6533b872fe1ef96bd12d38a1

RESEARCH PRODUCT

Magnetism of monomer MnO and heterodimer FePt@MnO nanoparticles

subject

description

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6--19 nm). 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 $(\ensuremath{\sim}25$ K) but $no$ feature around the N\'eel temperature at 118 K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118 K. 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 magnetic coupling to a ferrimagnetic ${\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ or ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ 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.