Neuromorphic Computing with Emerging Antiferromagnetic Ordering in Spin-Orbit Torque Devices.

Field-free switching (FFS) and spin-orbit torque (SOT)-based neuromorphic characteristics were realized in a W/Pt/Co/NiO/Pt heterostructure with a perpendicular exchange bias (HEB) for brain-inspired neuromorphic computing (NC). Experimental results using NiO-based SOT devices guided the development of fully spin-based artificial synapses and sigmoidal neurons for implementation in a three-layer artificial neural network. This system achieved impressive accuracies of 91-96% when applied to the Modified National Institute of Standards and Technology (MNIST) image data set and 78.85-81.25% when applied to Fashion MNIST images, due presumably to the emergence of robust NiO antiferromagnetic (AFM) ordering. The emergence of AFM ordering favored the FFS with an enhanced HEB, which suppressed the memristivity and reduced the recognition accuracy. This indicates a trade-off between the requirements for solid-state memory and those required for brain-inspired NC devices. Nonetheless, our findings revealed opportunities by which the two technologies could be aligned via controllable exchange coupling.

Large Thermo-Spin Effects in Heusler Alloy-Based Spin Gapless Semiconductor Thin Films.

Recently, Heusler alloy-based spin gapless semiconductors (SGSs) with high Curie temperature (TC) and sizable spin polarization have emerged as potential candidates for tunable spintronic applications. We report comprehensive investigation of the temperature-dependent ANE and intrinsic longitudinal spin Seebeck effect (LSSE) in CoFeCrGa thin films grown on MgO substrates. Our findings show that the anomalous Nernst coefficient for the MgO/CoFeCrGa (95 nm) film is ≈1.86 µV K-1 at room temperature, which is nearly 2 orders of magnitude higher than that of the bulk polycrystalline sample of CoFeCrGa (≈0.018 µV K-1) and almost 3 orders of magnitude higher than that of the half-metallic ferromagnet La1-xNaxMnO3 (≈0.005 µV K-1) but comparable to that of the magnetic Weyl semimetal Co2MnGa thin film (≈2-3 µV K-1). Furthermore, the LSSE coefficient for our MgO/CoFeCrGa (95 nm)/Pt (5 nm) heterostructure is ≈20.5 nV K-1 Ω-1 at room temperature, which is twice larger than that of the half-metallic ferromagnetic La0.7Sr0.3MnO3 thin films (≈9 nV K-1 Ω-1). We show that both ANE and LSSE coefficients follow identical temperature dependences and exhibit a maximum at ≈225 K, which is understood as the combined effects of inelastic magnon scatterings and reduced magnon population at low temperatures. Our analyses not only indicate that the extrinsic skew scattering is the dominating mechanism for ANE in these films but also provide critical insights into the functional form of the observed temperature-dependent LSSE at low temperatures. Furthermore, by employing radio frequency transverse susceptibility and broad-band ferromagnetic resonance in combination with the LSSE measurements, we establish a correlation among the observed LSSE signal, magnetic anisotropy, and Gilbert damping of the CoFeCrGa thin films, which will be beneficial for fabricating tunable and highly efficient Heusler alloy-based spin caloritronic nanodevices.

A folded π-system with supramolecularly oriented dipoles: single-component piezoelectric relaxor with NLO activity.

Organic molecules with an active dipole moment have a natural propensity to align in an antiparallel fashion in the solid state, resulting in zero macroscopic polarization. This primary limitation makes the material unresponsive to switching with electric fields, mechanical forces, and to intense laser light. A single-component organic material that bestows macroscopic dipole-driven electro-mechanical and optical functions, e.g., piezoelectric, ferroelectric and nonlinear optical (NLO) activity, is unprecedented due to the design challenges imparted by crystal symmetry and dipole orientations. Herein we report a crystalline organic material that self-assembles with a polar order (P 1), and is endowed with a high piezoelectric coefficient (d 33-47 pm V-1), as well as ferroelectric and Debye-type relaxor properties. In addition, it shows second harmonic generation (SHG) activity, which is more than five times that of the benchmark potassium dihydrogen phosphate. Piezoelectric force microscopy (PFM) images validated electro-mechanical deformations. Piezoresponse force spectroscopy (PFS) studies confirmed a signature butterfly-like amplitude and a phase loop. To the best of our knowledge, this is the first report of a folded supramolecular π-system that manifests unidirectionally oriented dipoles and exhibits piezoelectricity, ferroelectricity, and has excellent ability to generate second harmonic light. These findings can herald new design possibilities based on folded architectures to explore opto-, electro- and mechano-responsive multifaceted functions.

Enhanced magnetic performance in exchange-coupled CoFe2O4-LaFeO3nanocomposites.

Nanocomposite oxide system of (x)CoFe2O4-(100-x)LaFeO3with different weight percent of core-shell structured CoFe2O4(x = 0, 20, 40, 50, 80, 100) and LaFeO3were fabricated, via a two-step sol-gel wet-chemical synthesis technique. The phase formation of the composites was confirmed by x-ray diffraction and the structural parameters of both the phases were attained from the Rietveld refinement results of XRD patterns. The elemental composition and microstructure of the resulting nanocomposites were examined by using energy-dispersive x-ray spectroscopy and high-resolution transmission electron microscopy technique, respectively. The detailed magnetometry studies at 300 K and 5 K reveal that the inter-and intra-phase magnetic interactions affect the saturation magnetization (MS), remanence magnetization (MR) and coercivity (HC) values of this bi-magnetic system. The remarkable feature of 'pinched magnetic hysteresis loop' was evidenced in the [(50) CoFe2O4- (50)LaFeO3] composite, leading to a lesser magnetic loss factor and better magnetic performance of this sample. The report depicts an improved interfacial exchange coupling at 5 K, for the nanocomposites of core-shell morphology and offers an understanding or explanation of improved magnetic performance for the (50)CoFe2O4- (50)LaFeO3nanocomposite and opens up an important way to design new multiferroic applications in low magnetic fields.

Strain-tunable triple point Fermions in diamagnetic rare-earth half-Heusler alloys.

Topologically non-trivial electronic structure is a feature of many rare-earth half-Heusler alloys, which host atoms with high spin-orbit coupling bringing in the non-triviality. In this article, using the first-principles simulations, rare-earth half-Heusler YPdBi, ScPdBi, LaPdBi, LuPdBi, YPtBi and LuPtBi alloys are studied under strain to reveal multiple band inversions associated with topological phase transitions. From our simulations we find that, as a result of first band-inversion, the Brillouin zone of the diamagnetic half-Heusler alloys hosts eight triple points whereas, the second band inversion causes the emergence of sixteen more triple points. These band-inversions are observed to be independent of the spin-orbit coupling and are the reason behind increasing occupation of bismuth 7s orbitals as volume of the unit cell increases. The surface electronic transport in different triple point semi-metallic phases is found to evolve under strain, as the number of Fermi arcs change due to multiple band inversions. Once the second band inversion occurs, further application of tensile strain does not increase the number of triple points and Fermi arcs. However, increasing tensile strain (or decreasing compressive strain) pushes the triple point crossing to higher momenta, making them more effective as source of highly mobile electrons. These observations make a pathway to tune the bulk as well as surface transport through these semi-metals by application of tensile or compressive strain depending on the unstrained relative band-inversion strength of the material.

Strain driven emergence of topological non-triviality in YPdBi thin films.

Half-Heusler compounds exhibit a remarkable variety of emergent properties such as heavy-fermion behaviour, unconventional superconductivity and magnetism. Several of these compounds have been predicted to host topologically non-trivial electronic structures. Remarkably, recent theoretical studies have indicated the possibility to induce non-trivial topological surface states in an otherwise trivial half-Heusler system by strain engineering. Here, using magneto-transport measurements and first principles DFT-based simulations, we demonstrate topological surface states on strained [110] oriented thin films of YPdBi grown on (100) MgO. These topological surface states arise in an otherwise trivial semi-metal purely driven by strain. Furthermore, we observe the onset of superconductivity in these strained films highlighting the possibility of engineering a topological superconducting state. Our results demonstrate the critical role played by strain in engineering novel topological states in thin film systems for developing next-generation spintronic devices.

Thickness-dependent magneto-transport properties of topologically nontrivial DyPdBi thin films.

DyPdBi (DPB) is a topological semimetal which belongs to the rare-earth-based half-Heusler alloy family. In this work, we studied the thickness-dependent structural and magneto-transport properties of DPB thin films (20 to 60 nm) grown using pulsed laser deposition. The DPB thin films show (110) oriented growth on MgO(100) single crystal substrates. Longitudinal resistance data indicate metallic surface states dominated carrier transport and the suppression of semiconducting bulk state carriers for films ≤40 nm. We observe the weak antilocalization (WAL) effect and Shubnikov-de Hass (SdH) oscillations in the magneto-transport data. The presence of a single coherent transport channel (α∼ -0.50) is observed in the Hikami-Larkin-Nagaoka (HLN) fitting of WAL data. The power law temperature dependence of phase coherence length (LØ ) ∼ T-0.50 indicates the observation of the 2D WAL effect and the presence of topological nontrivial surface states for films ≤40 nm. The 60 nm sample shows semiconducting resistivity behavior at higher temperature (>180 K) and HLN fitting results (α∼ -0.72, LØ âˆ¼ T-0.68 ) indicate the presence of partial decoupled top and bottom surface states. The Berry phase ∼π is extracted for thin films ≤40 nm, which further demonstrates the presence of Dirac fermions and nontrivial surface states. Band structure parameters are extracted by fitting SdH data to the standard Lifshitz-Kosevich formula. The sheet carrier concentration and cyclotron effective mass of carriers decrease with increasing thickness (20 nm to 60 nm) from ∼1.35 × 1012 cm-2 to 0.68 × 1012 cm-2 and from ∼0.26 me to 0.12 me, respectively. Our observations suggest that samples with a thickness ≤40 nm have transport properties dominated by surface states and samples with a thickness ≥60 nm have contributions from both bulk and surface states.

Ionic Assembly, Anion-π, Magnetic, and Electronic Attributes of Ambient Stable Naphthalenediimide Radical Ions.

Organic spin-based molecular materials are considered to be attractive for the generation of functional materials with emergent optoelectronic, magnetic, or magneto-conductive properties. However, the major limitations to the utilization of organic spin-based systems are their high reactivity, instability, and propensity for dimerization. Herein, we report the synthesis, characterization, and magnetic and electronic studies of three ambient stable radical ions (1 a.+ , 1 b.+ , and 1 c.+ ). The radical ions 1 b.+ and 1 c.+ with BPh4 - and BF4 - counter anions, respectively, were synthesized in excellent yields by means of anion metathesis of 1 a.+ with Br- as its counter anion. Notably, synthesis of 1 a.+ was achieved in an ecofriendly, solvent-free protocol. The radical ions were characterized by means of single-crystal X-ray diffraction studies, which revealed the discrete nature of the radical ions and extensive hydrogen-bonding interactions within the radical ions and with the counter anions. Thus, radical ions can be organized to form infinite supramolecular arrays using weak noncovalent interactions. In addition, the Br- , BF4 - , and BPh4 - anions formed diverse types of anion-π interactions with the naphthalene and imide rings of the radical ions. The radical ions were characterized by means of X-band electron paramagnetic resonance (EPR) spectroscopy in solution and in the solid state. Magnetic studies revealed their paramagnetic nature in the range of 10 to 300 K. The radical ions exhibited high resistivity approaching the gigaohm (GΩ) scale. In addition, the radical ions exhibited panchromism.

Weak Antilocalization and Quantum Oscillations of Surface States in Topologically Nontrivial DyPdBi(110)Half Heusler alloy.

Recently, a number of ternary half-Heusler compounds have been predicted independently by several research groups as candidates for 3D topological insulators. In this work, we report the observation of a two-dimensional (2D) weak antilocalization (WAL) effect, one of the hall-marks of topological surface states, and Shubnikov-de Hass (SdH) quantum oscillations in <110> oriented DyPdBi (DPB) thin films grown on MgO (100) substrates. The films prepared by pulsed laser deposition technique under the optimized conditions, showed a textured structure with (110) planes parallel to the (100) plane of MgO. The measured WAL effect follows the Hikami-Larkin-Nagaoka (HLN) model and the extracted values of phase coherence length (lϕ) and α are ~420 nm and ~-0.52 respectively. The power law variation of lϕ (~T-0.46) indicates the presence of the 2D surface states in DPB film. The Dirac nature of the surface states is further confirmed by Landau-level fan diagram analysis of SdH oscillations of the magneto-transport data. This analysis shows a finite Berry phase of 0.90π ± 0.16, reasonably close to the expected π value. Sheet Carrier density, ns ~ 2.56 × 1012 cm-2, calculated from the SdH oscillations (fSdH ~ 106 T) and Hall measurements agree well with each other. These findings demonstrate that the half Heusler DPB thin films (~15-20 nm) can be used as a suitable material for investigating the novel intrinsic quantum transport properties of surface Dirac fermions.

Observation of low temperature metastable states in complex CaMn7O12.

The low temperature magnetic behaviour of the multiferroic quadruple perovskite CaMn7O12 is investigated. The magneto-caloric effect in this material is presented for the first time. Along with the established magnetic transitions, T N1 ~ 90 K and T N2 ~ 45 K, an anomaly at 20 K is observed in our dc magnetization, ac susceptibility and dielectric measurements; below which, an inverse magnetocaloric effect is also observed in our -ΔS M (T) plots. The neutron scattering measurements show minimal change between 10 K and 30 K in static correlations, but a clear change in energy and linewidth of the magnetic excitations is evident. The results suggest that only dynamic correlations change across T M ~ 20 K. The existence of multiple magnetic interactions below 45 K, with significant coupling between them, is demonstrated using an Arrott plot analysis of our magnetic data. Compatible conclusions are drawn from magnetocaloric plots. The peak change in isothermal magnetic entropy (-ΔS M) is 1.3 JK-1 kg-1 and the value of refrigeration capacity in CaMn7O12 is 34.5 J · kg-1 at 7 T.

A miniaturized nanobiosensor for choline analysis.

A novel reusable chemiluminescence choline nanobiosensor has been developed using aligned zinc oxide nanorod-films (ZnONR). The chemically fashioned ZnONR were synthesized by hybrid wet chemical route onto glass substrates and used to fabricate a stable chemiluminescent choline biosensor. The biosensor was constructed by co-immobilization of the enzymes choline oxidase and peroxidase. The covalent immobilization of the enzymes on the ZnONR was achieved using 16-phosphonohexadecanoic acid as a cross-linker. The phosphonation of the ZnONR imparted significant stability to the immobilized enzyme as against physisorbed enzyme. A lower value of Michaelis-Menten constant (Km), of 0.062 mM for the covalently coupled enzyme over the physisorbed enzymes facilitated enhanced stability of ZnONR nanobiosensor. The ZnONR-choline biosensor has been investigated over a wide range of choline from 0.0005 mM to 2 mM. Importantly, the recovery of choline in milk samples was close to 99%. Using the developed biosensor, choline was measurable even after 30 days with 60 repeated measurements proving the stability of the sensor (Intraday RSD%=2.83 and Interday RSD%=3.51).

Magnetic properties of nanocrystalline beta-SiC.

Four-hour ball-milled beta-SIC product synthesized by the thermal plasma technique shows room temperature ferromagnetism. The semi-lambda signature of the field-cooled magnetization (FCM) and zero field-cooled magnetization (ZFCM) curves suggest the possible signature of a glassy ferromagnetism state in the sample. The prominent fall in the magnetization value at around 50 K observed in ZFCM curve reveals the existence of a sharp transition from a ferromagnetic state to a glassy ferromagnetic state. The presence of glassy ferromagnetism at low temperature is confirmed from the M-H curve recorded at 5 K.

An all-organic steroid-D-π-A modular design drives ferroelectricity in supramolecular solids and nano-architectures at RT.

Confluence of a modular design approach and self-assembly with a 'steroid-D-π-A' module generates spontaneous polarization in solids and for the first time in nano-architectures constituted from organogels, at room temperature (RT).