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 (Tann) 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 Tann, reaches a minimum of α = 0.015 at Tann = 350 °C, and then increases to 0.020 after post-annealing at 400 °C. The minimum α observed at 350 °C is rationalized by two competing effects as Tann 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.
Highlights
Since the first demonstration of perpendicular magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) Ta/CoFeB/MgO stacks[1], interfacial PMA materials have been extensively studied as promising candidates for ultra-high-density and low-power consumption spintronic devices, including spin-transfer-torque magnetic random access memory (STT-MRAM)[2,3], electrical-field induced magnetization switching[4,5,6], and spin-orbit torque (SOT) devices[7,8,9]
While there have been a few scattered studies demonstrating the promise of fabricating SOT devices using the W/CoFeB/MgO stacks, special attention has been given to their PMA properties and functionalities as SOT devices[26,27], or the damping of in-plane W/CoFeB stacks[28]
The PMA in the W/CoFeB/MgO films is dominated by the interface anisotropy (Ki), which increases from 1.4 to 2.8 erg cm−2 with Tann up to 400 °C (Fig. 1g)
Summary
Since the first demonstration of perpendicular magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) Ta/CoFeB/MgO stacks[1], interfacial PMA materials have been extensively studied as promising candidates for ultra-high-density and low-power consumption spintronic devices, including spin-transfer-torque magnetic random access memory (STT-MRAM)[2,3], electrical-field induced magnetization switching[4,5,6], and spin-orbit torque (SOT) devices[7,8,9]. Spintronic devices need to sustain operation reliability for processing temperatures as high as 400 °C for their integration with existing CMOS fabrication technologies, providing the standard back-end-of-line process compatibility[14]. Based on this requirement, the magnetic properties of a PMA material should be thermally stable at annealing temperatures (Tann) up to 400 °C. Modifying the composition of thin-film stacks can prevent heavy metal diffusion, which is beneficial to both lowering α and improving thermal stability[20] Along this line, new interfacial PMA stacks have been developed, such as www.nature.com/scientificreports/. A systematic investigation is lacking on the effect of Tann on α of W/CoFeB/MgO PMA thin films with perpendicular anisotropy, as well as the physical mechanisms that alter α after post-annealing
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