Low magnetic moment and unconventional magneto-transport in half-Heusler alloy CoVGe

In the present work, we experimentally realize CoVGe for the first time and investigate its structural, magnetic, and transport properties, supported by theoretical calculations. The material crystallizes in a cubic structure and exhibits a very low magnetic moment of 0.13 μB per formula unit at 5 K. The temperature dependence of electrical resistivity suggests half-metallic behaviour. Magnetoresistance shows a positive, non-saturating linear field dependence at low temperature that gradually weakens with increasing temperature. The combination of low magnetic moment and unusual magnetotransport behaviour positions CoVGe as a promising platform for exploring spin-dependent transport in Heusler-based materials.

💡 Research Summary

In this work the authors report the first experimental synthesis of the half‑Heusler compound CoVGe and present a comprehensive investigation of its structural, magnetic, transport, and electronic properties, complemented by first‑principles calculations. Polycrystalline CoVGe was prepared by arc‑melting high‑purity Co, V and Ge under an argon atmosphere, followed by annealing at 850 °C for seven days and water‑quench. Powder X‑ray diffraction (Cu Kα) and Rietveld refinement confirm that the material crystallizes in the non‑centrosymmetric cubic C1b structure (space group 216) with a lattice constant a = 5.88 Å. The presence of (111) and (200) super‑lattice reflections demonstrates a high degree of atomic ordering, although weak impurity peaks are also observed.

Magnetic measurements performed with a vibrating‑sample magnetometer reveal a soft ferromagnetic transition at a Curie temperature Tc ≈ 43 K, identified from the derivative of the zero‑field‑cooled (ZFC) magnetization curve. Below Tc the M‑H loops are S‑shaped with a small hysteresis, while above Tc the response is linear and paramagnetic. The most striking result is the extremely low saturation moment of 0.13 μB per formula unit at 5 K, only slightly above the zero moment predicted by the Slater–Pauling rule for an 18‑valence‑electron half‑metal. This low net magnetization is highly desirable for spin‑tronic devices because it minimizes stray dipolar fields while still allowing spin‑polarized transport.

Electrical resistivity was measured from 2 K to 300 K in zero field. The material shows metallic behaviour (ρ increasing with temperature) with a residual‑resistivity ratio RRR ≈ 1.6, indicating modest crystallinity. In the low‑temperature range (2–43 K) the data are well described by ρ = ρ0 + ATⁿ with an exponent n = 1.4, deviating from the T² dependence typical of conventional ferromagnets where electron‑magnon scattering dominates. This reduced exponent signals a suppression of single‑magnon scattering, consistent with a half‑metallic scenario where the minority‑spin channel is gapped at the Fermi level. Above 43 K the resistivity follows ρ = ρ0 + BT + CT², indicating that electron‑phonon scattering (linear term) dominates, with a smaller quadratic contribution from magnon scattering.

Hall effect measurements show a clear anomalous Hall component only at 5 K, reflecting the ferromagnetic order, while at 100 K, 200 K and 300 K the Hall resistivity is linear in field, characteristic of ordinary Hall response in the paramagnetic regime. From the high‑field slope at 5 K the carrier concentration is estimated to be on the order of 10²¹ cm⁻³, comparable to other half‑metallic Heuslers. The anomalous Hall coefficient at 5 K is about 0.46 μΩ·cm.

Magnetoresistance (MR) was recorded at several temperatures with the magnetic field applied perpendicular to the current direction. At 5 K a pronounced, non‑saturating, positive linear magnetoresistance (LPMR) is observed up to ±5 T. The linear field dependence contrasts with the quadratic MR expected from classical Lorentz‑force orbital effects and with the negative MR typical of conventional ferromagnets where spin‑disorder scattering is suppressed by field. The authors attribute the LPMR to an intrinsic electronic structure featuring a near‑gapless or gapless band crossing close to the Fermi level, a situation that has been reported in other Heusler compounds (e.g., Fe₂CoSi, Mn₂CoAl). As temperature increases the linear MR rapidly diminishes and becomes negligible above ~100 K; at 300 K a weak negative MR reappears, consistent with ordinary spin‑disorder scattering.

First‑principles density‑functional calculations were performed using VASP with the GGA‑PBE functional, employing the experimentally determined lattice constant. The calculated density of states (DOS) shows a metallic majority‑spin (↑) channel with finite DOS at the Fermi energy, while the minority‑spin (↓) channel exhibits a pseudogap‑like suppression but retains a small finite DOS at EF. Thus, CoVGe is not a perfect half‑metal with a full gap in one spin channel; rather it is a “nearly half‑metallic” system where the minority channel is almost, but not completely, gapped. The authors note that previous theoretical work using a larger relaxed lattice constant (6.30 Å) predicted a full gap in the minority channel, indicating that the electronic structure is highly sensitive to lattice parameter and atomic ordering. The presence of a pseudogap in the ↓ channel can rationalize the observed low magnetic moment (partial compensation) and the linear MR (band‑crossing or near‑gapless dispersion).

In summary, CoVGe combines (i) a well‑ordered cubic half‑Heusler structure, (ii) an exceptionally low net magnetic moment of 0.13 μB/f.u., (iii) transport signatures of half‑metallicity (non‑quadratic low‑T resistivity) and a distinctive low‑temperature linear positive MR, and (iv) a calculated electronic structure featuring a metallic majority spin band and a pseudogapped minority spin band. These attributes make CoVGe a promising platform for spin‑tronic applications that require high spin polarization with minimal stray fields, and they open avenues for exploring unconventional charge transport arising from near‑gapless band topology in Heusler‑type materials.