Comparison of Magnetic Performance and
Magnetization Reversal Behavior between
Perpendicular Graded Anisotropy L10-FePt Films
Processed by Three Different Approiaches
Jen-Hwa Hsu1*, Yi-Hung Lin2, Po-Cheng Kuo2
1
2
Department of Physics, National Taiwan University, Taipei, 106, Taiwan
Graduate Institute of Material Science and Engineering, National Taiwan University, Taipei, 106 Taiwan
*
Corresponding author’s e-mail: jhhsu@phys.ntu.edu.tw, Tel.: +886-33665162; Fax: +886-3366-5892
Abstract
Perpendicular FePt-graded films were grown by three
approaches: gradient-temperature (Tg), working pressure
(Pg) and composition (Cg). Substantial reduction of
perpendicular coercivity was found in all cases, but with
different magnetization reversal mechanisms. Considering
reduction of the switching field and magnetic reversal
process, Pg-FePt film has the best gradient performance.
Keywords: magnetic films and multilayers; L10 FePt alloys;
magnetic perpendicular anisotropy; magnetic recording;
sputtering
Introduction
High magnetocrystalline anisotropy
constant (Ku) materials have been received much
attention due to the requirements of future ultra-high
density magnetic recording technology. L10 FePt thin
film is considered as one of the potential candidates
because of its large Ku of ~7x107 erg/cm3 and high
chemical stability. However, the material with such high
Ku usually requires a higher switching field in the
writing process. Several techniques have been proposed
to solve the writability issue. Among them one possible
way is to introduce gradations in Ku along the film plane
normal. In this study, approaches to realize graded L10FePt films, by means of gradient-temperature (Tg), working pressure (Pg) and -composition (Cg) have been
employed. The structures, magnetic properties and
magnetization reversal process were investigated and
compared in terms of their suitability in future recording
media.
superconducting quantum interference devise (SQUID)
and vibrating sample magnetometer (VSM) at room
temperature.
Results and Discussions
Structural studies revealed island-like
morphology with (001)-texture for all the graded films,
as similar to that of 5-nm thick L10-FePt hard layer,
while magnetic measurements on the graded layers
showed a significant reduction in their Hc values with
respect to L10-FePt hard layer (20 kOe). The Hc values
of 7.8, 7.2 and 6.1 kOe were obtained for the Cg-, Tgand Pg-L10 FePt layers, respectively (Fig. 1(a)). In the
case of L10-FePt, upon removal of the applied field (Hr)
during demagnetization process, only slightly increased
magnetization can be found, indicating the switching
mechanism is dominated by irreversible switching. For
Cg-FePt, the magnetization increases sharply upon
removal of the applied field (Fig. 1(b) and (c)). This
increment in the magnetization is due to the presence of
reversible magnetization switching arising from the
non-coherent rotation. Both Tg- and Pg-FePt have the
similar reversal behavior that indicates a large springback effect but without rigid coupling at the interface of
hard/graded layers. Therefore, considering the reduction
of the switching field and magnetic reversal behavior,
Pg-FePt film may exhibit best gradient performance
among these three structures.
Experimental Procedures
Samples were fabricated by dc
magnetron sputtering. Three types of graded structures
were all firstly grown onto a 5-nm-thick FePt hard
layer/10-nm-thick MgO underlayer. The composition of
the FePt layer was controlled by adjusting the sputtering
powers of the Fe and Pt targets, and was determined by
energy dispersion spectroscopy (EDS) to be Fe55Pt45. Xray diffractometer (XRD) was used to investigate the
structure of the films. The microstructure was studied
by transmission electron microscopy (TEM). The
magnetic
properties
were
determined
by
Fig. 1. (a) Out-of-plane hysteresis loops for the L10-FePt
and three graded FePt structures. The insert shows their
corresponding in-plane hysteresis loops. (b) Reversible
and (c) irreversible magnetization as a function of the
applied reversal field (Hr) for L10-FePt, Cg-FePt, TgFePt, and Pg-FePt.