Abstract
Ferromagnetic nanoparticles in the 10-14 nm size range are examined for their size and interaction dependent magnetic properties. From X-ray magnetic circular dichroism the orbital-to-spin magnetic moment ratio is determined and found to decrease significantly with particle size. This is in accordance with previous complementary studies on smaller particles and highlights the difficulty of fitting to a simple core-shell model. Vibrating sample magnetometry experiments on samples with more than 1000 particles per square micron show a wide distribution of blocking temperatures from 50 to greater than 650 K. This is attributed to the dipole-dipole magnetic coupling forces between particles. The blocking temperatures show an unexpected negative correlation with increasing particle density.
Highlights
Almost uniquely, micro and nanoscale magnetic particles have held cross-disciplinary interest for leading physicists, chemists and biologists alike for more than half a century [1,2,3]
To test the predictions of enhanced orbital moments in large nanoparticles, six samples with various average particle diameters D were deposited as described in the previous section, and measured with X-ray magnetic circular dichroism (XMCD) spectroscopy
The obvious model to fit to these data is a core-shell type structure whereby a central core of atoms have bulk-like moments, and a thin shell of surface atoms, with reduced coordination numbers, experience enhanced orbital moments
Summary
Micro and nanoscale magnetic particles have held cross-disciplinary interest for leading physicists, chemists and biologists alike for more than half a century [1,2,3]. Several studies in recent years have considered the enhanced orbital moment predicted [12, 13] and observed [14, 15] in sub 12 nm diameter single domain particles. For magnetic nanoparticles the orbital moment is expected to be enhanced at the surface atoms due to reduced coordination number. This compresses the d-band, enhancing spin (ms) and orbital (ml) moment through the spin-orbit (SO) coupling mechanism. We will consider the mechanism for enhanced orbital moments in light of the data presented in this paper. In the second part of this paper we use a superconducting quantum interference device (SQUID) VSM to investigate the anisotropies and interactions of densely packed nanoparticles. We find large in-plane anisotropies due to the dipole-dipole coupling forces between particles and a surprising negative correlation between anisotropy and particle density
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