Towards an understanding of microstructure of patterned FePt dots by magnetometry using pulse fields

• Patterned FePt dot arrays are studied by XMCD and MOKE with pulse fields.
• Results on varied timescale of the applied field indicate the dot microstructure.
• Hard magnetic grains are in the L10-phase with perpendicular cell c-axis.
• Hard magnetic dots with PMA fields around 90 kOe are found in the 100 and 30 nm dots.
• Exchange coupling between the hard and soft grains degrades the soft dot coercivity.

FePt nanodot arrays are patterned and investigated by X-ray magnetic circular dichroism measurements and magneto-optic Kerr effect magnetometry combined with pulse magnetic fields. The experimental results on varied timescale of the applied field are analyzed by Sharrock's formula, showing the emergence of hard and soft magnetic FePt grains with distinct perpendicular anisotropy. While the hard grains in L10-phase can construct the FePt dots with intrinsic perpendicular anisotropy fields around 90 kOe, the exchange coupling between the hard and soft grains in other dots degrades the dot perpendicular coercivity and widens the array switching field distribution, in both the 100 and 30 nm dot arrays. The dot size dependence of the proportion of the hard dot in the array demonstrates that the soft grains originate from the FePt grains of L10-phase with large c-axis misaligning and of fcc phase inside the dots, and the ion etching effects are insignificant.