Temperature dependence of magnetic resonance in NiO nanoparticles

Abstract

Magnetic resonance measurements of different size NiO nanoparticles were performed at frequencies below 33 cm−1 and in the temperature range 300–500 K. The samples were earlier investigated by superconducting quantum interference device magnetometry and electron spin resonance spectrometry. The spectra were scanned by changing the sample temperature at fixed frequencies. At room temperature, resonance frequency for bulk NiO is 36.5 cm−1. At higher temperatures, the magnetic resonance frequency becomes lower, and at the Néel temperature goes to zero. For the 435 Å nanoparticles, we detected only one resonance peak. Extrapolation of the dependence to zero frequency gives a Néel temperature of 492 K. For 57 Å NiO, we observed two different peaks. One of them was at the same place as for 435 Å NiO, and another one was at higher temperatures. There exists several mechanisms which determine the magnetic structure of NiO nanoparticles, and, respectively, its resonance spectra. We believe that the magnetic resonance in 435 Å NiO corresponds to a bulklike structure, and the detection of two separate peaks in 57 Å NiO by a “size effect,” which is consistent with a many-sublattice magnetic structure and corresponding additional exchange modes of magnetic resonance.