Micromagnetics of FePt-L10 Media with Thermally Insulating Magnetic Grain Boundaries

In heat assisted magnetic recording (HAMR), grain-to-grain Curie temperature variation presents a practical limit to grain size reduction hence, the scaling of recording area density capability (ADC), for the advancement of granular FePt-L10 media [1] [2]. At small grain sizes, a reduction of grain size yields an increase of surface-to-volume ratio, thereby, a reduction of Curie temperature [3]. The resulting grain size dependence of Curie temperature gives rise to the cause of the Curie temperature variation due to unavoidable grain size distributions with today’s film fabrication technique. Aiming at this grand challenge, a recent experimental study has shown a potentially practical solution by successfully fabricating granular FePt-L10 media with thermally insulating magnetic grain boundaries [4]. In this paper, we present a systematic modeling study on the micromagnetic recording characteristics of such media. In the study, both the FePt-L10 grain core and a thin layer of ferromagnetic grain boundaries are meshed and modeled using dynamic Landau-Lifshitz-Bloch (LLB) equations. The room temperature anisotropy field of the FePt core is assumed to be $H_{k}=80$ kOe and zero magnetocrystalline anisotropy is assumed for the soft magnetic grain boundary material. Gaussian geometric thermal profile is assumed along with realistic media motion are used to simulate practical recording process. Figure 1(a) shows the $M_{s}(\mathrm {T})$ curves for both the FePt grain core and the insulating magnetic grain boundary material assumed. Note that the assumed magnetic grain boundary has a Curie temperature significantly higher than that of the FePt core. For all the results presented in this digest, the diameter of the FePt core is assumed to be 6nm with 1nm thick magnetic grain boundary. The modeling simulation shows that the ferromagnetic grain boundary provide significant assist in the magnetization process of the FePt grain core during recording. With the grain boundary coupled to the FePt core, the magnetization process of the core becomes significantly more deterministic. Figure 1(b) shows the switching probability of the FePt grain core as a function of recording field amplitude with the medium moving at a speed of 15 m/s and a spatial thermal gradient of 8 K/nm. The case “Not Coupled” indicating no exchange coupling between the grain boundary and the FePt core is plotted for comparison. With the magnetic grain boundary exchange coupled to the core, full saturation recording can be reached at lower field values in comparison with the case of non-magnetic grain boundaries. The effect is even more enhanced for smaller size of FePt grain cores, which will be presented in the full paper. Please note the “Not Coupled” case is essentially the same as the case with non-magnetic grain boundaries. Figures 2(a) and 2(b) show the writing of a transition when the head field direction is reversed (black dashed lines). The case with the exchange-coupled grain boundary and FePt core shows more rapid and better defined magnetization switching with the magnetizations of the core and grain boundary switching together. Whereas for the case that the core and grain boundary are not coupled, the switching of the FePt core is much more gradual with noticeably fluctuations. Also note that the grain boundary actually magnetizes a little faster than the FePt core does. In summary, the magnetic grain boundary exchange coupled to the FePt core provides significant assistance to the magnetization process of the FePt core during recording, both in heat assisted magnetization process and in transition magnetization process with field reversal. Therefore, it is important for the Curie temperature of the grain boundary material to be higher than that of the FePt core for the obvious reasons. The level of grain boundary magnetization at the recording temperature is important and the higher the magnetization level, the more enhanced assisting effect will be during recording. It is also important to note that the exchange coupling between the grain boundaries of adjacent grains needs to be minimized in practice. The experimental study has shown that this objective could be achieved in practice [4]. In conclusion, granular FePt media with a thin layer of soft magnetic grain boundaries that are exchange coupled to the FePt cores not only can suppress or eliminate grain-to-grain Curie temperature variation, but also can assist the magnetization process, especially for small size grains, during recording in HAMR. The more deterministic magnetization processes helps to suppress thermal-induced transition jitter during recording process, consequently enhancing the scalability of both the linear recording density capability and the overall ADC.