confirming superior thermal stability of the W-seeded structure.
• The W/CoFeB/MgO system achieves a low damping constant (α = 0.016) while maintaining high thermal stability, making it suitable for high-temperature, low-power spintronic applications.
• The study demonstrates that interfacial engineering and controlled annealing can simultaneously optimize both α and thermal stability in PMA-based magnetic heterostructures.