The L10 typed FePt granular film with moderate Curie temperature (TC) is a promising candidate as a next-generation technology material for HAMR 1. In HAMR, since laser heat was utilized for decreasing switching field (Hsw) during recording, controlling the temperature dependence of Hsw and its distribution is required. The temperature where Hsw = 0 without and with taking into account of thermal fluctuation are TC and blocking temperature (TB), respectively. Therefore, TB/ TC and ΔTB/ TC show thermal fluctuation indexes.
In this presentation we propose a method to evaluate TB and its distribution by measuring thermal hysteresis of remanent magnetization (Mr) for FePt granular films. Fig. 1 shows a schematic diagram of TB evaluation method and its distribution. First, the magnetization of all the magnetic grains in a granular film is aligned in the same direction. As the temperature rises, the number of blocking grains increases, and the magnetization of those grains is frozen in the opposite direction due to dipole field from the surrounding grains. Here the Mr during temperature rise and fall are defined as Mrfor and Mrback, the percentage of blocking grains is expressed as (Mrfor - Mrback) / 2Mrfor, and its temperature dependence corresponds to the cumulative distribution of thermally demagnetized grains. The derivative of the cumulative distribution curve gives the frequency distribution of blocking grains, which corresponds to the TB distribution.
Fig. 2 shows the ratio of the average value of TB (TBave) to TC (TBave / TC) versus ΔTB for FePt granular films with various GBMs, where the minimum and maximum values of the TB variance are defined as TBmin and TBmax, respectively, and ΔTB = TBmax - TBmin. For each temperature, ΔTB varied from 86.4 to 182.7 K, depending on the GBM, and showed negative correlation with TBave/ TC with the present measurement time condition (τ) of thermal hysteresis of 60 s. Based on this result, the ΔTB at τ = 2 × 10-7 s, which corresponds to the recording time of HDD, was calculated to be ΔTB = 18.8- 53.9 K (ΔTB/ TC = 4.49× 10-2- 7.32 × 10-2).
1 D. Weller et al., Phys. Status Solidi A, 210, 1245 (2013).
Fig. 1 Evaluation method of blocking temperature.
Fig. 2 TBave / TC versus ΔTB for FePt granular films with various GBM.