Structure and magnetism in Fe-doped FeVSb and epitaxial Fe/FeVSb nanocomposite films

TL;DR

This paper addresses integrating magnetism into the semiconducting half-Heusler FeVSb and explores magnetic doping and epitaxial nanostructuring via Fe$_{1+x}$VSb films grown by MBE on MgO(001). It reports that for $x<0.1$, excess Fe dopes the FeVSb lattice to produce dilute ferromagnetism with $T_c \approx 5$ K, while for $x>0.1$ Fe precipitates form, creating Fe:FeVSb nanocomposites and enabling proximity-induced magnetism in FeVSb at $T_c \approx 20$ K; this is corroborated by XRD and STEM showing Fe/FeVSb interfaces. The study identifies two magnetic channels: intrinsic magnetism in FeVSb from proximity effects and strong magnetism from Fe nanoparticles, highlighting a tunable platform for magnetism in thermoelectric/spintronic materials. The work advances understanding of magnetic doping, epitaxial nanostructuring, and magnetic proximity effects in half-Heuslers, with potential implications for spin-dependent transport and thermoelectric performance.

Abstract

The combination of ferromagnetism and semiconducting behavior offers an avenue for realizing novel spintronics and spin-enhanced thermoelectrics. Here we demonstrate the synthesis of doped and nanocomposite half Heusler Fe$_{1+x}$VSb films by molecular beam epitaxy. For dilute excess Fe ($x < 0.1$), we observe a decrease in the Hall electron concentration and no secondary phases in X-ray diffraction, consistent with Fe doping into FeVSb. Magnetotransport measurements suggest weak ferromagnetism that onsets at a temperature of $T_{c} \approx$ 5K. For higher Fe content ($x > 0.1$), ferromagnetic Fe nanostructures precipitate from the semiconducting FeVSb matrix. The Fe/FeVSb interfaces are epitaxial, as observed by transmission electron microscopy and X-ray diffraction. Magnetotransport measurements suggest proximity-induced magnetism in the FeVSb, from the Fe/FeVSb interfaces, at an onset temperature of $T_{c} \approx$ 20K.

Figures (5)

• Figure 1: Structural evolution of epitaxial Fe$_{1+x}$VSb films by electron and x-ray diffraction. (a) RHEED pattern along the $<110>$ azimuth, showing strong streaky 2$\times$ reconstruction over all compositions studied. (b) Wide angle XRD (Cu $K\alpha$) showing the half-Heusler 00$l$ and Fe 002 reflections. Asterisks indicate the MgO substrate reflections. (c) High resolution scans of the FeVSb 004 reflections reveal the onset of a shoulder peak at composition $x = 0.14$, which we attribute to the 002 reflection of Fe (bcc). Shaded curves show the Gaussian fits. (d) Out of plane lattice parameter extracted from XRD as a function of excess Fe composition. Diamond and circle markers correspond to FeVSb and Fe respectively. Dotted lines show the lattice parameter of bulk FeVSb (half Heusler), and that of doubled body centered cubic Fe unit cells. Crystal structure models for FeVSb and Fe are shown. Black, red, orange and white spheres corresponds to Fe, V, Sb and interstitial respectively. For low $x$, excess Fe is expected to incorporate into the ($\frac{3}{4}, \frac{1}{4}, \frac{1}{4}$) tetrahedral interstitial sites of FeVSb (white spheres).
• Figure 2: Cross-sectional STEM image showing Fe precipitation in the Fe$_{1.46}$VSb film. (a) HAADF-STEM image of nanometer scale Fe nanoprecipitates embedded in the FeVSb matrix. (b) High resolution image of Fe/FeVSb interface. Individual atoms are identified by atomic models, confirming the half-Heusler and bcc crystal structures of the two phases.
• Figure 3: Transport measurements for films with varying Fe content. (a) Mobility $\mu$ and (b) electron concentration $n_{3d}$ as a function of excess Fe composition $x$ at 300K and at 2K.
• Figure 4: Magnetism from precipitated Fe nanoparticles, as measured by SQUID. (a) Magnetization $M(H)$ for films with varying excess Fe composition. The diamagnetic contribution from the substrate has been subtracted. $H$ was applied out of the sample plane ($H \parallel$ [001]). (b) Saturation magnetization, M$_{sat}$ as a function of $x$. The linear dependence of M$_{sat}$ on $X_{Fe}$ follows the Slater-Pauling curve for bcc Fe, as marked by the dotted line. Magnetization is expressed in units of Bohr magneton ($\mu_{B}$) per formula unit (f.u.) of FeVSb.
• Figure 5: Dilute magnetism and ferromagnetic proximity effect in the FeVSb matrix. (a-c) Magnetoresistance $[\rho_{xx}(H) - \rho_{xx}(0)]/\rho_{xx}(0)$ for Fe$_{1+x}$VSb samples with $x =$ 0.01, 0.05, and 0.37. (d) Width of the magnetoresistance hysteresis as a function of temperature. The $T_{c}$ for bulk Fe is from Ref. rosengaard1997finite.