Polarization In Thin Films Research Articles

Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co2(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co2(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey are fabricated on SiO2/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co2Mn0.5Fe0.5)1−yGey composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization.

Generation of out-of-plane polarized spin current by spin swapping

The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn3Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn3Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn3Sn layer and whose temperature dependence follows that of this layer’s antiferromagnetic order. On the other hand, the out-of-plane polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn3Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect.

Tuning surface spin polarization of CoFeB by boron diffusion detected by spin resolved photoemission

Abstract Investigation on the spin polarization of the magnetic CoFeB thin film is of practical importance in spintronic applications. Here, using a direct characterization technique of spinresolved photoemission spectroscopy, the surface spin polarization of amorphous Co40Fe40B20 thin films with different annealing temperatures from 100 ℃ to 500 ℃ prepared by magnetron sputtering is obtained. After high annealing temperature, a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion. While the global magnetization remains almost constant, the secondary electrons’ spin polarization, average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing of the annealing temperature above 100 ℃. These distinct surface properties are attributed to the enhanced Fe-B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films, which should be important for the development of high-performance magnetic random-access memory.

Fabrication of non-volatile memory transistor by charge compensation of interfacial ionic polarization of a ferroelectric gate dielectric

Lithium niobate (LiNbO3) is a popular lead free perovskite ferroelectric materials, but its implementation as a gate dielectric for developing a ferroelectric thin film transistor (FeTFT) has not been investigated much due to its low band gap. Low band gap introduces high gate leakage current that results in very low retention time of the device. In this report a solution processed Li5AlO4/LiNbO3/Li5AlO4 stacked layer has been used as a gate dielectric of a metal oxide thin film transistor which reduces gate leakage current by four orders of magnitude with respect to the single layer LiNbO3 gate dielectric device. Besides, due to the ionic polarization of Li5AlO4 thin film that originated from the mobile Li+, charge compensation of the depolarization field has been nearly removed. By reducing these factors, it becomes possible to fabricate metal oxide based FeTFT that has retention time of 7 h which is ∼200 times higher with respect to the LiNbO3 only FeTFT. The ratio of On and Off state of FeTFT is ∼103 which retains ∼ 95% after 7 h of operation. The carrier mobility, ON/OFF ratio and subthreshold swing (SS) of this FeTFT are 2.9 cm2 V − 1 s − 1,7.2 × 104 and 328 mV.decade−1 respectively.

Wideband circular polarizer for a photoconductive antenna.

We report a thin-film circular polarizer consisting of three metal-grid layers to be used with a photoconductive antenna (PCA) to generate terahertz (THz) circularly polarized (CP) radiation. The polarizer has a high transmission with a measured 3 dB axial-ratio bandwidth of 54.7% from 0.57 to 1 THz. We further developed a generalized scattering matrix approach to provide insight into the underlying physical mechanism of the polarizer. We revealed that the Fabry-Pérot-like multi-reflection among gratings enables the high-efficiency polarization conversion. The successful realization of the CP PCA can find widespread application, such as THz circular dichroism spectroscopy, THz Mueller imaging, and ultrahigh-speed THz wireless communications.

Solution processed Li-Al2O3/LiNbO3/Li-Al2O3 stacked gate dielectric for a non-volatile ferroelectric thin film transistor

Lithium niobate (LiNbO3) gate dielectric based SnO2 ferroelectric thin film transistor (FETFT) is fabricated by a simple solution processed technique. However, LiNbO3 alone is not a suitable candidate for a gate insulator of a TFT because of its low band gap. Therefore, Li-Al2O3/LiNbO3/Li-Al2O3 stacked gate dielectric has been used that reduces the gate leakage current by an order of magnitude compared to the LiNbO3 only device. Moreover, ionic polarization of Li-Al2O3 thin films that originated from mobile Li+ of Li-Al2O3, compensate for the ferroelectric charge polarization of LiNbO3 film. By reducing gate leakage current and compensating ferroelectric charge polarization, it becomes possible to achieve ferroelectric memory retention up to 7.2× 103 s of time with a difference of ON/OFF state by 3 times whereas the reference LiNbO3 device almost merges to each other very quickly. Besides, these ferroelectric TFTs (FETFT) can operate within 2 V operating voltage due to the strong ionic polarization of the gate dielectric. The carrier mobility of 1.9 cm2. V−1.s−1, current ON/OFF ratio of 1.6⨭104 and subthreshold swing (SS) of 167 mV.decade−1 has been achieved under 2 V operation of this FETFT, whereas memory retention time has been studied at 0 V gate and 1 V drain bias.

Synthesis of 2,5,8-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b′:5,6-b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.

X-ray magnetic circular dichroism spectroscopy at the Fe L edges with a picosecond laser-driven plasma source

Time-resolved x-ray magnetic circular dichroism (XMCD) enables a unique spectroscopic view on complex spin and charge dynamics in multi-elemental magnetic materials. So far, its application in the soft-x-ray range has been limited to synchrotron-radiation sources and free-electron lasers. By combining a laser-driven plasma source with a magnetic thin-film polarizer, we generate up to 30% circular polarization in the soft-x-ray regime, enabling the first XMCD spectroscopy at the Fe L edges in a laser laboratory, to our knowledge. Our approach can be readily adapted to other transition metal L and rare earth M absorption edges, and with a temporal resolution of <10ps, a wide range of ultrafast magnetization studies can be realized.

PZT ferroelectric polarization-enhanced ZnO thin film photodetector

ZnO is one of the most popular materials in UV photodetectors, but its low switching and long attenuation time seriously hinder its practical application. In this work, the performance of PZT/ZnO planar photodetectors is optimized by using the ferroelectric polarization of PZT thin films. The results show that the residual polarization field of PZT affects the electron arrangement in ZnO film, making ZnO in a depleted state, reducing the dark current of resulted photodetectors, and influencing the carrier life. By inhibiting recombination, the photocurrent is increased, and the switching ratio of the ZnO photodetector is increased by 5 times, and more importantly, the optical responsivity increases by 7 times to 1.015 A/W. This work provides a simple and feasible method for ferroelectric polarization to control the performance of two-dimensional material photodetectors.

Polarization switching dynamics simulation by using the practical distribution of ferroelectric properties

We investigated the internal bias field and coercive field in a typical ferroelectric thin-film capacitor and simulated polarization switching dynamics using Euler's method. The simulation results agreed well with the experimental results and reflected the well-known polarization domain switching model in which the polarization switching occurs on the order of nucleation, growth, and coalescence. The fit parameters (damping parameters affecting the polarization change rate) also followed a certain distribution. When the expected value was used instead of full distribution, the simulation results did not agree well with corresponding experimental results. The simulation results suggested no domain structure in the polarization switching dynamics, indicating that the polarization domain structure was affected by the distribution of the fit parameters. Our results demonstrate the possibility of simulation using realistic distribution of ferroelectric properties.

Lithium Niobate Thin Film Polarization Beam Splitter Based on Asymmetric Directional Coupling

Polarization devices on lithium niobate thin film (LNTF) are vital components for photonic integrated circuits (PICs) on the novel platform. Because both linear and nonlinear devices on the anisotropic platform are intrinsically polarization-dependent. Here, we demonstrate compact polarization beam splitters (PBS) on LNTF based on two asymmetric directional couplers with an effective length of only 180 μm. The PBS is fabricated via a single-step electron beam lithography (EBL) and inductively coupled plasma (ICP) etching process. Our device shows efficient PBS operation at the telecom bands with an extinction ratio over 26 dB and 13 dB for fundamental TE and TM modes, respectively. The device holds promise for efficient on-chip polarization division multiplexing and polarization manipulation, which would become an indispensable component in future LNTF PICs.

Polarization due to emergent polarity in elemental semiconductor thinfilms under bending

Polarization via strain engineering provides a facial way to functionalize materials. We investigate the origin of electronic polarization in the bent elemental semiconductor thinfilms by combining analytical modeling with quantum mechanical simulation. A bond orbital model reveals a polarity of covalent bonds induced by strain gradient such that polarization along the strain gradient dimension can be induced, giving rise to the flexoelectric effect. At strain gradient nm−1, the net charge differences between the two sides are , and for C, Si and Ge films respectively. On the other hand, due to the emergent bond polarity, the polarization can be effectively tuned by normal strain applied to the bent film, mimicking the piezoelectric effect. Simulations using the generalized Bloch theorem strongly support this revelation. Findings have important implications for delineating the formation of polarization and related phenomena in semiconductors.

Orientation Independent Growth of Uniform Ferroelectric Hf0.5Zr0.5O2 Thin Films on Silicon for High‐Density 3D Memory Applications

AbstractThe highly scalable ferroelectric hafnia‐based thin films can be easily integrated into ferroelectric field‐effect transistors (FeFETs) by existing Si technology, which are regarded as one of the promising candidates for fast read/write, energy‐efficient, and high‐density nonvolatile memories. However, device‐to‐device variation in threshold voltage (VTH) caused by non‐uniformity of ferroelectric properties is a serious challenge for implementing hafnia‐based FeFETs in high‐density nonvolatile memories. Here, the substrate‐orientation independent growth of Hf0.5Zr0.5O2 (HZO) thin films is realized with uniform ferroelectricity by using HfO2 seed layers. The HfO2 seed layers are beneficial to improving the ferroelectric polarization of HZO thin films grown on differently oriented Si substrates and reducing the variation in ferroelectric properties. Moreover, device simulation confirms that the proposed scheme contributes to realizing uniform memory properties in 3D vertical HZO‐based FeFETs. This study suggests the possibility to implement hafnia‐based FeFETs into 3D vertical high‐density memory.

Resonators with a continuously variable output coupling rate to enhance output performance of Yb:YAG thin-disk lasers.

Beam quality and average output power are two long-sought parameters of continuous-wave lasers. The operating characteristics of a Yb:YAG thin-disk laser based on a 72-pass pump module using output couplers with continuously variable coupling rates are reported. When the pump power is 450 W, the average power of more than 210 W is obtained, and the corresponding optical-optical efficiency is about 46.67%. The output beam is near diffraction limited with M2=1.20 and 1.18 on the horizontal and vertical directions respectively. The thin-disk laser maintains the beam quality near diffraction limited within the whole pump power range by continuously changing the coupling rate of the output coupler (the combination of thin film polarizer and quarter-wave plate) according to the incident pump power. The experimental results show that by continuously changing the coupling rate, combined with the dynamic change of thermal lens effect, the laser beam quality factor M2 can be reduced from 2.0 to 1.2, and the beam quality can be improved by about 40%. The improvement of and increase of average output power confirm that resonators with continuously variable coupling rate are a promising method for enhancing output performance of thin-disk lasers.

Exploring Physics of Ferroelectric Domain Walls in Real Time: Deep Learning Enabled Scanning Probe Microscopy.

The functionality of ferroelastic domain walls in ferroelectric materials is explored in real-time via the in situ implementation of computer vision algorithms in scanning probe microscopy (SPM) experiment. The robust deep convolutional neural network (DCNN) is implemented based on a deep residual learning framework (Res) and holistically nested edge detection (Hed), and ensembled to minimize the out-of-distribution drift effects. The DCNN is implemented for real-time operations on SPM, converting the data stream into the semantically segmented image of domain walls and the corresponding uncertainty. Further the pre-defined experimental workflows perform piezoresponse spectroscopy measurement on thus discovered domain walls, and alternating high- and low-polarization dynamic (out-of-plane) ferroelastic domain walls in a PbTiO3 (PTO) thin film and high polarization dynamic (out-of-plane) at short ferroelastic walls (compared with long ferroelastic walls) in a lead zirconate titanate (PZT) thin film is reported. This work establishes the framework for real-time DCNN analysis of data streams in scanning probe and other microscopies and highlights the role of out-of-distribution effects and strategies to ameliorate them in real time analytics.

Examining imprinted ferroelectric hysteresis loops and improved energy storage properties of Mn-doped epitaxial SrTiO3 thin films using heat treatment

This study investigated imprinted ferroelectric hysteresis loops and energy storage properties of Mn-doped epitaxial SrTiO3 (MSTO) thin films using heat treatment. The crystallinity of MSTO thin films was improved by reducing the deposition rates of epitaxial (001) MSTO thin films on single-crystal (100) Rh substrates from 0.15 to 0.03 nm/pulse. The polarization of MSTO thin films, with improved polarization by Mn doping, was further enhanced by improving film crystallinity. MSTO thin films exhibited an energy storage efficiency of approximately 90 % and an increase in energy storage density from 23 to 34 J/cm3. The energy storage density increased further to 39 J/cm3 using imprinted ferroelectric hysteresis loops and heat treatment.

Polarization of Bi2Se3 thin film toward non-volatile memory applications

In recent years, topological insulators have drawn growing interest as a unique electronic state of matter toward quantum information technology. Despite the logic devices with magnetization switching through spin–orbit torque or the topological magneto-electric effect, realizing memory devices based on topological insulators has been urged in quantum computing applications. In this work, we report the design and fabrication of a non-volatile memory device that employs polarization of Bi2Se3 thin films achieving fast memory speed, sufficient memory window, and good reliability. The Bi2Se3 film polarizes under an external electrical field with charges accumulated on the top and bottom surfaces separating the electrons and holes. Such polarization is much faster than the carrier tunneling in conventional floating-gate flash memory and ferroelectric-based memory devices. In addition, good memory retention and endurance properties have also been obtained, showing great potential in high-performance memory application in future topological insulator-involved information technology.

Interfacial Resistive Switching by Multiphase Polarization in Ion-Intercalation Nanofilms.

Nonvolatile resistive-switching (RS) memories promise to revolutionize hardware architectures with in-memory computing. Recently, ion-interclation materials have attracted increasing attention as potential RS materials for their ion-modulated electronic conductivity. In this Letter, we propose RS by multiphase polarization (MP) of ion-intercalated thin films between ion-blocking electrodes, in which interfacial phase separation triggered by an applied voltage switches the electron-transfer resistance. We develop an electrochemical phase-field model for simulations of coupled ion-electron transport and ion-modulated electron-transfer rates and use it to analyze the MP switching current and time, resistance ratio, and current-voltage response. The model is able to reproduce the complex cyclic voltammograms of lithium titanate (LTO) memristors, which cannot be explained by existing models based on bulk dielectric breakdown. The theory predicts the achievable switching speeds for multiphase ion-intercalation materials and could be used to guide the design of high-performance MP-based RS memories.

Continuous and fast magneto-ionic control of magnetism in Ta/Co/BiFeO3/SrRuO3 multiferroic heterostructure

Room temperature electric field controlled magnetism is extremely promising for the next-generation high-performance spintronic devices. Here, based on the ferroelectric switching driven oxygen ion migration in the Ta/Co/BiFeO3/SrRuO3 heterostructures, the magnetic moment, magnetic coercive field, exchange bias field, and junction resistance are reversibly manipulated by tuning the ferroelectric polarization of the BiFeO3 layer. All these phenomena are consistently explained by the oxygen ion migration induced CoOx/Co redox effect, which is evidenced by the synchrotron X-ray absorption spectroscopy measurements. Interestingly, owing to the controllable ferroelectric switching dynamics of the BiFeO3 thin film, the magnetic coercive field of the Co thin film can be continuously and precisely tuned by controlling the ferroelectric polarization of the BiFeO3 thin film, and the manipulating speed of the voltage control of magnetism can be fast to 100 ns. This nonvolatile, stable, reversible, fast, and reproducible voltage control of magnetism shows great potential for designing low-power and high-speed spintronics.

Manipulation of charged domain walls in geometric improper ferroelectric thin films: A phase-field study

Using phase-field simulations, we show how interfaces acting on the geometric-improper ferroelectric polarization of hexagonal manganite and ferrite thin films can be used to control the formation of charged domain walls. We modify the Landau expansion of the free energy valid in bulk to emulate interface effects known from previous cross-sectional experiments, and we verify our model by comparing our results with images obtained in these experiments. We then show how the interface affects the orientation of ferroelectric domain walls in the fully three-dimensional case. Furthermore, we demonstrate that interface effects combined with an external electric field enable us to specifically choose the dominant domain-wall type (head-to-head, tail-to-tail, or neutral). We also find that an electric field can stabilize a novel domain-wall type which only emerges in the improper ferroelectric order but not in the primary structural distortion. Since the domain walls have a conductivity that is different from the interior of the domains, the influence of the interfaces of a thin film on the type and distribution of the walls gives us the possibility to control the transport properties of a material by appropriate thin-film engineering.