Weak itinerant ferromagnetism in Heusler type Fe2VAl0.95

Abstract: We report measurements of the magnetic, transport and thermal properties of the Heusler type compound Fe2VAl0.95. We show that while stoichiometric Fe2VAl is a non-magnetic semi-metal a 5% substitution on the Al-site with the 3d elements Fe and V atoms leads to a ferromagnetic ground state with a Curie temperature TC = 33+-3 K and a small ordered moment ms = 0.12 mB/Fe in Fe2VAl0.95. The reduced value of the ratio ms/mp = 0.08, where mp = 1.4 mB/Fe is the effective Curie-Weiss moment, together with the analysis of the magnetization data M(H,T), show magnetism is of itinerant nature. The specific heat shows an unusual temperature variation at low temperatures with an enhanced Sommerfeld coefficient, g = 12 mJK-2mol-1. The resistivity, r(T), is metallic and follows a power law behavior r(T) = r0+AT^n with n = 1.5 below TC. With applying pressure, TC decreases with the rate of (1/TC)(dTC /dP) = -0.061 GPa-1. We conclude substitution on the Al-site with Fe and V atoms results in itinerant ferromagnetism with a low carrier density.

• The paper demonstrates that a 5% substitution on the Al site transforms Fe2VAl into a weak ferromagnet with a Curie temperature of 33 ± 3 K and a magnetic moment of 0.12 μB per Fe.
• The magnetization data indicates itinerant ferromagnetism, with a reduced saturation-to-paramagnetic moment ratio of 0.08, confirming non-localized electron behavior.
• Comprehensive specific heat and resistivity measurements reveal enhanced electronic contributions and metallic T1.5 dependence below TC, highlighting tunable magnetic properties under pressure.

Overview of Weak Itinerant Ferromagnetism in Heusler-type Fe$_{2}$VAl$_{0.95}$

The paper explores the intriguing magnetic, transport, and thermal characteristics of the Heusler-type compound Fe$_{2}$VAl$_{0.95}$, focusing on the emergence of weak itinerant ferromagnetism due to minor compositional deviations in the Al site. The study employs comprehensive experimental techniques, including dc magnetization, electrical resistivity, and specific heat measurements under varying conditions such as temperature, pressure, and applied magnetic field, to elucidate the origin and behavior of ferromagnetism within this system.

Key Findings

1. Ferromagnetic Transition: The paper reveals that non-magnetic semi-metallic Fe$_{2}$VAl undergoes a significant transformation upon 5% substitution on the Al-site with 3d transition metals, resulting in a weak ferromagnetic state with a Curie temperature $T_C = 33 \pm 3$ K. The substitution introduces a small ordered magnetic moment of $\mu_s = 0.12 \mu_B/\text{Fe}$.
2. Itinerant Nature of Magnetism: The reduced saturation to paramagnetic moment ratio ($\mu_s/\mu_p = 0.08$, where $\mu_p = 1.4 \mu_B/\text{Fe}$) indicates that the ferromagnetism is itinerant rather than localized. The magnetization data further corroborates this interpretation.
3. Specific Heat and Resistivity: The research reports an enhanced Sommerfeld coefficient, $\gamma = 12 \, \text{mJ K}^{-2} \text{mol}^{-1}$, in the specific heat, implying significant electronic contributions. The resistivity follows a metallic power-law behavior of approximately $T^{1.5}$ below $T_C$, displaying non-Fermi liquid characteristics.
4. Pressure Dependence: Application of pressure results in a decrease in the Curie temperature with a coefficient of $(1/T_C)(dT_C/dP) = -0.061 \, \text{GPa}^{-1}$, emphasizing the sensitivity of ferromagnetic ordering to external pressure.

Implications and Future Work

The findings have significant implications for understanding the mechanisms underlying itinerant ferromagnetism in transition metal compounds, particularly those nearing quantum critical points. The study’s demonstration of ferromagnetic ordering through a modest site substitution opens avenues for tuning magnetic properties via compositional engineering, potentially informing the design of magnetic materials for technological applications. Furthermore, the revelation of a non-Fermi liquid state in this low-carrier-density system parallels behaviors observed in heavy fermion systems, providing a fertile ground for exploring correlated electron behavior beyond traditional models.

Future research could explore temperature and magnetic field evolution of the density of states at the Fermi surface in Fe$_{2}$VAl$_{0.95}$, employing techniques such as ARPES or neutron scattering. There is also a promising potential for examining these phase transitions under extreme conditions to reach new magnetic phases or critical points, sharpening our theoretical models for itinerant electron magnetism. Additionally, exploring possible applications in thermoelectric devices through intentional doping and heat treatment merits investigation, given the observed enhancements in thermoelectric power.

In conclusion, the detailed experimental insights offered by the paper present a vital step forward in our understanding of itinerant ferromagnetism, offering pathways for exploiting such magnetic transitions in applied physics and material science.