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

TL;DR

This study combines template-free synthesis, structural/magnetic/transport characterization, and first-principles calculations to elucidate the properties of Ni$_2$FeAl Heusler nanoparticles. The nanomaterials crystallize in a tetragonal I4/mmm phase, exhibit high saturation magnetization, large magnetocrystalline anisotropy, and a notable magnetocaloric effect near the Curie temperature around 874 K. Transport measurements reveal a disorder-enhanced electron-electron interaction driven -$T^{1/2}$ upturn at low temperatures, supported by a low residual resistivity ratio; DFT confirms the sizable MCA and a finite spin polarization of ~40%, with surface- and finite-size effects captured in nanoclusters NC$_{43}$ and NC$_{79}$. The work demonstrates strong nanoscale tunability of magnetic and electronic properties via surface reconstruction and finite-size effects, highlighting potential for nano-spintronics, magnetocaloric cooling, and sensing applications.

Abstract

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 5~K, large magnetic anisotropy of 0.238 MJ/m$^3$, and a Curie temperature of 874~K. 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.

Figures (7)

• Figure 1: (a) The XRD pattern for Ni$_2$FeAl NPs recorded at room temperature, with the observed data points, simulated curve, difference in observed and simulated data and Bragg positions, denoted by open red circles, solid black line, solid blue line, and vertical green markers, respectively, (b) the unit cell (space group I4/$mmm$) where $a=$ 3.556 Å and $c/a=1.42$, with Ni (violet), Fe (red), Al (cyan) atoms. (c) The FE-SEM micrographs along with distribution of particle size in the inset, (d-f) the HR-TEM image and the SAED pattern.
• Figure 2: The thermo-magnetization curve recorded across the temperature range 300-1000 K at 100 Oe applied magnetic field for ZFC (open red squares) and FC (open blue circles) protocols. The inset illustrates low temperature thermo-magnetization curves across 2-300 K, recorded at 100 Oe. (b) The time dependence of iso-thermal remanent magnetization M(t) measured at 10 K. (c) Isothermal magnetization measured at 5, 300 K in low temperature (LT) assembly and 390 and 700 K in high temperature (HT) assembly, with inset showing the zoomed view of isotherms at 5 and 300 K. The high field region of the isotherms are fitted using the law of approach to saturation. (d) Thermo-magnetization curve recorded across 300-1000 K under an applied magnetic field of 10 kOe. The inset presents the normalized curve plotted as a function of T$^{3/2}$ and fitted using Bloch's function.
• Figure 3: (a) Isothermal magnetization curves for temperature range 800--925 K, up to 70 kOe applied magnetic field. (b) The magnetic entropy change $\Delta S_{\rm M}$ versus temperature plotted as a function of magnetic field (4--70 kOe). (c) The variation of maximum entropy change as a function of applied magnetic field for critical isotherm ($T = T_{\rm C}$) (d) Normalized slopes (NS) for different universality classes in the vicinity of $T_{\rm C}$ for Ni$_2$FeAl.
• Figure 4: (a) The temperature dependence (3--300 K) of longitudinal $\rho_{xx}$ under different magnetic field. The black solid curves represent the fit corresponding to linear temperature dependence. (b) the $\rho_{xx}$ versus $T$ (3--230 K), fitted using $T^{1/2}$+$T^2$ relation (solid black). The dashed curves represent the best fit corresponding to $T^{1/2}$ (green) and $T^2$ (blue) dependence. (c) The temperature dependent resistivity curves under applied field in the range of -70 to 70 kOe.
• Figure 5: (a, b) The electronic band structure near the Fermi level for minority and majority spin states, respectively. (c) The spin polarized total density of states of Ni$_2$FeAl for near Fermi level region. (d) The phonon dispersion and (e) projected phonon density of states for Ni, Fe and Al atoms for I4/$mmm$ phase.