Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid State Chemistry

Background:

• Manganese oxide (MnO) exhibits interesting magnetic and electronic properties.
• Strain engineering is a powerful tool to tune material properties.
• Understanding phonon behavior in strained thin films is crucial for device applications.

Purpose of the Study:

• To investigate the effect of biaxial and anisotropic compressive strain on MnO thin films.
• To analyze the influence of strain on phonon frequencies and magnetic-exchange interactions.
• To explore strain-induced structural phase transitions in MnO.

Main Methods:

• Pulsed laser deposition (PLD) for growing MnO thin films of varying thicknesses and strains on MgO substrates.
• X-ray diffraction (XRD) for structural characterization and strain analysis.
• Infrared reflectance spectroscopy for probing phonon behavior and magnetic-exchange interactions.

Main Results:

• Homogenous biaxial compressive strain in (001)-oriented films increases with reduced thickness, leading to enhanced phonon frequencies in the paramagnetic phase.
• Phonon splitting below the Néel temperature (T_N) is 20% larger in strained MnO films compared to bulk MnO.
• Anisotropic in-plane compressive strain in (110)-oriented films results in significant phonon splitting even at room temperature.
• Evidence of a spin-order-induced structural phase transition from tetragonal to a lower symmetry phase observed in (110)-oriented films.
• Total phonon splitting of 55 cm^-1 in (110)-oriented films, more than double that of bulk MnO.

Conclusions:

• Compressive strain enhances magnetic-exchange coupling in MnO thin films, supporting theoretical predictions.
• Strain engineering offers a viable route to tune the magnetic and structural properties of MnO.
• The observed phonon splitting provides insights into spin-order-induced phase transitions and magnetic interactions.