Quantification of site disorder and its role on spin polarization in the nearly half-metallic Heusler alloy NiFeMnSn

The electronic structure and magnetism of the quaternary Heusler alloy NiFeMnSn are studied using the full-potential linearized augmented plane-wave (FPLAPW) method. The calculation for the perfectly LiMgPdSn-type ordered crystal structure (type I) of NiFeMnSn shows a high spin polarization ($\sim 76$%) with a ferromagnetic ground state. The total spin magnetic moment is in good agreement with the Slater-Pauling rule. The structural investigations using neutron diffraction at 500 K, and Mössbauer spectroscopy at 300 K on the NiFeMnSn alloy, prepared using an arc melting, show the presence of atomic site disorder. The electronic structure calculation for the disordered structure shows that the site disorder destroys the nearly half-metallic nature of this alloy. The magnetization measurements indicate that the Curie temperature is well above room temperature ($\sim 405$ K) as desired for the spintronics application.

Figure 1

Observed room-temperature x-ray diffraction pattern of NiFeMnSn Heusler alloy is shown by open circles. Rietveld refined pattern is shown by solid line. Solid line at the bottom shows the difference between the observed and calculated patterns. The vertical bars indicate the position of allowed Bragg peaks. The $\left(hkl\right)$ values corresponding to the Bragg peaks are also shown.

Figure 2

Crystal structure of the quaternary Heusler compound NiFeMnSn. There are three nonequivalent structures for NiFeMnSn, depending on the occupation at the four different lattice sites $4a$ (light gray with larger radius), $4b$ (dark gray), $4c$ (medium gray), and $4d$ (black).

Figure 3

The Mössbauer spectrum for NiFeMnSn Hesuler alloy at 300 K.

Figure 4

Observed neutron diffraction patterns of NiFeMnSn Heusler alloy at (a) 5, (b) 300, and (c) 500 K are shown by open circles. Rietveld refined patterns [(a) and (b) nuclear and magnetic; (c) only nuclear] are shown by solid lines. Solid lines at the bottom show the difference between the observed and calculated patterns. The vertical bars indicate the position of allowed Bragg peaks.

Figure 5

(a) Temperature dependence of magnetization under the 200-Oe field for NiFeMnSn Heusler alloy. Inset shows the $d^{2}M/d{T}^2$ vs $T$ showing the Curie temperature $T_{\mathrm{C}}$. (b) Magnetization as a function of the applied magnetic field at 5 and 300 K.

Figure 6

Energy $(E-E_0)$ versus lattice parameter of three possible different crystal structures for both NM and FM spin configurations.

Figure 7

Spin-polarized densities of states plots for the NiFeMnSn Heusler alloy for (a) type-I, (b) type-II, and (c) type-III structures. (d) DOS plots for the structure with atomic site disorder as shown in Table 2.