A Room-Temperature Ferrotoroidic Material Exhibiting Magnetic Semiconductor Properties with Superhigh Hole Mobility

The design and fabrication of room-temperature ferrotoroidic materials and magnetic semiconductors are recognized worldwide as a great challenge, and of both theoretical and practical importance in the field of condensed matter physics and information storage. Reported herein are ferrotoroidic crystal powder and film formed by supramolecular self-assembly based on uranyl and cyclodextrin, with the Curie temperature above room temperature. Experimental measurements and calculations demonstrate spontaneous vortex-like alignment of magnetic moments and furthermore a macroscopic long-range arrangement in the crystal, which breaks simultaneously space-inversion and time-reversal symmetries, exhibiting strong superexchange, spin-orbit coupling as well as anomalous Hall effect (AHE). The electrical measurements show the film with a superhigh carrier mobility of 3200 cm2V-1s-1 and a Hall resistivity as high as 0.32 mVA-1cm at room temperature. This work is expected to pave greatly the applied research on new-generation magnetoresistive random access memory (MRAM), especially as flexible material.

💡 Research Summary

The authors report the design, synthesis, and comprehensive characterization of a room‑temperature ferrotoroidic magnetic semiconductor based on the supramolecular self‑assembly of uranyl (UO₂²⁺) ions and γ‑cyclodextrin (γ‑CD). By spin‑coating a precursor solution onto silicon substrates, they fabricate uniform polycrystalline U₂ films that retain the tetragonal “sandwich‑type” coordination structure observed in bulk crystals. Subsequent photoreduction or electron‑beam irradiation converts a large fraction of U⁶⁺ to U⁵⁺ (UO₂⁺), producing the U₁@U₂ material in both powder and film forms.

Magnetic measurements (EPR, ZFC/FC, AC susceptibility, and M‑H loops) demonstrate that the reduced material exhibits robust ferromagnetic ordering with a Curie temperature above 300 K. The hysteresis observed at room temperature (coercivity ≈ 80 Oe) confirms the persistence of magnetic order. Second‑harmonic generation (SHG) experiments reveal a strong non‑centrosymmetric response (intensity ∝ P¹·⁹), indicating that space‑inversion symmetry is broken; together with the magnetic ordering, this confirms simultaneous breaking of time‑reversal and inversion symmetries—a hallmark of ferrotoroidicity.

Density‑functional theory (DFT) calculations incorporating spin‑orbit coupling (SOC constant ζ ≈ 2164 cm⁻¹) show that UO₂⁺ ions possess a large orbital contribution, and the 5f‑5f superexchange constant J ≈ 7.8 cm⁻¹ is sufficiently strong to sustain the high Curie temperature. The calculated magnetic coupling leads to a vortex‑like arrangement of magnetic moments, generating a toroidal moment (𝐓 = ½ ∑ 𝐫ᵢ × 𝐦ᵢ). The authors propose a five‑step mechanism: (1) SOC aligns the total angular momentum of uranium; (2) superexchange creates local toroidal moments; (3) toroidal moments couple along the one‑dimensional tubular channels; (4) long‑range ferrotoroidic domains form due to the chirality of γ‑CD; (5) domains merge into a macroscopic ferrotoroidic state.

Electrical characterization of Hall‑device patterned U₁@U₂ films reveals p‑type semiconductor behavior with an exceptionally high hole mobility of (3.2 ± 0.2) × 10³ cm² V⁻¹ s⁻¹, far exceeding that of conventional high‑mobility p‑type materials such as SiC, InSe, or black phosphorene. Field‑effect transistor (FET) transfer curves display clear linear regimes, and logarithmic analysis confirms band‑like transport. Moreover, anomalous Hall effect (AHE) measurements show a Hall resistivity of 0.32 mΩ·cm at room temperature under alternating ±10 mA currents, evidencing strong spin‑polarized charge transport.

The combination of (i) room‑temperature ferrotoroidic order, (ii) strong SOC and superexchange, (iii) simultaneous inversion‑ and time‑reversal‑symmetry breaking, (iv) ultra‑high hole mobility, and (v) pronounced AHE makes this material a unique platform for spintronic and memory applications. Its flexible thin‑film form factor further suggests suitability for next‑generation magnetoresistive random‑access memory (MRAM) and other flexible electronic devices. The work thus opens a new avenue for designing multifunctional magnetic semiconductors where ferroic orders, charge transport, and symmetry properties can be co‑engineered at ambient conditions.