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

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.

💡 Research Summary

This paper presents a comprehensive experimental and theoretical investigation of the binary Heusler alloy Cr3Al, revealing its exceptional properties as the first experimentally verified material to simultaneously exhibit a fully compensated ferrimagnetic (FCF) state and spin-gapless semiconductor (SGS) transport behavior within a fully chemically disordered A2-type structure.

The study began with the successful synthesis of both polycrystalline and, for the first time, single-crystalline Cr3Al samples. Detailed structural characterization using single-crystal X-ray diffraction (XRD), synchrotron powder XRD, and neutron powder diffraction unequivocally demonstrated that Cr3Al does not adopt the ordered DO3 structure typical of Heusler alloys. Instead, it crystallizes in a fully disordered A2-type structure (space group Fm-3m), where all atomic sites are randomly occupied by a mixture of Cr and Al atoms, with a composition close to the ideal 3:1 ratio.

Remarkably, this pronounced chemical disorder does not destroy magnetic order. Magnetization measurements revealed a high Curie temperature of approximately 773 K. However, the saturation magnetization was found to be vanishingly small (~0.001 μB per formula unit), hinting at a compensated state. X-ray magnetic circular dichroism (XMCD) confirmed the presence of element-specific magnetic moments on Cr atoms. Crucially, temperature-dependent neutron diffraction provided direct evidence of long-range magnetic order and allowed for the quantification of a very small net ordered moment of about 0.1 μB/f.u., establishing a robust fully compensated ferrimagnetic ground state where opposing magnetic sublattices nearly cancel each other out.

The electronic and thermal transport properties displayed clear signatures of a spin-gapless semiconductor. The electrical resistivity showed a weak, semiconducting-like temperature dependence. The Seebeck coefficient (thermopower) was found to be very low (absolute value < 10 μV/K) across a broad temperature range (2-300 K), a hallmark of SGS materials where electron and hole contributions nearly compensate. Hall effect measurements revealed an unusual temperature dependence of carrier concentrations, indicative of a complex, disorder-modified electronic structure with contributions from both electrons and holes.

First-principles density functional theory (DFT) calculations, employing the SCAN meta-GGA functional, provided crucial theoretical support. Calculations on an ordered DO3 structure predicted a large net magnetic moment, inconsistent with experiments. However, calculations performed on a specially designed 16-atom special quasirandom structure (SQS) that accurately models the A2 chemical disorder yielded results in striking agreement with observations: a negligibly small net magnetization (0.0072 μB/f.u.) and an electronic band structure where the spin-up channel exhibits a vanishing band gap at the Fermi level, unambiguously confirming SGS behavior. Band unfolding techniques further illustrated how chemical disorder smears the sharp band edges of the ordered phase, creating the characteristic SGS density of states.

In conclusion, this work identifies Cr3Al as a groundbreaking material where chemical A2-disorder mediates rather than hinders the coexistence of two highly desirable spintronic properties: fully compensated ferrimagnetism and spin-gapless semiconductivity. This discovery challenges the conventional paradigm that such advanced quantum states require atomic-scale order. It positions Cr3Al as a robust, disorder-tolerant, and high-temperature platform for next-generation spintronic devices that require high spin polarization without stray magnetic fields.