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Journal Article
Collective magnetic response of CeO2 nanoparticles
2016
Overview
The magnetic response of nanoparticles made from wide-bandgap oxides that don’t contain any magnetic cations is somewhat of a mystery. Experiments with CeO
2
suggest that the origin may be due to vacuum fluctuations.
The magnetism of nanoparticles and thin films of wide-bandgap oxides that include no magnetic cations is an unsolved puzzle
1
. Progress has been hampered by both the irreproducibility of much of the experimental data, and the lack of any generally accepted theoretical explanation. The characteristic signature is a virtually anhysteretic, temperature-independent magnetization curve that saturates in an applied field that is several orders of magnitude greater than the magnetization. It would seem as if a tiny volume fraction, ≲0.1%, of the samples is magnetic and that the energy scale is unusually high for spin magnetism. Here we investigate the effect of dispersing 4 nm CeO
2
nanoparticles with powders of γAl
2
O
3
, sugar or latex microspheres. The saturation magnetization,
M
s
≍ 60 A m
−1
for compact samples, is maximized by 1 wt% lanthanum doping. Dispersing the CeO
2
nanopowder reduces its magnetic moment by up to an order of magnitude, and there is a characteristic length scale of order 100 nm for the magnetism to appear in CeO
2
nanoparticle clusters. The phenomenon is explained in terms of a giant orbital paramagnetism that appears in coherent mesoscopic domains due to resonant interaction with zero-point fluctuations of the vacuum electromagnetic field. The theory explains the observed temperature-independent magnetization curve and its doping and dispersion dependence, based on a length scale of 300 nm that corresponds to the wavelength of a maximum in the ultraviolet absorption spectrum of the magnetic CeO
2
nanoparticles. The coherent domains occupy roughly 10% of the sample volume.
Publisher
Nature Publishing Group UK,Nature Publishing Group
Subject
