Santanu Pakhira

Quaternary Heusler alloys have emerged as a versatile class of multifunctional materials owing to their highly tunable electronic structures and magnetic properties. Their crystal architecture provides exceptional compositional flexibility, enabling systematic control of electronic band filling, magnetic exchange interactions, and spin polarization. This tunability gives rise to a wide range of functional states, including half‐metallicity, spin‐gapless semiconductivity, bipolar magnetic semiconductivity, spin semimetallicity, compensated ferrimagnetism, and topological electronic phases. Among the descriptors governing these properties, the valence electron count (VEC) has proven particularly powerful, providing a predictive framework through the Slater–Pauling rule for correlating composition with magnetic and electronic behavior. In this Review, we comprehensively survey experimentally reported quaternary Heusler alloys and systematically classify them according to their VEC. We first discuss their crystal chemistry, atomic ordering, and disorder mechanisms, highlighting their influence on physical properties. We then establish clear correlations between VEC and magnetic ground states, electronic structure, magnetotransport, and thermoelectric performance, identifying overarching trends and unifying structure–property relationships across diverse compositional families. By integrating insights from structural, magnetic, electronic, and transport studies, this Review provides a coherent framework for understanding quaternary Heusler alloys and offers design principles for discovering next‐generation materials for spintronic, thermoelectric, and energy‐efficient electronic technologies.

Nontrivial topological spin‐textures, such as skyrmions, merons, bimerons, and skyrmioniums, are envisioned as robust building blocks for future memory and logic devices. Controllable transformations between these states require a quantum‐mechanical description of electronic degrees of freedom and atomic‐scale insight beyond existing phenomenological models. Here, we report an atomic‐scale investigation of topological phase transitions and their protection in the two‐dimensional van der Waals ferromagnet Fe ₃ GeTe ₂ (FGT) using a combined experimental‐theoretical approach. Synchrotron‐based Fourier transform holography directly images labyrinth domains, isolated skyrmions, mixed labyrinth‐skyrmion phases, and skyrmion bags with high spatial resolution. We compare these observations to simulations based on an electronic lattice Hamiltonian that captures both metallicity and relativistic spin‐orbit coupling in FGT. By systematically exploring a broad range of temperatures and magnetic fields, we map the mechanisms governing topological transitions and their stability. This sequential‐integrated experimental‐theoretical framework advances understanding of spin‐texture interactions and enables precise control of external tuning parameters. Our results establish a platform for creating, stabilizing, and manipulating topological states, paving the way for engineered spin‐texture transitions in next‐generation spintronic technologies.

We report the study of spin dynamics of Eu 2 + in the antiferromagnetic axion topological insulator EuSn 2 As 2 by means of antiferromagnetic resonance at 9.34 GHz. Below the Néel temperature ( T N ), two types of resonance modes, the conventional bulk antiferromagnetic resonance and additional surface ferromagnetic resonance, are observed. The latter turns out to be characteristic of the axion-insulating phase. Above T N , we prove the existence of a Kosterlitz-Thouless scenario that is relevant for the spin relaxation in the Eu 2 + layers. The absence of Korringa relaxation indicates the strong confinement of the conduction electrons at the Fermi level to the SnAs layers.

The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi energy ( E F ). In this study, we investigate this phenomenon in the Pauli paramagnet SrCo 2 As 2 , where an FB associated with Co e g orbitals lies close to E F . Remarkably, a minute substitution of the nonmagnetic element Pd onto the Co site ( ∼ 2%) induces antiferromagnetic order with a transition temperature as high as T N = 25 K. Temperature- and magnetic-field-dependent magnetic and transport measurements, complemented by zero-field neutron diffraction, reveal a helical magnetic order for x ≤ 0.10 in Sr(Co _{1− x} Pd _x ) ₂ As ₂ , transitioning to a complex ferromagnetic state at higher Pd concentrations. Spectroscopic evidence and theoretical band structure calculations demonstrate that electron doping shifts the flat band closer to E F , significantly enhancing the Stoner parameter. This enhancement drives a strong ferromagnetic instability, leading to helical magnetic ordering dominated by in-plane ferromagnetic interactions. The emergence of robust magnetic ordering through substitution with nonmagnetic elements is a unique phenomenon that underscores the pivotal role of flat-band instability in tuning magnetism in itinerant systems.

Understanding the origin of unconventional magnetic order is a central theme in the research field of functional quantum materials for spin-dependent applications. In general, a paramagnetic (PM) to antiferromagnetic (AFM) transition is of thermodynamically second order in nature in the absence of a magnetic field. In the magnetically frustrated corrugated-honeycomb lattice compounds ( Ca , Sr ) Mn 2 P 2 , electrical resistivity ρ measurements in the a b plane are reported to exhibit sharp discontinuities at the respective PM to AFM ordering temperatures T N = 69.5 and 53 K, respectively, signifying the unusual first-order nature of the AFM transition. To reveal the origin of such anomalous transitions, high-resolution linear thermal-expansion measurements are reported for both CaMn 2 P 2 and SrMn 2 P 2 in the a b plane and along the c axis using capacitance dilatometry. The thermal expansion measurements show a sharp discontinuity in Δ L / L at T N , yielding a diverging behavior in the thermal expansion coefficient α ( T ) associated with strong magnetoelastic coupling, driving the first-order AFM transition in CaMn 2 P 2 . However, this effect is weaker in SrMn 2 P 2 , consistent with the magnetic entropy changes S ( T ) obtained from the heat-capacity measurements. Temperature-dependent a b -plane x-ray diffraction measurements for CaMn 2 P 2 near its T N yield a volume thermal expansion coefficient consistent with literature data for other materials. The uniaxial pressure derivatives of T N in the two compounds are found to be respectively opposite in sign, presumably due to the different magnetic structures.

A new non-stoichiometric ternary compound, Gd₂Co_0.90Si_2.90, has been synthesized in single phase by introducing vacancies in the Co and Si sites. A spin-glass type of ground state is observed due to frustration and disorder.