Spin Filtering in Epitaxial Spinel Films with Nanoscale Phase Separation

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid State Chemistry

Background:

• Colossal magnetoresistance (MR) in oxides is often linked to coexisting ferromagnetic metallic and antiferromagnetic insulating phases.
• Existing models for MR in ferrites, like spin-polarized transport across antiphase boundaries, do not fully explain observed magnetic anomalies.

Purpose of the Study:

• To investigate the origin of large magnetoresistance in epitaxial spinel films (NiCo2O4-δ).
• To elucidate the role of phase separation and magnetic phase coexistence in achieving significant MR effects.

Main Methods:

• Epitaxial thin film deposition of NiCo2O4-δ.
• Characterization of magnetic properties and magnetoresistance.
• Development of a spin filtering model incorporating Zeeman effect and direct tunneling.

Main Results:

• Observed substantial magnetoresistance in NiCo2O4-δ spinel films, exceeding that of typical spinel ferrites.
• Attributed the MR to phase separation into coexisting ferrimagnetic metallic and ferrimagnetic insulating phases.
• Identified the magnetic insulating phase as crucial for spin filtering, enhancing the MR effect.

Conclusions:

• Phase separation in NiCo2O4-δ is the primary mechanism for its large magnetoresistance.
• The magnetic insulating phase acts as a spin filter, significantly contributing to the observed MR.
• A novel spin filtering model explains the MR behavior in these phase-separated spinel oxides.