CoFeB Films Research Articles

Double peaks of the permeability spectra of obliquely sputtered CoFeB amorphous films

Without applying an in situ magnetic field, soft magnetic Co40Fe40B20 films were successfully deposited onto rotating silicon substrates using the oblique sputtering technique. Different thicknesses could be achieved by controlling the sputtering time. The corresponding static and dynamic electromagnetic properties were then systematically investigated. Interestingly, as the thickness increased from approximately 50 nm to 250 nm, the imaginary part of the permeability spectra of CoFeB films changed from a single peak to double peaks and then to a broad single peak. Ferromagnetic resonance (FMR), crystalline structure and cross-section SEM analyses evidenced that the double peaks of the permeability spectra are attributed to double anisotropies and anisotropic diffusion. The overlapping double FMR peaks provide a wide absorption frequency band over 2 GHz, which is critical for the design of an electromagnetic interference suppressor.

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

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.

Effect of heavy metal interface on the magnetic behaviour and thermal stability of CoFeB film

Evolution of the structure and magnetic properties of an amorphous CoFeB layer in a heterostructure consisting of HM/CoFeB/HM (HM = Hf, W), with thermal annealing has been studied using magneto-optical Kerr effect (MOKE) and synchrotron based Grazing incidence x-ray diffraction (GIXRD) measurements. It is found that the interface with HM can significantly affect the thermal stability as well as magnetic properties of CoFeB layer. Crystallization temperature of CoFeB layer interfaced with Hf is about 50 °C lower than that of CoFeB interfaced with W. Further, while in Hf/CoFeB/Hf, the as-deposited film possesses a well-defined uniaxial magnetic anisotropy, in W/CoFeB/W the magnetization is almost isotropic in the film plane. Long range stresses in as-deposited film and possible bond orientational order are expected to cause the observed anisotropy. With onset of crystallization, random stresses and compositional inhomogenities are generated in CoFeB film, resulting in a large increase in magnetic coercivity, and disappearance of magnetic anisotropy.

Nanometer-scale etching of CoFeB thin films using pulse-modulated high density plasma

Pulse-modulated inductively coupled plasma reactive ion etching of nanometer-scale patterned CoFeB thin films was performed in CH4/O2/Ar gas mixture. As the pulse on-off duty ratio decreased, the etch selectivity of CoFeB/TiN slightly increased and the etch profiles were improved. Moreover, the etch selectivity of the CoFeB films and the etch profiles were improved with the increase in the pulse frequency of the plasma. X-ray photoelectron spectroscopy revealed that during the pulse-modulated etching in the CH4/O2/Ar gas mixture, some polymeric layers were formed on the CoFeB films, which helped prevent the lateral etching and increased the etch selectivity. Consequently, nanometer-scale etching of CoFeB thin films patterned with TiN hard masks could be achieved using pulsed-modulated plasma in CH4/O2/Ar gas mixture.

Effect of substrate roughness on the magnetic properties of CoFeB films

In this paper, a serious of CoFeB films of 30 nm prepared by a DC magnetron sputtering on Si substrates with different roughness etched by NaOH solution are investigated. It is found that the coercivity (Hc) and effective damping coefficient α are influenced by the substrate roughness of the films. As the substrate roughness increased to 73.9 nm, the Hc ranged from 14.4 Oe to 81.4 Oe; the α, from 0.012 to 0.11 at the same time. It is attributed to the function of magnetic moment pinning and magnon-magnon scattering caused by the rough substrate. Thus, the tunable performances of soft magnetism and high frequency of CoFeB films make them become potential candidates for microwave-based devices and other tunable microwave devices.

Exchange-torque-induced excitation of perpendicular standing spin waves in nanometer-thick YIG films

Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow magnetic damping are relevant for magnon-based spintronics and low-power wave-like computing. The excitation frequency of spin waves in YIG is rather low in weak external magnetic fields because of its small saturation magnetization, which limits the potential of YIG films for high-frequency applications. Here, we demonstrate how exchange-coupling to a CoFeB film enables efficient excitation of high-frequency perpendicular standing spin waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense PSSW modes up to 10th order. Strong hybridization between the PSSW modes and the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing behavior in broadband spin-wave spectra. We explain the excitation of PSSWs by exchange coupling between forced magnetization precessions in the YIG and CoFeB layers. If the amplitudes of these precessions are different, a dynamic exchange torque is generated, causing the emission of spin waves from the interface. PSSWs form when the wave vector of the spin waves matches a perpendicular confinement condition. PSSWs are not excited if exchange coupling between YIG and CoFeB is eliminated by a 10 nm Ta spacer layer. Micromagnetic simulations confirm the exchange-torque-driven mechanism.

Interlayer-assisted atomic layer deposition of MgO as a magnetic tunneling junction insulators

The growth behavior of MgO deposited on a Mg interlayer by plasma-enhanced atomic layer deposition (PE-ALD) was investigated to improve the MgO layer quality for magnetic tunneling junction (MTJ) applications. The Mg interlayer was inserted beneath the PE-ALD MgO film to prevent the oxidation of the bottom substrate layer. The chemical bonding status, elemental depth profile, and electron microscopy results for the PE-ALD Mg/MgO stack indicate that the Mg interlayer successfully blocked the oxidation of the substrate via its transformation to MgO. In addition, a change of the preferred orientation from (111) to (200) was observed in the X-ray diffraction pattern when the Mg interlayer was applied. This was attributed to the (200)-preferred orientation of the MgO formed through the oxidation of the Mg interlayer. The use of Mg interlayer is aim to prevent the degradation of the tunneling magneto-resistance properties by not only protecting the bottom layer (i.e., = CoFeB film in MTJs) from oxidation but also changing the preferred orientation from (111) to (200).

Interference induced enhancement of magneto-optical Kerr effect in ultrathin magnetic films

We have studied the magneto-optical spectra of ultrathin magnetic films deposited on Si substrates coated with an oxide layer (SiOx). We find that the Kerr rotation angle and the ellipticity of ~1 nm thick CoFeB thin films, almost transparent to visible light, show a strong dependence on the thickness of the SiOx layer. The Kerr signal from the 1 nm CoFeB thin film can be larger than that of ~100 nm thick CoFeB films for a given SiOx thickness and light wavelength. The enhancement of the Kerr signal occurs when optical interference takes place within the SiOx layer. Interestingly, under such resonance condition, the measured Kerr signal is in some cases larger than the estimation despite the good agreement of the measured and calculated reflection amplitude. We infer the discrepancy originates from interface states that are distinct from the bulk characteristics. These results show that optical interference effect can be utilized to study the magneto-optical properties of ultrathin films.

Tuning Magnetic Properties of Thick CoFeB Film by Interlayer Coupling in Trilayer Structured Thin Films

This study deals with tuning magnetic properties of a thick amorphous (a-)Co20Fe60B20 (CoFeB262) film by using interlayer magnetic coupling in trilayer structured films of [CoFeB262 (100 nm)/[Cr,Ta (x nm)]/CoFeB262 (y nm)] with y=2-50, xCr=0.75, 2 and xTa=1, 4. All the films are deposited directly on thermally oxidized Si substrate at ambient temperature using magnetron sputtering. The as-deposited a-CoFeB262 (100 nm) film exhibits magnetic stripe domain and transcritical hysteresis loop due to large effective magnetic anisotropy caused by stress induced during deposition of the films. On the other hand, the shape of magnetic hysteresis (M-H) loops in trilayer films transforms from transcritical to rectangular shaped one with enhanced remanence ratio (MR/MS) of ≥ 75% and single magnetization reversal behavior. This effectively reduces coercivity (HC) and field required to saturate magnetization (HS) in trilayer films. However, the changes in the loop shape and reductions in HC and HS depend strongly on x and y. Magnetic domain images obtained using Kerr microscopy in trilayer films show a rapid switching of large-sized domains along easy-axis and weak ripple domains along hard-axis. In addition, the magnetization reversal behavior along the hard-axis strongly depends on x(Cr,Ta). M-H loops obtained at different temperatures between 30 K and 300 K reveal no change in loop shape for trilayer films with small x and y, while the disappearance of shearing and formation of additional steps at low temperatures are observed for films with large x and y. The observed results are explained on the basis of change in interlayer coupling between CoFeB262 layers with x, y and temperature. Furthermore, these results clearly confirm that the magnetic properties of thick CoFeB262 film with stripe domain can easily be tuned into in-plane magnetization by this simple trilayer structured thin films.

Double Hard Axes of Hysteresis Loop in Wide-Angle Obliquely Sputtered CoFeB Amorphous Films

Soft magnetic Co40Fe40B20 films with different tilt angle were successfully deposited on silicon substrates by using oblique sputtering technique. Different oblique angles are achieved by controlling the position of samples. The corresponding static magnetic properties of these samples were then systematically investigated. Interestingly, with the oblique angle increasing from 38° to 55°, the MOKE hysteresis loop of the thin films displays a unique and special performance with double hard axes. Meanwhile, despite of the measurement magnetic field along PR or AR direction, both of the hysteresis loops have two-stage magnetization reversal that means there are two comparably strong anisotropies in the CoFeB films. Moreover, rotating samples from in-plane to out-of-plane, the hysteresis loops demonstrate the perpendicular anisotropy exist in CoFeB films. The cross-section SEM characterizations further verify that the residual field from the magnetic cylinder will strongly impact the microstructures of thin film.

The evolution of in-plane magnetic anisotropy in CoFeB/GaAs(001) films annealed at different temperatures

A considerable in-plane uniaxial magnetic anisotropy (UMA) field (Hu ∼ 300 Oe) could be achieved when the amorphous CoFeB film was deposited on the GaAs(001) wafer by magnetron-sputtering after proper etch-annealed procedure. In order to get deep insights into the mechanism of the UMA, the film was annealed at different temperatures and the evolution of the in-plane magnetic anisotropy was investigated carefully. With increasing the annealing temperature (TA), the UMA could be maintained well when TA reached 250°C, but became very weak at 300°C. However, when TA was elevated to 400°C, another UMA (Hu ∼ 130 Oe) was built accompanied with a fourfold magnetic anisotropy with its strength of about 50 Oe. In terms of the magnetic anisotropy evolution along with TA, the anelastic strain, which is thought to be resulted from the interfacial interaction between CoFeB and GaAs, may play a dominant role in producing the enhanced UMA based on the ‘bond-orientational’ anisotropy (BOA) model.

Effect of thickness on tuning the perpendicular coercivity of Ta/CoFeB/Ta trilayer

In the present investigation, a systematic study has been undertaken to explore the effect of magnetic layer film thickness on the structural, topographical, magnetic and electrical transport properties of the Ta(5 nm)/CoFeB(2–100 nm)/Ta(3 nm) trilayer. We show that the perpendicular coercivity (Hc┴) and the saturation magnetization (Ms) of Ta/CoFeB/Ta are very sensitive to the CoFeB film thickness. A large Hc┴ with small Ms is realized for 50 nm thick CoFeB film at a growth temperature (Ts) of 475 °C. Hc┴ started to develop at a considerable film thickness of 25 nm and reached a maximum at 50 nm and then decreased when the film thickness increased up to 100 nm. Magnetic studies of Ta/CoFeB (bottom) and CoFeB/Ta (top) interfaces confirm that the major contribution to the perpendicular coercivity of the stacks come from the bottom interface. The topography of the films is found to be affected by the film thicknesses. AFM displays larger grain sizes for thicker CoFeB films, however the nano crystallites embedded in the larger grains could not be detected by XRD analysis. The high resistivity values obtained from Hall resistivity measurements further confirms the non-crystalline nature of the CoFeB film. The presence of boron and other elements (Ta, Co, and Fe) are evident from the X-ray Photoelectron spectroscopic (XPS) studies. The Ta/CoFeB/Ta trilayer system being amorphous possessing high Hc┴, with small Ms can be enabled for the development of high-density data storage, spin logic gates and nonvolatile memories in low power consuming spintronics devices.

Determination of stress-coefficient of magnetoelastic anisotropy in flexible amorphous CoFeB film by anisotropic magnetoresistance

Flexible magnetic devices are one of the indispensable flexible devices. However, the deformation of the magnetic devices will change the magnetic anisotropy of magnetic materials due to magnetoelastic anisotropy, which will decrease the performance of the devices. Therefore, it is essential to determine the stress-coefficient of magnetoelastic anisotropy in magnetic materials. Here, the magnetic anisotropy constants of an amorphous CoFeB film on a flexible polyvinylidene fluoride (PVDF) substrate in different stress states were quantitatively investigated by anisotropic magnetoresistance (AMR). The enhanced magnetic anisotropy of the CoFeB film at reduced temperature is due to magnetoelastic anisotropy induced by anisotropic thermal expansion of the PVDF substrate. Through fitting the AMR curves under variant fields in different stress states, the stress-coefficient of magnetoelastic anisotropy in the amorphous CoFeB film is obtained to be 170.7 × 103 erg cm−3 GPa−1.

Microwave response to the magnetization switching of CoFeB/Ta/CoFeB spin valves and CoFeB films

Negative magnetoresistance modifying the quality factor of a microwave cavity under the magnetization switching of ferromagnetic layers has been discovered in a MgO/CoFeB/MgO/Ta film with a single ferromagnetic layer and a MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve consisting of two ferromagnetic CoFeB layers. The dependence of the first derivative dP/dH of the microwave absorption signal on the dc magnetic field of the spectrometer exactly reproduce the magnetic hysteresis loops of the sample. The slope of these dependences and the amplitude of dP/dH jumps under remagnetization of the layers are determined by the interplay of a negative magnetoresistance of individual layers and a positive giant magnetoresistance of the entire multilayer structure. The discovered phenomenon allows using microwave absorption for making a high-sensitivity contact-free indicator of the basic magnetization states of a spin valve.

Depth selective crystallization study of CoFeB film on MgO

Depth resolved study of crystallization of CoFeB film on MgO film has been done using GIXRD measurements. The quality and thermal stability of MgO film deposited using EB evaporation, was ensured using GIXRD measurements. Depth selectivity for studying the crystallization of CoFeB film has been achieved by performing the GIXRD measurements as a function of the angle of incidence on as-deposited as well as annealed samples. GIXRD measurements on partially crystallized sample annealed at 300 °C reveal that the crystallization of CoFeB film occurs preferentially at CoFeB/MgO interface with the thickness of crystallized layer being 7 nm. This is in conformity with suggestions made in some earlier studies. Anomalous XRD measurements near the Fe absorption edge on completely crystallized sample achieved after annealing at 450 °C shows the presence of (1 0 0) superlattice peak. This evidences the formation of ordered bcc CoFe phase. This result is important as it has been suggested that an ordered CoFe bcc phase may improve the symmetry filtering effect and hence the Tunnel Magnetoresistance value.

The effect of growth sequence on magnetization damping in Ta/CoFeB/MgO structures

Magnetization damping is a key parameter to control the critical current and the switching speed in magnetic random access memory, and here we report the effect of the growth sequence on the magnetic dynamics properties of perpendicularly magnetized Ta/CoFeB/MgO structures. Ultrathin CoFeB films have been grown between Ta and MgO but with different stack sequences, i.e. substrate/Ta/CoFeB/MgO/Ta and substrate/Ta/MgO/CoFeB/Ta. The magnetization dynamics induced by femtosecond laser was investigated by using all-optical pump-probe measurements. We found that the Gilbert damping constant was modulated by reversing stack structures, which offers the potential to tune the damping parameter by the growth sequence. The Gilbert damping constant was enhanced from 0.017 for substrate/Ta/CoFeB/MgO/Ta to 0.027 for substrate/Ta/MgO/CoFeB/Ta. We believe that this enhancement originates from the increase of intermixing at the CoFeB/Ta when the Ta atom layer was grown after the CoFeB layer.

Ferromagnetic resonance of CoFeB/Ta/CoFeB spin valves versus CoFeB film

In MgO/CoFeB/MgO/Ta (single layer) and MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve (bilayer), the orientation dependences of ferromagnetic resonance (FMR) have been analyzed. Magnetic anisotropy constants as well as damping constants have been extracted. The differences between the anisotropy constants in single-layer film and spin valve MgO/CoFeB/Ta/CoFeB/MgO/Ta are explained by dipole magnetic interaction between ferromagnetic layers. The main part of the linewidth corresponds to inhomogeneous widening, while smaller part is provided by interface exchange fluctuations. The FMR lineshape can be explained by standard FMR Lorenz line and nonresonant absorption provided by inverse spin Hall effect.

CoFeAlB alloy with low damping and low magnetization as a candidate for spin transfer torque switching

We investigate the effect of Al doping on the magnetic properties of the CoFeB alloy. Comparative measurements of the saturation magnetization, the Gilbert damping parameter α, and the exchange constant as a function of the annealing temperature for CoFeB and CoFeAlB thin films are presented. Our results reveal a strong reduction of the magnetization for CoFeAlB in comparison to CoFeB. If the prepared CoFeAlB films are amorphous, the damping parameter α is unaffected by the Al doping in comparison to the CoFeB alloy. In contrast, in the case of a crystalline CoFeAlB film, α is found to be reduced. Furthermore, the x-ray characterization and the evolution of the exchange constant with the annealing temperature indicate a similar crystallization process in both alloys. The data prove that CoFeAlB fulfills the requirements to be considered an attractive candidate for spin torque switching purposes and a reduction of the switching current in comparison with CoFeB is expected.

Control of perpendicular magnetic anisotropy and spin pumping damping in MgO/CoFeB/ Ta/Pt structures

The perpendicular magnetic anisotropy (PMA) and spin pumping induced magnetic damping of CoFeB films can be significantly tuned by the adjacent nonmagnetic layer (Ta or Pt). For the CoFeB/Pt(tPt) structure, both PMA strength and magnetic damping increase with tPt, due to the enhanced interfacial orbital hybridization and spin pumping effects, respectively. By inserting a thin Ta interlayer between CoFeB and Pt, the magnetic easy axis immediately turns into in-plane direction. Meanwhile, time-resolved magneto-optical Kerr effect measurement shows an obvious reduction in the magnetic damping, which can be explained as the increase of spin current backflow and the decrease of interfacial spin-mixing conductivity. As the Ta interlayer thickness (tTa) increases, the effective damping parameter αs measured at saturation field initially decreases and eventually reaches a constant value of 0.027. The αs value at tTa > 5 nm does not change regardless of the top Pt layer, implying the influence of Pt is completely isolated and the spin diffusion length of Ta is around 5 nm. Our findings provide more insights into the control of PMA strength and spin pumping contribution to magnetic damping.

Ferromagnetic resonance in monocrystalline spin valves CoFeB/Ta/CoFeB and CoFeB films with perpendicular magnetic anisotropy

Orientation dependences of ferromagnetic resonance in a MgO/CoFeB/MgO/Ta film with one ferromagnetic layer (monolayer) and in a MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve containing two single-crystal ferromagnetic CoFeB layers divided by a nonmagnetic Ta layer (bilayer) were investigated. Analysis of the orientation dependences of the structures with perpendicular magnetic anisotropy allowed calculating constants of magnetic anisotropy and damping factors. Physical reasons underlying the differences between these parameters in one- and two-layered structures are discussed.