Ananthakrishanan Srinivasan

A series of single-phase ternary and quaternary Co-Fe-Sn-(Ni) medium entropy alloy (MEA) films with configurational entropy (Sconf) varying from 1.0R to 1.4R (where R is the universal gas constant) were synthesized by electrodeposition followed by heat treatment. Phase purity and ferromagnetic nature of the heat-treated films were confirmed at room temperature by powder X-ray diffraction and magnetometry, respectively. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) exhibited a clear dependence on configurational entropy of the MEA films. The film with Sconf = 1.39R (designated as CFNS-4) delivered markedly reduced overpotentials of 100.3 mV for HER and 319.7 mV for OER at 10 mA/cm2 , accompanied by low Tafel slopes of 46.7 and 55.5 mV/dec, respectively, and stable operation for over 10 h. A monotonic decrease in charge-transfer resistance was observed with increasing Sconf, yielding minimum values of 3.0 Ω for HER and 6.7 Ω for OER for the CFNS-4 film. Correspondingly, the overpotential required to reach 10 mA/cm² also decreased progressively with increasing Sconf, indicating a strong correlation between Sconf and electrochemical activity. When employed as both anode and cathode, CFNS-4 film demonstrated overall water splitting at a cell voltage of 1.67 V, which is significantly lower than that of the ternary film (1.83 V) with S_conf = 1.1R. This work demonstrates the effectiveness of entropy engineering in enhancing bifunctional electrocatalytic performance as well as establishing the entropy-activity correlation in MEA films.

We report a systematic evaluation of the effects of post-deposition annealing temperature and structural ordering on the anomalous Nernst effect (ANE) in Fe2CoSi (FCS) Heusler alloy thin films. The study reveals that amorphous/disordered FCS films exhibit a larger anomalous Nernst coefficient (SANE) compared to their crystalline counterparts, in stark contrast to conventional Co-based Heusler alloys, which typically show maximum SANE in highly ordered structures. Furthermore, as a strategy for enhancing transverse thermoelectric properties, we explore the (FCS)100−xPtx composite alloy films by systematically varying the Pt concentration and optimizing the composition. This method effectively enhances the transverse thermoelectric performance of the FCS-based alloys. Our findings offer valuable insights into the design and development of next-generation high-performance ANE materials.

Electrodeposited Co _47.9 Fe _21.7 Sn _30.4 and Co _48.3 Fe _19.3 Cr _3.1 Sn _29.3 alloy films with thickness of 780 nm exhibited the ordered L 2 ₁ -type Heusler alloy structure after heat treatment at 550 °C. Both the films exhibit soft ferromagnetic nature with saturation magnetization of ∼4.5 μ _B /f.u. and effective anisotropy constant of ∼10 ⁶ erg/cc at 5 K. Ab initio calculations reveal that half-metallicity is induced in Co _48.4 Fe _21.9 Sn _29.7 alloy upon substituting ∼3.1 at.% of Cr in the place of Fe. The high value (∼7.1 meV) of minimum excitation energy required for the majority charge carriers to transit to empty minority states through spin–flip scattering obtained from DC electrical resistivity studies supports the half-metallicity in Co _48.3 Fe _19.3 Cr _3.1 Sn _29.3 alloy films. This study showcases a promising spintronic Heusler alloy composition apart from demonstrating electrodeposition as a low-cost method for producing high quality quaternary Heusler alloy films.

The reversible martensite ↔ austenite phase transition in Co₂NiGa nanoparticles is depicted schematically, demonstrating the Curie temperatures of both phases and structural evolution with heating and cooling.

The development of energy-efficient spintronic devices with enhanced magnetoresistance demands materials with low Gilbert damping constant (α) and high half-metallicity. In this study, we report a very low α in sputter-deposited off-stoichiometric Co2FeGa0.5Ge0.5 (CFGG) Heusler alloy thin films and investigate the relation between α and atomic ordering and half-metallicity. CFGG thin films with a composition of Co44.3Fe30.7Ga13.9Ge11.1 were epitaxially deposited on MgO (001) substrates by magnetron sputtering followed by in situ annealing. The density of states calculations revealed that this composition has higher half-metallicity than the stoichiometric composition, owing to Fermi energy tuning. As-deposited and 400 °C-annealed samples exhibited B2-type partially disordered structure, while annealing above 500 °C induced a L21-type ordered structure. The improvement in atomic order resulted in the reduction of α, as demonstrated by ferromagnetic resonance measurement, and enhancement in half-metallicity, as revealed by anisotropic and ordinary magnetoresistance measurements. The 600 °C-annealed samples exhibited an intrinsic α of (3.5 ± 0.3) × 10−4, the low value reported for metallic ferromagnetic materials, demonstrating the potential for its use in energy-efficient spintronic devices.

Single-phase, near stoichiometric X-type Fe₂CoGa nanowires with an average diameter of 253 ± 2 nm have been synthesized using the electrospinning route. The highly ordered polycrystalline nanowires with a lattice constant of 5.758 Å demonstrated ferromagnetic behaviour, with a substantial saturation magnetization of 4.61 μ_B f.u.⁻¹ at 5 K and a high effective anisotropy constant of ∼9.36 × 10⁶ erg cc⁻¹ at 5 K. The electronic density of states near the Fermi level, estimated using ab initio calculations, provides insight into the electronic behaviour of this compound. These findings highlight the potential of ferromagnetic Fe₂CoGa nanowires for nanomagnetic devices and magnetic data storage, and a scalable and versatile approach for fabricating nanoscale Heusler alloys via the electrospinning route.