[Paper Review] Designing Spin-driven Multiferroics in Altermagnets

Rongxing Cao, Ruizhi Dong+2 more|arXiv (Cornell University)|Dec 29, 2024

Multiferroics and related materials4 citations

TL;DR

This paper proposes that altermagnets enable strong spin-driven multiferroicity via a Heisenberg-like exchange striction mechanism, eliminating reliance on spin-orbit coupling. First-principles simulations show that LiMnO₂ and strained RuF₄ achieve spontaneous polarization exceeding 1.0 μC/cm² and magnetoelectric coupling one to two orders of magnitude stronger than conventional or spin-orbit-coupled multiferroics.

ABSTRACT

Spin-driven multiferroics exhibit strong magnetoelectric coupling, with notable polarization changes under a magnetic field, but these effects are usually limited to high-Z magnetic insulators with low electronic polarization. In this work, we introduce altermagnets as a promising platform for achieving strong magnetoelectric coupling in low-Z systems with substantial polarization. This large polarization arises from a design principle that utilizes the Heisenberg-like exchange striction mechanism, eliminating the reliance on spin-orbit coupling (SOC). This approach enables the Kramers-degenerate antiferromagnetic phase derived from altermagnetic insulators to achieve substantial polarization without spin splitting, providing a flexible platform for regulating spin-splitting phenomena. Through first-principles simulations and an effective Landau-Ginzburg Hamiltonian, we demonstrate that materials in the LiMnO2 family and strained RuF4 family can achieve polarization values exceeding 1.0 μC/cm2, an order of magnitude larger than those found in SOC-driven multiferroics. Moreover, their magnetoelectric coupling is one to two orders of magnitude stronger than that observed in conventional multiferroics and those driven by SOC.

Motivation & Objective

• To identify new platforms for strong magnetoelectric coupling in low-atomic-number (low-Z) materials without relying on spin-orbit coupling (SOC).
• To explore the potential of altermagnets as hosts for spin-driven multiferroics through a symmetry-based design principle.
• To demonstrate that the Kramers-degenerate antiferromagnetic phase in altermagnetic systems can host large spontaneous polarization via exchange striction.
• To achieve magnetoelectric coupling significantly stronger than in conventional or SOC-driven multiferroics.

Proposed method

• Employ first-principles density functional theory (DFT) calculations to investigate electronic structure and polarization in altermagnetic materials.
• Use an effective Landau-Ginzburg Hamiltonian to model the phase transitions and multiferroic order parameters.
• Design a spin structure in a 2×2 supercell with C₂||t symmetry to induce Kramers-degenerate antiferromagnetic order with broken inversion symmetry.
• Apply strain and external electric fields to tune the transition between altermagnetic and Kramers-degenerate multiferroic phases.
• Analyze the magnetoelectric coupling constant by computing the response of polarization to magnetic field changes.
• Focus on LiMnO₂ and strained RuF₄ as prototype systems due to their low symmetry and commensurate spin order.

Figure 1: Atomic structure showing only the magnetic atoms, with the spin pattern in (a) representing the altermagnetic phase and in (b) representing the Kramers-degenerate AFM phase. (c) Spin pattern along the y-direction in (b) showing a Peierls-like phase transition. Phase diagrams for the Kramer

Experimental results

Research questions

• RQ1 Can altermagnets support large spontaneous polarization through exchange striction without spin-orbit coupling?
• RQ2 What is the magnitude of magnetoelectric coupling in altermagnetic multiferroics compared to conventional and SOC-driven systems?
• RQ3 Can strain or electric fields induce a transition between altermagnetic and Kramers-degenerate multiferroic phases?
• RQ4 Why is the Kramers-degenerate phase energetically unfavorable in intrinsic altermagnets like MnO₂ and MnF₂?
• RQ5 How does the exchange striction mechanism in altermagnets differ from the Dzyaloshinskii–Moriya mechanism in terms of polarization and coupling strength?

Key findings

• LiMnO₂ and strained RuF₄ achieve spontaneous polarization exceeding 1.0 μC/cm², an order of magnitude larger than in SOC-driven multiferroics.
• The magnetoelectric coupling constant in these materials is one to two orders of magnitude stronger than in conventional multiferroics like BiFeO₃ and other SOC-driven systems.
• The Kramers-degenerate multiferroic phase in altermagnets exhibits a polarization of 0.25 μC/cm² in the intrinsic MnF₂ system, despite being higher in energy than the ground-state altermagnetic phase.
• Strain or compression up to 15% fails to stabilize the Kramers-degenerate phase as ground state, indicating that electric fields are more effective for phase switching.
• The exchange striction mechanism in altermagnets provides a robust, SOC-independent pathway to large polarization and strong magnetoelectric coupling.
• The phase transition between altermagnetic and Kramers-degenerate multiferroic states can be electrically controlled, enabling tunable multiferroic functionality.

Figure 2: (a) Band structure of the Kramers-degenerate AFM phase and (b) the altermagnetic phase. The green-highlighted plane in (b) indicates the spin-degenerate plane of the altermagnet. (c) Phase diagram of LiMnO 2 , showing the Kramers-degenerate AFM (K), altermagnetic (A), and ferromagnetic (F)