Abstract

In this work, it is our goal to investigate the structural and magnetic properties of core/shell magnetic nanoparticles synthesized by inert gas condensation technique. For that purpose, Fe/FeO, Fe/FeO/PMMA, Ni/NiO/CoO, and NiFe ₂O₄ have been chosen to study exchange bias phenomenon that is observed in these systems.

Two sets (small and large) of Fe/FeO nanoparticles with different particle sizes, (6.0/1.5nm and 9.0/3.0nm) have been prepared and the magnetic properties in terms of temperature dependencies of exchange bias field (H _EB, horizontal shift of the hysteresis loops) and magnetic viscosity were investigated. Small particles have shown superparamagnetic behavior above Blocking Temperature, TB and exhibited 1574±25Oe exchange bias whereas the large particles had 277±25Oe. It has been observed that HEB is inversely proportional with the particle size and exponentially decreases and vanishes as the temperature increases up to T_B. Along with the horizontal shift, vertical shift of the hysteresis loops due to pinned interface spins has also been realized. Dispersion of 14nm Fe/FeO particles in a non-magnetic polymer PMMA in order to study interparticle interactions has revealed that the magnetic response is in general nonmonotonic as a function of particle concentration in the polymer. The nonmonotonic behavior is linked to the competition between the exchange and dipolar interactions one of which being dominant above/below a threshold concentration.

In order to synthesize core/shell nanoparticles composed of different metal and metal oxides rather than metal and its native oxide forming the core/shell, two techniques, resistive evaporation and laser ablation, have been combined in our inert gas condensation system. Condensation of evaporated Ni and laser ablated CoO allowed us to prepare core/shell particles. Structural analyses have revealed that Ni/CoO nanoparticles with a thin (∼1nm) NiO intermediate layer in the form of Ni/NiO/CoO can only be formed under certain conditions within the limitation of the system. Furthermore magnetic measurements have shown that the coupling between the thin NiO and CoO layers causes the growth of a new anisotropy that in turn enhances the exchange coupling between Ni and NiO resulting in larger H_EB (778Oe at 5K) as compared to a similar Ni/NiO (80Oe at 5K) structure.

Exchange bias is also observed from nanomagnetic systems where there is structural disorder due to size confinement. Nanosize ferrites manifest loop shifts as a result of strong coupling between the collinear core spins and spin-glass like disordered surface spins. Therefore using inert gas condensation we have synthesized NiFe₂O₄ nanoparticles with different sizes in the range of 15 to 50 nm. Loop shift is observed from the smallest particle size sample. Besides investigating the size dependent magnetic properties, attention is focused on the 15 nm sample that showed peculiar cooling field dependencies in terms of temperature dependent magnetization and magnetic relaxation at low temperatures. It is realized that magnetic response of small NiFe₂O₄ particles is controlled by different mechanisms in the certain region of temperature-field space, namely below 50K and ∼3-4T.