# Magnetism of monomer MnO and heterodimer FePt@MnO nanoparticles

## TL;DR

This study investigates the magnetic properties of MnO nanoparticles and heterodimers with FePt, revealing complex behaviors including exchange bias effects and superparamagnetic fluctuations, advancing understanding of nanoscale magnetic interactions.

## Contribution

It provides detailed experimental insights into the magnetic behavior of MnO nanoparticles and FePt@MnO heterodimers, highlighting the interplay of superparamagnetism and exchange bias effects.

## Key findings

- MnO nanoparticles show a magnetization peak at ~25K but no clear Néel temperature feature.
- Polarized neutron scattering confirms AF order vanishing at 118K.
- Heterodimers exhibit exchange bias stabilizing FePt magnetic moments.

## Abstract

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6-19nm). Using a combination of polarized neutron scattering and magnetometry we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero field cooled (ZFC) magnetization curves at low temperatures (~25K) but no feature around the N\'eel temperature at 118K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118K. Moreover, hysteresis curves measured at various temperatures reveal an exchange bias effect indicating a coupling of an AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields exclude a purely superparamagnetic (SPM) scenario. We conclude that the magnetic behavior of MnO particles can be explained by a superposition of SPM-like thermal fluctuations of the AF-N\'eel vector inside the AF core and a strong magnetic coupling to a ferrimagnetic Mn$_2$O$_3$ or Mn$_3$O$_4$ shell. In addition, we have studied heterodimer ('Janus') particles, where a FM FePt particle is attached to the AF MnO particle. Via the exchange bias effect, the magnetic moment of the FePt subunit is stabilized by the MnO.