CoFeB Films Research Articles

Spin Texture Engineering by a Temperature Gradient in Ultrathin CoFeB Film

Magnetic texture in a chiral material is touted as one of the next-generation nonvolatile computing elements. Thermal manipulation of magnetic textures attracts considerable attention and has promising applications. Here, the temperature-dependent magnetic properties of Ta/CoFeB/MgO thin film are studied experimentally and theoretically. We report the controllable spin texture distribution with a temperature gradient in the Ta/CoFeB/MgO thin film. The nucleation area could be generated at a certain location through the cooperation of heat and magnetic field. This method offers a way to realize spin texture injection at specific positions in magnetic thin films and devices. This work provides useful guidelines for the thermal control of the spintronic device operation.

Tuning the magnetic properties of ultrathin magnetic films with MgO as the buffer layer

To minimize the screening effect of a metallic ferromagnetic film and improve the effectiveness of electric field modulation, high-quality ultrathin magnetic film is one of the critical prerequisites. Here ultrathin magnetic films and synthetic antiferromagnetics (SAFs) are deposited on SiO2 substrates at room temperature using the magnetron sputtering. Atomic force microscopy is used to characterize the roughness of MgO, Nb, Ru, W, and CoFeB films, revealing their atomic-level flatness, with root mean square roughness values below 0.3 nm, which are crucial in the subsequent preparation of ultrathin SAF to achieve high-quality interfaces. The results from the magneto-optical Kerr microscopy suggest that when MgO is 1.48 nm, ultrathin 0.6 nm CoFeB exhibits stable room-temperature perpendicular magnetic anisotropy (PMA) and there is a correlation between room-temperature ferromagnetism and MgO thickness. For SAFs, ultrathin 0.6 nm CoFeB-based SAF exhibits room-temperature antiferromagnetism via the Ruderman-Kittel-Kasuya-Yosida interaction mechanism. The exchange coupling field demonstrates that the transition between ferromagnetic coupling and antiferromagnetic coupling can be regulated by adjusting both the thickness of the non-magnetic spacer layer and the CoFeB layer. This work lays the ground for the potential applications of high-density storage and efficient electric field modulation of ferromagnetic multilayers for improving energy efficiency.

Strong magnon-magnon coupling in hexagonal magnetic elements

We achieve strong magnon-magnon coupling experimentally between edge and center modes in patterned hexagonal CoFeB films. The coupling strength is adjustable via the external magnetic field’s direction and the nanomagnet’s aspect ratio. Micromagnetic simulation indicates that the stable existence of the center mode determines the occurrence of coupling, which correlates with the direction of the edge demagnetization field. This work offers a novel perspective on the study of magnon-magnon coupling in a single material, which benefits magnon-based quantum information technology.

Influence of growth temperature on the magnetization dynamics of sputtered CoFeB thin films on various substrates and their microwave device functionality

In the present investigation, sputtering growth of 100 nm thick Co40Fe40B20 (CoFeB) film has been investigated in a low temperature range from room temperature (RT) to 400 ℃ on silicon (Si), silicon dioxide (Si/SiO2), and sapphire (Al2O3) substrates. The film surface morphology, static and dynamic magnetization were measured as a function of growth temperature and substrate. The grown films were further tested as a potential material for microwave device functionality in terms of filter and phase shifter. Surface roughness was lowest for the film grown on Si/SiO2 substrate at 200 ℃ (0.39 nm) measured from atomic force microscopy. Static magnetic properties measured by tracing magnetic hysteresis loop suggest a soft magnetic behaviour with in-plane magnetic anisotropy. Film saturation magnetization increases with the increase in growth temperature up to 200 ℃ and then decreases from 200 ℃ to 400 ℃. This suggest that the sputtering growth from room temperature to 200 ℃ offer a moderate to low interfacial stress resulting in low defects density. Gilbert damping constant (αeff) of the thin film is obtained from the ferromagnetic resonance measurements in a broad band of microwave frequencies. Damping was lowest (5.4×10−3) in the film grown at 200 ℃ on the Si/SiO2 substrate which correlates with the roughness and magnetization data. A low value of αeff is desirable for a material to be used in energy efficient spintronics and magnonic applications. The CoFeB films were also utilized in prototype microwave band-stop filter and phase shifter device applications. The filter has been tested in the frequency range from 6GHz to 25GHz with an applied magnetic bias up to 4.0 kOe. The frequency tunability with respect to the applied magnetic bias measured to be around 3.64 GHz/kOe. The maximum attenuation of −4dB was observed for the film grown on Si/SiO2 substrate at 200 ℃. Whereas, phase shifter fabricated on Si/SiO2/CoFeB film grown at 200 ℃ shows a differential phase shift up to 75°/cm. These results indicates that the CoFeB film deposited at low growth temperature can be a potential material in spintronics and CMOS applications where low temperatures are required for device fabrications.

Magnetic properties of CoFeB films grown on a single-layer graphene underlayer

Magnetic properties of CoFeB films grown on a single-layer graphene underlayer

“Tailoring thermal stability and in-plane magnetic anisotropy of CoFeB alloy thin films via growth conditions engineering”

The present work investigates the influence of growth conditions on the magneto-structural properties of magnetron-sputtered CoFeB alloy thin films. Films grown at room temperature (RT) are soft magnetic (coercivity <9 Oe), show amorphous microstructure, and possess a distinct uniaxial magnetic anisotropy (UMA) in the film plane. The amorphous microstructure and UMA persist up to a growth temperature (Ts) of 300 °C. CoFeB film grown at Si (100) substrate shows amorphous to crystalline phase transformation at Ts = 500 °C. In contrast, under the same growth conditions, the CoFeB film prepared on the SiO2 substrate maintains an amorphous structure up to Ts = 500 °C. The difference in the microstructures was explained in terms of the out-diffusion of B to the interfacial layer and segregation near the substrate for the films having crystalline and amorphous structure, respectively. On both the substrates Addition of W buffer layer facilitated the growth of smooth and nanocrystalline CoFeB films at Ts ∼300 °C. Upon crystallization, UMA evolution with a simultaneous increase in the coercive field is interpreted in terms of joint effect of random anisotropy model and surface/interface roughness-induced magnetization reversal mechanism. Unlike post-annealing, the CoFeB films prepared at high temperature shows superior thermal stability against crystallization. The relationship between the soft magnetic performance, UMA evolution, and thermal stability of CoFeB films with the growth conditions is established. The study reveals the role of growth condition engineering, to obtain CoFeB thin films with controlled microstructure and magnetic performance, which is crucial for designing the spintronics devices.

Magnetic domain structure and magneto-transport properties of laser ablated Co40Fe40B20 thin films

We report the magnetic domain structure, and electrical and magnetoresistance properties of laser ablated Co40Fe40B20 (CoFeB) (6 to 36 nm) thin films deposited on SiO2 / Si(001) substrates. Magnetic force microscopy performed on annealed CoFeB thin films revealed larger magnetic domains, which are formed due to strong exchange coupling between the grains. The temperature-dependent sheet resistance of as-deposited thin films revealed that the observed non-metallic behavior is due to intragrain-tunneling and SiO2 inclusions in the amorphous matrix. The metallic behavior of annealed CoFeB thin films is due to electron scattering from grain boundaries and granularity correlated to the formation of nano-crystallites. Thickness and field-dependent magneto-transport studies show higher magnetoresistance values for thinner annealed CoFeB films due to more scattering events upon crystallization, which is consistent with the granular nature of the annealed thin films.

A magnetoelectric memory device based on pseudo-magnetization

We propose a new type of magnetoelectric memory device that stores magnetic easy-axis information or pseudo-magnetization, rather than a definite magnetization direction, in magnetoelectrically coupled heterostructures. Theoretically, we show how a piezoelectric/ferromagnetic (PE/FM) combination can lead to non-volatility in pseudo-magnetization exhibiting overall ferroelectric-like behavior. The pseudo-magnetization can be manipulated by extremely low voltages especially when the FM is a low-barrier nano-magnet. Using a circuit model benchmarked against experiments, we determine the switching energy, delay, switching probability and retention time of the envisioned 1T/1C memory device in terms of magnetic and circuit parameters and discuss its thermal stability in terms of a key parameter called back-voltage vm which is an electrical measure of the strain-induced magnetic field. Taking advantage of ferromagnetic resonance measurements, we experimentally extract values for vm in CoFeB films and circular nano-magnets deposited on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 which agree well with the theoretical values. Our experimental findings indeed indicate the feasibility of the proposed novel device and confirm the assumed parameters in our modeling effort.

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.

Anisotropy of magnetic damping in Ta/CoFeB/MgO heterostructures

Magnetic damping controls the performance and operational speed of many spintronics devices. Being a tensor quantity, the damping in magnetic thin films often shows anisotropic behavior with the magnetization orientation. Here, we have studied the anisotropy of damping in Ta/CoFeB/MgO heterostructures, deposited on thermally oxidized Si substrates, as a function of the orientation of magnetization. By performing ferromagnetic resonance (FMR) measurements based on spin pumping and inverse spin Hall effect (ISHE), we extract the damping parameter in those films and find that the anisotropy of damping contains four-fold and two-fold anisotropy terms. We infer that four-fold anisotropy originates from two-magnon scattering (TMS). By studying reference Ta/CoFeB/MgO films, deposited on LiNbO3 substrates, we find that the two-fold anisotropy is correlated with in-plane magnetic anisotropy (IMA) of the films, suggesting its origin as the anisotropy in bulk spin–orbit coupling (SOC) of CoFeB film. We conclude that when IMA is very small, it’s correlation with two-fold anisotropy cannot be experimentally identified. However, as IMA increases, it starts to show a correlation with two-fold anisotropy in damping. These results will be beneficial for designing future spintronics devices.

The Effect of Size and Strain on Micro Stripe Magnetic Domain Structure of CoFeB Thin Films

The prerequisite for flexible magnetic electronic devices is the knowledge of the preparation technology of flexible magnetic films and the evolution of the film properties under strain. In this work, CoFeB amorphous ferromagnetic films with stripe domains were prepared on flexible polyimide (PI) substrates by oblique sputtering. The results show that oblique sputtering induces the formation of columnar crystal structure in CoFeB films, which increases the perpendicular magnetic anisotropy of the films, thus leading to the appearance of stripe magnetic domain structures. On this basis, the CoFeB films with stripe domains were processed on a microscopic scale to investigate the size effect and strain regulation on the microscopic domain structure of the magnetic films. The characterization of the magnetic domain structure shows that the stripe domain contrast is reduced by the striped structure prepared by lithography. The triangular, circular and ring patterns deflect the alignment of the stripe domain to different degrees. The experimental results show that the deflection of the stripe domains is caused by the anisotropy of the shapes produced by the different patterns and that the size of the microstructure needs to be close to the period of the stripe domains for the size effect to be significant. In addition, the strain-induced magnetoelastic anisotropy effectively rotates the orientation of the stripe domains, and the variation in domain contrast demonstrates that tensile/compressive strains vary the magnitude of the out-of-plane stray field of the film. Our results provide some insight into the modulation of the physical properties of flexible magnetic films.

Stabilizing High-Frequency Magnetic Properties of Stretchable CoFeB Films by Ribbon-Patterned Periodic Wrinkles.

The intrinsic nonstretchable feature of magnetic films has significantly limited its applications on wearable high-frequency devices. Recent studies have proved that the wrinkling surface structure based on the growth on polydimethylsiloxane (PDMS) is an effective route to obtain stretchable magnetic films. However, it is still a great challenge to simultaneously achieve a desired stretchability and stretching-insensitive high-frequency properties of magnetic films. Herein, we reported a convenient method to stabilize the high-frequency properties of stretchable magnetic films by depositing magnetic ribbon-patterned films on prestrain PDMS membranes. The ribbon-patterned wrinkling CoFeB films have far fewer cracks than the continuous film, which indicates a nice strain-relief effect and thus confers the stability of high-frequency properties for the films under stretching. However, the wrinkle bifurcation and the uneven thickness at the ribbon edge could adversely affect the stability of its high-frequency properties. The 200 μm wide ribbon-patterned film shows the best stretching-insensitive behaviors and maintains a constant resonance frequency of 3.17 GHz at strain from 10% to 25%. Moreover, a good repeatability has been demonstrated by performing thousands of stretch-release cycles, which did not significantly deteriorate its performances. The ribbon-patterned wrinkling CoFeB films with excellent stretching-insensitive high-frequency properties are promising for application in flexible microwave devices.

Strain-coupled domains in BaTiO3(111)−CoFeB heterostructures

The coupling of ferroelectric and ferromagnetic order holds promise for the realization of low-energy voltage-controlled multiferroic devices. Here, the authors show that using a (111)-oriented BaTiO${}_{3}$ substrate it is possible to create two distinct strain-coupled regions in a deposited CoFeB film, in which the in-plane magnetic easy axis rotates by either 60 or 120 degrees. These regions, which exist side by side in a sample heterostructure, contain different magnetic domain wall structures that exhibit different dependences on magnetic field.

Deterministic field-free switching of perpendicular magnetization by spin–orbit torques originating from in-plane magnetized Co2MnAl

Spin–orbit torques (SOTs) induced magnetization switching in a perpendicularly magnetized CoFeB film deposited on a Ti/in-plane magnetized Co2MnAl stack was investigated. Deterministic switching of the CoFeB magnetization was demonstrated by applying a current pulse to the stack in a direction parallel or antiparallel to the Co2MnAl magnetization. We found that the hysteresis loops of the anomalous Hall resistance for CoFeB under the constant current is shifted in the out-of-plane magnetic field axis direction depending on the directions of both the applied current and the magnetization of Co2MnAl. The shift amount exhibits an almost linear increase as the current magnitude increases. These results are consistent with the effects caused by SOTs originating from spin currents having an in-plane polarization orthogonal to the Co2MnAl magnetization.

Process-induced magnetic tunnel junction damage and its recovery for the development of spin–orbit torque magnetic random access memory

The generation of torques in heterostructures of heavy metals and ferromagnets (HM / FM) in spin–orbit torque magnetic random access memory (SOT-MRAM) makes them interesting for the development of magnetic memory technology. Currently, the emerging SOT-MRAM technology faces major challenges in etching based on current manufacturing tools. In that respect, we experimentally investigate the two different etching recipes (namely, CF-Etch, & NH-Etch) of the reactive ion etching (RIE) process on the performance of a Ta/CoFeB/MgO/Ta (hard mask) magnetic film stacks to develop the SOT-MRAM technology. The experimental results demonstrate that CF-Etch etches Ta very fast, and MgO etches very slowly. In contrast, NH-Etch etches those two materials at a comparable rate. Both etches are terminated before MgO is removed. Based on the experimental results, we found that the CF-Etch reduces the interfacial anisotropy of CoFeB/MgO films. Moreover, the CF-Etch also destroys the magnetic moment of CoFeB film, which further deteriorates after the annealing process. In contrast, the magnetic properties of the stack are only slightly affected when the NH-Etch recipe is used. We also discuss the advantages and disadvantages of these two recipes based on the experimental study. Finally, we developed the SOT-MRAM devices based on these two etching recipes of the RIE process to improve the etching yield of this technology. The current SOT-MRAM fabrication process enables a uniform etch stop on the thin tunnel barrier layer and the Ta heavy metal layer (HM). A wafer-level statistical study of SOT switching characteristics, including tunnel magnetoresistance (TMR), SOT switching thresholds, and parallel/antiparallel state resistances (RP/RAP), has been performed.

Effects of atomic layer etching on magnetic properties of CoFeB films: Reduction of Gilbert damping

Atomic layer etching (ALE) is an emerging technology to etch thin films with atomic level precision for microelectronics industry applications. This approach has been previously demonstrated to work on a number of materials; however, in most cases, only electronic properties of these materials following ALE are investigated. Since ALE of complex magnetic materials is extremely important for use in magnetic tunnel junctins (MTJs), it is imperative to understand how this etching approach affects the magnetic properties of the corresponding films. In this work, we studied the surface morphology, elemental composition, and most importantly, the magnetic properties of the technologically relevant magnetic alloy CoFeB before and after ALE treatment, and compared with the traditional ion milling etching technique. Through ferromagnetic resonance measurements, we find while the change in the saturation magnetization from ALE is small, the Gilbert damping of CoFeB is reduced by 11–35%, possibly due to the suppressed two-magnon scattering processes on the sample surface. Our results show that ALE can be used to etch CoFeB nondestructively and may even improve its magnetization dynamics properties.

Positive stretching dependence of resonance frequency in CoFeB films modulated by lateral growth pre-strain

We demonstrated a method via applying a lateral pre-strain to achieve a positive stretching dependence of resonance frequency in CoFeB films. An artificial wrinkling surface structure by adhering CoFeB films to biaxially pre-strained elastomeric polydimethylsiloxane (PDMS) supports was employed to fabricate stretchable magnetic films. The lateral growth pre-strain was used to adjust the longitudinal stretching dependence of resonance frequency of CoFeB films. For the sample fabricated on an elastomeric support that can freely contract in the lateral direction, the resonance frequency fr monotonously decreases with applying a tensile strain εapp. For the sample fabricated on an elastomeric support that can partially contract in the lateral direction, the fr first decreases and then increases as the εapp increases. The sample fabricated on an elastomeric support in which the lateral contraction is strictly limited display a positive stretching dependence of fr in the whole range of εapp. Thousands of fatigue tests show that the stretching dependence of microwave properties have a good repeatability.

Tailoring structural and magnetic viz. static and dynamic properties of soft magnetic CoFeB film with Ta incorporation

Present study provides an insight of tailoring magnetic properties (static and dynamic) driven by structural modifications, with controlled incorporation of Ta in soft magnetic CoFeB films. Unlike conventional thermal annealing, stress induced in-plane uniaxial magnetic anisotropy of CoFeB have observed to be release with incorporation of Ta, as confirmed from in-plane rotation dependent magneto-optical Kerr effect (MOKE). Increasing atomic percent of Ta in CoFeB is accompanied with decreasing saturation magnetization of soft-magnetic layer and film densification, as confirmed from polarized neutron and X-ray reflectivities. Ferromagnetic resonance (FMR) confirms the reduction of effective magnetization of the samples with Ta incorporation, with rise of damping parameter because of increasing magnetic inhomogenities. Further, the effect of vacuum annealing on these films has been investigated. Post annealing measurements suggest rapid elimination of magnetic anisotropy for Ta doped CoFeB films. Annealing at a temperature beyond CoFeB crystallization temperature results in an increase of magnetization, relative to as-prepared samples, amplitude of which modifies with incorporated Ta concentration. Such enhancement is accompanied with the structural changes as confirmed from X-ray diffraction (XRD) measurements, resulting an increase of magnetic coercivity. The results are important in view of tailoring the functional properties of CoFeB films for wide range of applications.

Magnetic properties in soft (CoFeB)/hard (Co) bilayers deposited under different Ar gas pressure

We have studied the effect of deposition pressure on the magnetization reversal, domains, anisotropy and Gilbert damping constant in the ferromagnetic (CoFeB and Co) single and bilayer samples. Hysteresis measured by magneto-optic Kerr microscopy for the single layer films prepared at higher deposition pressure indicate no change of loop shape i.e. isotropic behaviour. An enhancement of anisotropy has been observed in the bilayer samples than the single layer samples prepared at a particular deposition condition. However, increasing the deposition pressure to 50 sccm for the bilayer samples, anisotropy gets reduced. For single layer Co film deposited at 10 sccm exhibits branch and patch like domains for different angle between the easy axis and the external magnetic field. However, the Co film deposited at 50 sccm exhibits ripple like domains. In the case of single layer CoFeB, branch and patch like domains are observed deposited at 10 sccm. Patch like domains are found for the CoFeB films deposited at 50 sccm. Pinned labyrinth and ripple kind of magnetic domains along with the big branch domains are found in the bilayer samples. The pinned domains may be due to the interfacial exchange coupling. Similar values of damping constants have been observed for different thin films prepared at different deposition pressure.

Role of Nb content in tailoring the microstructure and magnetic anisotropy of soft magnetic W/CoFeB alloy thin films prepared with varying the substrate temperature

The work describes the role of Nb addition on the magneto-structural properties of magnetron sputtered Si/SiO2/W/(CoFeB)1−xNbx (x = 0, 3, 5 and 10) metal metalloid alloy thin films. Films were grown at substrate temperature (Ts) of RT, 300 °C and 500 °C to realise the influence of Nb content in the amorphous and nano-crystalline states of (CoFeB)1−xNbx thin films. RT deposited films are soft magnetic (coercivity (Hc) ≤ 10 Oe), exhibits amorphous microstructure and display a well-defined uniaxial magnetic anisotropy (UMA) in the film plane. At 300 °C, devitrification occurs, and the film (x = 0) becomes partially crystalline with precipitation of FeCo nano-crystals, and UMA nearly disappears. With increasing Nb content, the mean grain size and amount of FeCo nano-crystals decreases, and partially crystalline CoFeB films transform to a completely amorphous microstructure with the simultaneous appearance of in-plane UMA and systematic improvement in soft-magnetic nature (Hc decreases from~22–5 Oe). Nb addition on at Ts= 500 °C results in grain growth inhibition and magnetically isotropic (CoFeB)1−xNbx films (except x = 3). The present study demonstrates that thermally stable amorphous CoFeB films with desired soft magnetic properties can be prepared by appropriate selection of Nb content and suitable growth temperature.