Magnetic, transport and electronic properties of Ni$_2$FeAl Heusler alloy nanoparticles: Experimental and theoretical investigation

We present a comprehensive investigation of structural, magnetic and transport properties of Ni$2$FeAl Heusler alloy nanoparticles (NPs) synthesized via template-less chemical route. The NPs exhibit high saturation magnetization of 3.02 $μ {\rm B}$/f.u. at 5K, large magnetic anisotropy of 0.238 MJ/m$^3$, and a Curie temperature of 874K. Magnetocaloric analysis reveals a magnetic entropy change of 3.1 J.kg$^{-1}$K$^{-1}$ at 70 kOe. Low-temperature transport measurements show a weak resistivity upturn, following a $-T^{1/2}$ dependence, indicative of disorder-enhanced electron-electron interactions. First-principles calculations based on density functional theory yield a magneto-crystalline anisotropy energy of 0.987 MJ/m$^3$, consistent with experiment and demonstrate pronounced surface and finite-size effects through comparison of bulk and nanocluster geometries. The combination of high Curie temperature, sizable perpendicular magnetic anisotropy, and moderate spin polarization and magnetic entropy change make the Ni$_2$FeAl as promising candidate for various applications.

💡 Research Summary

This work presents a comprehensive experimental and theoretical investigation of Ni₂FeAl Heusler alloy nanoparticles synthesized by a template‑free chemical route. Structural characterization by X‑ray diffraction, Rietveld refinement, SEM, TEM, and EDS confirms a single‑phase tetragonal I4/mmm structure with lattice parameters a = 3.556 Å, c/a = 1.42. The average crystallite size derived from peak broadening is ~25 nm, while the particle diameter observed in microscopy is ~45 nm, indicating well‑defined nanocrystals with moderate size dispersion.

Magnetic measurements reveal a high saturation magnetization of 3.02 μ_B per formula unit at 5 K and a Curie temperature of 874 K, comparable to bulk Heusler alloys. Zero‑field‑cooled (ZFC) and field‑cooled (FC) curves display a pronounced bifurcation and a low‑temperature drop below ~50 K, suggesting the coexistence of ferromagnetic (FM) clusters and antiferromagnetic (AFM) interactions that give rise to spin‑glass or cluster‑glass behavior. Frequency‑dependent AC susceptibility peaks, long‑time relaxation of isothermal remanent magnetization, and the temperature dependence of the magnetization following Bloch’s T³⁄² law further support the presence of correlated spin dynamics and inter‑particle interactions.

The hysteresis loops measured at 5 K and 300 K show soft ferromagnetic behavior with coercivities of 140 Oe and 80 Oe, respectively. By fitting the high‑field region to the law of approach to saturation (M = M_s