Ultrathin CoFeB 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.

Large voltage-controlled magnetic anisotropy effect in magnetic tunnel junctions prepared by deposition at cryogenic temperatures

We investigated the influence of the buffer material and a cryogenic temperature deposition process on the voltage-controlled magnetic anisotropy (VCMA) effect for an ultrathin CoFeB layer in bottom-free type MgO-based magnetic tunnel junctions prepared by a mass production sputtering process. We used Ta and TaB buffers and compared the differences between them. The TaB buffer enabled us to form a flat and less-contaminated CoFeB/MgO interface by suppressing the diffusion of Ta with maintaining a stable amorphous phase. Furthermore, the introduction of cryogenic temperature deposition for the ultrathin CoFeB layer on the TaB buffer improved the efficiency of the VCMA effect and its annealing tolerance. Combining this with interface engineering employing an Ir layer for doping and a CoFe termination layer, a large VCMA coefficient of −138 ± 3 fJ/Vm was achieved. The developed techniques for the growth of ultrathin ferromagnet and oxide thin films using cryogenic temperature deposition will contribute to the development of high-performance spintronic devices, such as voltage-controlled magnetoresistive random access memories.

Low magnetic damping in an ultrathin CoFeB layer deposited on a 300 mm diameter wafer at cryogenic temperature

We deposited an ultrathin CoFeB(1.1 nm) layer, which functions as a storage layer of MgO-based magnetic tunnel junctions for spin-transfer-torque (STT) magnetoresistive random-access memory (MRAM), on ϕ300 mm wafers at 100 K and investigated its effect on the magnetization dynamics of CoFeB. We observed clear reductions in both the inhomogeneous linewidth and total magnetic damping parameter for the CoFeB layer deposited at 100 K compared to those deposited at 300 K through the improvement in the interfacial quality. The results show that deposition at cryogenic temperatures is an effective manufacturing process for high-quality magnetic thin films with low magnetic damping.

Improvement in perpendicular magnetic anisotropy and its voltage control efficiency in CoFeB/MgO tunnel junctions with Ta/Mo layered adhesion structures

By utilizing Ta/Mo layered adhesion structures, thermally robust perpendicular magnetic anisotropy and voltage-controlled magnetic anisotropy (VCMA) effects were achieved in magnetic tunnel junctions (MTJs) with ultrathin CoFeB films grown on MgO. After annealing at 400 °C, MTJs with Ta/Mo layered adhesion exhibited VCMA coefficients of 48 fJ/Vm. The combination of Ta and Mo improved the crystalline orientation and flatness of the CoFeB/MgO tunneling barrier interfaces, as determined by cross-sectional scanning transmission electron microscopy. Additionally, we demonstrate that the small interdiffusion between Mo and CoFe enables effective scavenging of B from CoFeB by increasing the thickness of the B sink layer without impairing the device performance due to atomic diffusion.

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.

Analyzer-free, intensity-based, wide-field magneto-optical microscopy

In conventional Kerr and Faraday microscopy, the sample is illuminated with plane-polarized light, and a magnetic domain contrast is generated by an analyzer making use of the Kerr or Faraday rotation. Here, we demonstrate possibilities of analyzer-free magneto-optical microscopy based on magnetization-dependent intensity modulations of the light. (i) The transverse Kerr effect can be applied for in-plane magnetized material, as demonstrated for an FeSi sheet. (ii) Illuminating that sample with circularly polarized light leads to a domain contrast with a different symmetry from the conventional Kerr contrast. (iii) Circular polarization can also be used for perpendicularly magnetized material, as demonstrated for garnet and ultrathin CoFeB films. (iv) Plane-polarized light at a specific angle can be employed for both in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetization and to a domain boundary contrast. (vi) Domain contrast can even be obtained without a polarizer. In cases (ii) and (iii), the contrast is generated by magnetic circular dichroism (i.e., differential absorption of left- and right-circularly polarized light induced by magnetization components along the direction of light propagation), while magnetic linear dichroism (differential absorption of linearly polarized light induced by magnetization components transverse to propagation) is responsible for the contrast in case (v). The domain–boundary contrast is due to the magneto-optical gradient effect. A domain–boundary contrast can also arise by interference of phase-shifted magneto-optical amplitudes. An explanation of these contrast phenomena is provided in terms of Maxwell–Fresnel theory.

Novel Tunnel Magnetoresistive Sensor Functionalities via Oblique-Incidence Deposition.

Controlling the magnetic properties of ultrathin films remains one of the main challenges to the further development of tunnel magnetoresistive (TMR) device applications. The magnetic response in such devices is mainly governed by extending the primary TMR trilayer with the use of suitable contact materials. The transfer of magnetic anisotropy to ferromagnetic electrodes consisting of CoFeB layers results in a field-dependent TMR response, which is determined by the magnetic properties of the CoFeB as well as the contact materials. We flexibly apply oblique-incidence deposition (OID) to introduce arbitrary intrinsic in-plane anisotropy profiles into the magnetic layers. The OID-induced anisotropy shapes the magnetic response and eliminates the requirement of additional magnetic contact materials. Functional control is achieved via an adjustable shape anisotropy that is selectively tailored for the ultrathin CoFeB layers. This approach circumvents previous limitations on TMR devices and allows for the design of new sensing functionalities, which can be precisely customized to a specific application, even in the high field regime. The resulting sensors maintain the typical TMR signal strength as well as a superb thermal stability of the tunnel junction, revealing a striking advantage in functional TMR design using anisotropic interfacial roughness.

Perpendicular magnetic anisotropy and its voltage control in MgO/CoFeB/MgO junctions with atomically thin Ta adhesion layers

We study the perpendicular magnetic anisotropy (PMA) and the voltage-controlled magnetic anisotropy (VCMA) effect in MgO/Ta/CoFeB/MgO junctions. A few monolayers (3–5 Å) of Ta acts as an adhesive layer, allowing for the formation of flat and ultrathin (∼10 Å) CoFeB films with large PMA on top of MgO. In the MgO/Ta/CoFeB/MgO junctions, the diffusion of Ta to the upper CoFeB/MgO interface is effectively eliminated, and superior thermal stability up to 400 ∘C is demonstrated. Detailed nanostructural analyses reveal the appearance of Co–Fe compositional variation in the CoFeB layer associated with the insertion of Ta layer, which explains the observed changes in PMA and VCMA. Our results will facilitate the development of VCMA-based memory devices and also pave the way toward controlling the magnetic and electric properties of CoFeB thin films through subnanometer-scale compositional modulation.

Perpendicular Magnetic Anisotropy and its Voltage Control in MgO/CoFeB/MgO Junctions with Atomically Thin Ta Adhesion Layers

We study the perpendicular magnetic anisotropy (PMA) and the voltage-controlled magnetic anisotropy (VCMA) effect in MgO/Ta/CoFeB/MgO junctions. A few monolayers (3 − 5 A) of Ta acts as an adhesive layer, allowing for the formation of flat and ultrathin (∼ 10 A) CoFeB films with large PMA on top of MgO. In the MgO/Ta/CoFeB/MgO junctions, the diffusion of Ta to the upper CoFeB/MgO interface is effectively eliminated, and superior thermal stability up to 400◦C is demonstrated. Detailed nanostructural analyses reveal the appearance of Co-Fe compositional variation in the CoFeB layer associated with the insertion of Ta layer, which explains the observed changes in PMA and VCMA. Our results will facilitate the development of VCMA-based memory devices and also pave the way toward controlling the magnetic and electric properties of CoFeB thin films through subnanometer-scale compositional modulation.

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 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.

Ultrathin perpendicular magnetic anisotropy CoFeB free layers for highly efficient, high speed writing in spin-transfer-torque magnetic random access memory

Perpendicular magnetic anisotropy (PMA) ferromagnetic CoFeB with dual MgO interfaces is an attractive material system for realizing magnetic memory applications that require highly efficient, high speed current-induced magnetic switching. Using this structure, a sub-nanometer CoFeB layer has the potential to simultaneously exhibit efficient, high speed switching in accordance with the conservation of spin angular momentum, and high thermal stability owing to the enhanced interfacial PMA that arises from the two CoFeB-MgO interfaces. However, the difficulty in attaining PMA in ultrathin CoFeB layers has imposed the use of thicker CoFeB layers which are incompatible with high speed requirements. In this work, we succeeded in depositing a functional CoFeB layer as thin as five monolayers between two MgO interfaces using magnetron sputtering. Remarkably, the insertion of Mg within the CoFeB gave rise to an ultrathin CoFeB layer with large anisotropy, high saturation magnetization, and good annealing stability to temperatures upwards of 400 °C. When combined with a low resistance-area product MgO tunnel barrier, ultrathin CoFeB magnetic tunnel junctions (MTJs) demonstrate switching voltages below 500 mV at speeds as fast as 1 ns in 30 nm devices, thus opening a new realm of high speed and highly efficient nonvolatile memory applications.

Electrical Initialization of Electron and Nuclear Spins in a Single Quantum Dot at Zero Magnetic Field.

The emission of circularly polarized light from a single quantum dot relies on the injection of carriers with well-defined spin polarization. Here we demonstrate single dot electroluminescence (EL) with a circular polarization degree up to 35% at zero applied magnetic field. The injection of spin-polarized electrons is achieved by combining ultrathin CoFeB electrodes on top of a spin-LED device with p-type InGaAs quantum dots in the active region. We measure an Overhauser shift of several microelectronvolts at zero magnetic field for the positively charged exciton (trion X+) EL emission, which changes sign as we reverse the injected electron spin orientation. This is a signature of dynamic polarization of the nuclear spins in the quantum dot induced by the hyperfine interaction with the electrically injected electron spin. This study paves the way for electrical control of nuclear spin polarization in a single quantum dot without any external magnetic field.

Ultrathin CoFe-Borate Layer Coated CoFe-Layered Double Hydroxide Nanosheets Array: A Non-Noble-Metal 3D Catalyst Electrode for Efficient and Durable Water Oxidation in Potassium Borate

It is highly desired to develop earth-abundant catalyst materials for efficient and durable water oxidation under benign conditions. In this Letter, we report on the development of ultrathin CoFe-b...

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.

Excitation of coherent propagating spin waves in ultrathin CoFeB film by voltage-controlled magnetic anisotropy

Spin waves (SWs) may be used as potential information carriers in next generation low-power spintronics devices. Here, we report an experimental study on the excitation of propagating magnetostatic surface SWs by voltage-controlled magnetic anisotropy in a 2 nm thick CoFeB film. The SWs are detected by a pico-second time-resolved longitudinal Kerr microscope with a spatial resolution of 600 nm. We found a linear increase in the SW amplitude with the applied rf voltage. We show that in this ultrathin film, the voltage excited SWs can propagate up to micrometer distances which decrease with the increase in the bias magnetic field value. This is also supported by micromagnetic simulation results. Furthermore, we show that voltage excitations are spatially localized as opposed to conventional microstrip antenna induced Oersted field excitations. We discuss about the advantage of voltage excitation compared to the Oersted field excitation. We believe that voltage excitation of SWs will be more suitable and useful for the development of all-voltage-controlled nanoscale spintronics devices with a high density of integration.

Temperature dependence of the interfacial magnetic anisotropy in W/CoFeB/MgO

The interfacial perpendicular magnetic anisotropy in W/CoFeB (1.2 ∼ 3 nm)/MgO thin film structures is strongly dependent on temperature, and is significantly reduced at high temperature. The interfacial magnetic anisotropy is generally proportional to the third power of magnetization, but an additional factor due to thermal expansion is required to explain the temperature dependence of the magnetic anisotropy of ultrathin CoFeB films. The reduction of the magnetic anisotropy is more prominent for the thinner films; as the temperature increases from 300 K to 400 K, the anisotropy is reduced ∼50% for the 1.2-nm-thick CoFeB, whereas the anisotropy is reduced ∼30% for the 1.7-nm-thick CoFeB. Such a substantial reduction of magnetic anisotropy at high temperature is problematic for data retention when incorporating W/CoFeB/MgO thin film structures into magneto-resistive random access memory devices. Alternative magnetic materials and structures are required to maintain large magnetic anisotropy at elevated temperatures.

Effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film

Voltage or electric field induced magnetization dynamics promises low power spintronics devices. For successful operation of some spintronics devices such as magnetic oscillators and magnetization switching devices a clear understanding of nonlinear magnetization dynamics is required. Here, we report a detailed experimental and micromagnetic simulation study about the effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film. Experimental results show that the resonance line-width and frequency remains constant, whereas cone angle of the magnetization precession increases linearly with square-root of excitation power below threshold value, known as linear excitation regime. Above threshold power, the dynamics enters into nonlinear regime where resonance line-width monotonically increases and resonance frequency monotonically decreases with increasing excitation power. Simulation results reveal that a strong nonlinear and incoherent magnetization dynamics are observed in our experiment above the threshold power which reduces dynamic magnetic signal by suppressing large cone angle of magnetization precession. Moreover, a significant transfer of spin angular momentum from uniform FMR mode to its degenerate spin waves outside of excitation area further restrict the cone angle of precession within only few degrees in our device. Our results will be very useful to develop all-voltage-controlled spintronics devices.

Magneto-ionic effect in CoFeB thin films with in-plane and perpendicular-to-plane magnetic anisotropy

The magneto-ionic effect is a promising method to control the magnetic properties electrically. Charged mobile oxygen ions can easily be driven by an electric field to modify the magnetic anisotropy of a ferromagnetic layer in contact with an ionic conductor in a solid-state device. In this paper, we report on the room temperature magneto-ionic modulation of the magnetic anisotropy of ultrathin CoFeB films in contact with a GdOx layer, as probed by polar micro-Magneto Optical Kerr Effect during the application of a voltage across patterned capacitors. Both Pt/CoFeB/GdOx films with perpendicular magnetic anisotropy and Ta/CoFeB/GdOx films with uniaxial in-plane magnetic anisotropy in the as-grown state exhibit a sizable dependence of the magnetic anisotropy on the voltage (amplitude, polarity, and time) applied across the oxide. In Pt/CoFeB/GdOx multilayers, it is possible to reorient the magnetic anisotropy from perpendicular-to-plane to in-plane, with a variation of the magnetic anisotropy energy greater than 0.2 mJ m−2. As for Ta/CoFeB/GdOx multilayers, magneto-ionic effects still lead to a sizable variation of the in-plane magnetic anisotropy, but the anisotropy axis remains in-plane.