Nanostructure and Magnetic Properties $L$1$_{0}$ FePt Films with Additions of Au and Cu

Non-epitaxially grown $L$1$_{0}$ FePt:Au and FePt:Cu films have been fabricated and investigated.~ All films are initially deposited with the structure [FePt/X]$_{n}$ and have individual layer thicknesses from about 0.1 nm to 1 nm. The $L$1$_{0}$ phase is achieved by post-deposition annealing at temperatures from 500 to 600 \r{ }C for varying times.~ XRD and TEM show that Cu enters the $L$1$_{0}^{ }$lattice whereas Au segregates at the grain boundaries. Both types of films exhibit a decrease in $M_{s}$, due to magnetic dilution. The coercivity ($H_{c})$ increases and decreases with the addition of Au and Cu, respectively. These changes are due to reduced anisotropy (Cu) and to reduced inter-granular exchange coupling (Au). In the FePt:Au films, MFM shows a decrease in magnetic coherence length (L$_{M})$ from 90 to 74 nm and the $M-H$ slope $\alpha $ = (d$M$/d$H)_{Hc}$ decreases from 5.7 to 0.9 for Au contents from zero to 32 vol{\%}. A simple interaction model quantifies these trends by considering that interparticle exchange cooperatively enhances both $\alpha $ and L$_{M}$. In the FePt:Cu films, the addition of Cu yields a decrease in Curie temperature (574 K at 20 vol{\%}). Mean-field calculations qualitatively reproduce this decrease in $T_{c}$ but indicate deviations from random solid-solution behavior. - This research is supported by INSIC, NSF-MRSEC and NCMN.