CoFeB Thickness Research Articles

Comparative Study of Current-Induced Torque in Cr/CoFeB/MgO and W/CoFeB/MgO.

Orbital torque (OT) in magnetic heterostructures has been actively discussed in terms of its actual existence and usefulness in comparison to the spin-orbit torque (SOT) that shows promise for next-generation magnetoresistive random access memories. The objectives of this study are 2-fold: (i) making an apples-to-apples comparison in two representative stacks where OT and SOT are expected to dominate and (ii) examining the potential emergence of OT in archetypal SOT stacks. Cr/CoFeB/MgO and W/CoFeB/MgO are chosen as the OT- and SOT-dominant systems, respectively. Systematic variations in each layer's thicknesses reveal that (i) Cr/CoFeB/MgO exhibits substantial torque comparable to or even exceeding that of the W/CoFeB/MgO stack when Cr and CoFeB layers are especially thick and (ii) the torque in W/CoFeB/MgO changes sign with increasing W and CoFeB thicknesses, suggesting a crossover of the dominant mechanism from SOT to OT. The findings clarify the opportunities and challenges of devices leveraging SOT and OT.

Thickness dependence on dynamical spin injection driven by thermal effects in CoFeB/Pt bilayer

In ferromagnetic metal (FM)/non-magnetic metal (NM) bilayer structures, dynamical spin injection is primarily attributed to spin pumping at the interface. However, thermal effects, such as the spin (dependent) Seebeck effect (S(d)SE) caused by FMR heating effect, are also expected to contribute, particularly farther from the interface within the FM layer. In this study, the detailed mechanism of dynamical spin injection in CoFeB/Pt bilayer films has been investigated. We demonstrate the proper evaluation of the CoFeB thickness dependence of inverse spin Hall voltage by subtracting the signal from single CoFeB system. The dynamical spin injection and FMR heating effect were observed to significantly depend on thickness. Additionally, we separated the contributions of SSE and SdSE by focusing on their diffusion properties and found that SSE is larger than SdSE in the CoFeB/Pt bilayer film.

Interfacial Magnetic Anisotropy Controlled Spin Pumping in Co60Fe20B20/Pt Stack

Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of the CoFeB thin film by tilting the magnetization from in-plane to out-of plane direction due to interfacial anisotropy from higher thickness of CoFeB thin film. The ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 1.10[Formula: see text]MA/m2 to 2.40[Formula: see text]MA/m2, with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices.

Towards fully electrically controlled domain-wall logic

Utilizing magnetic tunnel junctions (MTJs) for write/read and fast spin-orbit-torque (SOT)-driven domain-wall (DW) motion for propagation, enables non-volatile logic and majority operations, representing a breakthrough in the implementation of nanoscale DW logic devices. Recently, current-driven DW logic gates have been demonstrated via magnetic imaging, where the Dzyaloshinskii-Moriya interaction (DMI) induces chiral coupling between perpendicular magnetic anisotropy (PMA) regions via an in-plane (IP) oriented region. However, full electrical operation of nanoscale DW logic requires electrical write/read operations and a method to pattern PMA and IP regions compatible with the fabrication of PMA MTJs. Here, we study the use of a Hybrid Free Layer (HFL) concept to combine an MTJ stack with DW motion materials, and He+ ion irradiation to convert the stack from PMA to IP. First, we investigate the free layer thickness dependence of 100-nm diameter HFL-MTJ devices and find an optimal CoFeB thickness, from 7 to 10 Å, providing high tunneling magnetoresistance (TMR) readout and efficient spin-transfer torque (STT) writing. We then show that high DMI materials, like Pt/Co, can be integrated into an MTJ stack via interlayer exchange coupling with the CoFeB free layer. In this design, DMI values suitable for SOT-driven DW motion are measured by asymmetric bubble expansion. Finally, we demonstrate that He+ irradiation reliably converts the coupled free layers from PMA to IP. These findings offer a path toward the integration of fully electrically controlled DW logic circuits.

Single-nanometer CoFeB/MgO magnetic tunnel junctions with high-retention and high-speed capabilities

Making magnetic tunnel junctions (MTJs) smaller while meeting performance requirements is critical for future electronics with spin-transfer torque magnetoresistive random access memory (STT-MRAM). However, it is challenging in the conventional MTJs using a thin CoFeB free layer capped with an MgO layer because of increasing difficulties in satisfying the required data retention and switching speed at smaller scales. Here we report single-nanometer MTJs using a free layer consisting of CoFeB/MgO multilayers, where the number of CoFeB/MgO interfaces and/or the CoFeB thicknesses are engineered to tailor device performance to applications requiring high-data retention or high-speed capability. We fabricate ultra-small MTJs down to 2.0 nm and show high data retention (over 10 years) and high-speed switching at 10 ns or below in sub-5-nm MTJs. The stack design proposed here proves that ultra-small CoFeB/MgO MTJs hold the potential for high-performance and high-density STT-MRAM.

Size and density control of skyrmions with picometer CoFeB thickness variations—observation of zero-field skyrmions and skyrmion merging

The controlled formation and adjustment of size and density of magnetic skyrmions in Ta/CoFeB/MgO trilayers with low Dzyaloshinskii–Moriya interaction is demonstrated. Close to the out-of-plane to in-plane magnetic spin reorientation transition, we find that small energy contributions enable skyrmion formation in a narrow window of 20 pm in CoFeB thickness. Zero-field stable skyrmions are established with proper magnetic field initialization within a 10 pm CoFeB thickness range. Using magneto-optical imaging with quantitative image processing, variations in skyrmion distribution and diameter are analyzed quantitatively, the latter for sizes well below the optical resolution limit. We demonstrate the controlled merging of individual skyrmions. The overall demonstrated degree of comprehension of skyrmion control aids to the development of envisioned skyrmion based magnetic memory devices.

Ta interfaced CoFeB: Role of CoFeB thickness and thermal annealing in modification of structural and magnetic properties

CoFeB interfaced with heavy metals is a potential system for spintronic applications. In the present work, evolution of magnetic and structural properties of CoFeB interfaced with Ta have been studied as function of increasing thickness of magnetic film and increasing annealing temperature. While the evolution of structural properties was investigated using X-ray scattering measurements i.e., X-ray reflectivity (XRR) and grazing incidence X-ray Diffraction (GIXRD), the modification of magnetic properties were studied using vibrating sample magnetometer (VSM) and magneto-optical Kerr effect (MOKE) measurements. The detailed XRR analysis shows the diffusion of Boron to the bottom Ta layer with annealing, which increases significantly for post-crystallized phase. Such enriched B diffusion also increases the thickness of interfacial magnetic dead layer as confirmed from VSM results. The synchrotron XRD show an increased in-plane CoFe grain size relative to out-of-plane grain size, while CoFeB is interfaced with Ta. MOKE measurements confirm the presence of stress-induced uniaxial magnetic anisotropy in as-deposited CoFeB films, which reduces for Ta-interfaced CoFeB. Vacuum annealing well above the crystallization temperature eliminates such magnetic anisotropy with concurrent increase of magnetic coercivity (Hc) because of increasing grain size. CoFeB interfaced with Ta shows an abrupt enhancement of Hc, as compared to CoFeB without Ta interface.

Influence of CoFeB layer thickness on elastic parameters in CoFeB/MgO heterostructures

The surface acoustic waves, i.e., surface phonons may have huge potential for future spintronic devices, if coupled to other waves (e.g., spin waves) or quasiparticles. In order to understand the coupling of acoustic phonons with the spin degree of freedom, especially in magnetic thin film-based heterostructures, one needs to investigate the properties of phonons in those heterostructures. This also allows us to determine the elastic properties of individual magnetic layers and the effective elastic parameters of the whole stacks. Here, we study frequency versus wavevector dispersion of thermally excited SAWs in CoFeB/MgO heterostructures with varying CoFeB thickness by employing Brillouin light spectroscopy. The experimental results are corroborated by finite element method-based simulations. From the best agreement of simulation results with the experiments, we find out the elastic tensor parameters for CoFeB layer. Additionally, we estimate the effective elastic parameters (elastic tensors, Young’s modulus, Poisson’s ratio) of the whole stacks for varying CoFeB thickness. Interestingly, the simulation results, either considering elastic parameters of individual layers or considering effective elastic parameters of whole stacks, show good agreement with the experimental results. These extracted elastic parameters will be very useful to understand the interaction of phonons with other quasiparticles.

Investigation of perpendicular magnetic anisotropy in Pt/Co20Fe60B20/Pt multi-layer structures

Multi-layers of Co20Fe60B20 and Pt with large perpendicular magnetic anisotropies (PMA), comparable to those obtained with the more widely utilized Ta/CoFeB/MgO heterostructures, are reported. CoFeB thickness of approximately 0.8nm is necessary to maximize PMA in Pt/CoFeB/Pt. Strong PMA can only be obtained upon careful tuning of the material the CoFeB layer is deposited on. Specifically, a Pt layer thickness of at least 3nm is necessary to obtain PMA in CoFeB due to the emergence of (111) texture in thicker Pt films. The absence of a magnetic dead layer at the interface between CoFeB and Pt (as opposed to an interface between CoFeB and Ta) maximizes the amount of magnetic content per unit volume. Furthermore, this study investigates the magnetic properties of multi-layer structures where four CoFeB layers are deposited between Pt and Pt/Ta/Pt buffers. While the out-of-plane remanent magnetization of the structures is low, they present small nucleation field down to 30Oe and large anisotropy energy in the range of 106erg/cc.

Depth-resolved magnetization profile of MgO/CoFeB/W perpendicular half magnetic tunnel junctions

In this work, we used the soft X-ray resonant magnetic reflectivity to study the depth-resolved out-of-plane (oop) magnetization profile of a CoFeB/MgO sample with W/Ta cap layer after annealing at 400°C. It is a powerful technique to probe buried magnetic interfaces of ultra-thin films by combining the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism. It allowed us to resolve the oop magnetization within a 1.36 nm thick CoFeB layer by the measurement of angle-dependent specular reflectivity at large scattering angles (up to 80°). We determined a graded magnetic distribution for both Fe and Co with a 20% increase at the interface with MgO, decreasing slightly over a thickness of 0.7 nm from MgO before it rapidly decreases to 50% at the interface with W. After applying a non-saturating magnetic field in the plane of the sample, we also quantified a similar magnetization profile with an inclined moment configuration. This indicates that the magnetization gradient is a robust property of the CoFeB layer in the studied sample.

The thickness dependence of the field-like spin–orbit torque in heavy metal/CoFeB/MgO heterostructures

We investigated the ferromagnet (FM) and heavy metal (HM) thickness dependence of the electric current-induced spin orbit torque (SOT), especially the field-like (FL) torque component in HM/CoFeB/MgO heterostructures. For Pt/CoFeB/MgO and Ta/CoFeB/MgO structures, after subtracting the dead-layer thickness of CoFeB, the damping-like (DL) effective field follows 1/tFM dependence, while the FL effective field deviates from 1/tFM dependence at the ultra-thin FM thickness range, indicating that an extra origination of FL torque, i.e., spin backflow at the FM/MgO interface, is responsible for the large FL torque in HM/CoFeB/MgO structures with a ultra-thin CoFeB layer. For Ta/Pt(tPt)/CoFeB(1)/MgO structures, the FL-SOT exhibits a gradual change similar to the DL-SOT, suggesting that the spin Hall effect is the dominant origination of spin current, which enhances the FL-SOT in the HM/CoFeB/MgO structures by the spin backflow effect when tCoFeB is less than the spin dephasing length. We also demonstrated that the obvious dead-layer thickness at the Ta/CoFeB interface reduces the effective CoFeB thickness and enhances the spin backflow effect further.

Structural Phase-Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin-Film Heterostructures.

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (Geff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co20Fe60B20 (d)/SiO2 (2 nm) thin-film heterostructures in the beta-tungsten (β-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the β-W/CoFeB interface. A stark variation of Geff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

Field-driven domain wall creep motion in ferrimagnetic Tb/CoFeB/MgO microwires

We studied field-driven domain wall (DW) creep motion in ferrimagnetic Tb/CoFeB/MgO with respect to CoFeB thickness by using a real-time DW detection method. The DW velocity for Tb(5 nm)/CoFeB(1.0 ∼ 1.8 nm)/MgO microwires was measured at room temperature. The DW velocity increases with increasing the CoFeB thickness, which is contrary to a general trend in the ferromagnetic wire. From the creep-scaling analysis, the characteristic velocity at which the DW moves when the energy barrier vanishes is found to be a dominant contribution to the DW velocity. Our results clarify the ferrimagnetic DW dynamics in the creep regime.

Investigation of the correlation between perpendicular magnetic anisotropy, spin mixing conductance and interfacial Dzyaloshinskii–Moriya interaction in CoFeB-based systems

Correlation between interfacial Dzyaloshinskii–Moriya interaction (iDMI), perpendicular magnetic anisotropy (PMA) and spin pumping-induced damping was investigated in CoFeB-based systems grown by sputtering on Si substrates, using Pt, Ta, Cu, W and MgO capping layers. Vibrating sample magnetometer, Brillouin light scattering (BLS) and broadband ferromagnetic resonance techniques were combined for this aim. The CoFeB thickness dependence of iDMI and PMA constants, in CoFeB/X (where X = Pt, Cu/Pt, Ta/Pt or W/Al), revealed that only the CoFeB/Pt system presents a measurable iDMI and that the interfacial PMA is mostly similar except for the Ta/CoFeB/Ta/Pt system. Therefore, no clear correlation between the above-mentioned interfacially-driven and spin-orbit coupling related quantities was observed due to their different origins in our systems. An efficient sample design involving various spacer layers of variable thicknesses in Ta/CoFeB(1.5 nm)/Y/Pt (where Y = Cu, Ta, MgO) allowed evidence of a linear correlation between iDMI, PMA constants and the effective spin mixing conductance. The linear dependence, which could result from the narrow variation range of PMA and/or iDMI, is attributed to the similar interface orbital hybridizations involved in PMA, iDMI and spin pumping-induced damping.

Exchange stiffness and damping constants of spin waves in CoFeB films

Exchange stiffness and damping of spin waves in a soft ferromagnet of CoFeB are investigated using a time-resolved pump-probe technique. A pump pulse excites coherent spin waves in CoFeB via optical spin-orbit torque from Pt, and a probe pulse detects the phase of the spin waves via magneto-optic Kerr effect. From the frequency of spin waves, exchange stiffness is determined to be 13 pJ m−1, which is nearly independent of the CoFeB thickness from 6 to 16 nm and the annealing temperature of 400 °C. Damping constant of spin waves is determined from the relaxation of spin waves. Importantly, the spin wave damping increases with decreasing the CoFeB thickness, suggesting the spin pumping effect at the interface between CoFeB and Pt. The thickness dependence of damping becomes stronger after annealing at 400 °C, and the spin wave damping of >0.08 is obtained at a CoFeB thickness of 6.3 nm. Such large damping of spin waves would have an important effect on the operation of the CoFeB-based memory devices.

Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurement of Spin-Orbit Torque

Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayer structures. In the course of a measurement, other resonant processes such as spin pumping (SP) and heating can cause spin current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects (NE). In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOTs. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in Pt (6 nm)/CoFeB samples the contribution from the artifacts is small compared to the SOT rectification signal for CoFeB layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of CoFeB thickness that we suggest may be due to a competition between a resonant heating effect and the SP/ISHE contribution.

Underlayer material dependent symmetric and asymmetric behavior of voltage-controlled magnetic anisotropy in CoFeB films

Voltage-controlled magnetic anisotropy (VCMA), observed at the interfaces of ultrathin ferromagnetic metallic films and oxide layer, has proven to be a useful tool for the development of all-electric field controlled spintronics devices. Here, we have studied the symmetric and asymmetric behavior of VCMA in CoFeB/MgO heterostructures, grown on different underlayer materials, by measuring ferromagnetic resonance using spin pumping and inverse spin Hall effect technique. We observe symmetric behavior of VCMA in CoFeB films with Ta underlayer, whereas a systematic transformation from symmetric to asymmetric behavior of VCMA with decreasing CoFeB thickness is observed for Pt underlayer. We speculate that the increased interfacial roughness, defects and strain of ultrathin CoFeB films with Pt buffer layer probably leads to the complicated band structure at CoFeB/MgO interface resulting in asymmetric behavior of VCMA. The observed symmetric behavior of VCMA in control samples justifies the role of interfacial roughness, defects and discards the role of oxide overlayer on the observed asymmetric behavior of VCMA in ultrathin CoFeB films.

Enhancing perpendicular magnetic anisotropy through dead layer reduction utilizing precise control of Mg insertions

Using an ultrathin metallic layer of Mg at the CoFeB|MgO interface has been emerged as an appealing solution to efficiently enhance the tunneling magnetoresistance and thermal stability while reducing the resistance-area product and effective damping of CoFeB films for efficient and low-power dissipated MTJ. However, it is still a great challenge to significantly enhance the perpendicular magnetic anisotropy (PMA) using Mg interlayer in CoFeB|MgO structures and reveal the behind mechanism, for continuously scaling down the MTJ structure. Here, we investigate the effect of ultrathin Mg insertions of 0.2 nm, 0.4 nm and 0.6 nm on PMA at CoFeB|MgO interface for the CoFeB thickness range of 1–1.5 nm and compare the results with an identical structure without insertion. We observed that upon inserting ultrathin Mg layer, the effective anisotropy turned to out-of-plane even for thicker 1.5 nm CoFeB layers which favors in-plane anisotropy without Mg insertion. We also observe a reasonable increase in the interfacial anisotropy energy along with the effective CoFeB thickness for the structures with Mg insertion. The effect that Mg insertion layer helps reducing the dead layer was revealed accounting for the PMA enhancement mechanism. Our results provide a more detailed explanation for the enhancement of PMA through Mg insertion, which may offer a guidance for the development of CoFeB|MgO based MTJ structure with higher thermal stability and lower switching current.

Terahertz emission generated from Bi<sub>2</sub>Te<sub>3</sub>/CoFeB heterostructures grown by magnetron sputtering

High-performance terahertz emitters, which convert the femtosecond laser pulses into terahertz pulses, are essential for terahertz spectroscopy technology and terahertz wireless communication. Spintronic terahertz emitters based on ferromagnet/nonmagnet bilayers have attracted tremendous attention due to their high efficiency, ultra-broadband, low cost and high flexibility. Here, we systematically investigate the terahertz emission from polycrystalline topological insulator Bi<sub>2</sub>Te<sub>3</sub>/ferromagnetic CoFeB heterostructure grown by magnetron sputtering. The Bi<sub>2</sub>Te<sub>3</sub>/CoFeB heterostructure exhibits high efficiency of terahertz emission, and the polarization of terahertz waves can be controlled by the external magnetic field direction. The performance of Bi<sub>2</sub>Te<sub>3</sub>/CoFeB heterostructure is almost comparable to that of the Pt/CoFeB bilayer. In contrast, no terahertz emission is observed in the pure Bi<sub>2</sub>Te<sub>3</sub> or CoFeB film driven by femtosecond laser pulses, probably because the Bi<sub>2</sub>Te<sub>3</sub> prepared by sputtering is polycrystalline and the thickness of CoFeB is too thin. We also compare the performances of Bi<sub>2</sub>Te<sub>3</sub>/CoFeB grown on MgO, glass and high-resistivity silicon substrates, and find that the samples grown on MgO substrates exhibit the best emission performances. The glass substrate absorbs more terahertz waves than MgO substrate, resulting in a slightly weaker terahertz signal emitted from the Bi<sub>2</sub>Te<sub>3</sub>/CoFeB grown on the glass substrate. Although the absorption coefficient of high-resistivity silicon to terahertz waves is very small, the residual pump light excites the high-resistivity silicon to generate the photo-generated carriers, which change the conductivity of the high-resistivity silicon and reduce the transmittance of terahertz wave. We attribute the mechanism of the terahertz emission to the spin-charge conversion at the interface of Bi<sub>2</sub>Te<sub>3</sub>/CoFeB. The terahertz emission efficiency of our sample is expected to be able to be further improved by optimizing the samples. Moreover, with the sputtering method, it is possible to fabricate large area samples at low cost, which is critical for commercial applications.

Composition dependence of the second-order interfacial magnetic anisotropy for MgO/CoFeB/Ta films

The CoFeB thickness, t dependence of the effective first- and second-order magnetic anisotropy, K1eff and K2, for MgO/(Co1-xFex)80B20/Ta films (x=0.3-1.0) is investigated. As Co40Fe40B20 thickness decreases, K1eff increases and shows a perpendicular magnetic anisotropy for t=1.2 nm. On the other hand, in-plane magnetic anisotropy is observed for t≥1.4 nm. Also, a 1.3-nm-thick CoFeB sample demonstrates an easy-cone behavior, which suggests that the magnitude of K1eff and K2 becomes comparable. By plotting the product of K2 and t-td as a function of t-td, where td is a magnetic dead layer thickness, linear dependences with negative y-axis intercepts are displayed for all ranges of x. The extracted interfacial K2, Ki(2) are varied depending on the compositions in the range of-0.024 to −0.042 erg/cm2 for x=100% and 30, 50%, respectively. A magnetic phase diagram summarizing the results of K1-2πMs2 and K2 suggests that the ratio of K2 against K1-2πMs2 is varied depending on the compositions. These results give us a guideline to achieve the desired magnetic properties of CoFeB for spintronic applications.