[Paper Review] Altermagnetic Polar Metallic phase in Ultra-Thin Epitaxially-Strained RuO2 Films

Seung Gyo Jeong, In Hyeok Choi+13 more|arXiv (Cornell University)|May 9, 2024

Magnetic and transport properties of perovskites and related materials12 citations

TL;DR

The paper establishes a phase diagram for RuO2/TiO2 (110) ultra-thin films, revealing an altermagnetic metallic polar phase in 2 nm strained films and identifying a magnetic phase transition near 500 K unique to ultrathin films.

ABSTRACT

Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in non-trivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic order parameter remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray and optical spectroscopies, we establish a phase diagram in hybrid molecular beam epitaxy-grown RuO2/TiO2 (110) films, mapping symmetries along with altermagnetic/electronic/structural phase transitions as functions of film thickness and temperature. This features a novel altermagnetic metallic polar phase in epitaxially-strained 2 nm films, extending the concept of multiferroicity to altermagnets. Such a clear signature of a magnetic phase transition at ~500 K is observed exclusively in ultrathin strained films, unlike in bulk RuO2 single crystals. These results demonstrate the potential of epitaxial heterostructure design to induce altermagnetism, paving the way for emergent novel phases with multifunctional properties.

Motivation & Objective

• Map symmetries and phase transitions in RuO2/TiO2 (110) films as a function of thickness and temperature.
• Identify signatures of altermagnetic order in transport, X-ray, and optical spectroscopy.
• Extend the concept of multiferroicity to altermagnetic systems via epitaxial strain.
• Demonstrate the potential of heterostructure design to induce altermagnetism.

Proposed method

• Combine theoretical analysis with electrical transport, X-ray spectroscopy, and optical spectroscopy on RuO2/TiO2 (110) films grown by hybrid molecular beam epitaxy.
• Systematically vary film thickness (notably down to 2 nm) and temperature to map phase behavior.
• Extract symmetry-related signatures of altermagnetic, electronic, and structural transitions.
• Identify a polar metallic altermagnetic phase in ultrathin films through experimental observables.

Experimental results

Research questions

• RQ1 What are the symmetry-allowed altermagnetic, electronic, and structural phases in RuO2/TiO2 (110) films as thickness and temperature vary?
• RQ2 Can epitaxial strain and extreme thinness stabilize an altermagnetic polar metallic phase not observed in bulk RuO2?
• RQ3 What signatures across transport, X-ray, and optical spectroscopy indicate a transition to altermagnetic order around 500 K in ultrathin films?
• RQ4 How does the observed phase diagram extend the concept of multiferroicity to altermagnetic materials?

Key findings

• A novel altermagnetic metallic polar phase is observed in 2 nm epitaxially strained RuO2 films.
• A clear magnetic phase transition signature appears near ~500 K only in ultrathin strained films, not in bulk RuO2 crystals.
• The study maps symmetries and phase transitions as functions of film thickness and temperature.
• Epitaxial heterostructure design can be used to induce altermagnetism in oxide films.
• The work combines theory with multiple spectroscopic and transport measurements to establish the phase diagram.