Evolution of ferromagneticlike order in ${\text{Fe}}_2{\text{V}}_{1-x}{\text{Cr}}_x\text{Al}$ Heusler alloys

Structural, magnetic, and magnetotransport behaviors of ${\text{Fe}}_2{\text{V}}_{1-x}{\text{Cr}}_x\text{Al}$ Heusler-type alloys have been investigated in the present work. From the detailed analysis of data it is observed that increasing Cr content promotes site disorder of Fe and Al, which in turn leads to destabilization of $L{2}_1$ superstructure of ${\text{Fe}}_2\text{VAl}$. This site disorder seems to enhance the magnetic moment through spatially confined magnetic entities and intercluster interactions, transforming the alloy system into a magnetically ordered state in the limit $x\to 1$. Low-temperature magnetoresistance (MR) values drop quite rapidly from $\sim 10\%$ for $x=0$ to 0.4% for $x=0.8$, suggesting that magnetic interactions have a role in MR deterioration. Further the MR exhibits a maximum at a particular temperature in the vicinity of magnetic transition temperature, where interaction effects weaken. Our magnetization data support MR results and suggest that undoped ${\text{Fe}}_2\text{VAl}$ exhibits cluster-glass behavior with optimum superparamagnetic (SPM) contribution, developed possibly due to the random anisotropy with the presence of V. However, in Cr-substituted alloys anisotropy reduces due to coupling between the clusters, which, in turn, results in reduced SPM contribution and low MR values.

Figure 1

X-ray diffraction patterns (Normalized Intensity) of ${\text{Fe}}_2{\text{V}}_{1-x}{\text{Cr}}_x\text{Al}$ $\left(x=0–0.8\right)$ alloys.

Figure 2

(Color online) (a) (Main panel) $M\text{−}H$ hysteresis loops for ${\text{Fe}}_2{\text{V}}_{1-x}{\text{Cr}}_x\text{Al}$ at 300 K. Insets: (upper) Normalized magnetization as a function of $H$. (lower) Separated SPM and PM components of $x=0.2$ and 0.8 from the fits to Eq. (1). Symbols used in (a) and (b) represent same composition. Legend is given in (b). (b) (Main panel) $M\text{−}H$ hysteresis loops for ${\text{Fe}}_2{\text{V}}_{1-x}{\text{Cr}}_x\text{Al}$ at 7 K. Insets: (upper) Normalized magnetization as a function of $H$. (lower) Low-field hysteresis loops for better view of magnetic parameters. Solid lines through the data points represent fits to the Eq. (3).

Figure 3

(Color online) (Main panel) ZFC and FC $\left(H=100\text{Oe}\right)$ curves as a function of temperature of all the compositions. Insets: (upper) $M\text{−}T$ curves for $x=0.8$ and (lower) for $x=0$ alloys are shown for better clarity.

Figure 4

(Color online) $M\text{−}H$ plots of the alloys at 7 K along with the fits (solid line) to the Eq. (4).

Figure 5

Inverse magnetic susceptibility as a function of temperature. Solid lines represent fits to the modified Curie-Weiss law.

Figure 6

(Color online) (a) Magnetoresistance as a function of applied field at 300 K. Solid lines represent fits to Eq. (6). (b) Magnetoresistance as a function of applied field at 5 K. Solid lines represent fits to the Eq. (6).

Figure 7

Magnetoresistance (at 50 kOe magnetic field) as a function of temperature of all compositions. Inset: Variation of magnetoresistance and fractional coefficient of SPM component at 5 K with Cr concentration $\left(x\right)$.

Figure 8

Magnetization and the estimated site disorder parameter (XRD) as a function of Cr concentration. Inset: Variation of transition temperature and magnetic moment with Cr concentration.