Engineering altermagnetic symmetry to enable anomalous Hall response in Cr$_{1-x}$Mn$_x$Sb

Altermagnets are a promising class of materials for spintronic applications. However, compounds that simultaneously combine the symmetry required to support an anomalous Hall effect with good metallic conductivity and magnetic ordering temperatures well above room temperature remain elusive. Here, we demonstrate that partial substitution of Cr by Mn in epitaxial CrSb(100) thin films provides a viable route to engineer the combined structural and magnetic symmetry necessary to enable an otherwise symmetry-forbidden anomalous Hall effect. By systematically exploring the magnetic phase diagram Cr${1-x}$Mn${x}$Sb thin films, we identify a pronounced anomalous Hall effect in Cr${0.75}$Mn${0.25}$Sb. Guided by Landau theory, we model the field-driven reorientation of the Néel vector and the resulting anomalous Hall response, achieving good qualitative agreement with the experimental observations.

💡 Research Summary

The authors address a central challenge in the emerging field of altermagnetism: how to combine the large altermagnetic spin splitting and high magnetic ordering temperature of CrSb with the symmetry conditions required for an anomalous Hall effect (AHE). In pristine CrSb the Néel vector points along the crystallographic c‑axis, a direction that preserves the magnetic‑space symmetry forbidding an AHE despite strong spin‑orbit coupling. By partially substituting Cr with Mn, the authors engineer the magnetocrystalline anisotropy and break the restrictive symmetry, enabling a field‑driven reorientation of the Néel vector into a configuration that allows a Hall response.

Epitaxial Cr₁₋ₓMnₓSb(100) thin films were grown on GaAs(110) by DC sputtering. Three compositions (x ≈ 0.13, 0.25, 0.38) were characterized by TEM‑EDX, temperature‑dependent magnetometry, and anisotropic magnetoresistance (AMR). The magnetic phase diagram reproduces that of bulk single crystals, confirming that the thin films retain the same magnetic structures. For x = 0.25 the Néel vector lies in the film plane at zero field, as inferred from a two‑fold AMR symmetry caused by epitaxial strain. When a magnetic field exceeding ~5 T is applied with a slight out‑of‑plane tilt (≈ 2°), the Néel vector undergoes a spin‑flop‑like transition, acquiring a finite z‑component.

Hall measurements at 300 K reveal a pronounced, nonlinear Hall voltage with two hysteresis loops in the 3–5 T range, only for the 0.25 composition and only when the field is tilted. This signal is absent in the other compositions and in the parent CrSb, ruling out contributions from ferromagnetic impurities or strain‑induced ordinary Hall effects. The authors therefore attribute the observed nonlinearity to an intrinsic AHE arising from the altermagnetic order.

To rationalize the data, a phenomenological Landau free‑energy model is constructed. The free energy includes an exchange term enforcing antiparallel sublattice magnetizations, a Zeeman term coupling the net magnetization to the external field, and Dzyaloshinskii–Moriya interaction (DMI) terms. One DMI term respects the hexagonal symmetry of the parent lattice and generates a twelve‑fold degeneracy of Néel‑vector orientations; a second, strain‑induced DMI lifts this to four energetically equivalent minima. Energy landscapes projected onto a unit sphere show that at zero field the four minima are equally populated, yielding zero net Hall response. As the field increases, the minima shift, and at ~5 T the system preferentially occupies a state with a positive z‑component of the Néel vector, producing the observed AHE. The Hall resistivity is expressed as ρ_AHE = α Q_AM n_z + β m_y, where Q_AM is the altermagnetic order parameter, n_z the Néel‑vector z‑component, and m_y a small field‑induced magnetization component. Numerical evaluation of this expression reproduces the sign change and hysteresis seen experimentally.

The work demonstrates three key advances: (i) symmetry engineering via Mn substitution can unlock an AHE in a high‑temperature altermagnet; (ii) epitaxial strain and DMI critically control domain selection and Néel‑vector reorientation; (iii) the combination of metallic conductivity, large altermagnetic spin splitting, and an intrinsic AHE is achievable above room temperature. These findings open a pathway toward altermagnetic spin‑orbit torque devices and Hall‑based readout schemes, though further work is needed to achieve deterministic electric control of the Néel vector and to integrate such films into functional heterostructures.