Griffiths phase–like behavior in nearly half-metallic CoFeVAl: Theory and experiment

We report a combined theoretical and experimental study of the quaternary Heusler alloy CoFeVAl hosting various interesting electronic/magnetic features including an extended Griffiths phase–like behavior. The room temperature x-ray diffraction pattern confirms a cubic structure (space group No. 216) with a B2 disorder between V and Al atoms. The dc magnetization measurements indicate a ferromagnetic-like behavior with a magnetic transition temperature $\left(T_C\right)$ $\sim 50$ K. Above $T_C$ an unusual magnetic phase (extended Griffiths like) is observed which is carefully analyzed from the dc and ac magnetic susceptibility data. Longitudinal resistivity data confirm a metallic-like behavior with a negative nonsaturating magnetoresistance in the magnetically ordered region. The anomalous Hall effect mediated by a non-negligible intrinsic mechanism is found in the total Hall resistivity whereas negative slope in the ordinary Hall resistivity indicates the electron-mediated conduction in CoFeVAl. First-principles calculated results corroborate well with the experimentally observed crystal structure and nearly half-metallic ferromagnetic (FM) behavior. A high spin polarization $(\sim 86\%)$ is observed in the FM ground state. Inclusion of B2 disorder retains the nearly half-metallic behavior of CoFeVAl. Such combined studies for the discovery of new quantum materials are the backbone of a future path toward the spintronic-based device applications.

Figure 1

For CoFeVAl: (a) The primitive unit cells of three possible nondegenerate ordered configurations. (b) Room temperature XRD pattern along with the Rietveld refinement considering B2-type antisite disorder between the octahedral sites V and Al only while keeping Fe and Co sites ordered. Inset shows the primitive cell of this B2 disordered structure. (c) The zoomed-in view of the refinement of two peaks (111) and (200) considering (i) completely ordered structure and (ii) $12.5\%$ and (iii) $25\%$ disordered structure between V and Al atoms. (d) Energy-dispersive x-ray spectroscopy results with elemental mapping and atomic compositions.

Figure 2

For CoFeVAl: (a) Magnetization (M) vs temperature (T) for ZFC and FCC protocol under applied magnetic field $H=100$ Oe. Inset shows the M vs H curves for temperatures 2, 75, and 300 K. (b) Arrott plots for different temperatures from 2 to 100 K. (c), (d) Inverse dc susceptibility vs T for 100 Oe and 2 kOe along with the Curie-Weiss (C-W) fitting. Insets of (c) and (d) show a zoomed-in view of the respective plots. (e) Imaginary part of the ac susceptibility vs T under 1 Oe bias field and 997 Hz excitation frequency. Inset shows a zoomed-in view of the same plot. (f) Magnetic phase diagram of the FCC data under 500 Oe which highlights the extended Griffiths phase region along with the C-W fitting.

Figure 3

For CoFeVAl: (a) Longitudinal resistivity (${\rho }_{xx}$) vs temperature (T) for zero and 50 kOe applied field. Inset (i): Zoomed-in view of ${\rho }_{xx}$ vs T to highlight the hump near 40 K. Inset (ii): Low-temperature fitting of the zero-field ${\rho }_{xx}$ data. (b) Magnetoresistance (MR) vs applied field (H) for various temperatures.

Figure 4

For CoFeVAl: (a) Hall resistivity ${\rho }_{xy}$ vs H for various temperatures up to 250 K. ${\rho }_{xy}$ vs H along with separated ordinary (${\rho }_{xy}^{\mathrm{OHE}}$) and anomalous (${\rho }_{xy}^{\mathrm{AHE}}$) Hall contributions for (b) 5 K, (c) 10 K, (d) 50 K, and (e) 100 K. (f) Intrinsic anomalous Hall conductivity (${\sigma }_{\mathrm{int}}^{\mathrm{AHE}}$) vs H for 5, 10, and 50 K.

Figure 5

For CoFeVAl: Spin-polarized band structure and density of states (DOS) for the (top) completely ordered structure and (bottom) B2 disordered structure.