Temperature-Dependent Growth and Magnetic Characterization of FePt Thin Films for Advanced Data Storage Applications

This content is from a 2025 Stanford Advanced Materials College Scholarship submission by Frank Efe.

Abstract

Artificial Intelligence (AI) continues to reshape modern technology, placing high demands on data processing and storage capabilities. Improving the speed and capacity of electronic data storage systems, particularly hard disk drives (HDDs), is essential to meet these demands. Iron-platinum (FePt) thin films have emerged as promising materials due to their exceptional properties, such as high magnetic anisotropy, strong magnetization, large coercivity, and high thermal and chemical stability. These qualities make FePt thin films ideal candidates for advanced storage technologies, including Heat-Assisted Magnetic Recording (HAMR), which is designed to significantly enhance HDD data density. While FePt has been widely studied, a notable gap remains in understanding the mechanism behind the double magnetic switching behavior observed when these films are deposited on silicon substrates. This research explores the synthesis and characterization of FePt thin films grown on glass, silicon, and oxidized silicon substrates at room temperature, 250 °C, and 450 °C using DC magnetron sputtering. Surface morphology and crystalline structure were examined using Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD), while magnetic characteristics were assessed through Magnetic Force Microscopy (MFM) and Vibrating Sample Magnetometry (VSM). Investigating the impact of growth temperature on the structural and magnetic properties of FePt films provides valuable insight for tailoring their performance in next-generation data storage systems and industrial applications.

Introduction

Neodymium alloy films have been studied extensively and widely employed for data storage applications over the years (Emmelius et al., 1989; He et al., 2022). However, because they are rare earth elements, they are expensive and easily demagnetize at very high temperatures, with little information on their electrical and magnetic properties for device fabrication (Baloni et al., 2023; Shkir et al., 2022; Yumnam et al., 2020). Ferromagnetic iron alloy films have significantly increased memory storage applications due to their well-defined structure and intriguing magnetic properties. Several studies have investigated the fascinating features of binary iron alloy thin films for device applications such as spintronics, permanent magnets, and magnetic recording media (Appel et al., 2019; Krupinski et al., 2019; Preller et al., 2020).

Among the binary iron alloys, iron platinum (FePt) films have exceptional magnetic properties such as high magnetic anisotropy, exchange coupling features, double-switching phenomena, thermal and chemical stability, and much more. These are significantly influenced by their growth conditions, such as temperature, growth time, and gas flow rate. As a result, choosing the right growth condition is critical for achieving suitable magnetic characteristics of FePt thin films (Suzuki et al., 2021). To enhance the data storage capacity of memory data storage devices, the bit alignment of magnetic recording must be changed from longitudinal to perpendicular alignment, as seen in the heat-assisted magnetic recording. However, current research is being conducted to grow high texture and associated perpendicular magnetic anisotropy in FePt thin,films (Liu et al., 2022; Shen et al., 2018; Yang et al., 2019).

Exchange bias coupling between the hard and soft phases of FePt thin films arises from the interdiffusion of transfer contact at the grain boundary and magnetostatic coupling caused by stray fields present in the hard phase (Singh et al., 2018). Depending on the growth conditions, FePt films can have two phases: cubic phase and ordered L1₀ phase with a randomly oriented grain structure. Unlike the granular L1₀ FePt films, there is an increase in the ferromagnetic resonance of the film at high temperatures. Thermal treatment has been shown to increase the perpendicular magnetic anisotropy of FePt films, resulting in increased coercivity and enhanced areal density for data storage applications (Li & Wang, 2022; Liu et al., 2022). Furthermore, increased temperature above a certain temperature may result in undesirable grain formation due to nanoparticle clumping (Goyal et al., 2019). In addition, Vashisht et al., (2021) co-deposited FeCo/FePt multilayer films on Si substrates, showing an upsurge in the crystal size of FePt grains after annealing, as well as confirmation of soft-phase magnetic behavior. The pinning dominated by domain walls is responsible for the increase in coercivity in the out-of-plane axis.

Sample preparation and Experimental details

FePt thin films were deposited using DC magnetron sputtering onto 5 × 5 mm glass substrates at substrate temperatures of room temperature (23 °C), 250 °C, and 450 °C. The glass substrates were ultrasonically cleaned in acetone for 90 minutes at 25 °C to remove surface contaminants, followed by air drying. Before deposition, substrates were preheated at 100 °C for 5 minutes to enhance adhesion. The heater was mounted within the sputtering chamber, which was evacuated to a base pressure of 10-7 Torr. Deposition was carried out at 5 mTorr argon pressure and 50 W gun power for 15 minutes, with a constant target-to-substrate distance of 40 cm. Following each deposition, the system was cooled to room temperature. These growth parameters were consistent with those reported in related studies (Alqhtany, 2017; Efe, 2023; Lisfi et al., 2017).

Results and Discussions

The surface morphology and topography of the demagnetized films were analyzed using atomic force microscopy (AFM), while magnetic domain structures were assessed using magnetic force microscopy (MFM). X-ray diffraction (XRD) was used to investigate the crystallographic structure and phase composition, and vibrating sample magnetometry (VSM) was used to evaluate magnetic properties under in-plane fields ranging from –20 to 20 kOe.

AFM revealed that at 23 °C, films showed grain clustering with some cracks and voids, suggesting poor surface diffusion. At 250 °C, grains appeared more homogeneously distributed, forming spherical features without visible cracks. At 450 °C, a uniform, crack-free surface with an average roughness of 10 nm was achieved. These results indicate that increasing the substrate temperature enhances the microstructural quality of FePt films, making them promising for device applications, particularly in magnetic storage technologies. The observed trends align with previously reported findings (Skok et al., 2022; Weisheit et al., 2004).There was no magnetic force detected between the film and the cantilever's tip, as seen in Figure 2a. This is due to the low deposition temperature of 23 ℃, which is insufficient to align the magnetic moment. As a result, at room temperature, the film features a soft phase of disordered cubic FCC phase structure properties. When the temperature was increased to 250 ℃, an island structure of the magnetic domains was discovered, which are randomly oriented out-of-plane, as illustrated in Figure 2b. Furthermore, as the substrate temperature increased to 450 ℃, there was an increase in magnetic domain contrast in the film’s magnetic image, which consists of black and white contrast that represents magnetic structures with strong interactions of either positive or negative response with the cantilever tip, as shown in Figure 2c. These domains were found to typically point to the out-of-plane component of magnetization.

Figure 1(a-c): AFM image of the synthesized FePt films showing the topography of the grains as
substrate temperature increased from (a) 23 ℃, (b) 250 ℃, to (c) 450 ℃

In addition, the brownish portion of the magnetic domain reflects weak domains, which could be due to magnetic elements with an almost in-plane magnetization easy axis that interact weakly with the cantilever tip. As a result, the film’s entire magnetization structure is altered. This is due to the high perpendicular magnetic anisotropy of the deposited film, where the magnetization direction is aligned up and down within the domain wall. The ordered L1₀ face-centered tetragonal (FCT) structure of the grown films could explain the films’ significant perpendicular anisotropy at higher substrate temperatures (Lisfi et al., 2017).

Figure 2 (a-c): MFM image of the synthesized FePt thin film showing the magnetic domains at
(a) 23 ℃ (b) 250 ℃ (c) 450 ℃

Conclusion

FePt thin films have been successfully deposited on a glass substrate at three different temperatures: room temperature, 250°C, and 450°C. Increased deposition temperature leads to an increase in grain growth with no voids and pinholes, as observed by the AFM and SEM. The magnetic force microscopy showed that the magnetic moments are oriented perpendicularly to the plane of the film. As the substrate temperature increases in the closed system containing the inert gas, the magnetic phases of the soft phase fcc-FePt film, whose atoms are randomly oriented, transition to the formation of an ordered L10 fct-FePt film on the glass substrate.

Recommendations

This study focuses on the synthesis and characterization of FePt thin films, a promising
rare metal alloy, tailored for industrial applications in magnetic data storage, particularly Heat-Assisted Magnetic Recording (HAMR). By optimizing growth conditions through substrate
temperature variation, we enhance the film’s structural and magnetic properties (work in progress),
making them suitable for high-density storage devices. The work aligns with current development
trends in rare metal utilization, addressing global demands for durable, high-performance materials
in electronics. Advancing FePt-based technologies supports the strategic shift toward efficient,
miniaturized, and energy-saving devices in the evolving data-driven industrial landscape.

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