Finite Size Confined Magnetic Phases in Antiferromagnetic NiO Nanoparticles
Ashish Chhaganlal Gandhi1*, Jauyn Grace Lin1
1Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
* presenting author:Ashish Gandhi, email:acg.gandhi@gmail.com
In recent years, finite size effects on magnetic, optical and electrical properties of antiferromagnetic (AF) NiO nanoparticles (NPs) have gained enormous attention in both fundamental and applied research owing to their peculiar physical and chemical properties. At nanoscale, its property varied depending on sample size and synthesis methods in association with the modifications of morphology, surface defects and cation/anion vacancies. Among several factors, nickel and oxygen vacancies play key roles for the changes of atomic structures, electrical, optical and magnetic properties. For an in-depth understanding, the critical characterization of nanostructured NiO is crucial. I will present my recent findings on a series of NiO NPs (4 to 80 nm) which are characterized by advance probes such as synchrotron radiation X-ray diffraction, Raman spectroscopy, static and dynamic (ferromagnetic resonance, FMR) magnetic techniques. The size of the NiO NPs during synthesis was controlled by a single annealing temperature parameter. The ferromagnetic (FM) Ni phase only exists in NiO NPs with the size ranging from 8 to 20 nm, attributed to the oxygen vacancies in core structure. A considerable exchange bias of ~ 100 Oe is observed at 50 K in 8 nm NPs ; but it drops abruptly with increasing temperature and vanishes above 150 K, in association with the reduction of frozen spins. FMR data indicate a strong interaction between FM and AF phases below 150 K, consistent with the picture of isolated FM clusters in AF matrix. Our findings provide important information for controlling the magnetic properties of NiO NPs.

References:
1A. C. Gandhi et. al., Mate. Res. Express 2016, 3 (3), 035017. 2A. C. Gandhi et. al., J. Magn. Magn. Mater. 2017, 424, 221-225.

*Corresponding author: jglin@ntu.edu.tw (J.G.L.)

Acknowledgement: This work is partly supported by the National Taiwan University with the project no. 104R4000 (A.C.G.) and partly supported by Ministry of Science and Technology at Taiwan via the project number MOST 102-2112-M-002-012-MY3 (J.G.L.).


Keywords: NiO, FMR, Local field, Ni cluster, Frozen spins, Defects , Exchange bias