Abstract
The electron spin resonance (ESR) spectra of antiferromagnetic nanoparticle NiO specimens have been investigated as a function of temperature at x-band (microwave) frequencies. Below the nominal Néel temperature, the x-band resonances arising from the bulk antiferromagnets, including NiO particles with diameters greater than 100
Å, all vanish due to the emergence of large molecular exchange fields. The ESR resonance signals of 60
Å antiferromagnetic nanoparticles, however, persist to the lowest temperatures. These nanoparticle resonance lines shift to lower fields rapidly as the temperature is decreased, while the lineshapes broaden and distort.