Size-induced exchange bias in single-phase CoO nanoparticles

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Exchange bias (EB) in nanoparticles is crucial for various applications.
• Understanding size-dependent magnetic phase transitions is key to controlling EB.
• Single-phase materials exhibiting multiple magnetic phases offer unique properties.

Purpose of the Study:

• To demonstrate and investigate exchange bias (EB) in single-phase cobalt oxide (CoO) nanoparticles.
• To explore the emergence of distinct magnetic phases as a function of crystallite size.
• To elucidate the role of surface spin disorder in the observed EB phenomenon.

Main Methods:

• Synthesis of CoO nanoparticles with controlled crystallite sizes (34.6 nm and 10.8 nm).
• Magnetic measurements including zero-field cooled (ZFC) and field-cooled (FC) magnetization.
• Analysis of ac-susceptibility, thermoremanent magnetization (TRM), and iso-thermoremanent magnetization (IRM).

Main Results:

• Two magnetic phases emerge in CoO nanoparticles as size decreases, influencing the Néel temperature (T_N).
• Magnetization irreversibility observed above T_N in smaller nanoparticles indicates an additional magnetic phase.
• Evidence of a core-shell structure with an antiferromagnetic core and a disordered ferromagnetic shell.

Conclusions:

• Exchange bias in these CoO nanoparticles originates from the exchange coupling between an antiferromagnetic core and a disordered surface spin shell.
• Size reduction leads to unconventional surface spin dynamics and the absence of a true paramagnetic regime.
• The tunable EB in CoO nanoparticles is attributed to the interplay between core antiferromagnetism and shell spin disorder.