arXiv subjects

Nanhe Kumar Gupta

Publications and source records attributed to Nanhe Kumar Gupta.

7 recordsLinked to original sources

arXiv2026

Room-Temperature Polarity Control of the Anomalous Nernst Effect in a High-Magnetic-Anisotropy Topological Nodal-Line MnAlGe

Controlling the polarity of anomalous Nernst thermopower is a promising strategy for enhancing the performances of thermoelectric applications. However, realizing such control at room temperature (RT) in topological ferromagnets with high magnetic anisotropy (Ku) remains challenging. Here, we report RT polarity control of the anomalous Nernst effect (ANE) in quasi-two-dimensional nodal-line MnAlGe epitaxial thin films through Al/Ge compositional tuning while preserving robust high Ku. This polarity reversal originates from intrinsic Berry curvature contributions modulated by sublattice-selective carrier doping, as supported by spin-resolved electronic band structure analysis and hard X-ray photoemission spectroscopy. To demonstrate the practical feasibility, we also fabricated a meander-structured device combining MnAlGe with positive and negative polarity enhanced the thermoelectric output. Our results demonstrate that tuning the Fermi level relative to the nodal-line electronic structure while preserving high Ku enables controllable ANE polarity reversal within a single material, providing a route toward RT transverse thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2026

Nodal-Surface and Flat-Band Driven Large Anomalous Nernst Effect in Epitaxial Ferromagnetic Weyl Metal Fe5Si3

Magnetic topological materials such as Weyl and Dirac magnets exhibit unconventional electronic properties arising from the interplay between magnetic order and band topology, leading to remarkable thermomagnetic and thermoelectric effects. Here, we investigate the ANE in epitaxial thin films of the Weyl ferromagnet candidate Fe5Si3. A pronounced transverse Nernst response exceeding approximately 1.50 microvolt per kelvin is observed at room temperature, together with a giant anomalous Nernst angle of about 0.56, indicating highly efficient conversion between thermal gradients and transverse electric fields. Beyond the anomalous contribution, a sizable topological Nernst signal of approximately 0.43 microvolt per kelvin persists above room temperature, suggesting the possible presence of real-space Berry curvature associated with nontrivial spin textures. First-principles density functional theory calculations combined with symmetry analysis reveal an unconventional electronic structure in which Weyl nodal lines, nodal surfaces, and nearly flat bands coexist near the Fermi level. This rare concurrence of multiple topological band features produces a strongly enhanced and sharply energy-dependent Berry curvature, which governs both the magnitude and temperature evolution of the observed Nernst response. The close quantitative agreement between calculated anomalous Nernst conductivity and experimental results establishes the topological electronic structure as the dominant origin of the observed thermomagnetic transport, highlighting Fe5Si3 as a chemically simple, low-cost binary topological magnet for exploring both real-space and momentum-space Berry-curvature-driven thermoelectric phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Emergent Anomalous and Topological Hall Responses in an Epitaxial Ferromagnetic Weyl Nodal-Line metal Fe5Si3

The interplay between real and reciprocal space topology yields intrinsically linked transport phenomena in magnetic Weyl systems, wherein the broken time-reversal symmetry, strong Dzyaloshinskii-Moriya interaction, and pronounced uniaxial anisotropy stabilize the momentum-space Berry-curvature monopoles (Weyl nodes) and real-space chiral spin textures. We present a combined first-principles and experimental study of epitaxial Fe5Si3 thin films, establishing them as a magnetic Weyl nodal-line material. First-principles Density Functional Theory (DFT) calculations unambiguously reveal that Fe5Si3 hosts a topologically nontrivial electronic structure containing six pairs of Weyl nodes at or near the Fermi level, accompanied by pronounced Berry curvature at high-symmetry points of the Brillouin Zone. High-quality epitaxial films exhibit robust ferromagnetism with a Curie temperature of ~370 K and strong magneto crystalline anisotropy. The magneto transport measurements on epitaxial films reveal the corresponding Berry curvature-driven responses, including a significantly large intrinsic anomalous Hall conductivity of 504 S/cm and a high anomalous Hall angle of 5.5%, which is in good agreement with DFT calculations. A negative and non-saturating longitudinal magnetoresistance is observed, consistent with a chiral-anomaly contribution from Weyl fermions near the Fermi level (EF). Furthermore, a substantial topological Hall resistivity of 1.6 μΩ cm robust across a wide temperature range, indicating the possibility of robust chiral spin textures in the thin-film geometry. These combined theoretical and experimental results establish Fe5Si3 as a unique, low-cost, centrosymmetric magnetic Weyl nodal-line material, providing a versatile platform for exploring coupled real and reciprocal space topologies in topological spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Tailoring magnetic properties of CoFeB films via tungsten buffer and capping layers

Controlling the interface between W and CoFeB-based buffer or capping layers at an appropriate temperature is essential for modifying the strength of magnetic anisotropy. In this work, we systematically explore the impact of W buffer and capping layers on the structural, topological, and magnetic anisotropy properties of W (5 nm)/CoFeB(10 nm) and CoFeB(10 nm)/W(3 nm) bilayers sputtered at room temperature (RT) and annealed at an optimal annealing temperature (TA) of 400 C. Our findings demonstrate that the bilayer films uniaxial magnetic anisotropy (UMA) with out-of-plane coercivity (Hcp) is highly influenced by the W buffer, capping layers, and TA. Specifically, the Hcp of the CoFeB layer with the buffer and capping layers annealed at 400 C samples exceed several times the coercivity of those unannealed. CoFeB buffered with W and annealed at 400 C shows larger Hcp, two-fold UMA, and higher in-plane UMA energy density (Keff) than the CoFeB/W bilayers, which can be attributed to the W buffer layer inducing the crystallization of CoFeB during annealing. The W buffer, capping layers, and the TA for W and CoFeB-based bilayer samples significantly alter the surface morphology, grain sizes, and surface roughness. The XRD analysis reveals nano-crystallites embedded in the larger grains of the 400 C annealed samples. Hence, this work offers a promising approach to achieving high thermal stability of UMA in W and CoFeB-based spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Large Orbital to Charge Conversion in Weak Spin Orbit Coupling Element Zr via Spin Orbital Pumping and Spin Orbital Seebeck Effect

The generation of spin-orbital currents is crucial for advancing energy-efficient spintronic devices. Here, the intricate process involved in the generation and conversion of spin and orbital to charge currents in Zr(t=2, 3, 4.5, 6, &10nm)/Co60Fe20B20(CFB), Zr/Pt/CFB, and Zr/Pt/CFB/Pt heterostructures are investigated using spin-orbital pumping ferromagnetic resonance and longitudinal spin-orbital Seebeck effect measurements. The moderate spin-orbit coupling (SOC) in the CFB layer facilitates the simultaneous generation of spin and orbital currents, which are transferred into adjacent Zr and Pt layers. Different spin-orbital to charge current contributions, namely, Inverse spin Hall effect (ISHE), Inverse orbital Hall effect (IOHE), and Inverse orbital Rashba-Edelstein effect (IOREE) are analyzed. Notably, introducing a single Pt layer increases the spin-orbital to charge current conversion via combined effects: ISHE in Pt, IOREE in Zr/Pt interface. An enhanced effective spin-orbital Hall angle (θ_eff) of 0.120 {\pm} 0.004 is observed for Zr/Pt/CFB, compared to that of 0.065 {\pm} 0.002 for the Zr/CFB, and 0.077 {\pm} 0.003 for the Zr/Pt/CFB/Pt heterostructures. These findings provide new insights into orbital-moment dependent phenomena and offer promising avenues for developing advanced spintronic devices exploiting both spin and orbital degrees of freedom, even in materials with lower SOC.

physics.app-ph↗

arXiv2024

Effects of Interfacial Oxygen Diffusion on the Magnetic Properties and Thermal Stability of Pd/CoFeB/Pd/Ta Heterostructure

We investigated the effects of annealing temperatures (TA) on a Pd (5 nm)/CoFeB (10 nm)/Pd (3 nm)/Ta (10 nm) multilayer structure. The as-deposited sample showed an amorphous state with in-plane uniaxial magnetic anisotropy (UMA), resulting in low coercivity and moderate damping constant (α) values. Increasing TA led to crystallization, forming bcc-CoFe (110) crystals, which increased in-plane coercivity and introduced isotropic magnetic anisotropy, slightly reducing the α. The two-fold UMA persists up to 600 C, and the thermal stability of the in-plane magnetic anisotropy remains intact even TA = 700 C. The TA significantly influenced the magnetic properties such as in-plane saturation magnetization (Ms//), in-plane and out-of-plane coercivities, and in-plane effective magnetic anisotropy energy density (Keff). Above 600 C, Keff decreased, indicating a transition towards uniaxial perpendicular magnetic anisotropy. Interfacial oxidation and diffusion from the Ta capping layer to the Pd/CoFeB/Pd interfaces were observed, influencing chemical bonding states. Annealing at 700 C, reduced oxygen within TaOx through a redox reaction involving Ta crystallization, forming TaB, PdO, and BOx states. Ferromagnetic resonance spectra analysis indicated variations in resonance field (Hr) due to local chemical environments. The α reduction, reaching a minimum at 300 C annealing, was attributed to reduced structural disorder from inhomogeneities. Tailoring magnetic anisotropy and spin dynamic properties in Pd/CoFeB/Pd/Ta structures through TA-controlled oxygen diffusion/oxidation highlights their potential for SOT, DMI, and magnetic skyrmion-based spintronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Impact of Annealing on Perpendicular Magnetic Anisotropy in W/MgAl2O4/CoFeMnSi/W/CoFeMnSi/MgAl2O4/W. Double Storage Layers for Upcoming MTJs

In this study, we achieved the improvement of uniaxial perpendicular magnetic anisotropy (PMA) in the W/MgAl2O4/CoFeMnSi/W/CoFeMnSi/MgAl2O4/W heterostructure by manipulating the annealing temperature (TA) [350 C, 450 C, and 550 C]. We observed a maximum effective PMA energy density (Keff) of = 1.604 x 106 erg/cc with low saturation magnetization (Ms) at the specified TA. The enhancement of Keff with Ms is significantly influenced by structural variations at the interfaces of CoFeMnSi and MgAl2O4, attributed to sufficient interfacial oxidation dependent on the TA. The TA was identified as a critical factor affecting the surface morphology, grain size, and surface roughness of the multilayer. Fourier-transform infrared (FT-IR) measurements were employed to confirm the presence of Co-O or Fe-O bond in the multilayer structures, elucidating the true origin of PMA. The control of interfacial oxidation at the interface during annealing is crucial for regulating the strength of PMA. Therefore, this double CoFeMnSi/MgAl2O4-based multilayer presents a promising avenue, serving as a favorable candidate for future p-MTJs-based spintronic devices with enhanced thermal stability.

cond-mat.mtrl-sci↗