Tunable magnetic properties of metal/metal oxide nanoscale coaxial cables

Abstract

We report the synthesis and magnetic characterization of metal/metal oxide one-dimensional heterostructures with a radial geometry---coaxial nanocables. High-density arrays of cobalt and magnetite- $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ based nanocables with the diameter of $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and length of $\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ have been synthesized within the pores of anodized aluminum oxide membranes using a supercritical fluid inclusion-phase technique. All investigated heterostructures demonstrate ferrimagnetic/ferromagnetic properties at room temperature. We show that the magnetic properties of cobalt-magnetite nanocables can be tuned by varying the thickness of the sheath and core layers. The exchange bias effect, due to formation of $\mathrm{CoO}$ thin layers at the nanocable interfaces, was observed at low temperatures. The strength and manifestation of this effect depends on the structural characteristics and oxidation state of the Co layer. We show that the vast increase of the coercive field (up to 450 times) at low temperatures in samples containing the large fraction of magnetite is caused by a governing influence of the temperature-dependent magnetoelastic term of the anisotropy.