Transition-metal-oxide/transition-metal nanocomposites,
such as
NiO/Ni, FeO/Fe, and CoO/Co, have been the subject of much recent investigation
(i) because of their potential applications and (ii) because they
are good model systems for studies of some effects on the nanoscale.
They are used, for example, as catalysts, fuel-cell electrodes, magnetic
memories, etc. When a nanocomposite is composed of both ferromagnetic
(FM) and antiferromagnetic (AFM) nanoparticles, interesting physical
properties can occur, such as the phenomenon of exchange bias (EB).
A Ni/NiO nanocomposite obtained by the thermal decomposition of nickel(II)
acetate tetrahydrate, Ni(CH3COO)2·4H2O, at 300 °C is composed of NiO (62%) and Ni (38%) with
crystallite sizes of 11 and 278 nm, respectively. We observed an increase
in the crystallite size for NiO and decrease of crystallite size for
Ni, a decrease in the microstrain for both and an increase in the
NiO phase content with thermal annealing in air, while high-energy
ball milling leads to a decrease of the crystallite size, an increase
in the size of the agglomerates, and microstrain as well as reduction,
NiO → Ni. The lattice parameters of the nanosized NiO and Ni
show a deviation from the value for the bulk counterparts as a consequence
of crystallite size reduction and the grain-surface relaxation effect.
The exchange bias found in a milled sample with particles of 10 nm
(NiO) and 11 nm (Ni) disappears for larger particles as a consequence
of a coupling-area decrease between the antiferromagnetic and ferromagnetic
particles. Due to reduction/oxidation (NiO ↔ Ni) and size as
well as surface-relaxation effects the saturation magnetization value
increases/decreases with milling/annealing, respectively. Having in
mind the effect of size on the exchange bias, coercivity, and magnetization
values, it is possible, by annealing/milling, to tailor the composition
and particle size and then control the exchange bias and improve the
other magnetic properties of the Ni/NiO.
