Ta Buffer Layer Research Articles

Effects of Buffer and Capping Layers on Thermal Stability of CoFeB/MgO Frames at Various Temperatures

The utilization of CoFeB thin films in spintronic devices has attracted significant attention due to their exceptional magnetic properties, which include high saturation magnetization and spin polarization. However, the effect of ambient temperature on the magnetic properties of CoFeB/MgO frames, particularly those with different buffer and capping layers, remains unexplored. Therefore, in this study, the magnetostatic and dynamic properties of CoFeB/MgO frames were investigated at various temperatures. Using vibrating sample magnetometry and ferromagnetic resonance spectroscopy, changes in key parameters such as saturation magnetization, the Gilbert damping constant, magnetic anisotropy field, in-plane uniaxial magnetic anisotropy energy, and thermal stability factor were investigated. Furthermore, the thermal stabilities of CoFeB/MgO frames with Ta buffer and capping layers were compared with those of CoFeB/MgO frames with W buffer and capping layers by examining the changes in the key parameters at various temperatures. These results reveal that the thermal stability of the latter surpassed that of the former. This study provides significant insights for the development of thermally robust spintronic devices capable of operating above room temperature.

Effect of Ta buffer layer on the structural and magnetic properties of stoichiometric intermetallic FeAl alloy

The magnetostructural phase transition in Fe50Al50 alloy with chemically ordered paramagnetic B2 and disordered ferromagnetic (FM) A2 phase has applications in spintronics such as phase-change magnetic memory and magnonic devices. We first conducted a systematic first-principles density functional theory study of the A2 and B2 phases in the Fe50Al50 alloy. A theoretical understanding of this equiatomic alloy’s electronic and spin-dynamical properties leads us to the experimental exploration of the FM A2 phase. Therefore, we deposit the 50 nm Fe50Al50 alloy thin film using sputtering and investigate the influence of the Ta buffer layer on its structural and magnetic properties. Our results reveal that the film with a buffer layer exhibits the A2 phase with appreciably higher saturation magnetization (848 emu/cc) than the film without a buffer layer (576 emu/cc). However, the surface roughness and Gilbert damping (α) slightly increase with the presence of the buffer layer from 0.41 to 0.56 nm and 4.35 × 10−3–4.94 × 10−3, respectively. The enhancement in α is due to extrinsic contributions induced by surface inhomogeneities.

Interfacial DMI in Fe/Pt thin films grown on different buffer layers

We study the interfacial Dzyaloshinskii–Moriya interactions (i-DMI) of Fe/Pt bilayers grown on Si substrates with MgO, SiO2, or Ta each as a buffer layer on the basis of wave-vector-resolved Brillouin light scattering (BLS) measurement. The obtained i-DMI energy values for Fe/Pt on MgO, Ta, and SiO2 buffer layers are 0.359, 0.321, and 0.274 mJ/m2, respectively. The large i-DMI value observed in Fe/Pt system on the MgO buffer layer can be attributed to the good interfacial quality and the Rshaba effect at the MgO/Fe interface. Moreover, the MgO/Fe/Pt system, benefiting from better sample quality, exhibits a lower damping factor. Furthermore, layer-resolved first-principles calculations are carried out to gain a more in-depth understanding of the origin of the i-DMI in the Fe/Pt system. The results indicate that in the Fe(110)/Pt(111) system, the substantial DMI energy between Fe spins at the interface is related to a significant change in spin–orbit coupling (SOC) energy in the neighboring Pt layer. In contrast, for the MgO(002)/Fe(002) system, both the DMI and its related SOC energy are concentrated at the interfacial Fe layer. Our investigation will provide a valuable insight into the spintronic community in exploring novel devices with chirality dependence.

Achievement of High Perpendicular Anisotropy and Modification of Heat Treatment Peeling in Micron-Thickness Nd-Fe-B Films Used for Magnetic MEMS.

Thick Nd-Fe-B permanent magnetic films with good perpendicular anisotropy have important applications in magnetic microelectromechanical systems (MEMSs). However, when the thickness of the Nd-Fe-B film reaches the micron level, the magnetic anisotropy and texture of NdFeB film will become worse, and it is also prone to peeling during heat treatment, which seriously limits their applications. In this paper, Si(100)/Ta(100 nm)/NdxFe91-xB9(x = 14.5, 16.4, 18.2)/Ta (100 nm) films with thicknesses of 2-10 μm are prepared by magnetron sputtering. It is found that gradient annealing (GN) could help improve the magnetic anisotropy and texture of the micron-thickness film. When the Nd-Fe-B film thickness increases from 2 μm to 9 μm, its magnetic anisotropy and texture do not deteriorate. For the 9 μm Nd-Fe-B film, a high coercivity of 20.26 kOe and high magnetic anisotropy (remanence ratio Mr/Ms = 0.91) are achieved. An in-depth analysis of the elemental composition of the film along the thickness direction is conducted, and the presence of Nd aggregation layers at the interface between the Nd-Fe-B and the Ta layers is confirmed. The influence of thicknesses of the Ta buffer layer on the peeling of Nd-Fe-B micron-thickness films after high-temperature annealing is investigated, and it is found that increasing the thickness of the Ta buffer layer could effectively inhibit the peeling of Nd-Fe-B films. Our finding provides an effective way to modify the heat treatment peeling of Nd-Fe-B films. Our results are important for the development of Nd-Fe-B micron-scale films with high perpendicular anisotropy for applications in magnetic MEMS.

Ferromagnetic resonance excited by interfacial microwave electric field: the role of current-induced torques

Excitation of magnetization dynamics in magnetic materials, especially in ultrathin ferromagnetic films, is of utmost importance for developing various ultrafast spintronics devices. Recently, the excitation of magnetization dynamics, i.e. ferromagnetic resonance (FMR) via electric field-induced modulation of interfacial magnetic anisotropies, has received particular attention due to several advantages, including lower power consumption. However, several additional torques generated by unavoidable microwave current induced because of the capacitive nature of the junctions may also contribute to the excitation of FMR apart from electric field-induced torques. Here, we study the FMR signals excited by applying microwave signal across the metal-oxide junction in CoFeB/MgO heterostructures with Pt and Ta buffer layers. Analysis of the resonance line shape and angular dependent behavior of resonance amplitude revealed that apart from voltage-controlled in-plane magnetic anisotropy (VC-IMA) torque a significant contribution can also arises from spin-torques and Oersted field torques originating from the flow of microwave current through metal-oxide junction. Surprisingly, the overall contribution from spin-torques and Oersted field torques are comparable to the VC-IMA torque contribution, even for a device with negligible defects. This study will be beneficial for designing future electric field-controlled spintronics devices.

Strong uniaxial magnetic anisotropy in Fe3Si thin films

Thin Fe3Si films grown on Si/SiO2 substrates by ion-beam and pulsed laser deposition possess soft ferromagnetic behavior and strong in-plane uniaxial magnetic anisotropy. The material of the capping layer and deposition of the buffer layer do not affect the presence and character of obtained anisotropy. The structural studies showed that Fe3Si film deposited on amorphous SiO2 layer is polycrystalline while the same layer grown on polycrystalline Ta buffer layer is amorphous. It has also been found that uniaxial anisotropy is similar for amorphous and crystalline DO3 Fe3Si films. Magnetic properties yield a magnetization saturation of both amorphous and crystalline films as 1050 emu/cm3 at room temperature and coercive field not exceeding 5 Oe. The origin of such an anomalous behavior were discussed in terms of deposition technique features and parameters, substrate and capping layer type and resulting film parameters.

Transformation of coercivity mechanism and evolution of microstructure in high performance Nd-Dy-Fe-Co-B thick films

It is important to understand the relationship between coercivity and microstructure for the development of high performance rare-earth permanent magnets. In this paper, the effects of film thickness on perpendicular magnetic anisotropy, coercivity mechanism, magnetization reversal process and microstructure of Nd-Dy-Fe-Co-B thick films are systematically investigated by varying film thickness from 1 to 12 µm. It is found that the coercivity mechanism transforms from a mixed type dominated by pinning to another mixed type dominated by nucleation. In addition, the evolution of grain shape from irregular spherical crystals to columnar ones occur at film thickness of 3 µm. The columnar crystals form on the capping layer and grow toward the buffer layer. When the film is thicker than 6 µm, Nd elements accumulate on the Ta buffer layer. With increasing the film thickness, the columnar crystal structure can grow throughout the whole thick film, and the Nd-rich phase is extruded onto the buffer layer, while the pinning effect weakens and the nucleation mechanism is dominated. This work can help us better understand the relationship between coercivity mechanism and microstructure of thick rare-earth permanent magnetic films.

Asymmetric modification of the magnetic proximity effect in Pt/Co/Pt trilayers by the insertion of a Ta buffer layer

The magnetic proximity effect in top and bottom Pt layers induced by Co in Ta/Pt/Co/Pt multilayers has been studied by interface sensitive, element specific x-ray resonant magnetic reflectivity. The asymmetry ratio for circularly polarized x-rays of left and right helicity has been measured at the Pt $L_3$ absorption edge (11567 eV) with an in-plane magnetic field ($\pm158$ mT) to verify its magnetic origin. The proximity-induced magnetic moment in the bottom Pt layer decreases with the thickness of the Ta buffer layer. Grazing incidence x-ray diffraction has been carried out to show that the Ta buffer layer induces the growth of Pt(011) rather than Pt(111) which in turn reduces the induced moment. A detailed density functional theory study shows that an adjacent Co layer induces more magnetic moment in Pt(111) than in Pt(011). The manipulation of the magnetism in Pt by the insertion of a Ta buffer layer provides a new way of controlling the magnetic proximity effect which is of huge importance in spin-transport experiments across similar kind of interfaces.

Role of the Ta buffer layer in Ta/MnN/CoFeB stacks for maximizing exchange bias

Ta/MnN/CoFeB systems show high exchange bias of about 1800 Oe at room temperature; however, their thermal stability is limited by nitrogen diffusion that occurs during annealing processes [Quarterman et al., Phys. Rev. Mater. 3, 064413 (2019) and Dunz et al., AIP Adv. 8, 056304 (2018)]. In this study, we investigate the consequences of nitrogen diffusion in Ta/MnN/CoFeB exchange bias stacks in dependence on the Ta buffer layer thickness. Furthermore, we test the effects of introducing a TaNx layer between MnN and Ta as a diffusion barrier. Our findings show that the Ta buffer layer plays a decisive role in determining the exchange bias in the Ta/MnN/CoFeB system. It acts as a crystallographic seed layer for better growth of MnN and as a nitrogen sink during the annealing process. We show that both of these functions are crucial for the outcome of high exchange bias. Additionally, our results reveal that the measures decreasing nitrogen diffusion, even though being beneficial in terms of thermal stability, mostly lead to decreased crystallinity and thus weaker exchange bias.

Detection of ferromagnetic resonance from 1\xa0nm-thick Co

To explore the further possibilities of nanometer-thick ferromagnetic films (ultrathin ferromagnetic films), we investigated the ferromagnetic resonance (FMR) of 1 nm-thick Co film. Whilst an FMR signal was not observed for the Co film grown on a SiO2 substrate, the insertion of a 3 nm-thick amorphous Ta buffer layer beneath the Co enabled the detection of a salient FMR signal, which was attributed to the smooth surface of the amorphous Ta. This result implies the excitation of FMR in an ultrathin ferromagnetic film, which can pave the way to controlling magnons in ultrathin ferromagnetic films.

The thickness of buffer layer and temperature dependent magneto dynamic properties of Ta/FeGaB/Ta tri-layer

In this paper, we have deposited a tri-layer film stack of Ta (25, 50, 75 and 100 nm)/FeGaB (100 nm)/Ta (5 nm) onto Si substrates, and studied ferromagnetic resonance (FMR) properties of the Ta buffer layer thickness and temperature dependence. The surface morphology of the tri-layer film stack is captured by Atomic Force Microscopy (AFM). The static magnetic properties for different buffer layer thickness reveal that both coercive field and remanent magnetization decrease for increasing the Ta buffer layer thickness due to the disturbance of magnetic moments from the FeGaB layer. The dynamic ferromagnetic properties of all samples suggest that narrow inhomogeneous linewidth and low damping factor are obtained for different Ta buffer layer thickness. Ta (100)/FeGaB (100)/Ta (5) film stack shows the lowest damping from 100 K to 300 K. The buffer layer thickness and temperature dependence studies indicate that the interface damping dominates at low temperature 100 K. Hence, these studies are of great importance for development of magnetostrictive microwave devices applications.

The buffer layer effect of the exchange stiffness constants on Co thin film

The exchange stiffness of Cobalt thin films with various buffer layers (Pd, Pt, Ta, and no buffer) has been studied by Brillouin scattering in the applied magnetic field of up to 175 mT. We found that Pd and Pt buffer layers led to higher exchange stiffness constants more than 12% compared to the Ta buffer layer, whereas, it was difficult to determine the exchange stiffness constant with no buffer layer due to broad spinwave spectra with poor signal-to-noise ratios. The determined exchange stiffness constant value is 25.7 ± 0.1, 25.6 ± 0.2, and 22.7 ± 0.1 pJ/m for Pd, Pt, and Ta buffer layers. We also observed that the Pd-underlayered sample gives us the largest saturation magnetization and in contrast the Ta underlayer shows the relatively smaller values. This observation indicates that the exchange stiffness constant and the saturation magnetization exhibit the same trend on various buffer layers.

Realization of Epitaxial NbP and TaP Weyl Semimetal Thin Films.

Weyl semimetals (WSMs) exhibit an electronic structure governed by linear band dispersions and degenerate (Weyl) points that lead to exotic physical phenomena. While WSMs were established in bulk monopnictide compounds several years ago, the growth of thin films remains a challenge. Here, we report the bottom-up synthesis of single-crystalline NbP and TaP thin films, 9 to 70 nm thick, by means of molecular beam epitaxy. The as-grown epitaxial films feature a phosphorus-rich stoichiometry, a tensile-strained unit cell, and a homogeneous surface termination, unlike their bulk crystal counterparts. These properties result in an electronic structure governed by topological surface states as directly observed using in situ momentum photoemission microscopy, along with a Fermi-level shift of −0.2 eV with respect to the intrinsic chemical potential. Although the Fermi energy of the as-grown samples is still far from the Weyl points, carrier mobilities close to 103 cm2/(V s) have been measured at room temperature in patterned Hall-bar devices. The ability to grow thin films of Weyl semimetals that can be tailored by doping or strain, is an important step toward the fabrication of functional WSM-based devices and heterostructures.

Impacts of Ta Buffer Layer and Cu–Ge–Te Composition on the Reliability of GeSe-Based CBRAM

We analyze the switching and retention properties of 65-nm integrated Cu(-Ge-Te)/(Ta)/GeSe conductive bridge random access memory devices operated at $50~\mu \text{A}$ . We evidence the crucial role played by a Ta buffer layer inserted between the Cu alloy and GeSe layers in decreasing the preforming current and in significantly improving the low-resistance state retention. Cu alloys of different compositions are tested to reveal lower device variability and longer retention with Cu2GeTe3 active electrode.

Magnetic properties of Co-W thin films

In this paper, the magnetic properties and structure of the Co-W films of different compositions prepared by magnetron sputtering on glass substrates without and with Ta or Ru buffer layers were investigated. The dependences of coercive force on the W content for the studied films were non-monotonic. The peak value of the coercive force of about 400 Oe was observed for the films on glass and Ta layers with W content of 8-9 at. %. The films deposited on Ru layers had a coercive force of 4 times less, and it was practically independent of the W content in a wide range from 6 to 20 at. %.

Enhancement of the Dzyaloshinskii-Moriya interaction and domain wall velocity through interface intermixing in Ta/CoFeB/MgO

The interfacial Dzyaloshinskii-Moriya interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light ${\mathrm{He}}^{+}$ irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.

Spin-orbit torques acting upon a perpendicularly magnetized Py layer

We show that Py, a commonly used soft ferromagnetic material with weak anisotropy, can become perpendicularly magnetized while depositing on a Ta buffer layer with Hf or Zr insertion layers (ILs) and an MgO capping layer. By using two different approaches, namely, harmonic voltage measurement and hysteresis loop shift measurement, the dampinglike spin-orbit torque (DL-SOT) efficiencies from Ta/IL/Py/IL/MgO magnetic heterostructures with perpendicular magnetic anisotropy are characterized. We find that though Ta has a significant spin Hall effect, the DL-SOT efficiencies are small in systems with the Ta/Py interface compared to that obtained from the control sample with the traditional Ta/CoFeB interface. Our results indicate that the spin transparency for the Ta/Py interface is much less than that for the Ta/CoFeB interface, which might be related to the variation of spin mixing conductance for different interfaces.

Variation of Magnetization Dynamics of Co/Ni Multilayer by Capturing Magnetic Nanoparticles

[Co (0.29 nm)/Ni (0.71 nm)]10 multilayers were deposited on Ta (30 nm) and Pt (30 nm) buffer layers, and the variation of the magnetization dynamics by the adsorption of Fe3O4 nanoparticles was studied by vector network analyzer—ferromagnetic resonance method. The surface roughness of the Co/Ni multilayer was 0.52 nm, and significantly increased after the adsorption of the Fe3O4 nanoparticles which was captured by large field gradient on the demagnetized Co/Ni multilayer. The anisotropy field $H_{k}$ , damping constant $\alpha $ , and anisotropy distribution $\Delta H_{k}$ of the Co/Ni on Ta buffer layer were 2.6 kOe, 0.012, and 0.32 kOe, respectively. By adsorbing of the Fe3O4 nanoparticles, the stray field from the nanoparticles $H_{{\text {np}}}$ was applied to the Co/Ni, and $H_{k} + {H}_{{\text {np}}}$ was estimated to increase to 3.7 kOe. Moreover, $\alpha $ and $\Delta H_{k}$ also increased to 0.020 and 0.42 kOe by the adsorption, respectively. Similarly, $H_{k} + H_{\mathrm{np}}$ , $\alpha $ , $\Delta H_{k}$ of Pt buffered Co/Ni also increased from 2.4 kOe, 0.033, 0.43 kOe to 4.2 kOe, 0.055, 0.53 kOe, respectively. These results suggest that the adsorption of Fe3O4 nanoparticles significantly modifies the magnetization dynamics of Co/Ni multilayer.

Effect of magnetic layer thickness on magnetic properties of Ce-Fe-B thin films

The Ce2Fe14B thin films with a notable out-of-plane c-axis texture were prepared by DC magnetron sputtering on a Ta buffer layer. The morphological and magnetic properties were investigated. The thickness of the magnetic layer had a dramatic effect on the formation of Ce2Fe14B phase, and excellent magnetic properties (Hci ≈ 4.25 kOe, Mr/Ms ≈ 0.81) were observed for the Ce-Fe-B film with the thickness dm = 200 nm. The results of the hysteresis loops for Ce-Fe-B film (dm = 200 nm) at various measured temperatures show that a decoupling between the hard and the soft phases is observed at low temperatures, which is due to the regions with quite low anisotropy provided by the α-Fe. Moreover, it is clear that significantly various magnetization behaviors between the films with dm = 200 and 300 nm were observed with a similar trend due to the existence of the α-Fe soft phase.

MnNi-based spin valve sensors combining high thermal stability, small footprint and pTesla detectivities

Magnetoresistive sensors with high thermal robustness, low noise and high spatial resolution are the answer to a number of challenging applications. Spin valve sensors including MnNi as antiferromagnet layer provide higher exchange bias field and improved thermal stability. In this work, the influence of the buffer layer type (Ta, NiFeCr) and thickness on key sensor parameters (e.g. offset field, Hf) is investigated. A Ta buffer layer promotes a strong (111) texture which leads to a higher value of MR. In contrast, Hf is lower for NiFeCr buffer. Micrometric sensors display thermal noise levels of 1 nT/Hz1/2 and 571 pT/Hz1/2 for a sensor height (h) of 2 and 4 μm, respectively. The temperature dependence of MR and sensitivity is also addressed and compared with MnIr based spin valves. In this case, MR abruptly decreases after heating at 160°C (without magnetic field), contrary to MnNi-based spin valves, where only a 10% MR decrease (relative to the initial value) is seen at 275°C. Finally, to further decrease the noise levels and improve detectivity, MnNi spin-valves are deposited vertically, and connected in parallel and series (in-plane) to create a device with low resistance and high sensitivity. A field detection at thermal level of 346 pT/Hz1/2 is achieved for a device with a total of 300 SVs (4 vertical, 15 in series, 5 in parallel).