Structural origin for low-temperature relaxation features in magnetic nanoparticles

Abstract

In addition to superparamagnetic relaxation associated with coherent spin reversal, magnetic nanoparticles often also exhibit additional relaxation features in the magnetic dissipation at low temperatures. Our studies show that the incorporation of boron, gadolinium and lanthanum into iron oxide (Fe3O4) nanoparticles substantially enhances these low-temperature magnetic relaxation properties. The structural and morphological studies of these nanoparticles were conducted using x-ray diffraction, transmission electron microscopy and Raman spectroscopy. The doped samples have retained the crystal structure of the parent Fe3O4 nanoparticles, although the shape and size of some of the nanoparticle samples have changed. Using ac magnetic susceptibility measurements, we parameterized the low-temperature magnetic features, with the amplitude of the associated magnetic relaxation showing a dramatic increase for certain dopants. The enhanced frequency-dependent magnetic relaxation features can be attributed to structural, rather than magnetic, defects in these doped Fe3O4 nanoparticles. These results strongly suggest that the low-temperature magnetic relaxation typically observed in magnetic nanoparticles is a single-particle effect produced by structural defects and is not significantly influenced by interparticle interactions.