Fabrication and characterization of YIG nanotubes

We have successfully synthesized Y3Fe5O12 (yttrium iron garnet, YIG) nanotubes (NTs) of different diameters using a conventional sol-gel method in anodized aluminum oxide (AAO) templates. An annealing process at 800 degrees C is performed to get a pure phase of YIG. The room-temperature magnetization measurements show that the coercivity decreases significantly as the diameter of nanotubes increases. The temperature dependence of magnetic moments in field-cooled (FC) and zero-field-cooled (ZFC) modes indicates that YIG NTs transform from ferromagnetic-like (FM-like) state to superparamagnetic (SPM) state when temperature decreases (in the region of 310 K-400 K). The Blocking temperature dramatically decreases as the size of YIG NTs reduces. In the FM-like state, the temperature-dependent coercivity in three different samples yields a 1/2 power law of the temperature. The temperature dependence of saturation magnetization in three different samples can be well fitted to Bloch's T-3/2 law. Characterization of ferromagnetic resonance (FMR) is conducted, which indicates the Gilbert damping parameter of YIG NTs is around 7-9 x 10(-3).