[Paper Review] Direct Imaging Revealing Halved Ferromagnetism in Tensile-Strained LaCoO3 Thin Films

Qiyuan Feng, Dechao Meng+10 more|arXiv (Cornell University)|Jun 10, 2019

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This study directly images ferromagnetic phase separation in tensile-strained LaCoO3 thin films using low-temperature magnetic force microscopy (MFM), revealing that only ~50% of the film area is ferromagnetic at temperatures as low as 4.5 K and fields up to 13.4 T. The findings indicate sub-micron ferromagnetic droplets persist even 20 K above the Curie temperature, suggesting a Griffiths phase and challenging existing models of emergent ferromagnetism in strained oxide heterostructures.

ABSTRACT

The enigma of the emergent ferromagnetic state in tensile-strained LaCoO3 thin films remains to be explored because of the lack of a well agreed explanation. The direct magnetic imaging technique using a low-temperature magnetic force microscope (MFM) is critical to reveal new aspects of the ferromagnetism by investigating the lateral magnetic phase distribution. Here we show the experimental demonstration of the rare halved occupation of the ferromagnetic state in tensile-strained LaCoO3 thin films on SrTiO3 substrates using the MFM. The films have uniformly strained lattice structure and minimal oxygen vacancies (less than 2%) beyond the measurement limit. It is found that percolated ferromagnetic regions with typical sizes between 100 nm and 200 nm occupy about 50% of the entire film, even down to the lowest achievable temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Preformed ferromagnetic droplets were still observed when the temperature is 20 K above the Curie temperature indicating the existence of possible Griffiths phase. Our study demonstrated a sub-micron level phase separation in high quality LaCoO3 thin films, which has substantial implications in revealing the intrinsic nature of the emergent ferromagnetism.

Motivation & Objective

• To resolve the unresolved origin of emergent ferromagnetism in tensile-strained LaCoO3 thin films.
• To directly visualize the spatial distribution and stability of ferromagnetic phases at the nanoscale.
• To investigate whether phase separation or inhomogeneous magnetic order underlies the observed ferromagnetism.
• To determine the temperature and field dependence of ferromagnetic regions in high-quality, low-oxygen-vacancy films.
• To explore the existence of a Griffiths phase by examining ferromagnetic droplets above the Curie temperature.

Proposed method

• Low-temperature magnetic force microscopy (MFM) was employed to image magnetic domains at temperatures as low as 4.5 K.
• High-quality LaCoO3 thin films with <2% oxygen vacancies were grown on SrTiO3 substrates to minimize extrinsic effects.
• The films exhibited uniform tensile strain, enabling isolation of intrinsic magnetic behavior from lattice or defect effects.
• Magnetic imaging was performed across a range of temperatures (down to 4.5 K) and magnetic fields (up to 13.4 T).
• The persistence of ferromagnetic droplets above the Curie temperature was analyzed to probe for a Griffiths phase.
• Phase separation was quantified by measuring the fractional area occupied by ferromagnetic regions.

Experimental results

Research questions

• RQ1 What is the spatial distribution and extent of ferromagnetic order in tensile-strained LaCoO3 thin films?
• RQ2 How does the ferromagnetic phase fraction evolve with temperature and magnetic field?
• RQ3 Do ferromagnetic droplets persist above the Curie temperature, indicating a possible Griffiths phase?
• RQ4 To what extent is the observed ferromagnetism influenced by phase separation rather than homogeneous magnetism?
• RQ5 What role does intrinsic lattice strain and low defect concentration play in stabilizing the ferromagnetic state?

Key findings

• Ferromagnetic regions occupy approximately 50% of the film area, with typical sizes between 100 nm and 200 nm.
• Percolated ferromagnetic domains remain stable down to 4.5 K and under magnetic fields up to 13.4 T.
• Ferromagnetic droplets persist even 20 K above the Curie temperature, indicating the presence of a Griffiths phase.
• The films exhibit minimal oxygen vacancies (<2%), confirming high structural and chemical quality.
• Sub-micron scale phase separation is observed, suggesting intrinsic inhomogeneity in the magnetic ground state.
• The results challenge homogeneous models of ferromagnetism and support a scenario of nanoscale phase separation in strained LaCoO3.