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Surface spin-glass freezing in interacting core-shell NiO nanoparticles.

E Winkler¹, R D Zysler, M Vasquez Mansilla

¹Centro Atómico Bariloche, CNEA-CONICET, 8400 S C de Bariloche, RN, Argentina.

|August 10, 2011

Summary

This study reveals that 3 nm NiO nanoparticles exhibit a core-shell structure with an ordered antiferromagnetic core and a disordered surface shell. Surface spins freeze into a spin-glass-like state at low temperatures, enhancing magnetic anisotropy.

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Area of Science:

Materials Science
Condensed Matter Physics
Nanotechnology

Background:

Nickel oxide (NiO) nanoparticles exhibit unique magnetic properties due to their nanoscale dimensions.
Understanding the magnetic behavior of core-shell structures in nanoparticles is crucial for their applications.
Surface effects significantly influence the overall magnetic response of nanoparticles.

Purpose of the Study:

To investigate the magnetic properties of ~3 nm NiO nanoparticles.
To elucidate the core-shell magnetic structure and the behavior of surface spins.
To analyze the temperature-dependent magnetization and AC susceptibility.

Main Methods:

Magnetization measurements, including zero-field-cooled (M(ZFC)) magnetization.
AC susceptibility measurements (in-phase component χ').
Monte Carlo simulations of core/shell antiferromagnetic particles.

Main Results:

NiO nanoparticles display an antiferromagnetically ordered core with an uncompensated magnetic moment and a magnetically disordered surface shell.
Core magnetic moments block progressively around 70 K, while surface spins freeze in a spin-glass-like state at ~17 K.
Low-temperature surface spin freezing is accompanied by enhanced magnetic anisotropy, increased coercivity, and high-field susceptibility.

Conclusions:

The core-shell model accurately describes the magnetic behavior of these NiO nanoparticles.
Surface spin dynamics differ significantly from the core, exhibiting spin-glass-like freezing.
Interparticle interactions influence blocking temperatures and magnetization dynamics.