FePt-based perpendicular graded films for ultra-high density magnetic recording

L10-FePt alloy is a good candidate material for future ultra-high density perpendicular magnetic recording media (5 - 10 Tbits/in2) because of its high uniaxial magnetocrystalline anisotropy (Ku = 4 -7 MJ/m3), which enables it to be thermally stable even at grain sizes down to 3 nm [1]. However, the high coercive field (m0Hc >= 5.5 T) of these materials makes the writing process difficult using currently available magnetic write heads [2]. To combine thermal stability and write-ability, different technological approaches have been proposed, such as exchange coupled media (ECM) [3], energy assisted media and bit patterned media. Among them, ECM are very promising candidates since they can be easily introduced in the disk manufacturing process. In the so-called exchange-spring media [4] a soft layer is strongly exchange coupled at the interface with a hard phase: the soft layer ensures low write fields, while the hard phase provides thermal stability. A better approach for reducing the writing field, while retaining the high thermal stability, is through the use of so-called graded media [3], a new class of nanocomposite material proposed for ultrahigh density recording media. Graded media with a spatially varying anisotropy Ku(z) offer improved characteristics in comparison to homogeneous, constant K hard/soft bilayer media. Here we report on the investigation of magnettic properties of L10-FePt/graded-FePt(L10÷A1) ECM with perpendicular magnetic anisotropy[5]. Ramping the substrate temperature during the deposition from high to low values affects the transformation from the soft fcc A1 phase to the hard fct L10 phase, thus changing the order degree and then the anisotropy constant. Using this procedure we fabricated a "phase" graded tetragonal-cubic structure where the soft/hard phase ratio changes along the film thickness depending on the final deposition temperature. Such "phase graded media" appear the same features as "averaged stacked" conventional graded systems [6]. Indeed, magnetic measurements revealed a significant reduction of the switching fields without affecting thermal stability making this system suitable for ultra-high density magnetic recording. [1]D. Weller et al., IEEE Trans. Magn. 36 (2000), 10 [2]Y. K. Takahashi and K. Hono, Appl. Phys. Lett. 83 (2004), 383 [3]D. Suess et al., JMMM 321 (2009), 545 [4]G. Varvaro et al., New Journal of Physics 14 (2012), 073008 [5] V. Alexandrakis et al., J. of Appl. Phys. 109 (2011), 07B729 [6]A. J. Lee et al., APL 98 (2011), 222501