Magnetic properties in FePt-based nanoparticle system

Abstract

Perpendicular magnetic recording media designs $require^{1-4}$ high perpendicular anisotropy ($K_u$) energy to resist the large demagnetization effects from the perpendicular recording geometry and to maintain thermal stability with ever-smaller grain volumes. The thermal stability is dependent on the factor $K_uV/k_BT$, where V denotes the grain volume. Clearly, the reduction in grain volume required for noise reduction can be balanced by the increase in anisotropy, as long as the coercivity of the medium remains within the tolerable limit. As a result, it is essential to consider high anisotropy materials such as FePt. $L1_0$ ordered equiatomic FePt alloy thin $films^5$ have high saturation magnetization $(~ 1100 emu/cm^3)$ and large $K_u(7 \times 10^7 ergs/cm^3)$. So, we obtained ordered FePt alloy thin films and studied the variation of magnetic properties as well as structural change. $L1_0$ ordered FePt thin film which was obtained from high temperature annealing showed out-of-plane magnetic anisotropy both in the low and high temperature. However, disordered FePt exhibits in-plane anisotropy in the room temperature and spin glass like phase in the low temperature below 50 K. The effects of film thickness on the degree of ordering and the switching field distribution have been studied. As the film thickness increased, the degree of ordering decreased and the switching field distribution got worse by the existence of disordered region and irregular granular structure. For the aspect of low media noise, the optimum film thickness is 7 nm where we can get the most narrow switching field distribution. To reveal the domain reversal behavior of FePt thin films, static and dynamic domain patterns were investigated. Static domain patterns are affected by both the magnetic anisotropy and surface morphology as the substrate temperature increases. However, domain evolution patterns are affected by only magnetic anisotropy dominantly and changed from nucl...