The effects of Ag underlayer and Pt intermediate layers on the microstructure and magnetic properties of epitaxial FePt thin films
Introduction
The popularity of the computer internet and mobile digital recording devices has further increased the demand of high recording density to store data. In order to increase storage density, the volume of each bit cell has reduced dramatically and consequently heightened the concerns related to recording medium thermal stability. If recording media are in a state of thermal instability, the media can lose stored data easily. The product of the anisotropy energy (Ku) and the volume of the magnetic switching unit (V) represents the energy barrier to magnetization reversal in the presence of a thermal fluctuations. If KuV/kT<∼60, the magnetic medium is considered thermally unstable (T: temperature and k: Boltzmann constant) [1]. At high recording density, the volume of the magnetic switching unit (V) becomes smaller. Hence, it is essential to search for new materials that have higher anisotropy energy (Ku) to continue to increase the magnetic recording density.
L10 FePt is known to have an anisotropy energy (Ku) 15 times higher than the current HCP Co magnetic recording material [2]. It is this large magnetocrystalline anisotropy that has drawn considerable attention to the L10 FePt thin films as a potential high-density magnetic recording material [3]. FePt has FCC phase above 1300°C [4]. At temperatures below 1300°C, the FePt transforms to the L10 phase, which has its c-axis slightly smaller than its a- and b-axis as shown in Fig. 1. The easy axis of the L10 FePt phase is along the c-axis direction. The FePt thin film deposited directly onto an amorphous substrate tends to have the FCC structure. This FCC FePt thin film is (1 1 1) textured because the (1 1 1) plane is its close-packed plane. After the FCC (1 1 1) textured FePt thin film is annealed and becomes L10 ordered, the easy axis is tilted 35° out of the film plane. This was shown by Ristau et al. where FePt thin films deposited onto glass substrates were FCC disordered and (1 1 1) textured [5]. In that work, the FePt film had to be annealed at a temperature greater than 550°C in order to obtain the L10 ordered phase [5].
In order to use FePt as recording media, the L10 FePt ordering temperature needs to be reduced and the easy axis should be either perpendicular or parallel to the film plane for perpendicular or longitudinal recording, respectively. In other words, the FePt thin film has to be (0 0 1) textured to function as perpendicular media or (1 0 0) textured to function as longitudinal media.
In this work, underlayers are used to induce the proper texture in FePt thin films and to align the FePt easy axes. We take advantage of the moderate lattice mismatch (7.1%) between Ag and FePt (Fig. 2) and use Ag (0 0 1) underlayers to induce the properly oriented FePt epitaxial films. As reported by Yang et al. [7], single crystal Si (0 0 1) substrates can be used to induce epitaxial growth of Ag (0 0 1) thin films based on the orientational relationship of Ag (0 0 1) [1 1 0]∥Si (0 0 1) [1 1 0]. Furthermore, the lattice misfit of FePt with Pt (2.6%) is even smaller than that with Ag (7.1%). Hence, a Pt intermediate layer has been deposited between the FePt and Ag layers to reduce the film to film lattice misfit. It can be seen that FePt has the cube-on-cube orientational relationship with both Ag and Pt as shown in Fig. 2. In order to further understand the effects of using Ag underlayers and Pt intermediate layers, the microstructural and magnetic properties of the FePt thin films using Ag underlayers as well as Pt intermediate layers were investigated.
Some of the results shown in this paper have been briefly discussed in a previous paper published by the same author [8]. However, this paper is intended to give a very detailed discussion on the effects of the Ag underlayer as well as the Pt intermediate layer.
Section snippets
Experiment
There are four variables studied in this work. These four variables are FePt thickness, Ag thickness, substrate temperature and the use of intermediate layer. To be able to focus on the texture study and exclude the effects of grain size, thin films were all deposited onto single crystal Si (0 0 1) substrates. To remove the oxide layers, the Si (0 0 1) substrates were hydrofluoric acid (HF)-etched [7].
Thin films were deposited by RF diode sputtering in a Leybold-Heraeus Z-400 system. The base
Results and discussion
First, the epitaxial relationships between the FePt magnetic layer, Ag underlayer and the Pt intermediate layer are discussed. Secondly, the results of the effects of FePt thickness, Ag thickness, substrate temperature and the Pt intermediate layer on the FePt microstructure are presented, followed by a discussion of the mechanism of FePt L10 ordering and texture development. In this section, some hypotheses are proposed to explain the microstructure results at various FePt thickness, Ag
Conclusion
FePt thin films directly deposited onto HF-etched single crystal Si substrates are (1 1 1) textured and FCC disordered. On the other hand, the use of Ag underlayers has been shown to induce epitaxial growth of the FePt thin film. Because Ag has a slightly larger unit cell than FePt, the in-plane direction of the FePt unit cells were stretched by the Ag underlayers during epitaxial growth. This causes a contraction of the FePt unit cells along the plane normal direction, which enhances the L10
Acknowledgements
The authors gratefully thank Dr. B. Lu and Dr. T. Klemmer in Seagate Research for valuable discussions. This research is supported by CMU Data Storage Systems Center under an NSF grant No. ECD-89-07068.
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Enhancement of magnetic anisotropy for L1<inf>0</inf>-(0 0 1) FePt films grown on SrTiO<inf>3</inf> substrate
2016, Chemical Physics LettersCitation Excerpt :Dong et al. showed that large tensile strain resulted in large Ku for FePt on MgO compared with that on SrTiO3 [12]. Other experiments also have proved that PMA of L10-FePt film decreases with increasing the substrate lattice parameter, such as silver [17] or platinum [18,19]. Therefore, the contradictory results make the effect of strain on PMA more confused by now.
Evolution of microstructure and residual stress on L1<inf>0</inf> ordering in FePt thin films with different initial stress states
2016, Journal of Magnetism and Magnetic MaterialsCitation Excerpt :Taking practical manufacturing into consideration, the annealing or deposition temperature (>500 °C) is still too high. Many approaches, therefore, have been attempted to reduce the ordering temperature, such as off-stoichiometry [5], third element addition [1,6], multilayer process [7], underlayer insertion [8–10], ion irradiation [11], etc. Besides, the fundamental aspect on underlying disordered–ordered phase transition is of interest and importance, which has been explored by using differential scanning calorimetry (DSC) [12,13], x-ray diffraction (XRD) [14], and extend x-ray absorption fine structure (EXAFS) [15].
Thickness-dependent (001) orientation and surface morphology of rapid-annealed FePt thin films on a glass substrate
2015, VacuumCitation Excerpt :Therefore, numerous efforts have been performed to obtain a highly ordered L10 FePt thin film with an (001) texture along the film normal, which can be roughly divided into two groups: (i) epitaxial and (ii) non-epitaxial growth. Epitaxial growth is the most frequently used strategy to prepare L10 FePt films with (001) texture on preheated substrates [1–9]. For simplicity, a single-crystal MgO (100) substrate has been adopted to enhance FePt (001) orientation [1–3].
The structural transition from epitaxial Fe/Pt multilayers to an ordered FePt film using low energy ion beam sputtering deposition with no buffer layer
2014, Thin Solid FilmsCitation Excerpt :The other is to lower the L10 FePt ordering temperature to avoid grain growth. In the literatures, several selected substrates and buffer layers, such as MgO [4], Ag [5], Pt [5–8], CrMo [9], and CrRu [10] have been used to grow the L10 FePt film with (001)-preferred orientation. The buffer layer was used to reduce the lattice mismatch between the FePt film and the substrate to enhance the growth of FePt (001) orientation and lower the ordering temperature.