Novel Antiferromagnets for Spintronic Devices

Spin electronic or spintronic devices which are used in hard disk drive (HDD) read heads are expected to replace the current silicon based transistor technology used in volatile memories. A prime example for the net advantage of employing spin rather than electric charge manipulation is found in the newly developed magnetic random access memory (MRAM) which is proposed as a replacement for the dynamic random access memory (DRAM) based on three terminal metal-oxide-semiconductor (MOS) devices. Besides the decrease of energy consumption by a factor three arising from manipulating electron angular momentum, the magnetic memories are non-volatile hence they do not require constant power to store information. This allows for additional energy saving due to data stability when the storage device is powered off

Integration of antiferromagnetic Heusler compound Ru2MnGe into spintronic devices

We report on the integration of an antiferromagnetic Heusler compound acting as a pinning layer into magnetic tunneling junctions (MTJs). The antiferromagnet Ru2MnGe is used to pin the magnetization direction of a ferromagnetic Fe layer in MgO based thin film tunneling magnetoresistance stacks. The samples were prepared using magnetron co-sputtering. We investigate the structural properties by X-ray diffraction and reflection, as well as atomic force and high-resolution transmission electron microscopy. We find an excellent crystal growth quality with a low interface roughnesses of 1–3 A ̊ , which is crucial for the preparation of working tunneling barriers. Using Fe as a ferromagnetic elec- trode material, we prepared magnetic tunneling junctions and measured the magnetoresistance. We find a sizeable maximum magnetoresistance value of 135%, which is comparable to other common Fe based MTJ systems

Growth and Crystallisation of Ferromagnetic and Antiferromagnetic Fe2+xVyAl Heusler Alloy Films

We investigated growth, annealing conditions and magnetic properties of the Heusler alloy Fe2+xVyAl by means of X-ray diffraction, magnetic hysteresis and exchange-bias measurements. Ferromagnetic Heusler alloy films were obtained by sputtering Fe2VAl and Fe3VAl targets and performing post-growth annealing. The characteristic (220) Heusler alloy peaks were seen in the X-ray diffraction measurements and corresponding ferromagnetic behaviours were observed. In addition, antiferromagnetic Heusler alloy films were deposited by employing Al pegs on Fe3VAl sputtering targets. The deposited films had elemental ratios close to the predicted Fe2.5V0.5Al phase, and a 16 Oe exchange-bias was measured in a Fe2.5V0.5Al/Co60Fe40 system at 100 K

Development of Antiferromagnetic Heusler Alloys for the Replacement of Iridium as a Critically Raw Material

As a platinum group metal, iridium (Ir) is the scarcest element on the earth but it has been widely used as an antiferromagnetic layer in magnetic recording, crucibles and spark plugs due to its high melting point. In magnetic recording, antiferromagnetic layers have been used to pin its neighbouring ferromagnetic layer in a spin-valve read head in a hard disk drive for example. Recently, antiferromagnetic layers have also been found to induce a spin-polarised electrical current. In these devices, the most commonly used antiferromagnet is an Ir-Mn alloy because of its corrosion resistance and the reliable magnetic pinning of adjacent ferromagnetic layers. It is therefore crucial to explore new antiferromagnetic materials without critical raw materials. In this review, recent research on new antiferromagnetic Heusler compounds and their exchange interactions along the plane normal is discussed. These new antiferromagnets are characterised by very sensitive magnetic and electrical measurement techniques recently developed to determine their characteristic temperatures together with atomic structural analysis. Mn-based alloys are found to be most promising based on their robustness against atomic disordering and large pinning strength up to 1.4 kOe, which is comparable with that for Ir-Mn. The search for new antiferromagnetic films and their characterisation are useful for further miniaturisation and development of spintronic devices in a sustainable manner

Factors controlling segregation tendency of solute Ti, Ag and Ta into different symmetrical tilt grain boundaries of tungsten: First-principles and experimental study

In previous reports, experimental studies have shown that both thermal stability and strength can be controlled by grain boundary (GB) segregation. In this study, we investigate the segregation behavior of solute (Ti, Ag and Ta) atoms to low/high-angle symmetric tilt grain boundaries (STGBs) of W using density functional theory (DFT) calculations and supported by TEM experiments. We found no segregation preference for Ti or Ta at low-angle STGBs; however, they exhibit a slight segregation tendency to the core of high-angle STGBs. In contrast, Ag is more prone to segregate in and all around the GB plane. We estimated the mechanical and electronic contributions to solution energy and found that the electronic contribution is dominant. Furthermore, the role of valence electrons of solute and W atoms, was analyzed using the local density of states (PDOS). We found that substantial valence electrons hybridization in the case of Ta plays an important role in stabilizing W-Ta bonds, while the anisotropic nature of W-Ti bond contributes to stabilize surrounding W atoms. Charge transfer analysis revealed that Ti and Ta lose electrons to W atoms. Contrary to the electronegativity rule, Ag atoms gain charge from neighboring W atoms and excellent hybridization may explain the increased GB segregation of Ag atoms

Fabrication of Epitaxial Fe3O4 Film on a Si(111) Substrate

The application of magnetic oxides in spintronics has recently attracted much attention. The epitaxial growth of magnetic oxide on Si could be the first step of new functional spintronics devices with semiconductors. However, epitaxial spinel ferrite films are generally grown on oxide substrates, not on semiconductors. To combine oxide spintronics and semiconductor technology, we fabricated Fe3O4 films through epitaxial growth on a Si(111) substrate by inserting a gamma-Al2O3 buffer layer. Both of gamma-Al2O3 and Fe3O4 layer grew epitaxially on Si and the films exhibited the magnetic and electronic properties as same as bulk. Furthermore, we also found the buffer layer dependence of crystal structure of Fe3O4 by X-ray diffraction and high-resolution transmission electron microscope. The Fe3O4 films on an amorphous-Al2O3 buffer layer grown at room temperature grew uniaxially in the (111) orientation and had a textured structure in the plane. When Fe3O4 was deposited on Si(111) directly, the poly-crystal Fe3O4 films were obtained due to SiOx on Si substrate. The epitaxial Fe3O4 layer on Si substrates enable us the integration of highly functional spintoronic devices with Si technology

Nanotribological investigation of sliding properties of transition metal dichalcogenide thin film coatings

Transition metal dichalcogenide (TMD)-based coatings are known for their low friction performance, which is attributed to the formation of a tribolayer consisting almost exclusively of pure well-ordered TMD. However, the formation of such a tribolayer and its wear track coverage is still unknown. In this study, we employed surface mapping and nanotribological techniques to study the properties of the wear tracks of composite W-S-C coatings. Our analysis revealed that the as-deposited coating consisted of two phases, with significantly different nanoscale frictional properties. We attributed the phases to nanocrystalline WS2 (low friction) and amorphous solution of carbon and WS2 (high friction). The two phases wear at different rates, especially at lower loads, where we observed faster depletion of nanocrystalline WS2. In the wear track, sparse flat WS2 flakes were identified, suggesting that the recrystallization of the WS2 phase occurs only at the spots where the contact pressure is the highest

Deformation-controlled design of metallic nanocomposites

Achieving the theoretical strength of a metallic alloy material is a demanding task that usually requires utilizing one or more of the well-established routes: (1) Decreasing the grain size to stop or slow down the dislocation mobility, (2) adding external barriers to dislocation pathways, (3) altering the crystal structure, or (4) combining two of the previous discrete strategies, that is, implementing crystal seeds into an amorphous matrix. Each of the outlined methods has clear limitations; hence, further improvements are required. We present a unique approach that envelops all the different strength-building strategies together with a new phenomenon–phase transition. We simulated the plastic deformation of a Zr–Nb nanolayered alloy using molecular dynamics and ab initio methods and observed the transition of Zr from hexagonal close-packed to face-centered cubic and then to body-centered cubic during compression. The alloy, which was prepared by magnetron sputtering, exhibited near-theoretical hardness (10.8 GPa) and the predicted transition of the Zr structure was confirmed. Therefore, we have identified a new route for improving the hardness of metallic alloys

Fabrication of nanocrystalline supersaturated W–Al alloys with enhanced thermal stability and high sinterability

In this work, nanocrystalline W–Al alloys (up to 20 at.% of Al) were produced by high energy ball milling and powder microstructural evolution was investigated as a function of milling time. It was found that, regardless of the composition, alloys crystallite size progressively decreases and stabilizes around a value of about 10–15 nm after 70–100 h of mechanical treatment. The aluminum dissolution into the bcc W lattice was confirmed by DSC, SEM, and TEM. The formation of intermetallic compounds was detected neither during ball milling nor after thermal treatments up to 1450 °C. Sintering behavior of mechanically alloyed W–Al alloys was tested under pressureless conditions, and a significant improvement in terms of sinterability with respect to pure W was observed. Along with favoring the sintering process, the addition of Al also resulted in a notable enhancement of the coarsening resistance. Indeed, the analysis of ball-milled pure tungsten after thermal treatment at 1450 °C provided an estimated average crystallite size of about 2 μm, while W80Al20 and W90Al10 alloys retained an average crystallite size of about 70 nm and 60 nm, respectively. Although further work is required to optimize sintering conditions for achieving full density samples, the retaining of the nanostructure marks a significant advancement in the field of W-based alloys.</p

Elucidating the role of TiCl₄post-treatment on percolation of TiO₂electron transport layer in perovskite solar cells

The ideal electron transport layer of a high performance perovskite solar cell should have good optical transparency, high electron mobility, and an energy level alignment well-matched with the perovskite material. In this work, we investigate the role of TiCl4 post-treatment of the mesoporous TiO2 electron transport layer by varying the concentration of TiCl4 and characterizing optical and electrical properties, charge carrier dynamics, and photovoltaic performance of mesoscopic CH3NH3PbI3 solar cells. It is found that the TiCl4 treatment provides an additional interconnection between the TiO2 particles, leading to better percolation as evident from high resolution cross-section images and chemical maps. This enhances effective electron mobility in the material as well as significantly reduces average sub-bandgap absorption due to defects and electronic disorder determined by photothermal deflection spectroscopy. Moreover, improvement of interfacial contact due to a smoother surface contributes to more efficient charge extraction and suppressed charge recombination and reduced hysteresis. As a result, the optimized device based on TiCl4 post-treated mesoporous TiO2 achieved the highest conversion efficiency of 17.4% compared with 14.1% for the device with pristine mesoporous TiO2.</p