Disorder mediated fully compensated ferrimagnetic spin-gapless semiconducting behaviour in Cr3Al Heusler alloy

TL;DR

This work demonstrates that Cr$_3$Al can simultaneously host spin-gapless semiconductor transport and fully compensated ferrimagnetism within a fully A2-disordered Heusler structure, with a high Curie temperature near $T_C \\approx 773$ K. Using combined structural probes (SCXRD, synchrotron XRD, neutron diffraction), XMCD, magnetization, transport, and first-principles calculations with SQS disorder modeling, the authors show that Cr/Al site mixing does not destroy magnetic order but instead enables SGS via disorder-induced modification of the spin-up band gap. The ordered DO$_3$ phase is a magnetic semiconductor with finite spin gaps, while A2 disorder collapses the spin-up gap to near zero, yielding almost zero net moment ($|oldsymbol{ extmu}_ ext{tot}| \\approx 0.0072 \, oldsymbol{ extmu}_B/ ext{f.u.}$) and SGS transport; theory and experiment are in strong agreement. Overall, Cr$_3$Al emerges as a robust, disorder-tolerant platform for high-temperature spintronic devices that benefit from stray-field-free operation and spin-selective transport, with disorder engineering proposed as a design lever for future work.

Abstract

Spin-gapless semiconductors (SGSs) that simultaneously host fully compensated ferrimagnetism are highly sought for energy-efficient and stray-field-free spintronic technologies, yet their realization in chemically disordered systems has remained elusive. Here, we demonstrate that the binary Heusler alloy Cr3Al despite adopting a fully A2-disordered structure exhibits a rare coexistence of SGS transport and a fully compensated ferrimagnetic (FCF) ground state. Single-crystalline and polycrystalline Cr3Al samples were synthesized, and comprehensive structural analyses using single crystal XRD, synchrotron powder XRD, and neutron powder diffraction reveal complete Cr/Al site mixing. Remarkably, this chemical disorder does not disrupt magnetic order; instead, magnetization, X-ray magnetic circular dichroism (XMCD), and temperature-dependent neutron diffraction establish a robust compensated ferrimagnetic state with a vanishingly small ordered moment of 0.1(1) muB/f.u and a high Curie temperature of 773(2) K. Electrical and thermal transport measurements uncover clear SGS characteristics, including weak temperature-dependent conductivity, very low Seebeck coefficients, and electron-hole compensated transport. Hall measurements show unusual temperature-dependent carrier concentrations consistent with disorder-modified electronic states. First-principles calculations on an A2-disordered SQS structure reproduce the experimentally observed negligibly small magnetization (0.0072 muB/f.u) and reveal a vanishing spin-up band gap unambiguously supporting SGS behavior driven by chemical disorder. Our results identify Cr3Al as the first experimentally verified A2-disordered Heusler alloy exhibiting both fully compensated ferrimagnetism and spin-gapless semiconducting transport, positioning it as a robust and disorder-tolerant platform for next-generation, high-temperature spintronic devices.

Figures (13)

• Figure 1: Schematic density of states (DoS) for (i) HMF (ii) SSM (iii) SGS and (iv) fully compensated ferrimangetic (FCF) SGS, where $N_v\uparrow$ and $N_v\downarrow$ are the majority and minority spin valence electrons and $E_{\textit{F}}$ is the Fermi energy.
• Figure 2: (a) Reciprocal lattice reconstructions of the diffraction data of Cr$_3$Al single crystal. (b) As synthesized Cr$_3$Al single crystal. The Rietveld refinement of the room-temperature synchrotron XRD pattern of polycrystalline Cr$_3$Al sample, showing the fit with (c) FCC ($Fm\overline{3}m$) structure having A2-type disorder and (d) BCC ($Im\overline{3}m$) structure. The respective insets show the unit cells.
• Figure 3: Magnetic susceptibility ($\chi$) vs. temperature ($T$) of (a) polycrystal, (c) single crystal Cr$_3$Al from 300 to 900 K at 1000 Oe. The insets show the $T$-dependent magnetic susceptibility from 2 K to 300 K. (b) Magnetization ($M$) vs. magnetic field ($H$) of polycrystalline Cr$_3$Al at 2 K, 600 K and 900 K. The inset shows a zoomed-in view of the same. (d) $M$ vs. $H$ of single crystal at 2 K and 300 K. The inset shows a zoomed-in view of the same at 300 K, indicating the coercive field.
• Figure 4: (Top) XAS measured with left circularly polarized X-rays under opposite directions of magnetic field and (bottom) the corresponding XMCD at the Cr L$_2$ and L$_3$ edges (black line).
• Figure 5: Observed neutron powder pattern of Cr$_3$Al at 300 K with the Rietveld refinement fit of the nuclear and magnetic structures. The green markers of the Bragg reflections for the nuclear (top lines) and magnetic reflections (bottom lines), along with the difference curve are shown at the bottom. The inset shows the alignment of the Cr1(0, 0, 0), Cr2(0.25, 0.25, 0.25), Cr3(0.5, 0.5, 0.5), and Cr4 (0.75, 0.75, 0.75) magnetic moments along the crystallographic axis.