Abstract
Perpendicular magnetic materials with low damping constant and high thermal stability have great potential for realizing highdensity, nonvolatile, and lowpower 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 (T_{ann}) is determined via an alloptical TimeResolved MagnetoOptical Kerr Effect method. We find that α of these W/CoFeB/MgO PMA films decreases with increasing T_{ann}, reaches a minimum of α = 0.015 at T_{ann} = 350 °C, and then increases to 0.020 after postannealing at 400 °C. The minimum α observed at 350 °C is rationalized by two competing effects as T_{ann} becomes higher: the enhanced crystallization of CoFeB and deadlayer growth occurring at the two interfaces of the CoFeB layer. We further demonstrate that α of the 400 °Cannealed 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 Wseeded CoFeB films.
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Introduction
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 ultrahighdensity and lowpower consumption spintronic devices, including spintransfertorque magnetic random access memory (STTMRAM)^{2,3}, electricalfield induced magnetization switching^{4,5,6}, and spinorbit torque (SOT) devices^{7,8,9}. An interfacial PMA stack typically consists of a thin ferromagnetic layer (e.g., CoFeB) sandwiched between a heavy metal layer (e.g., Ta) and an oxide layer (e.g., MgO). The heavy metal layer interface with the ferromagnetic layer is responsible for the spin Hall effect, which is favorable for SOT and skyrmion devices^{10,11}. The critical switching current (J_{c0}) should be minimized to decrease the power consumption of perpendicular STTMRAM and SOT devices. Reducing J_{c0} requires the exploration of new materials with low Gilbert damping constant (α) and large spin Hall angle (θ_{SHE}). Meanwhile, a large effective anisotropy (K_{eff}) is also favorable to maintain thermal stability^{12,13}.
In addition, 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 backendofline process compatibility^{14}. Based on this requirement, the magnetic properties of a PMA material should be thermally stable at annealing temperatures (T_{ann}) up to 400 °C. Unfortunately, Ta/CoFeB/MgO PMA films commonly used in spintronic devices cannot survive with T_{ann} higher than 350 °C, due to Ta diffusion or CoFeB oxidation at the interfaces^{15,16,17}. The diffusion of Ta atoms can act as scattering sites to increase the spinflip probability^{18} and lead to a higher Gilbert Damping constant (α), a measure of the energy dissipation from the magnetic precession into phonons or magnons^{19}.
Modifying the composition of thinfilm 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 Mo/CoFeB/MgO, to circumvent the limitation on device processing temperatures^{21,22}. While Mo/CoFeB/MgO films can indeed exhibit PMA at temperatures higher than 400 °C, they cannot be used for SOT devices due to the weak spin Hall effect of the Mo layer^{21,22}. Recently, W/CoFeB/MgO PMA thin films have been proposed because of their PMA property at high postannealing temperature^{23}, and the large (negative) spin Hall angle of the W layer (θ_{SHE} ≈ −0.30)^{24}, which is twice that of a Ta layer (θ_{SHE} ≈ −0.12 ~ −0.15)^{9,25}. 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 inplane W/CoFeB stacks^{28}. A systematic investigation is lacking on the effect of T_{ann} on α of W/CoFeB/MgO PMA thin films with perpendicular anisotropy, as well as the physical mechanisms that alter α after postannealing.
Methods
Sample preparation and magnetic characterization
In this work, we grow a series of W(7)/Co_{20}Fe_{60}B_{20}(1.2)/MgO(2)/Ta(3) thin films on Si/SiO_{2}(300) substrates (thickness in nanometers) with a magnetron sputtering system (<5 × 10^{−8} Torr). These films are postannealed at varying temperatures (T_{ann} = 250–350 °C for 1 hour, 400 °C for 30 minutes) within a highvacuum furnace (<1 × 10^{−6} Torr). After postannealing, the magnetic properties and damping constants of these films are systematically investigated as a function of T_{ann}. For comparison, a reference sample of Ta(7)/Co_{20}Fe_{60}B_{20}(1.2)/MgO(2)/Ta(3) is also prepared to examine the effect of seeding layer to the damping constant of these PMA films. The effective saturation magnetization (M_{s}) and anisotropy of these films are measured with the Vibrating Sample Magnetometer (VSM) module of a Physical Property Measurement System (Quantum Design, DynaCool).
TRMOKE measurements and data reduction
The magnetization dynamics of these PMA thin films are determined using the alloptical TimeResolved MagnetoOptical Kerr Effect (TRMOKE) method^{29,30,31,32,33,34}. This pumpprobe method utilizes ultrashort laser pulses to thermally demagnetize the sample and probe the resulting Kerr rotation angle (θ_{K}). In the polarMOKE configuration, θ_{K} is proportional to the change of the outofplane component of magnetization^{35}. Details of the TRMOKE setup are provided in Section S2 of the SI.
The TRMOKE signal is fitted to the equation
where A, B, and C are the offset, amplitude, and exponential decaying constant of the thermal background, respectively. D denotes the amplitude of oscillations, f is the resonance frequency, φ is a phase shift (related to the demagnetization process), and τ is the relaxation time of magnetization precession. Directly from TRMOKE measurements, an effective damping constant (α_{eff}) can be extracted based on the relationship α_{eff} = 1/(2πfτ). However, α_{eff} is not an intrinsic material property; rather, it depends on measurement conditions, such as the applied field direction (θ_{H}), the magnitude of the applied field (H_{ext}), and inhomogeneities of the sample (e.g. local variation in the magnetic properties of the sample)^{36,37}.
To obtain the Gilbert damping constant, the inhomogeneous contribution needs to be removed from α_{eff}, such that the remaining value of damping is an intrinsic material property and independent of the measurement conditions. To determine the inhomogeneous broadening in the sample, the effective anisotropy field (\({H}_{k,\mathrm{eff}}=2{K}_{{\rm{eff}}}/{M}_{{\rm{s}}}\)) needs to be predetermined from either (1) the magnetic hysteresis loops; or (2) the fitting results of f vs. H_{ext} obtained from TRMOKE. The resonance frequency, f, can be related to H_{ext} through the SmitSuhl approach by identifying the second derivatives of the total magnetic free energy, which combines a Zeeman energy, an anisotropy energy, and a demagnetization energy^{38,39,40}. For a perpendicularly magnetized thin film, f is defined by Eqs 1–4^{40}.
This set of equations permits calculation of f with the material gyromagnetic ratio (γ), H_{ext}, θ_{H}, H_{k,eff}, and the angle between the equilibrium magnetization direction and the surface normal (θ, determined by Eq. 4). The measured values of f as a function of H_{ext} can be fitted to Eq. 1 by treating γ and H_{k,eff} as fitting parameters. To minimize the fitting errors resulting from the inhomogeneous broadening effect that is pronounced at the low fields, we use measured frequencies at high fields (H_{ext} > 10 kOe) to determine H_{k,eff}.
With a known value of H_{k,eff}, the Gilbert damping constant of the sample can be determined through a fitting of the inverse relaxation time (1/τ) to Eq. 5. The two terms of Eq. 5 take into account, respectively, contributions from the intrinsic Gilbert damping of the materials (first term) and inhomogeneous broadening (second term)^{36}:
where H_{1} and H_{2} are related to the curvature of the magnetic free energy surface as defined by Eqs 2 and 3^{40,41}. The second term on the right side of Eq. 5 captures the inhomogeneous effect by attributing it to a spatial variation in the magnetic properties (ΔH_{k,eff}), analogous to the linewidth broadening effect in Ferromagnetic Resonance measurements^{42}. The magnitude of dω/dH_{k,eff} can be calculated once the relationship of ω vs. H_{ext} is determined with a numerical method. Both α and ΔH_{k,eff} (the inhomogeneous term related to the amount of spatial variation in H_{k,eff}) are determined via the fitting of the measured 1/τ based on Eq. 5. In this way, we can uniquely extract the fieldindependent α, as an intrinsic material property, from the effective damping (α_{eff}), which is directly obtained from TRMOKE and dependent on H_{ext}.
It should be noted here that the inhomogeneous broadening of the magnetization precession is presumably due to the multidomain structure of the materials, which becomes negligible in the highfield regime (H_{ext} \(\gg \) H_{k,eff}) as the magnetization direction of multiple magnetic domains becomes uniform. This is also reflected by the fact that the derivative in the second term of Eq. 5 approaches zero for the highfield regime^{43}.
Results and Discussion
Figure 1 plots the magnetic hysteresis loops and associated magnetic properties extracted from VSM measurements. With the increase of T_{ann}, M_{s} for the W/CoFeB/MgO films decreases from ~780 to ~630 emu cm^{−3} (Fig. 1e). The PMA in the W/CoFeB/MgO films is dominated by the interface anisotropy (K_{i}), which increases from 1.4 to 2.8 erg cm^{−2} (excluding the deadlayer thickness effect) with T_{ann} up to 400 °C (Fig. 1g). If the film thicknesses are corrected by subtracting the dead layer, K_{i} will change from 1.3 to 1.6 erg cm^{−2} as T_{ann} increases from 250 to 400 °C, which agrees better with literature values^{44}. Details about the determination of K_{i} including the deadlayer effect are provided in Section S1 of the Supplementary Information (SI).
We attribute the decrease of M_{s} at high T_{ann} to the growth of a dead layer at the CoFeB interfaces, which becomes prominent at higher T_{ann}. To quantitatively determine the thickness of the dead layer as T_{ann} increases, we prepare four sets of PMA stacks of W(7)/CoFeB(t)/MgO(2)/Ta(3). One set contains five stacks with varying thicknesses for the CoFeB layer (t = 1.2, 1.5, 1.8, 2.2, and 2.5 nm) and is postannealed at a fixed T_{ann}. Four T_{ann} of 250, 300, 350, and 400 °C are used for four sets of the PMA stacks, respectively. The annealing conditions are the same as those for the W(7)/CoFeB(1.2)/MgO(2)/Ta(3) samples discussed previously. We measure the magnetic hysteresis loops of these samples using VSM and plot their saturation magnetization area product (M_{S} × t) as a function of film thickness (t) in Fig. 2. Linear extrapolation of the M_{S} × t data provides the deadlayer thickness, at which the magnetization reduces to zero as illustrated by the xaxis intercept in Fig. 2. The slope of the linear fit also provides intrinsic saturation magnetization (M_{s,0}), which corresponds to the saturation magnetization after the removal of the deadlayer effect. The values of M_{s0} (Fig. 1f) show an increasing trend with T_{ann} from ~1300 to ~1600 emu cm^{−3}, which agrees well with previous measurement results for W/CoFeB/MgO films^{44}.
Figure 3a depicts the schematic of polar TRMOKE experiments with the definition of several parameters and angles that are important for the data reduction (Section 2.2). Figure 3b plots the TRMOKE signals (symbols) and the model fitting of θ_{K} (black lines) as functions of time delay for the 400 °C W/CoFeB/MgO sample at an external field angle of θ_{H} = 76°. To determine the values of H_{k,eff} and α, the TRMOKE measurements are conducted at varying H_{ext} from 2 to 20 kOe. Both the f and τ of the measured oscillations, resulting from the magnetization precession, depend greatly on H_{ext} as predicted by Eqs 1 and 5.
By repeating this measurement at varying θ_{H}, we can show that α is an intrinsic material property, independent of θ_{H}. Figure 4a plots the resonance frequencies derived from TRMOKE and model fittings for the 400 °C sample at two field directions (θ_{H} = 76° and 89°). For the data acquired at θ_{H} = 89°, a minimum f occurs at H_{ext} ≈ H_{k,eff}. This corresponds to the smallest amplitude of magnetization precession, when the equilibrium direction of the magnetization is aligned with the applied field direction at the magnitude of H_{k,eff}^{40}. The dip at this local minimum diminishes when θ_{H} decreases, as reflected by the comparison between the red (θ_{H} = 89°) and blue (θ_{H} = 76°) lines in Fig. 4a. With the H_{k,eff} extracted from the fitting of frequency data with θ_{H} = 89°, we generate the plot of theoretically predicted f vs. H_{ext} (θ_{H} = 76° theory, blue line in Fig. 4a), which agrees well with experimental data (open squares in Fig. 4a).
The inverse relaxation time (1/τ) should also have a minimum value near H_{k,eff} for θ_{H} = 89° if the damping was purely from Gilbert damping (as shown by the solid lines in Fig. 4b,d); however, the measured data do not follow this trend. Adding the inhomogeneous term (dotted lines in Fig. 4b,d) more accurately describes the field dependence of the measured 1/τ (open symbols in Fig. 4b,d) It should be noted that the dip of the predicted 1/τ occurs when the frequency derivative term in Eq. 5 approaches zero; however, this is not captured by the measurement. Figure 4c,e depict the fielddependent effective damping (α_{eff}) calculated using the Gilbert damping (α) extracted from fitting the measured 1/τ.
With the knowledge that the value of α extracted with this method is the intrinsic material property, we repeat this data reduction technique for the annealed W/CoFeB/MgO samples discussed in Fig. 1. The symbols in Fig. 5 represent the resonance frequencies and damping constants (both effective damping and Gilbert damping) for all samples measured at θ_{H} ≈ 90°. The fittings for the resonance frequency based on Eq. 1 (red lines) are also shown to demonstrate the good agreement between our TRMOKE measurement and theoretical prediction. The uncertainties of f, τ, and H_{k,eff} are calculated from the leastsquares fitting uncertainty and the uncertainty of measuring H_{ext} with the Hall sensor.
The summary of the anisotropy and damping measured via TRMOKE is shown in Fig. 6. Figure 6a plots H_{k,eff} obtained from VSM (black open circles) and TRMOKE (blue open squares), both of which exhibit a monotonic increasing trend as T_{ann} becomes higher. Discrepancies in H_{k,eff} from these two methods can be attributed to the difference in the size of the probing region, which is highly localized in TRMOKE but sampleaveraged in VSM. Since H_{k,eff} determined from TRMOKE is obtained from fitting the measured frequency for a localized region, we expect these values more consistently describe the magnetization precession than those obtained from VSM. The increase in H_{k,eff} with T_{ann} has previously be partially attributed to the crystallization of the CoFeB layer^{37}. For temperatures higher than 350 °C, this increasing trend of H_{k,eff} begins to lessen, presumably due to the diffusion of W atoms into the CoFeB layer, which is more pronounced at higher T_{ann}. The W diffusion process is also responsible for the decrease in M_{s} of the CoFeB layer as T_{ann} increases (Fig. 1e). Subsequently, the decrease in M_{s} leads to a furtherreduced demagnetizing energy and thus a larger H_{k,eff}.
Similar observation of M_{s} has been reported in literature for Ta/CoFeB/MgO PMA structures and attributed to the growth of a dead layer at the heavy metal/CoFeB interface^{1}. Figure 6b summarizes t_{dead} as a function of T_{ann} with t_{dead} increasing from 0.17 to 0.53 nm as T_{ann} changes from 250 to 400 ^{ο}C, as discussed in Section II.
Figure 6c depicts the dependence of α on T_{ann}, which first decreases with T_{ann}, reaches a minimum of 0.015 at 350 °C, and then increases as T_{ann} rises to 400 °C. Similar trends have been observed for Ta/CoFeB/MgO previously (minimum α at T_{ann} = 300 °C)^{37}. We speculate that this dependence of damping on T_{ann} is due to two competing effects: (1) the increase in crystallization in the CoFeB layer with T_{ann} which reduces the damping, and (2) the growth of a dead layer, which results from the diffusion of W and B atoms and is prominent at higher T_{ann}.
As the amorphous asdeposited CoFeB film begins to form ordered phases at elevated temperatures, the number of scattering sites in the film tend to decrease^{45,46}. The increase in crystallinity of the W/CoFeB/MgO film with T_{ann} is demonstrated by the XRD analysis detailed in Section S5 of the SI. At T_{ann} = 400 °C, the deadlayer formation leads to a larger damping presumably due to an increase in scattering sites (diffused W atoms) that contribute to spinflip events, as described by the ElliotYafet relaxation mechanisms^{18}. Additionally, W atoms can increase the spinorbit coupling and thus the damping as the interdiffusion increases with T_{ann}^{47}. The observation that our Wseeded samples still sustain excellent PMA properties at T_{ann} = 400 °C confirms their enhanced thermal stability, compared with Ta/CoFeB/MgO stacks which fail at T_{ann} = 350 °C or higher.
While the damping constants are comparable for the W/CoFeB/MgO and Ta/CoFeB/MgO films annealed at 300 °C, our work focuses on the enhanced thermal stability of Wseeded CoFeB PMA films that can maintain a relatively low damping constant (0.020 at 400 °C). Such an advantage enables Wseeded CoFeB layers to be viable and promising alternatives to Ta/CoFeB/MgO, which is currently widely used in spintronic devices.
Conclusion
In summary, we deposit a series of Wseeded CoFeB PMA films with varying annealing temperatures up to 400 °C and conduct ultrafast alloptical TRMOKE measurements to study their magnetization precession dynamics. The Gilbert damping, as an intrinsic material property, is proven to be independent of measurement conditions, such as the amplitudes and directions of the applied field. The damping constant varies with T_{ann}, first decreasing and then increasing, leading to a minimum of α = 0.015 for the sample annealed at 350 °C. Due to the deadlayer growth, the damping constant slightly increases to α = 0.020 at T_{ann} = 400 °C, which demonstrates the improved enhanced thermal stability of W/CoFeB/MgO over the Ta/CoFeB/MgO structures. This strongly suggests the great potential of W/CoFeB/MgO PMA systems for future spintronic device integration that requires materials to sustain a processing temperature as high as 400 °C.
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Acknowledgements
This work is supported by CSPIN (award #: 2013MA2381), one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA. X.H. thanks Niron Magnetics, Inc. for financial support. The sample structural characterizations (XRD and XRR) were performed at the University of Minnesota Characterization Facility (CharFac), which received capital equipment funding from the University of Minnesota MRSEC under NSF Award DMR1420013. The authors would like to thank Prof. Paul Crowell and Dr. Changjiang Liu for valuable discussions.
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D.Z. and D.L. conceived the idea and designed the measurements, under the supervision of X.W. and J.W., D.Z. synthesized the CoFeB samples and conducted VSM measurements. X.H. performed all the XRD and XRR characterizations. D.L. carried out the TRMOKE measurements and led the manuscript writing with a portion of the materials provided by D.Z., J.Z., D.L. and D.Z. contributed to analyzing the data and revising the manuscript. All authors have reviewed and discussed the results and conclusions of this work.
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Lattery, D.M., Zhang, D., Zhu, J. et al. Low Gilbert Damping Constant in Perpendicularly Magnetized W/CoFeB/MgO Films with High Thermal Stability. Sci Rep 8, 13395 (2018). https://doi.org/10.1038/s41598018316429
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DOI: https://doi.org/10.1038/s41598018316429
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