| Hard magnetic nanoparticles have many applications, such as high density recording media, and magnetic nano-devices. Finite size effects dominate many properties of magnetic nanoparticles. In this work, Tb, Gd, and SmCo 5 nanoparticles with different sizes (< 10 nm) embedded in Cr and W matrices were fabricated via annealing multilayer precursors made by magnetron sputtering. Their structural and magnetic properties were studied with an emphasis on the dependence on particle size and the matrix used.;In Tb and Gd nanoparticles, their thermomagnetic properties (M vs. T) and low-temperature hysteresis behavior were investigated. The Tb nanoparticles showed only one magnetic transition on their M vs. T curves, comparing with bulk samples which have two transitions. This magnetic transition was found to be a true magnetic ordering transition instead of due to superparamagnetic behavior. It was found that both the Curie temperature and the coercivity at T = 5 K increased with the average particle size. As for matrix effect, it was proposed that Tb particles grew much more slowly but uniformly in W matrix than in Cr matrix. The properties of Tb and Gd nanoparticles were compared, and most of the differences could be attributed to the huge differences in their magnetocrystalline anisotropy constants.;As for the SmCo5 nanoparticles, their magnetocrystalline anisotropy and hysteresis behavior at different temperatures were studied with a focus on the room-temperature behavior. The magnetocrystalline anisotropy constants were found to be much lower than bulk values. Giant coercivities were observed. The shapes of their hysteresis loops were related to the magnetic interactions among the SmCo5 grains. The mechanism for magnetization reversal was investigated via initial magnetization curves, Hc vs. Happ plot, and magnetic viscosity measurements. The reversal was found to be by magnetization rotation. |
