Tunable polar distortions and magnetism in Gd$_x$La$_{1-x}$PtSb epitaxial films

Hexagonal $ABC$ intermetallics are predicted to have tunable ferroelectric, topological, and magnetic properties as a function of the polar buckling of $BC$ atomic planes. We report the impact of isovalent lanthanide substitution on the buckling, structural phase transitions, and electronic and magnetic properties of Gd$x$La${1-x}$PtSb films grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates. The Gd$x$La${1-x}$PtSb films form a solid solution from x = 0 to 1 and retain the polar hexagonal structure ($P6_3 mc$) out to $x \leq 0.95$. With increasing $x$, the PtSb buckling increases and the out of plane lattice constant $c$ decreases due to the lanthanide contraction. While hexagonal LaPtSb is a highly conductive polar metal, the carrier density decreases with $x$ until an abrupt phase transition to a zero band overlap semimetal is found for cubic GdPtSb at $x=1$. The magnetic susceptibility peaks at small but finite $x$, which we attribute to Ruderman Kittel Kasuya Yosida (RKKY) coupling between localized $4f$ moments, whose concentration increases with $x$, and free carriers that decrease with $x$. Samples with $x\geq 0.3$ show antiferromagnetic Curie-Weiss behavior and a Neel temperature that increases with $x$. The Gd$x$La${1-x}$PtSb system provides opportunities to dramatically alter the polar buckling and concentration of local $4f$ moments, for tuning chiral spin textures and topological phases.

💡 Research Summary

In this work the authors investigate how isovalent substitution of Gd for La in the hexagonal‑type intermetallic GdₓLa₁₋ₓPtSb (0 ≤ x ≤ 1) can be used as a “chemical pressure” knob to continuously tune the polar buckling of the Pt–Sb layers, the electronic band structure, and the magnetic interactions of the system. Epitaxial films were grown by molecular‑beam epitaxy on c‑plane sapphire, and a comprehensive suite of structural, transport, and magnetic measurements was combined with density‑functional‑theory (DFT) calculations.

Structural analysis (X‑ray diffraction, φ‑scans, HAADF‑STEM) shows that the films retain the polar hexagonal LiGaGe‑type space group P6₃mc up to x ≈ 0.95, beyond which the end‑member GdPtSb adopts the cubic half‑Heusler (F ¯43 m) structure. As Gd content increases, the out‑of‑plane lattice constant c contracts from 8.37 Å (x = 0) to 7.53 Å (x = 1) because of the lanthanide contraction, and the Pt–Sb buckling amplitude grows from ~35 pm to ~78 pm (DFT predicts a factor‑of‑two increase; STEM measures ~40 % smaller values, likely due to strain relaxation and defects). The buckling increase is directly linked to a reduction of the interlayer spacing and stronger Pt–Sb bonding.

Electronic transport measurements reveal metallic temperature dependence (dρ/dT > 0) for all compositions, but the resistivity rises sharply with Gd substitution, reflecting a systematic decrease in hole carrier density. Hall effect data show a hole‑dominated conduction with carrier concentration dropping by roughly an order of magnitude from x = 0 to x ≈ 0.9. This trend is attributed to the enhanced buckling, which narrows the bandwidth and reduces the band overlap at the Fermi level. DFT band structures corroborate this picture: LaPtSb is a semimetal with ~1 eV band overlap, while the hypothetical hexagonal GdPtSb shows a much smaller overlap; the real cubic GdPtSb (x = 1) is a zero‑overlap semimetal.

Magnetically, LaPtSb is diamagnetic. Introducing Gd creates localized 4f⁷ moments. For low Gd concentrations (0.1 ≤ x ≤ 0.5) the magnetization versus field curves are nonlinear and resemble a tanh‑shaped super‑paramagnetic response, suggesting dilute, weakly interacting Gd moments or small clusters. At higher Gd content (x ≥ 0.7) the response becomes linear and positive, and the Curie‑Weiss analysis yields a negative Weiss temperature, indicating antiferromagnetic (AFM) correlations. The Néel temperature extracted from the kink in 1/χ(T) rises monotonically with x, reaching ≈15 K for the Gd‑rich films.

The magnetic susceptibility χ shows a non‑monotonic dependence on x: it rises sharply from the diamagnetic LaPtSb, peaks around x ≈ 0.1–0.3, and then declines for larger x. The authors explain this behavior within an RKKY framework: the RKKY exchange strength is proportional to both the concentration of localized 4f moments (which increases with x) and the density of itinerant carriers (which decreases because of buckling). Consequently, the product of these two factors maximizes at an intermediate Gd concentration, giving the observed χ peak. For x > 0.5 the increased 4f density and reduced carrier density drive the RKKY interaction toward antiferromagnetic coupling, consistent with the observed AFM Curie‑Weiss behavior and the linear M(H) curves.

Additional observations include a crossover from weak positive quadratic magnetoresistance at low x to increasingly negative magnetoresistance at higher x. The authors suggest that the negative MR may arise from field‑induced alignment of Gd moments, which reduces spin‑dependent scattering of the remaining carriers. They also note that a chiral‑anomaly contribution cannot be excluded, given theoretical predictions of Weyl points in buckled AB C compounds, but further angular‑dependent studies are required.

Overall, the study demonstrates that (i) chemical substitution provides a robust, continuous handle on polar buckling, (ii) buckling directly modulates the electronic structure and carrier density, and (iii) the interplay between buckling‑controlled carriers and Gd‑derived 4f moments yields a tunable RKKY exchange that can be switched from ferromagnetic‑like super‑paramagnetism to antiferromagnetism. This dual‑parameter tuning platform opens pathways for engineering chiral spin textures, exploring topological semimetal phases, and realizing polar‑metal or hyperferroelectric behavior in 4f‑containing intermetallics.