CoFeB Films Research Articles

Impact of Complex Permeability Measurements Up to Millimeter-Wave Frequency Range

A highly sensitive microstrip-line-type probe using a flexible substrate was developed to measure thin-film permeability continuously up to the millimeter-wave frequency range (over 30 GHz). The probe enables broad bandwidth and highly sensitive permeability measurements without sample size limitations. The complex permeability of the CoFeB film (45 mm × 25 mm and 0.5 μm in thickness), CoNbZr film (25 mm × 25 mm and 3 nm in thickness), and nickel ferrite film (8 mm × 5 mm, and 1.2 μm in thickness) were optimized. The measured permeability spectrum and ferromagnetic resonance frequency showed good agreement with the theoretical values based on the Landau-Lifshitz-Gilbert equation and eddy current generation up to 67 GHz.

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.

Effect of the stray field of Fe/Fe3O4 nanoparticles on the surface of the CoFeB thin films

Fe/Fe3O4 nanoparticles have been deposited on the surfaces of ultrathin CoFeB film and CoFeB/Ta/CoFeB hetero-structure to be detected due to the stray field generated by one particle or a cluster of particles. Exchange biased Fe/Fe3O4 core-shell nanoparticles have been used to stabilize the particles magnetization. Comparison between the Atomic Force and Magnetic Force Microscope images and subtraction of corresponding phase contrasts allows visualization of the film magnetization affected by the particles. Spectra of Ferromagnetic Resonance of the ultrathin films with deposited particles allow one to estimate particle/film dipolar interaction. The results will be useful for the development of lab-on-chip sensors of magnetically labeled cells. Estimation of particles number by magnetic response of the CoFeB heterostructure is demonstrated.

Electric-Field Control of Spin-Orbit Torques in Perpendicularly Magnetized W/CoFeB/MgO Films.

Controlling magnetism by electric fields offers a highly attractive perspective for designing future generations of energy-efficient information technologies. Here, we demonstrate that the magnitude of current-induced spin-orbit torques in thin perpendicularly magnetized CoFeB films can be tuned and even increased by electric-field generated piezoelectric strain. Using theoretical calculations, we uncover that the subtle interplay of spin-orbit coupling, crystal symmetry, and orbital polarization is at the core of the observed strain dependence of spin-orbit torques. Our results open a path to integrating two energy efficient spin manipulation approaches, the electric-field-induced strain and the current-induced magnetization switching, thereby enabling novel device concepts.

Large anisotropy of magnetic damping in amorphous CoFeB films on GaAs(001)

Amorphous CoFeB films grown on GaAs(001) substrates demonstrating significant in-plane uniaxial magnetic anisotropy were investigated by vector network analyzer ferromagnetic resonance. Distinct in-plane anisotropy of magnetic damping, with a largest maximum–minimum damping ratio of about 109%, was observed via analyzing the frequency dependence of linewidth in a linear manner. As the CoFeB film thickness increases from 3.5 nm to 30 nm, the amorphous structure for all the CoFeB films is maintained while the magnetic damping anisotropy decreases significantly. In order to reveal the inherent mechanism responsible for the anisotropic magnetic damping, studies on time-resolved magneto-optical Kerr effect and high resolution transmission electron microscopy were performed. Those results indicate that the in-plane angular dependent anisotropic damping mainly originates from two-magnon scattering, while the Gilbert damping keeps almost unchanged.

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.

Enhanced electrical switching of magnetism in the artificial multiferroic CoFeB/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure

Multiferroic heterostructure CoFeB/Pb(Mg1/3Nb2/3)O3–PbTiO3 is prepared by depositing CoFeB films on Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal through direct current magnetron sputtering. The electric field induced magnetism change of CoFeB film was studied by magneto-optical Kerr effect measurement. The magnetic hard/easy axis change was investigated under direct current electric field. Alternative current voltage magneto-optical Kerr effect test indicates the different magnetoelectric mechanism with the change of film thickness. The microwave frequency tunability was observed under the electric field and the tunable ferromagnetic resonance field reaches 190 Oe. Our study provides great opportunities for the application of electric-field tunable information storage devices.

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.

Electric field control of spin waves in ultrathin CoFeB films

Recently, investigation and control of the properties of propagating spin waves (SWs) in ultrathin ferromagnetic films have attracted considerable attention because of their possible technological impact on future nanoscale magnonic devices. Control of SW properties using voltage-controlled magnetic anisotropy (VCMA) may reduce the power consumption of future magnonic devices significantly. Here, we report an experimental study of manipulation of uniform ferromagnetic resonance (UFMR) and dipole-exchange SWs by VCMA in ultrathin $\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ (CoFeB) films with thicknesses of 1.6, 1.8, and 2.0 nm. The UFMR and the SWs are excited using microwave antennae and detected by spin pumping and inverse spin Hall effect techniques. A significant change in the SW frequency caused by VCMA is observed, particularly in the 1.6-nm-thick CoFeB film, where the effective value of the net anisotropy is quite small because of the presence of strong interfacial perpendicular magnetic anisotropy. Additionally, micromagnetic simulations are performed to demonstrate that the SWs in the 1.6-nm-thick CoFeB film with its in-plane easy axis of magnetization can be guided through virtual nanochannels formed by VCMA.

Signature of spin-dependent Seebeck effect in dynamical spin injection of metallic bilayer structures

The dynamical spin injection in a ferromagnetic/paramagnetic bilayer with various paramagnetic layers has been examined by using the inverse spin Hall effect. We adapt a CoFeB film as a ferromagnetic layer, which has a large spin dependent Seebeck coefficient. The contribution of the spin pumping was evaluated from the line-width change of the ferromagnetic resonant spectra while that of the thermal spin injection was evaluated from the heat conductivity for the paramagnetic layer. We find that the spin Hall voltage does not show the systematic change with respect to the line-width change. However, the normalized spin Hall voltage is found to increase with the heat conductivity for the paramagnetic layer. These results suggest that the thermal spin injection is a major contribution for the dynamical spin injection in CoFeB/paramagnetic bilayer systems.

Determination of spin pumping effect in CoFeB/Ir bilayer

The spin pumping effect of CoFeB/Ir bilayers has been investigated via the broadband ferromagnetic resonance measurement. With increasing the thickness of heavy metal Ir layer, the Gilbert damping factor of 8 nm CoFeB film increases from 7 × 10−3 to 1.1 × 10−2, and the additional damping factors induced by the spin pumping effect quickly reaches its saturation value. According to the spin pumping theory, the spin mixing conductance of CoFeB/Ir and the spin diffusion length of Ir were obtained. The spin mixing conductance is 2.14 × 1019 m−2. Furtherly, the spin diffusion length is only 1.34 nm, which is quite shorter compared with the heavy metal Pt. Our results reveal the existence of the strong spin-orbit coupling in the heavy metal Ir, which can be a potential material for the application in spintronic devices.

Tuning of interfacial perpendicular magnetic anisotropy and domain structures in magnetic thin film multilayers

We investigate the magnetic domain structures and the perpendicular magnetic anisotropy (PMA) arising in CoFeB films interfaced with selected heavy metal (HM) layers with large spin Hall angles in HM/CoFeB/MgO (HM = W, Pt, Pd, WxTa1−x) stacks as a function of CoFeB thickness and composition for both as-deposited and annealed materials stacks. The coercivity and the anisotropy fields of annealed material stacks are higher than for the as-deposited stacks due to crystallisation of the ferromagnetic layer. Generally a critical thickness of MgO > 1 nm provides adequate oxide formation at the top interface as a requirement for the generation of PMA. We demonstrate that in stacks with Pt as the HM, the PMA depends crucially on the alloy composition of the ferromagnet requiring a Co rich alloy to achieve PMA, i.e. it is important to have a large number of Pt–Co hybrid bonds at the interface compared to Pt–Fe hybrid bonds. In Pd/CoFeB/MgO layers, we observe for appropriate PMA a special stripe domain configuration that also includes circular skyrmion-like spin structures. Three different means of selective tuning of the PMA are discussed apart from the conventional FM thickness variation: Changing the number of repetitions of a multilayer stack is shown to change the PMA and strong PMA is found for certain compositions of WxTa1−x that also result in very low coercive fields (<1 mT) of the CoFeB. The PMA has also been shown to be tuned via annealing, very strong anisotropy fields of up to 1 T and low pinning of domain structures were measured in W/CoFeB/MgO, making these multilayers stacks a promising candidate for MRAM development.

Femtosecond laser-heating effect on the magnetization dynamics in perpendicularly magnetized Ta/CoFeB/MgO film

We have studied the effect of ultrafast laser-heating on the magnetization dynamics of perpendicularly magnetized CoFeB film by means of the time-resolved magneto-optical Kerr rotation effect. The effective perpendicular magnetic anisotropy field is significantly decreased with enhancing the pump laser-fluence in a moderate range of 5–12 mJ cm−2. The Gilbert damping, however, is found to be independent of the pump fluence. These findings provide a new method of separately manipulating the Gilbert damping and perpendicular magnetic anisotropy.

Thickness dependent magneto-static and magneto-dynamic properties of CoFeB thin films

The authors report thickness dependent magneto-static and magneto-dynamic properties of amorphous Co40Fe40B20 films with thickness, t = 10–200 nm deposited on thermally oxidized Si substrates using magnetron sputtering technique at ambient temperature. Magnetic hysteresis loops of films with t ≤ 20 nm transformed from rectangular or flat type shape to transcritical nature for t ≥ 50 nm. This is attributed to a change in the magnetic domain structure from in-plane magnetization with uniaxial anisotropy in thinner films to stripe domains with effective magnetic anisotropy in thicker films. Magnetization dynamics in the films has been investigated using ferromagnetic resonance spectra. Out-of-plane angular dependent resonance field (Hr) and linewidth (ΔH) data have been interpreted using the Smit–Beljers formula. It is found that ΔH contains large extrinsic contribution with the major one being two-magnon scattering. An inverse scaling of Gilbert damping constant (α) with the film thickness over the entire thickness range of 10–200 nm is observed with the lowest α value of 0.004. The results reveal a cost effective strategy for developing films for spintronics based microwave devices and spin-transfer-torque based magnetic devices.

Bolometric ferromagnetic resonance techniques for characterising spin-Hall effect at high temperatures

Abstract We report on current-induced ferromagnetic resonance techniques to characterise spin-Hall effect at high temperatures. A microwave current was injected into a patterned CoFeB/Pt bi-layer grown on a glass substrate, simultaneously exerting spin-transfer torques through the spin-Hall effect and also causing Joule heating enabling the control of the device temperature. We measured the device temperature by using the device itself as a local temperature sensor. A clear reduction of CoFeB magnetisation was observed as the device temperature was increased allowing us to estimate the Curie temperature of our CoFeB film to be 920 K. The spin-Hall angle of Pt was quantified as (1.72 ± 0.03) × 10−2 at 300 K and was slightly increased to (1.75 ± 0.02) × 10−2 at 410 K. This simple method can be widely used for quantifying the spin-Hall angle of a large variety of materials at high temperatures.

CoFeB/MgO-based magnetic tunnel junction directly formed on a flexible substrate

We succeeded in fabricating a CoFeB/MgO-based magnetic tunnel junction (MTJ) directly on an organic flexible substrate. The MTJ shows good strain endurance while keeping the tunnel magnetoresistance (TMR) ratio of ∼100% under various strained conditions; the TMR ratio is almost unchanged up to a tensile strain of 1.2%. Because of the magnetoelastic effect, the magnetic anisotropy fields for the top and bottom CoFeB layers are linearly proportional to strain with almost the same rate as that in a single CoFeB film, suggesting that the expected strain is added on both CoFeB layers in the MTJ pillar from the stretched flexible substrate.

Giant non-volatile magnetoelectric effects via growth anisotropy in Co40Fe40B20 films on PMN-PT substrates

Uniaxial magnetic anisotropy was imposed on a CoFeB film by applying an in-plane magnetic field during growth. Electrically driven strain from a ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (011) substrate resulted in giant magnetoelectric effects, whose coupling constant peaked at a record value of ∼8.0 × 10−6 s m−1. These large magnetoelectric effects arose due to non-volatile 90° rotations of the magnetic easy axis, reflecting a competition between the fixed growth anisotropy and the voltage-controlled magnetoelastic anisotropy. In contrast to previous work, our non-volatile rotations did not require the assistance of an applied magnetic field or the setting of an in-plane substrate polarization prior to deposition.

Substrate heating effects on composition, structure and ferromagnetic resonance properties of CoFeB thin films

Abstract We investigate the ferromagnetic resonance and crystallinity properties of CoFeB thin films grown on Si (1 0 0), at substrate temperatures from 20 °C to 800 °C. The X-ray diffraction data show that the CoFeB films are amorphous when deposited at a substrate deposition temperature of less than 500 °C. X-ray diffraction peak analysis matched the crystalline peaks of films grown at 500 °C to a face-centered cubic (1 1 0) Co7Fe3 phase. Increasing the substrate deposition temperatures increased above 600 °C leads to the formation of a face-centered cubic silicide FeCo2Si (2 2 0) and oxide Fe2CoO4 (4 0 0). Upon crystallization into different phases at a substrate deposition temperature above 500 °C, the soft ferromagnetic properties of CoFeB are lost. The coercivity of the deposited thin films increases from 20 Oe at room temperature to a maximum of 275 Oe at 700 °C. The peak-to-peak ferromagnetic resonance linewidth at 15 GHz, increases from 80 Oe at room temperature to 790 Oe at 500 °C. The grain size of the as-deposited samples increases with substrate temperature from 20-nm at room temperature to 241-nm at 800 °C as determined by atomic force microscopy.

Control of spin-wave transmission by a programmable domain wall

Active manipulation of spin waves is essential for the development of magnon-based technologies. Here, we demonstrate programmable spin-wave filtering by resetting the spin structure of pinned 90° Néel domain walls in a continuous CoFeB film with abrupt rotations of uniaxial magnetic anisotropy. Using micro-focused Brillouin light scattering and micromagnetic simulations, we show that broad 90° head-to-head or tail-to-tail magnetic domain walls are transparent to spin waves over a broad frequency range. In contrast, magnetic switching to a 90° head-to-tail configuration produces much narrower and strongly reflecting domain walls at the same pinning locations. Based on these results, we propose a magnetic spin-wave valve with two parallel domain walls. Switching the spin-wave valve from an open to a closed state changes the transmission of spin waves from nearly 100 to 0%. Active control over spin-wave transport through programmable domain walls could be utilized in magnonic logic devices or non-volatile memory elements.

Magnetic Anisotropy and Damping Constant in CoFeB/Ir and CoFeB/Ru Systems

Co20Fe60B20 thin films of various thicknesses (3 nm $\le t_{\text {CFB}} \le 14$ nm) have been sputtered on thermally oxidized Si substrates and capped with 8 nm-thick Ir or Ru layers and then annealed at 400 °C. A vibrating sample magnetometer has been used to measure the CoFeB thickness dependence of the saturation magnetic moment per unit area in order to straightforwardly determine the magnetization at saturation and the magnetic dead layer thickness, found to be 1115 ± 50 emu/cm3 (930 ± 50 emu/cm3) and 1.53 nm (0.76 nm) for the as-deposited CoFeB/Ir (CoFeB/Ru) sytems, respectively. These values change to 1185 ± 50 emu/cm3 (1110±50 emu/cm3) and 2.93 nm (1.62 nm) for CoFeB/Ir (CoFeB/Ru) films annealed at 400 °C. Microstrip ferromagnetic resonance (MS-FMR) was used to investigate the Gilbert damping parameter and the magnetic anisotropy as a function of the CoFeB thickness. While the in-plane anisotropy field for the as-deposited films does not show a regular behavior versus the CoFeB thickness, a clear linear behavior of this field versus the reciprocal CoFeB effective thickness can be observed for the annealed samples. Moreover, the effective magnetization varies linearly with the inverse effective thickness of CoFeB due to the perpendicular interface anisotropy. This surface anisotropy constant, which was estimated to be 0.84 and 0.89 erg/cm2 for the as-deposited CoFeB films capped with Ir or Ru, respectively, reinforces the perpendicular easy axis. The corresponding anisotropy constants for the annealed Ir and Ru capped films are found to be 1.07 and 0.57 erg/cm2, respectively. Moreover, MS-FMR results revealed that the damping constant increases linearly with the reciprocal effective thickness of CoFeB, most probably due to spin pumping. The effective spin mixing conductance of the as-deposited (annealed) CoFeB/Ru and CoFeB/Ir has been estimated to be 12.93 nm−2 (22.4 nm−2) and 29.3 nm−2 (43.85 nm−2), respectively.